[organophosphorus chemistry] organophosphorus chemistry volume 17 || ylides and related compounds

8 Ylides and Related Compounds

BY B. J. WALKER

1 Introduction Further detailed studies of the mechanism of the Wittig reaction and of a-lithio ylides have provided interesting new information. However, in neither case has a full understanding been achieved.

2 Methvlenephosphoranes 2.1 Preparation and Structure.-An X-ray structure of 2-(fluorenyl- idene)-l-(triphenylphosphorany1idene)-ethene suggests that it has a significant contribution from structure (1). The diphosphine ylide (2) and the related alkene ( 3 ) have been compared using n.m.r. spectroscopy and g-ray structural analysis in an attempt to understand the steric and electronic requirements of olefinic and ylidic bonds.' than isopropylidene - ylides would be expected from base treatment of the salts ( 4 ) . 3 However, isopropylidene ylides are formed in every case and this is explained in terms of the less favoured pyramidal geometry of the cyclopropylidene carbanion.

On the basis of PIC, values cyclopropylidene - rather

The dispute over the exact nature and reactivity of a-lithio ylides has intensified. Streitwieser and McDowell have carried out an ab initio SCF-MO study on the parent (a-1ithiomethylene)- phosphorane (5) .' ionic lithium-carbon bond and so would be expected to exist as aggregates in solution. The results predict that (5) would have both increased nucleophilicity a, due to its geometry, increased reactivity towards sterically hindered substrates compared to methylene ylide.

This suggests that the molecule has a largely

Schlosser and his coworkers5 have investigated

316

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8: Ylides and Related Compounds 317

t Ph,P - C ,c =3 \ /

A+ PPr d

( 4 )

H,P=CH Li*

( 5 )

'+ 3-?H2

R I

Phj6-eHLi Ph3P=CH2 PhZP=CHZ

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318 Organophosphorus Chemistry

the reaction of methylenetriphenylphosphorane with lithium alkyls and, on the basis of n.m.r. studies' and the products obtained from hydrogen chloride quenching, suggest that the two predominant reactions are ligand exchange to give (6) and 2-metallation of the phenyl group to give (7); the proportions of these reactions depend on the alkyl-lithium used. These results suggest that the formation of the a-lithio ylide ( 8 ) is at the most a minor pathway. Corey has repeated and published6 experimental details of reactions which he claims, and Schlosser disputes, are possible with the more reactive a-lithio ylide ( 8 ) but not with the normal methylene ylide (9). Corey suggests that a possible solution to the contradictory results is exchange between the various lithiated species observed by Schl~sser,~ but with ( 8 ) still the predominant reacting species: this is supported to some extent by Streitwieserls work.4 ligand exchange species such as (10). However, none of this resolves the quite different reactivities reported by Schlosser and Corey in what appear to be the same experiments. In his most recent publication' Schlosser reports very clear cut evidence that while the a-lithio ylide ( 8 ) can be generated from bromomethylene- triphenylphosphorane (11) by base treatment, it is not formed when methylenetriphenylphosphorane (9) is further reacted with base. However, he uses different conditions to those of Corey and does admit that ( 8 ) formed (albeit only 10%) when ( 9 ) is treated with butyl-lithium in the presence of lithium bromide. We await further developments.

An additional possibility is the enhanced reactivity of

One restriction on the use of the convenient "instant ylide" (dry sodarnide-phosphonium salt) mixtures is that they may only be used for salts containing functional groups of, at most, moderate polarity since vigorous reactions can occur in storage when more

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PhLi ButLi + - P h , k i , B r 6r- + Ph36-CH6r Ph,P-CHLi

( 1 1 ) ( 8 )

R,Cu Li

(12) R = CH=CH2 , 6u Ph ,or CH=CHBu

Ph,P=CHR

(13)

Reagents : i , ButOK ; i i , HCOOEt

i,ii NR Ph,P=C,

CHO (14)

Scheme 1

H0F4 i Ph,P=CHCOR + ArIXz

X,= 0 , F, or (OAc) ,

R = O E t , Ph or p - t o l y l ( 1 5 1

Me3P=C= PPh,

(1 61

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highly acidic protons are available. This has now been partially overcome by coating the sodamide with paraffin or a polymer blend.' This allows dry mixing and storage with phosphonium salts carrying hydroxy and possibly amino groups. The method has been applied to the synthesis of (Z)-alkenols with 97% stereoisomeric purity (reactions of this type require two moles of base). Care must be taken when preparing hydroxyalkylphosphonium salts from hydroxyalkyl halides to avoid contamination with biphosphonium salt. A useful route to ylides and hence alkenes is provided by conjugate addition of nucleophiles to vinylphosphonium salts. However, the addition of alkyl-lithium reagents does not give alkylidene ylides, apparently because elimination of phosphine to give acetylenes is preferred. This problem has now been overcome by the use of the less basic alkyl or aryl cuprates (12).'

An improved procedure for the preparation of a-formyl- alkylidenetriphenylphosphorane (14) has been reported. lo tertiary butoxi.de is added to the ylide (13) before addition of ethyl formate, thereby circumventing the necessity for trans ylidation and increasing the yield of (14, R=Me) from 37 to 84% (Scheme 1). A variety of stable mixed phosphonium - iodonium ylides (15) have been prepared by various methods." analysis suggests that (15) is the major contributing canonical form. Unsymmetrical methyl/phenyl carbodiphosphoranes (ed. 16) have been prepared by standard procedures. More interesting is the formation of the carbodiphosphorane (18) by spontaneous rearrangement of' the dimethylene ylide (17) formed from the corresponding phosphonium salt by transylidation with triethylethylidene y1ide.l' (20)14 have been'prepared and used in the Wittig reaction apparently without racemisation at the chiral centre.

