20

Biogenesis of limonoids:

The main features of triterpene biogenesis are

now v;ell established and are summarised in scheme "1, The

limonoids are def raded triterpenes and constitute a group

which derives its name from limonin, the bitter principle

3 9 present m most citrus species '•". Determination of the

structure'of this principle was a magor problem in the mid

fifties v/hich v;as solved through the efforts of several

10-12 internationally reno\ med chemists , Since NPIR

spectroscopy had not yet arrived on the scene, the structure

was deduced largely from chemical transformations of the

molecule. X-ray analysis clarified such doubts as still

existed and established besides the relative stereochemistry,

The absolute configuration of limonin and other members

of the group was assumed on the basis of biogenetic

relationship to the tetracyclic triteri ene tirucallol/euphol

which had been previously related to compounds of known

absolute configuration. Further confirmation v as soon

1^ available through apolication of ORD -''.

Evolution of the limonin carbcai skeleton from

euDhol (3) involves elimin&tion of four carbon atoms of the

21

side chain, cleavage of Gp-C., bond of ring A, migration of

C^^ methyl to G^ and modification of ring D. The limonin

nucleus emerges through oxidative cyclisation leading to

tv;o 5 membered and tv/o 5 membered heterocyclic rings. The

changes can be represented diagrammatically as shovm in

scheme - I.

[ SCHB\/!E I ]

Scission of r ing A as "oostulated in the 'blorcr.etlc

schene v/as o r i g i n a l l y observed in dammarenolic acid and

• 1 5 i s of v;ide occurrence among t r i t e r p e n e s e .g . shoreic acio - ,

22

. . 16 putranjivic acid and migration of the methyl group occurs

with introduction of double bond between C^^ and C. - during

oxidation of butyrospermyl acetate (5) to the 7-keto

compound (5) , Conversion of ring D to the 6 membered lactone

can occur through an in vivo enuivalent of the Baeyer -

Villiger oxidation. Many compounds have been isolated in the

intervening years which lie either on the pathv/ay to limonin

from euphol, or have resulted through scission and modifi­

cation of other ring. '\

NZO'^'

H

ACO

(5) (6)

In the scheme that follows limonoids or their

immediate precursors have been so arranged as to place the

more primitive, and therefore siiapler ones, before the

23

structurally more complex. Protolimoaoids are compounds in

which structural changes of the precursor triterpene moiety

are confined to the side chain. Thus flindissol (7)

turraeanthin (8) aphanamixin (9) melianone (10) contain all

the carbon atoms of tirucallol/euphoi but cyclisation of

AcO

R = H r OH

(9)

do;

Lhe side chain to the Cdiurabcd furan has already occurred.

24

Cedrelone (11) and grandifolone (12) are representative of

compounds in v/hich methyl migration is accompanied by loss

of 4-carbon atoms from the side chain. Gedunine (13) type

t<=A

Kry o

(II)

ACO-

;i2)

R = H

of compounds are readily derivable from these through

Baeyer-Villiger rearrangement v/hile nomilin (I' O and

obacunone (15) are more obviously related to limonin (l&)

and ichangin (17) must be its immediate precursor.

25

(14) (15)

O^^'

;i6} (17

Also regarded as members of the limonoid group

arc andirobin (18) and methyl angolensate (19) in v/tiich

26

ring B undergoes scission and ring A remains intact, v/hereas

in nimbin (20) both rings A and B survi\-e and it is ring C

that is modified. Though more complex structurally

swietenolide (21) and mexicanolide (22) are readily accommo­

dated in the scheme as products formed through cyclisation of

ring A with the methylene resulting fron scission of ring C,

^ COOCH

US) [19)

CH OC 2 II O

20) 21

COXI^

^

27

2 2 )

28

Structure elucidation of Limonoids;

NMR opectroscQ-py:

The structural problem of liraonin, as already

pointed out, v/as resolved largely through chemical trans­

formations and PMR spectroscopy, applied at a later stage

of the work, served only to confirm the number of methyls.

The published data was meagre and only resonances of the

furanoid protons v;ere assigned. The first comprehensive

18 study of PI'IE spectra of limonoids was made by D,L.Dryer ,

vjho also employed it in confirming the structures assigned

to products resulting from deep-seated rearrangement of the

molecule.