Potassium

An X-ray structural

Optically active ylides ( 19)13 and

Phosphorus esters and amides react with dimethyl acetylene-

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Ph2

OA c 1

( 1 7 )

1

Ph3P* Me P h 3 P v O H 0 H Me

+

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dicarboxylate in the presence of a proton donor to give ylides (21) and/or phosphoranes (22) .I5 2.2 Reactions of Methylenephosphoranes. -Work on the mechanism of the wittig reaction has been reviewed. l6 2.2.1 Aldehydes. - Further investigations of the Wittig reaction continue to reveal fundamental information about the mechanism. A theoretical study of the reactions of the simplest phosphonium and sulphonium ylides with formaldehyde offers an explanation for the observed predominance of olefin formation in the former case and oxirane formation in the latter case.17 for all four possible modes of reaction indicate that while the initially formed oxaphosphetane (23) can pseudorotate easily to (25). thus weakening the P-C bond to be broken in olefin formation, the oxathietane (24) cannot. This results in a much higher activation energy for alkene formation in the latter case and so oxirane formation via (26) predominates. In spite of all the past changes of opinion and disappointments it is tempting to suggest that an understanding of the mechanism controlling Wittig stereochemistry is close at hand. Maryanoff has carried out a detailed kinetic study on the Wittig reaction of butylidene- triphenylphosphorane (27, R=Ph) with benzaldehyde using 31P, 'H, and 13C n.m.r. spectroscopy.18 (E)- and (Z)-alkene formation rates (Is5 and kc) are similar, but k3 is much greater than k4 which leads to oxaphosphetane equilibration from the (assumed) kinetically favoured (28). In a l8salt-freen reaction the phosphetane (28) again predominates initially: however, under these conditions (28) and ( 2 9 ) do not equilibrate betaine reversibility to the ylide and aldehyde. A similar reaction of the butylidenetri-n-butylphosphorane (27, R=n-Bu) under "salt-free" conditions did show betaine reversibility, indeed at -4OOC it was

Energy profiles produced

In the presence of lithium bromide

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X=CHZ 4- HCHO

+ L/ \ X-CH,

I CH,-0-

( 2 6 )

1

X - CH, I I 0- CH,

(23) X = H,P (24) X = H2S

( 2 5 )

4 H,P=O + H,C=CH,

PhCHO + &

(C H , 1 ,M c

Ph

( 2 8 )

puH2)zMe +

R,P =O

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possible to stop alkene formation from (28) and (29) while equilibration of (28) to (29) still occurred, thus allowing a separate kinetic study of this latter process. Perhaps the most important conclusion to be drawn from these results is that the changes in alkene ratios recently obtained l9 by varying phosphorus substituents are due partly to variations in Isl and k2 (the rates of formation of ( 2 8 ) and (29)) and partly to enhancement of betaine reversibility (probably an increase of lc3/k4 through an increase in kc). have been put forward pre~iously.'~ with nucleophilic groups in the alkyl side-chain give enhanced amounts of (E_)-alkene in Wittig reactions. Maryanoff and his coworkers have now made a systematic study of the effect of the type and position of the nucleophilic group on the extent of (E)-alkene formation." ylides react with benzaldehyde to give enhanced (E)-alkene formation up to p 6 , with a maximum when g=1 or 2. Reactions with dimethylamino-ylides (32) give much less enhancement as do all reactions with aliphatic aldehydes. These results, together with a variety of labelling and quenching experiments, lead to the suggesticrn that enhancement of (E)-alkene is again due to enhancement of betaine reversibility. Further support for this explanation comes from the first reported observation (by use of cross-over experiments and 31P n.m. r. spectroscopy) of reversible oxaphosphetane formation in reactions of non-stabilised ylides with aliphatic aldehydes.'l way contain oxido groups in the alkyl side-chain.

This clearly raises doubts over the steric arguments which It is well known that ylides

They find that hydroxy-(30) and carboxylate-(31)

The ylides ( 3 3 ) observed to behave in this

The Wittig reaction of reactive ylides can be carried out --selectively ("salt-free") or trans-selectively (betaine-ylides in the presence of lithium salts). Schlosser has applied the less- used latter reaction to syntheses with o-hydroxyalkyl ylides

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8: YIides and Related Compounds 325

Ph,P=C H(CH,),X Ph,P=CH (CHt),.,O- Li'

(30) X = 0- ( 3 1 ) X=COO-

( 3 2 ) X = NMe2

( 3 3 ) n = 1 of 2

Br- I I

Li Br

i i , i 1 R CHOLi

R ... I I I , i v + I (CH2),CH20H f--- Ph3P-S (CH,),C H,OL i

I

I I

Li Br

Reagents: i , L i C 6 H 5 . L i B r i i t , RCHO; i i i , H C l ; i v , K O B u t

S c h e m e 2

6 u,P- 0

+ Bu,P-

F + C-PBu3

R = A r ( 3 4 )

+ - RCHO

J 6 ~ 3 6 - CFZCHR X-

( 3 6 )

1 NaoH ' H20

PBu,

Bu33$+ H F Ar F ( 3 5 1 Bu-P-0

\

Ar J&:Bu3 F

Bu,P=O + FCH=CHR

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3 26 Organophosphorus Chemistry

(Scheme 2) .22 reproducibility in these reactions (which is a problem familiar to many of us) are the use of self-prepared phenyl-lithium (which unlike the commercial product does contain stoichiometric amounts of lithium bromide) and assuring solubility of the betaine/lithium bromide adduct by adding extra solvent. Wittig reactions of the fluorinated phosphoranium salt ( 3 4 ) give the vinyl salt (36) and, on hydrolysis, vinyl fluorides. 23 reactions is the reversal of stereochemistry through the use of aromatic (mainly (Z)-alkene) instead of aliphatic (mainly (E)-a1kene)aldehydes. An explanation for the predominant formation of (Z)-alkene is presented on the basis of preferential formation of the oxaphosphetan (35) due to charge transfer from the arene ring to the phosphonium centre. The opportunity for such charge transfer is obviously not available in reactions with aliphatic aldehydes.

Two useful tips to improve stereochemical

What is surprising about these

Base-induced ring-opening of thiosubstituted bicycloheptenols (e3.37) to isomeric aldehyde is accompanied by major amount of epimerisation. 24 However, in the presence of methylenetriphenylphosphorane the &-aldehyde can be almost exclusively trapped to give the alkene (38). The thiol ester-stabilised ylide (39) is reported to have several advantages over its oxygen analogue ( 4 0 ) when used in the Wittig reaction. Much higher (g):(z) ratios are obtained from (39) and the a.0-unsaturated thiolesters produced, unlike their oxygen analogues, can be quantitatively isomerised to the ( E ) form by base. The almost unknown 1.1-diiodoalkenes have now been prepared by the Appel reaction using triphenylphosphine- tetraiodomethane and carbonyl compounds. 26 [cyclobut-1-enyl]triphenylphosphonium salt (41) with diethyl phosphonate anion followed by Wittig reactions of the resulting ylide ( 4 2 ) . offers a convenient route to 1.2-bisylidene- cyclobutanes ( 4 3 ) (Scheme 3 ) . 27

The reaction of

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B U ~ OK

Ph3PMe Br-

9 : l

Ph,P=CHCOX E t

( 4 1 )

0 II

Reagents : i , ( Et0 I2PLi ; i i , RCHO

(44)

(42)

Scheme 3

i , i i 1

( 4 3 )

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328 Organophosphorus Chern is try

The difunctionalised dienes (44) have been prepared by the reaction of aldehydes with alkoxyalkyl ylides (to give (E),(z)-alkene mixtures) and with allylic ylides (to give stereospecif ically (B)-alkene) . used to prepare new types of the now familiar n-donors (45) and (46) , various trienes?Oa31 and a large number of o,o-biazulenylpolyenes (e.g. 47). 32

The Wittig reaction has also been

Continuing his investigation of the reactions of azinyl ylides, Schweizer has used33 Wittig reactions of 2-hydrazonopropylidene ylides (48) to prepare a variety of substituted pyrazoles through spontaneous cyclisation of the initially formed l-oxo-3.4-diaza-2.4.6-heptatrienes. The ylides (48) also react with isocyanates to give mixtures of 4,9-dihydropyrazolo[5,1-~]- quinazolines (49) and 2,3-dihydro-1J-imidazo[l,2-~]pyrazol-2-ones (50). 34

Both Wittig and phosphonate olefinations have been carried out on alumina or potassium fluoride and alumina.35 solvent is unnecessary the rate of both reactions is increased by the addition of small amounts of water. The Wittig reaction of the polymer-bound ylide (51) has been used to prepare a variety of polymer-supported crown ethers (e.g. 5 2 ) . 36 2 . 2 . 2 Ketones. - Michael addition of enolate anions to triphenyl- vinylphosphonium salts followed by intramolecular Wittig reaction provides an efficient one-pot synthesis of cyclohex-3-enylphosphine oxides ( 5 3 ) . 37 (55) or isocoumarins ( 5 6 ) can be obtained from the ylide (54) which is prepared by the reaction of ethoxycarbonylmethylidene ylide with - o-benzoyl benzoic acid derivative^.^^ heterocycles ( 5 8 ) aye available through reaction of phosphacumulene ylides ( 5 7 ) with ketones OK aldehydes carrying an a- or B-OH, -NHR. or -SH group.39

Although the use of

Depending on the conditions used either indenones

Routes to a variety of

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O N

1

phY CoPh

X

( 4 9 )

+ R*CHO

( 5 0 )

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Except in one case in very low yield, reactions of Q-quinones with bisphosphonium salts (59) in the presence of lithium ethoxide do not give bis-Wittig reactions, but instead a wide range of products dependent on the phosphonium salt used.40 experienced in synthesising the diene (60) by a Wittig reaction of 1.4-cyclohexanedione and methoxycarbonylmethylenephosphonium ylide have been overcome by the use of potassium Carbonate as a catalyst in benzene.41 obtained from reactions of ethoxycarbonylmethylene ylides with trifluoroalkyl(alky1) ketones (61). similar reactions with ethoxycarbonylmethylphosphonates give @,y-unsaturated esters through isomerisation of the a,@-unsaturated ester formed initially.42 2-trimethylsilylethylideneylide with ketone (62, R=H) gave some diene (63). a similar reaction with(62, R=Me) gave exclusively (64) from intramolecular Michael addition of (62). 43

Difficulties

Although the expected a, @-unsaturated esters are

While a Wittig reaction of

The reaction of allylidene ylides (65) with diphenylcyclo- propenone gives bicyclic phosphoranes (66) or the allene (67) depending on the nature of (65).44 - P-chloroylides (68) give vinylphosphine oxides rather than the expected a l k e n e ~ . ~ ~ alternative is a pathway y& (69) analogous to the rearrangement of betaines (70) derived from triphenylphosphine and styrene oxide. 2.2.3 Miscellaneous Reactions. - The synthesis and chemistry of lanthanide compounds containing ylide ligands has been reviewed.