In spite of the comparatively poor resolving

pov;er of the 60 KHz instruments, the spectra of limonin

and its derivatives reported by him are fa.irly clear, a

consequence of the wide difference in chemical shifts of

structurally significant protons and presence of but fev;

contiguous methylonos. It was thus possible to make

assignments with some certainty, the important ones being

of (a) methyl resonances (b) epoxidic protons (c) the C-1

and C-19 methine and methylene (d) furfuryl proton and (e)

90

the furanoid protons.

Methyl Resonance:

The C-8 methyl bei.ng farthest av;ay from deshielding

influences resonates at highest field followed by the

gem-dimethyls which are B to the ether ox;}'-gen and the / • — • ' '

18-methyl v/hich is in proximity to the luran ring. That the

resonance of the C-8 methyl is correctly identified follows

from the fact that it is the one most affected by reduction

01 carbonyl or its conversion to oxime. Similarly in obacunone

methyl assignments could be made on the basis of shifts

30

produced by opening of the 7-niembered lactone ring.

Protons under oxygen:

Of these there are five and of the five only the

one at C-1 is further coupled. The 0-19 methylene gives

rise to a singlet In some compounds and in others to AB

doublets. The singlet of the epoxide profcons v;as found to

overlap in some limonoids v/ith the mulfciplet of the C-1

proton but there is no ambig\,iity about the furfuryl proton

as, apart from its being on a carbon under oxygen, it is

further deshielded by the adjacent furan ring.

The epoxide proton appears at much lov;er field

than in similar compounds, because of uhe 0-7 carbonyl group.

The effect is similar to that observed, in case of 0-1

hydrogen in 11-ketosteroids ' nd 0-7 hydrogen in some

diterpenes e.g. fibleucin (23). Reduction of the 0-7 carbonyl

23)

31

gives two compounds, liraonol and epilimonol v/hich are

epimeric at C-7. The position of the epo;.dde proton

differs by about 40 Hz in the two compounds, Spilimonol,

the reduction product in which the epoxide hydrogen appears

at lower field, has been assigned equatorial [ OH as in

this case the hydroxy group ojid epoxide proton are in

proximity and the latter is thus exposed to the deshielding

influence of C-0 bond. An interesting fact that emerges

from a study of different limonoids is that the epoxidic

proton in compounds in which ring A is 7-Kiembored e.g.

obacunone (15) is at higher field than in limonin. Besides

serving to distinguish compounds of the two tj'~oes this

provides evidence of conformational changes in ring B on

expansion of ring A. Further the epoxidic ring also

influences the environment of the C-7 hydrogen in C-7

alcohol causing it to resonate at lovier field when it is

equatorial, as in limonolo Models shov/ that the epoxide

ring is also close tc the fu.riuryl proton and excercises

a deshielding influence on it too, for in dooxylimonin the

proton is shifted upfield hy about 20-30 Hz.

The stereochemistry at C-I7 was established

through recourse to Klyne's modification of Hudson's

32

19 lactone rule , The axial i.e. orientation of the

-fupfuryl proton is clearly discernible in columbin by the

larger coupling constant with the axial proton of the

adjacent methylene in columbin (24-) 20

24

33

Sfcructure of Janfpraolide:

The defatted bark of Jlacourtia cataphracta v;as

extracted v ich ethariol and acetone :..nd from the extract after

the usual purification a v.'hite cry£i:alline solid v/as

isolated. It had sharp melting poini; and initially appeared

pure on tic but the mass spectrum vras not very clear about

the position of M ' and the HMR too seemed to belong to a

mixture. The NHR spectrum shov/ed 8 t art-methyls atj high

field which suggested that the compound was a triterpene but

the mass spectrum had its most prominent ion at 5'J-8 with

peaks of very lov/ intensity only at higher mass values. It

was therefore presumed that the isolated solid was a mixture

of tv/o diterpenes. The IR spectrum (?ig-I) shov/ed a broad

-1 C^O band betv/een 17^0-. 1690 cm vi^n fine structure indicating

the presence of an ester or lactone grouping. Further

information about the structural features of vhc constituent

compounds was available from the ?'70 MHz HMR spectrum

(J ig-I- ). The presence of epoxide appeared likely but

integration over this region and the .uode of coupling v;as

not such as could be accorimodated within the fraraev/ork of

a di-or triterpene molecule and presence of other impurities

was susnccted„ The signals at lo'.'er field wore, ho\/ever,

34

35

f^^-^^ ^V^t j,'VS?"S>)^?a^t^1^:5trfliA'v?-'"^i^^J 'fe'iaf' e*'S2^^ ?(Kw*9Enaitei* RJ^^»_I 'ks?''MS' C£i-5iacL^-*v ^C£i-5iacL *v - i —-tvJ^ i'*