A convenient synthesis of cyclopentadienones (71) and their dimers ( 7 2 ) is available from the reaction of allylidenephosphoranes with ketocarbonyl chloride^.^' (71) undergoes competitive Michael addition reactions. Wittig reactions of the anhydride (73, X=O) with stabilised ylides take place at the carbonyl group remote from the 1-aryl substituent togive

Attempted Wittig reactions with

A mechanism is suggested, although an

46

47

In the presence of excess ylide,

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8: Ylides and Related Compounds 33 1

COO t

&h3 COAr

( 5 4 ) /

4 Ar

( 5 5 )

0

Q!& COO+ Ar

( 5 6 )

V = S, NR, or 0 X=NPh, 0 , S ,

or

Ph,P= C H , C I

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+

Z X - Y /CHZPPh3 \ +

C H2 P P h,

( 5 9 ) Y = C H 2 , S , O , o r C O

CHCOOMe =# MeOOCCH

( 6 0 )

P h 3P= CH COO E t I * C =C H COOE t

F3C\

c=o

/ RCH2

\ C H ,COOE t

b C H 2 5 i M e 3

Ph3P=CHCH2Si Me3

(63) R:H

( 6 2 )

R = M e Ph3P=CHCH2SiMe3 I + (64, R = H ) 0

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Ph

YPh 0 + R17pph3 R L ( 6 5 )

, CH=C Mez CH

C II

II h,C,fHCH=C Mez

(67)

Ph

+ \ \ CR3,

-OH

0 R2PCR-CR3, 1 11 t R1ZhCH2R2 CI-

I CI

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(73, X=CRZ). 49 Allene carboxanilides (75) have been synthesised by addition of isocyanates to ylides (74) followed by Wittig reaction with ketene (Scheme 4 ) . 5 0

The first authentic example of a thermal Stevens rearrangement of a phosphonium ylide has been reported.51 1-f luoren-9-ylidene-l,2, 5-triphenyl-h5-phosphole (76) undergoes quantitative rearrangement to the spiro-1.2-dihydrophosphorin (77). the structure of which has been confirmed by X-ray analysis of the corresponding phosphine oxide. Attempts to observe CIDNP phenomena by lH or 31P n.m.r. spectroscopic studies provided no evidence for radical intermediates. The dihydrophosphorin (77) is also involved in the reaction of ylide (76) with dimethyl acetylenedicarboxylate which gives, in addition to the expected adduct (78). the bicyclic ylide (79),the structure of which has been confirmed by &-ray crystallography. 52 The formation of (79) from (77) was established by separate experiments. Methylenetrimethylphosphorane reacts with 1-alkyl-2-vinylphosphiranes (80) to give isomeric mixtures of phosphinosubstituted ylides (81) and (82). 53 of a variety of anions to butadienylphosphonates (83) has been reported.54 cyclopropylphosphonate ( 8 4 ) and triphenylphosphine. A method of acylating Grignard reagents is provided by their reaction with ketenylidenetriphenylphosphorane (85) to give, after hydrolysis, B-ketophosphoranes (86). 55 to the corresponding methyl ketone or reacted with aldehydes to give a,@-unsaturated ketones. The thermolysis and alcoholysis of ylides (e.g. 87) derived from phosphites has been in~estigated.~~

In refluxing toluene

The Michael addition

In the case of phosphonium ylides the products are the

These last compounds can be hydrolysed

The interaction of ylides with transition-metal complexes continues to be actively investigated. Examples include the

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R'

Ph,P LPh R 2

+ Ph GRl2 ph&R2 R'

R' R *C 0 C OC I 0

Ar

( 7 3 )

Scheme 4

P h a P h

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synthesis and structural determination of nickel ( 8 8 ) , 57 uranium (89),58 lutetium (90): and chromium (91)60 and (92)61 ylide complexes. The zwitterionic product (93) has been obtained from reaction of methylenetriphenylphosphorane with the appropriate fulvalene metal complex.

3 Reactions of Phosphonate Anions The structure of lithium (94) and potassium (95) bis(diethoxyphosphony1)methane has been investigated by a variety of methods both in solution and in solid state.63

The enantiomeric chiral bicyclic phosphonamide reagents (96) and (97) have been prepared and their anions reacted with a variety of c y c l o h e ~ a n o n e s . ~ ~ alkenes with remarkably high stereodifferentiation (up to 90% optical purity). A similar reaction with (+)-3-methylcyclohexanone was highly stereoselective favouring the (E_)-alkene (93%); a similar reaction with ethylidenetriphenylphosphorane gave much lower stereoselectivity (60% (E)). Alkylation of (96) also gave predominantly one diastereomer (80%) and the relative configuration in one case (99) was established by g-ray crystallography (Scheme 5). Studies on the synthesis of stilbenes (101), which have potential value in the treatment of skin diseases, indicate that olefination with the phosphonate (100) initially leads to kinetically controlled mixtures of ( 4 ) - and (g)-alkenes. Base-catalysed isomerisation to the thermodynamically stable (g)-alkene then occurs depending on the nature of the base used. The authors suggest that this isomerisation should be taken into account when studying isomer ratios from phosphonate-based syntheses of other trisubstituted alkenes carrying conjugated e-withdrawing groups. A comparison of the stereochemistry of phosphonate-based with that observed for silicon-based olefination has been

In appropriate cases ( e z . 98) they give

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( 7 6 ) xc- cx

______.) P h e P h

xc=cx ( 7 7 ) -

X = C O O M e Ph

X Ph x ( 7 9 )

R I

( 8 0 ) R = But or cyclohcxyl (81) ( 82)

0 II

-k Ph3P=CR'R2

ROOC /C=f Ph \ H

+ Ph,P

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Ph,P=C=C=O 4- RMgX L [Adduct] H20 Ph,P=CHCOR

( 8 5 ) ( 8 6 )

CH (0Me)COOMe ( 8 7 )

R3PCH2Ni (olkenel2

(88 ) R = Me or Ph

PPh, / \ /

Ph P

'CH, / \ CH, CH2 CH2

\ / P Ph,

(89)

Ph

(91) X = NR Mo (Cog (92) X * C=PR, (93)

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Me

( R , R )

( 9 6 )

' i i ( 9 6 1

i , i i i ( 9 6 1 -

i i v ( 9 6 1 _____)

Me

(S,S)

(97)

T + "p;' 95 : 5

Me

93 : 7 8LMe Me

( 9 9 )

Reagents : i K D A THF, - 7 8 O C ; ii, 0 0 ; iii, 6 ; i v , MeCH2X Me

(9 81 Scheme 5

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340 Organ oph osp h orus Chemistry

reported. 66 Attempted intramolecular Wittig reactions of

y-acyloxy-B-keto-phosphonate anions (102) give either the expected 3(2l)-dihydrofuranone (103) or the rearranged 2(33)-dihydrofuranone (104) depending on the reaction conditions. 67 1 3 - S u b s t i t u t e d - 8 ~ - d i b e n z o [ a . q ] q u i n o l i z i n - 8 - o n e s (106) have been prepared by intramolecular Wittig reaction of the 2-(~-acylbenzoyl)-l,2-dihydro-l-isoquinolylphosphonates (105) . 6 8 A new,improved synthesis of quinoline-2.3-dicarboxylic acid esters (107) has been reported. 69 Reaction of succinylphosphonate with 2-nitrobenzaldehydes provides the quinoline N_-oxides,which are readily deoxygenated to give (107) (Scheme 6).

The reaction of the trichloro-tert-butyloxycarbonate-protected phosphonate (108) with aldehydes followed by depK0teCtiOn and hydrolysis provides an efficient two-carbon homologation of aldehydes to a-ketOeSteKS (109) (Scheme 7) .70 A new synthesis of y-oxoacrylates (111) in poor-to-good yields and involving alkenation with the phosphonate anion (110) followed by hydrolysis has been reported.71 5-Amino-l,3-pentadienes (112) have been prepared in moderate yield by condensation of aldehydes and ketones with 4-aminobut-2-enylphosphonate. 72 However, significant isomerisation of the phosphonate double bond occurs during the reaction. Olefinations using the oxocyclo-pentan-2-ylphosphonate (113) offer routes to 2-methyleneoxocyclopentanes ( e d . 114) and, through isomerisation, endo cyclic alkene~.~ reported that olefination of unstable aldehydes (e.g. hydroxy-, nitro-,and keto-aldehydes) with phosphonates is possible in good-to-

It is

excellent yields through the use of weak bases (ed. potassium carbonate) in heterogeneous media.74 The method has been applied to the synthesis of Royal Jelly acid (115) and the queen substance (116) of the honey bee (Scheme 8 ) .

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COOEt + ArCOMe 4 base Me\ / C=CH 0 COOEt AT

(100) (101)

0 (102)

But (104)

(105) (106)

i t ( EtOlpCHCOOEt

CHZCOOEt I

COOEt

COOEt

(107 1

Scheme 6

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342 Organophosphorus Chem is 1 ry

Highly substituted phosphonates ( e 2 . 117) have been prepared and successfully used in olefin synthesis.75 cyclohexylidene derivatives which show smectic liquid-crystal properties have been prepared by reaction of phosphonates (118) with substituted cyclohe~anones.~~

A variety of

The reaction of phosphonate carbanions with en01 lactones provides a potential method of synthesising cyclic enones (121). However, the major pathway appears to be formation of the dianion (119), which cannot undergo intramolecular olef inati~n.~ solution to this problem involves using two moles of phosphonate

A simple

anion to form (119) and then adding one mole of acid to form (120), which gives the required enone in good yield.

Reports of reactions of a-phosphoryldiazoalkane anions continue to appear. Reactions with 4-phenyl-1.2.3-triazolin- 3,5-dione in ethanol give urazoles (122), 78 while 4-(diazomethyl)-4&thiapyrans are formed by reaction with thiapyrylium salts (123) .79 2.4.6-triphenylthiapyrilium salt (124) follows a quite different pathway to give the bicyclic pyrazole dimers (125). The mechanism of the base-catalysed reaction of diethyl (diazomethy1)-phosphonate with N,N-dialkylated pyruvamides to give mixtures of 2-pyrrol-2-ones and 2-butynamides has been investigated.

However, a similar reaction with

8 0

The reactions of carbanions of allylic phosphonic acid derivatives (126)81 and ( 127)82 with electrophiles give mixtures of a- and y-substitution products. In the case of reactions of (127) with diary1 Schiff bases the regiochemistry is shown to depend on both the steric bulk of the Schiff base and the reaction conditions. The kinetically favoured y-product usually predominates, but the thermodynamically favoured a-adduct can become the major product under certain conditions.

The phosphonate (129) has been prepared by acylation of the

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0 II

( M e O ) , P ~ C O O M e

OTCBOC I

( 1 0 8 )

. .. I , I I

___) [RcH+]

OMc

i i i 1 0 II

RCH2C COOM e

( 1 0 9 )

Reagents: i , R C H O ; ii, Z n d u s t ; iii , H20

Scheme 7

0

R3HCY-CWR2 COOR' II

f R3CH0

~ T H P

(110)

OTHP

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344 Organophosphorus Chem islry

0 II i - i i i

( E .to ),PCH,COOE t RCH=CH C OOH

Reagents : i , RCHO, K 2 C 0 3 , H 2 0 ; ii , KOH, EtOH , H20 ; i i i , H,Ot

Scheme 8

0 0

( E t0)2PCHC(Mc)2(CH2)3Me II R ~ C O C H 2 P ( O M e 1 2 It I CN

(11 7) (118)

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( 123 1

Ph

phQ - Ph +

(124)

Ph

0

HCPR, II

II N2

0

HCPR2 II

N2 It

0

- N 2 E t O H

0

1 0 O E t II 1

N Ph hJ4 R,P-CH HA*

0

(122)

0 !i2 II H C-PR2

CHC13

Ph

( 1 2 5 )

(126) X = OEt , SMc, 0 II

O C O B U ~ , or OP(OR),

0 11

( Me,N I2P C H z C H =CH2 * * t

Li

(127 1

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copper phosphonate carbanion (128); the corresponding lithium derivative gave much lower yields.83 the copper alkyl is the reason for its superiority is not clear, but variation of the cation is obviously worth considering in phosphonate reactions which do not give good yields. Stepwise alkylation and deprotection of the phosphonate (130) provides the 1-hydroxyalkylphosphonate (131) and, this latter compound, offers convenient routes to ketones, phosphates,and vinylphosphonates (Scheme 9). 84 with 5-formylmorpholine followed by acid hydrolysis provides a new route to (1-formylalky1)phosphonates (132) . 8 5

Whether the lower basicity of

Reaction of phosphonate carbanions

The B-oxyanion alkylphosphonates (133, X = halogen), generated by base-treatment of the corresponding B-hydroxyalkylphosphonate, are intermediates in a new synthesis of diethyl 1, 2-epoxyethylphosphonate.86 competes when X = C1 and in reaction in aprotic solvents.