36

clearly resolved and left no doubt of the presence of a

j^-substituted furan and c<. p -unsaturated carbonyl moities.

Especially to be noted are the doublets at 6.6 and 6."15

(^ v/hich were assigned to a CHp=CH-C- grouping and the

doublets at 4.45 and 4.75 v/hich suggested a -CHp~0~C-

grouping« The integration of these doublets is different,

from that of the doublets at 5*5 and of the doublets of

furanoid protons at 6c35« These features of the HMS spectrum

directed attention tov/ards either a colu&bin or a limonin

type of molecule v;ith the mass spectrum favouring the latter

possibility. In any case, no progress could be made v/ithout

resolution of the mixture and to this end extensive tic on

various types of plates v;as attempted. Separation was

ultimately achieved through preparative tic and supplied.

tvjo compounds one of \/hich was identical v/ith limonin. The

position, and multiplicity of the signals, in the off

resonance C spectrum of the isolated limonin agreed

21 closely v;ith values i eported_ m literatu- e " and the tv/o

are compared in the table - 1 .

All chemical shifts are reported in 6 values.

24 2

37

( 16 )

Found

" 0

21

20

1?

141

143

110

120.

11

16v6,

53.

65.

.2

A

»7

.7

.2

.3

>1

A

.8

6

9

8

T a b l e - 1

ReDorted

29.6

21.3

19.6

17.1

I 4 l e 8

1''i3.2

1 1 W ct I

120.2

77.^

167.2

53.7

66.7

Assip;nments

quar te ts of t e r t i a r y meth3/

doublets of furanoid carbo

s ing le t of furanoid carbon

17

16

15

14

38

Zound Re-Qorted Assir^nments

38.0 57 ,6 15

30 .8 29 .6 12

-18.9 17-6 11

5 1 . ^ 45 .2 10

^5 .1 ^ -6.2 9

^-6.0 50o3 8

206.1 207.8 7

35.7 36.0 6 ^

60 ,6 58.0 5

8 0 . 3 7 9 . ^ 4

169.7 170.0 3

36 .4 55.6 2

79o2 78 .4 1

6 5 . 4 19

1x

Comparison of t h e - C s p e c t r a of t h e new compound

d e s i g n a t e d as .iangomolide (PLp-TIl) ( l^lacourt ia Ca taphrac ta

i s s^/nonyaiOLis v/ith F l a c o u r t i a J&ngomoceG'S) and l imonin

(Tal)le-<'5) b r i n g s out c l e a r l v bliat d i f f e rence bet\'Gen t h e

two r e s i d a s only in t h e s t r u c t u r a l f e a t u r e s of . r i n g A.

39

'aJptS^ i^rfc? '^— •

LL

> f -C

40

. > o

o o

o in

JD

o LL

O O CO

rj

41

{ 2 )

Table - 2

Limonin

"50,?„

21.4

20.7

17.7

141,2

145.3

110«1

120.1

77c8

166.6

53«9

65.8

33cO

cra.np;onolide Assip;nments

31.5

24.7 quartets oT t e r t i a r y methyl;

20.0

15.5

150.5

143.;

1^1,1

119e3

77-5

166 c 3

54.1

67.2

38»4

doublets of furanoivl carbons

singlet of furanoid carbon

17

16

15

14

13

42

Jane:omolide Assignment;

12

11

10

9

8

7

6

5

3

2

1

19

Thus -resonance of the recons t i tu ted r ing A

carbons occur at 79.2, 36.4, 169.7, 80.0 axnd 65.4

(carbons 1 ,2 ,3 ,4 , of modified r ing A and 19 methyl carbon)

in the limonin spectrum. .Signals close to these values in 15

the -C NMR s'pectrum of the jangoraolide are more appropriate.

assigned to other carbons of the limonoid nucleus and in

any case one has to find a viable explanation of the

signals at 150,8, 118.3, 166.8, 36,3 a?id 104.8 which, except

Li i

30

18

31

45 .

4 6 .

nonin

. 8

.9

. 4

.1

.0

206 .1

35.

6 0 .

8 0 .

16S

3 6 .

79.

6 5 .

.7

,6

• 3

K7

,4

2

4

28

18

49.

53.

48 .

209.

37.

4 1 .

36 .

166.

118.

150,

104.

. 0

. 0

. 2

. 2

.3

,1

.5

.6

3

.8

3

,8

0

43

for the signal at I6608 have no counterpart in

the limonin spectrum. The NI-IR spectrum 01 jangomolide

is critical in establishing the nature of the ring

system in jangomlide arid is discussed below.

The PKR spectrum of jangomolide (Fig, IV) shows

4 tertiary methyls at 1.3, 1.28, 1,16, od ,p"Substituted

furan at 7.41 (m,H-21 and H-23) and 6,33 (m~H-22), epoxy

lactone at 5.99 (3H-15) & 3.53 (S-H-l?). The presence

of an i: ,|3~unsaturated lactone grouping is evident from

signals at 6,5 and 6,14 "(both doublets, J=9.6 Hz). The

most significant feature of the FI-iR spectrum is the

singlet at 6.06 Since the -C Mi;R allows only one C-C

double bond in the molecule the singlet must arise from

protons en a carbon under two oxygens i.e., an acetal

carbon, ^uch a linkage can come into existence if the

C°19 methyl gets oxidised to an aldehyde group which then

from a haniacetal, vath the C-4 hydroxy group and the newly

formed hydroxyl is inolved in esterification of the

carbonyl groiio as illustrated belo ;*

44

•ffiss^ - N f

m

o

=5

IN

;'Mm*lO<SS»B3»aiVTr5CC3!*'1S'«a!JF£'^

45

HON

The most logica l str^ic-fcure on th i s basis i s (2)

which i s i n complete accord with a l l spec t r a l features of

the compound. Thus the band at 1700 in the i r , spectrum

of the mixture resolves in to bands at 1773, 1750 and 1706

and the high resolut ion mass spectrum shows N"^'- 15 a t

453-15^0 correspondingly Cpj-Hpc-O'*'*o and the molecular

formula C^ H^ Oo 25 2o 8.

The above correlation with limonin fixes up the

stereochemistry of jangomolide at all centres except C~19

i^ei the Junction of the 5 and 6 membered heterocyclic

ringsc The 3-orientation of the hydrogen is based on IWR

evidence of slight coupling between the C-19 and the 0-1

46

hydrogens0 Such long range coupling is a feature of

compounds e.ge steroids (25) in v/hich the concerned

(25)

hydrogens form an M arrangement, Models shov/ that

in jangoraolide such an arrangement is possible only if

the hydrogens is |i , hence it must-have the stereo­

chemistry shown as belov; i.26)6

261

47

The family Gnctaceae of the plant kinp;dom is

comparatively unexplored and comprises of but two genera,

Ephedra and Gnetum. While Ephedra vulp;aris is a commercial

22 source of ephedrine , isolation of the alkaloid from Gnetun

2-5 species has also been reported ^, The work on the chemical

constituents of Gnetum ula was taken up in this laboratory

to find out if it also contained phenylethylamines, which

are invariably of therapeutic value. It showed that the

plant did not contain alkaloids but was rich in phenolic

secondary metabolites. One of the 3 6 V/ SL S Q.^3 signed the

stilbcne structure (2?) which was later revised to (28) ""'c

The remainin g three v/ere unambiguously characterised as the

iJesacetophenon benzyl ether (29) and diraer (30) and the

isocoumarin C-glycoside bergenin (3' )»

( 27 )

48

HOx^.-'C^-v^OH

( 2 9 )

o

! 3! )

x u i U ; . . c x V. l ix u i c i o u [ , ^ c j i i i C ciiicL-L^'^-'-'^ ^-^ C/iit C27UU.9

ext rac t led to the i s o l a t i o n of a ncv product uhich was

df r ""nat }d as gneT:ol ard idoncified as the sbiloene (32) on

the basjs of evidence preseatod below*

49

(32 )

M " at m/z 24^ (100:0 positive ferric chloride

reaction, and absence of OMe signal in the NHR spectrum

sugj-ested a tetrahydroxy stilbene structure for ^netol, a

conclusion supported by UV absorption at 309, 318 and 33' nni

(32) and strong hydroxylic absorption v/ithout any indication

of a carbonyl band in the IR spectrum.