The alternative olef ination reaction

4 Selected Applications in Synthesis The use of the Wittig reaction in the synthesis of fine chemicals has been reviewed. 87 many examples of the use of the Wittig reaction in their synthesis.88 4.1 Carotenoids and Retinoids. -Reactions of the dialdehyde (134) with Wittig reagents ( e d . 135) have been used to prepare (3Rs.- 3'HS)-alloxanthin (136) and related acetylenic carotenoids, thus confirming their structures.

A review of cross-conjugated polyenes includes

(13Z)-Retinoic acids (138) have been prepared by Wittig reactions of the aldehyde (137) followed by opening of the pyranyl ring. The 13,l4-cyclopropyl analogues of ethyl all-trans- and 13-&-retinoate (140) and the analogous 13-desmethyl compounds have been prepared by reaction of the phosphonium ylide (139) with the

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0 nx + CuCH2P(0Mel2 I I T $ ( O M e ) 2 0

( 1 2 8 ) (129)

0 R2 0 R 2 I I i - i v I I I I I I

1 I

0 (Et0)2PCH20R (Et0I2P-C-OR * (Et0I2P-C-OH

R3

R= C H,CH,Si Me, (131)

( 1 3 0 ) R 3

Reagents: i , B u s L i ; i i , R 2 X ; i i i , B u S L i ; i v , R 3 X

Scheme 9

0 I I

( ROI~PCRRCHO

(132)

0 I t

(Et0l2P-CHX

-O-CH2 1

( 1 3 3 )

CH=PPh,

(134 )

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appropriate cyclopropyl aldehyde.91 reaction is rather slow and leads to mixtures of ll-(l3)- and ll-(Z)-alkenes and to isomerisation of the cyclopropyl aldehydes; however,it is possible to isolate all these isomers by a combination of isomerisation and h.p.1.c. Other examples of retinoid analogues synthesised by Wittig or phosphonate methods include amino retinal6 (e.g. 142) from (141)," 7,8-acetylenic compounds,93 and a variety of fluorinated compounds. 4.2 A-Lactam8.- Although the bicyclic ketone (143) undergoes the Wittig reaction with some stabilised ylides to give mixtures of endo (144) and exo (145) olefins, attempts to use this as a general route to C-3 carbon-substituted carbapenams were not successful. 6a-Alkenyl derivatives (146) of penicillin have been prepared by Wittig reactions of 6a-formyl derivatives. 9 6

Unfortunately the Wittig

95

Intramolecular Wittig reactions continue to be used to form the second ring in bicyclic f3-lactams. for example in the synthesis of a variety of carbapenams (147). 97 Wittig reactions are involved in the synthesis of 3-aminoalkyl- substituted carbapenams (148)" and (Z)-Z-(Z-methoxyethylidene)- l-oxaceph-3-em-4-carboxylates ( e 2 . 149). 99 cephalosporins (152) and (153) have been prepared by reaction of the cephalosporin 3'-triphenyl-phosphonium ylides (150) and (151). . respectively, with acylaldehyde. loo cephalosporin 3'-triphenylphosphonium ylides (154) offer a potential method of carbon-carbon elongation at C-3'. However, these reactions are slow and give mixtures of C-2 and C-4 substitution as by-products. A detailed study has now shown that the regioselectivity is temperature dependent and the reaction can be carried out with high regioselectivity. lo' component was glyoxal a tricyclic cephalosporin derivative (155) was obtained.

Both intra- and intermolecular

The tricyclic

Wittig reactions of

When the carbonyl

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Ph3P=CHR 4- 0 ~ c H o * R ~ o 0

"" Ph3 + Me

ecH=cH4 Me (139) L O H C (160)

(141) n = O or 1

c10,-

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R

D H X

0 I

CHZCHX

RcoHNEJ 0 k O O R 2

( 1 4 6 ) X=CN,CHO, or COOMc

OR H---C.,

I OR I N2

95- 112 *c

COOPN B COOPNB

( 147)

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(01, I

Ph CH,COH N

0 X & P P h 3

0 Jif: coo- COOR (149)

PhCHZCOH N 0 E& ROOC

( 1 5 2 )

COOCH P h

( 1 5 0 ) n = 0 (151) n = 1

HO H

COOCHPhZ

OH I

( 1 56 )

OH

( 1 5 7 )

OR

( 1 5 8 )

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4.3 Leukotrienes and Related Compounds. -The synthesis of leukotrienes and lipoxygenase products has been reviewed. lo2

The Wittig reaction has been used extensively in syntheses of a variety of HETE derivatives, e.g. (&)-5-HETElo3 and 5.15- and 8,15-diHETE. lo4 been prepared by Wittig reactions of the protected chiral dialdehyde (158). which is readily obtained in an optically pure form from D-a~abin0se.l~ several novel analogues uses olefination with the phosphonate (159) followed by partial reduction and hydrolysis.lo6

11g-( 156) and 12g-( 157) HETE methyl esters have

A new total synthesis of leukotriene B4 (160) and

The Wittig reaction followed by hydrolysis has been used to prepare the conjugated trihydroxy eicosatetraenoic acid (16l),which is identical with, and so confirms the stereochemistry of, Lipoxin A.lo7 The four isomers of lipoxin B (e.g. 164) have been synthesised by two rather different approaches: one introduceslo8 the ( g , E, Z)-triene component in one step through use of the enyne ylide (162). while the other is based on Wittig reactions with the 8-ethoxycarbonylhexylidene ylide (163) .Io9 A variety of 8.15-dihydroxy arachidonic acid isomers (165) have been synthesised by methods which make extensive use of the Wittig reaction. 12~-0xido-5~,7lj,9lj,14~-eicosatetraenoic acid (11g,12g-LTA4) (166)

115.

has been synthesised by two alternative approaches using phosphonate- and ylide-based olefination procedures. The latter approach was more efficient. Inhibitors, e.g. ( 167)12 and (l68I1l3, of 5-lipoxygenase have been synthesised; in the latter case stereospecific (g)-alkenylation was achieved using the phosphonate (169).

Other uses of the Wittig reaction in this area include the synthesis of the two C(8) diastereomers of (170),which are potential hormone releasing agents. 114

4.4 Macro1ides.- Phosphorus-based olefination continues to be the

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0 (M eO), P- II

C * (CH2)$OOMe e ( C H2 ),COO H Y

OSi 8ufPh2 (159) (160 1

(CH )&Me (CH2)3COOEt P h , P w I + O H C V

OR

OH

(C H,),CO 0 H

OH

( 1 64)

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method of choice for generating the two (E,E)-diene fragments in syntheses of cytotoxic maytansinoid macrocycles. '15 f3 analogue (173) has been prepared by a Wittig reaction of the ylide (171) and 5-hydroxy-5H-furan-Z-one (172) followed by various steps.'" this can be readily isomerised by iodine in benzene to the required ( z , E ) isomer. 4.5 Pheromones.- A wide variety of insect pheromones, e.g. that of the dried-bean beatle (174) ,117 glossyplure isomers (175) 0118'119 and juvenile hormone analogues (e2. 176) , Izo have been prepared using standard methods of phosphorus-based olefin synthesis.

The milbemycin

The reaction gives mainly the (z,Z_) diene isomer, but

Schlosser has applied his "instant ylide" method (dry phosphonium salt-sodamide mixture) to the synthesis of muscalure (177), the common house-f ly sex attractant. As usual in these syntheses - cis selectivity is very high (approximately 97%), but difficulties were experienced in determining the isomer ratio accurately.

The ant substance (Z,E_)-a-farnesene (179) and its (Z,z)-isomer have been synthesised by a Wittig reaction of the ylide (178),lZ2 while both Wittig- and phosphonate-olefination have been used in routes to the naturally occurring insecticide spilanthol (180) and the termite trail pheromone (181) .Iz3 The pheromones (183) of the Douglas fir tussock moth and (184) of the olive fly have been prepared using Wittig reactions of the 2-tetrahydropyranylidene ylide ( 1 8 ~ ~ ' ~ and, in the case of (183). phosphonate-based methods.lZ5 three Lepidoptera pheromones, have been prepared using methods based on the phosphonium ylides (185) and (186),which were prepared in Fitu from the corresponding a-silyl phosphonium salts . I z 6 pheromone (191) of the California red scale has been synthesised using as the key step a Wittig reaction

A number of dienes and trienes, e d . (187), including

The

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mcooH > CH= CHCH =CHC H=CHC H ( 0 H 1 ( C H,),Me HO

(165 1

(168) R:CONHOH or CH2CONHOH

(1 69)

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Ph3P t-Q OR 'Me OMc

(171)

OMc

( 1 7 3 )

( 1 7 2 )

M c( C H,),CH = C = CH C H=C H CO 0 Me

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8: Ylides and Related Compounds

Ph,P ( 1 7 8 ) I

357

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CONHBU' - \ ( 1 8 0 )

Me( C H 2)6 ( CH, ),CO ( C H )$ie UPPh3 ( 1 8 2 ) (183 1

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of the o-alkoxyalkyl ylide (190) with the B-hydroxy-a- phenylsulphenyl ketone (188)127. retroaldol reaction under the reaction conditions to give the required acyclic aldehydo-enolate (189). The Wittig reaction is highly stereoselective to give (Z)-alkene,which might seem unusual (see for example ref -20); however, Corey has also observedlZ8 high (z) selectivity in Wittig reactions where both the ylide and the carbonyl compound carry negative charge. Dienic fluorinated analogues (193) of insect sex pheromones have been synthesised stereoselectively using Wittig reactions of fluoroaldehydes with o-hydroxyalkylidene ylides or, for increased yields, their tetra- hydropyranyl ethers (192).

This last compound undergoes

4.6 Miscellaneous Arwlications. - Pure samples of straight-chain paraffins containing 102,150,198,246.and 390 carbon atoms have been prepared f o r the first time by a procedure involving repeated partial hydrolysis and Wittig reaction, with end-group reduction as the final step (Scheme All four possible stereoisomers of the marine invertebrate fatty acid 5.9-hexacosadienoic acid have been prepared. 13 obtained by Wittig reactions of the ( 4 2 ) - and (IE_)-aldehydes, respectively (Scheme 11). Methyl octadeuterio-oleate (197) has been synthesised (for use in studies of the biosynthesis of conjugated triene fatty acids) by a Wittig reaction of the specifically deuteriated aldehyde (196). 13 (198) have been applied to the synthesis of (199), which contains a structural unit commonly occurring in lipid amides. 133 phosphonium salt (200) and aldehyde (201) are readily prepared from alcohol (199) and have been used to prepare seven natural amides (e.g. 2 0 2 ~ ~ ~ ~ Routes from the salt (200) rather than the aldehyde (201) were preferred since they gave a higher (Zl-alkene content in the product.

The (5Z.9Z) (194) and (52_,9E) (195) isomers were

Phosphonate-based olef inations using

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360 Organophosphorus Chemisfry

r L i + d

(190) I

0

( 1 8 8 )

5.

JcoMr A

( 1 9 1 )

H (192)

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8: YIides and Related Compounds 36 1

B r C H2 (C H , lloC H= CH (C H 21,&() 0

ii-v 1

vi - i x I Reagents : i , base ; i i , separat ion of h a l f mater ia l ; i i i , hydrolys is of one h a l f

m a t e r i a l , r e a c t i o n of o ther h a l f w i t h Ph3P ; i v , recombinat ion and t r e a t m e n t w i t h base ; v , r e p e a t s teps ii- v ; vi , hydrolys is ;

vii,Ph3$R Br; b a s e ; vi i i , LiBHEt3; i x , H2, c a t a l y s t , 130% Scheme 10

i , i i P$6(CH2),COOH

i, iii 1 y ( C H 2 l 3 COOH

(196) R = n -C,$i33

( 1 9 5 )

i , KH , M%SO, R . T . a 2 h ; i i , R

Reagents :

Scheme 11

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The Wittig reaction has been used to prepare (9E)-strobilurin (203) and this has led to a change in the assigned SteKeOChemiStKy of all natural strobilurin derivatives to (9g).135 method of C-glycoside synthesis involving the Wittig reaction followed by cyclisation has been applied to a variety of pyranoses. In the case of glucopyranoses, but not Other pyranoses, competitive elimination, initially to give (204), was observed.13' acid-containing disaccharide6 have been prepared by a method

The widely used

Sialic

involving reaction of the phosphonate (205) with the aldehyde (206) as the key step.137

Attempts to prepare derivatives (207) of the antibiotic pseudomonic acid'by introducing the double bond through phosphonate olef ination were only partly successful. 138 Both ylide- and phosphonate-based olefinations have been used extensively in new syntheses139' 140 of the ionophore antibiotic X-14547A. including use of the novel phosphonate ( 2 0 8 ) Phosphonate olef ination, including the reaction of phosphonate (209) with malealdehydic acid to give (210). has been used extensively in the synthesis of the cytotoxic verrucarin J and its isomers.141 the optically active ylide (211) has been used to introduce part of the chiral side chain in a total synthesis of pumiliotoxin B (212). a highly biologically active alkaloid isolated from a Panamanian poison frog. lg2

A Wittig reaction with

A variety of cyclooctaannellated biphenylenes have been prepared in very low yields through Wittig reactions of the tetraylide (213) with dicarbonyl compounds. 143 By using step-wise reaction of (213) with two different dicarbonyl compounds it was possible to prepare unsymmetrical polycycles ( e 3 . 214). again in very low yield. The ylide (215), generated by reaction of three equivalents of n-butyl- lithium with 6-hydroxycarbonylhexyltriphenylphosphonium bromide, has been used to construct the side chain in a synthesis of

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0 1 1

( EtO),PCH2CONHBui

( 1 9 8 )

( 2 0 0 )

0 ( 1 9 9 )

COOMc

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B Z : CH2Ph

MeOOC 0 I II

0-CH --P(OMel2 1

B z O m 820

620 I OMe

+ RCHO ( 2 0 6 )

620

( 2 0 4 )

COOMe ~ 6 I HR

N a H , T H F ~~0 BzO

OMe

7!-0 0 OBz

R

A0 N H C O M e

OH COO R

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COO H 0 II ( E t0I2PCH2COOR 4-

( 2 0 9 ) ( 2 1 0 )

Ph3P&Me

Me OSiR3

( 2 1 1 )

PPh,

3"' ' 0 H

Me ( 2 1 2 )

( 2 1 3 )

\ / O L i LiO

( 2 1 5 )

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( 2 1 6 ) ( 2 1 7 ) R:H

( 2 1 8 ) R = ( P O M e

Me0

0 0 NHCHO 0 i , i i II

1 NH2

(HO),PCHMe I1 II

I I

- R-pPl...l, ( Me0 ),PCH P(OMe1, NH

CHO

H 11 0

( 2 2 0 )

Reagents : i , base ; ii, R C H O ; i i i , H2 , c a t a l y s t

Scheme 12

i (HO 1,PCHMeNH COCH MeNH,

( 2 2 2 )

MOy COOR ( 2 23)

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0

CH,Mc II

2- (McO),PCH,COCH

( 2 2 7 )

qH

j7cooH OH OH Me ( 2 2 8 )

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(2)-isoprosopinine B.144 would be interesting to know details of the stereochemistry of this reaction; however,it is only given as IIa mixture of ( g ) and ( z ) isomers" in this ~aper!~~The synthesis of tetramic acid (216) should be possible by use of the phosphonate (217). However, olefination of unsaturated aldehydes with (217) gives poor yields and requires harsh conditions. This problem has been largely overcome by use of the N-protected phosphonate (218) where prototropic rearrangement of the N-H proton is not an available pathway.145 applied to the synthesis of tirandamycins A and B. 1-(Aminoethy1)phosphonic acid (220). a segment of the antibiotic phosphadipeptide (221). has been prepared by reaction of the diphosphonate (219) with paraf ormaldehyde. lg6 The phosphonate (222) has been used to prepare the trienes (223) and (224) en route to the naturally occurring cytochalasans. 147

In view of Maryanoff ' 6 work18*20*21 it

The method has been

The ylide (225) has been used to prepare (226) in a new synthesis of tetrahydrocannabinols. 14' CZ2-prostaglandins in the E and F series (9. 228) which uses the phosphonate (227) to introduce the $-side-chain has been ~ep0rted.l~' been synthesised using intramolecular phosphonate olefination as the cyclisation step. 150

A total synthesis of the

The cytotoxic bis(bibenzy1) marchantin A (229) has

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1984, 49, 3517. a., Chem. Commun., 1985, 755. - 26, 51.

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[organophosphorus chemistry] organophosphorus chemistry volume 17 || ylides and related compounds

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