Bue to its poor solubility in chloroform the PHR

soectrum of gnetol had to be run in DM50-d^. The 60 MHz

spectrum (ii'ig-V) initially obtained was too poorly resolved

to allow any inferrence of the aromatic substitution«

Since subsequent v/orli shov/ed that gnetol forms a

tetraacetate the rise of the integral over signals in the

50

;> en

51

offset must be assignoci to four protons and on this basis

bhe rise over the aromatic region, as expected for tetra-'

substituted stilbene; accounts for eight protons. Though

the J'IIR spectrum of gnetol and the stilbene isolated earlier

differ in detail the presence in both of a 111 triplet at 6.03

is notev/orthy and structure assigned to the stilbene isolated

earlier 'ind gnetol must provide for a proton absorbing at

this value and having the required multiplicity. It v/as

natural to suspect at this stage that gnetol is the

deraethylation product of (2?) but this possibility was

eliminated when permethylation products of the two v/ere

empaced. It v;as also noted that whereas methylation of (27)

proceeded smoothly and was over in a few hours that of gnetol

took several days. The acetate, ho\/ever, formed readily but

the PlUl spectra of both methyl ether and acetates are

singularly uninformative as multix)lets of the arom.atic

protons merge to a multiplet between 3 and 4 . The acetate

methyls give rise to two singlets at 2.31 and 2.30 each

integrating for 6 protons.

•'.'he mass s ooctrum of gnetol is also of not much

value, Tbis is in line with the behaviour of stilbenes in

v/hich, because of extended, conjugation, the molecular ion

52

is quite stable. The only breakdovnis besides random

cleavages can be attributed to loss of water and,CO from

the molecular ion. In contrast the spectra of dihydrostil-

benes are extremely useful in 'deciding about the distribution

of substituents in the two benzene rings as cleavage of the

central bond gives rise to benzylic (tropylium) cations.

The mass spectrum of dihydrognetol shows M ' at 245 amu and

base peak at 125 ^smi indicating clearly that each ring

carries tv;o hydroxyls. Several possibilities exist for the

structure of gnetol on the basis of data recorded above.

The PHR spectra in hand did not permit a clear cut choice to

be made, the general difficulty ivith stilbenes as opposed

to the flavonoids, v;hich also contain tv/o benzene rings. In

flavonoids, however, the chemical shift of benzenoid".

protons are not uniformly efiecocd by the carbonyl group

and since the chemical shift difference is large compared

to J values the spectra are well resolved and approximately

first order» This mad e it necessary to obtain from abroad

a 560 Mtlz spectrum of gnetol in K-'ISO-d and DMSO-d^ benzene.

The signals of the aromatic protons vvere clearly resolved

in these tv/o spectra and the substitution pattern could now

be inferred.

53

The spectrum (l ig-VI) in DKSO-d^ shov/s two doublets

at 7»37 and 7o25. Because of the large coupling constants

(J:= 17 Hz) it can only be assigned to olefinic protons. The

signals at 6.6 and 6.2 are, however, not as easily assignable

in this spectrum. Addition of benzene leads to further

resolution, the spectrum now showing a partially merged

double doublet at 6.7 and a clear 2H doublet at 6.88. This

means that the two nrotons must have identical chemical

shifts and are coupled to the same protono These tv;o signals

can therefore only be assigned to the contiguous protons of

a 1,2,3 trisubstituted benzene ring. The hydroxyls in one

ring are therefore located ortho to the stilbene double bond.

The 2H doublet at 5.4 must similar ly be assigned to

identical nrotons which are coupled to a proton in meta

position to both. The other beazene ring must ha.ve a

symmetrical substitution and structure (52) follows logically

from this. The triplet at highest field is nov; understandatble.

It must arise from a uroton which is betv/Gen two oxygens

and it must be meta coupled to t\;o identical protons. This

assignment was further confirmed through double resonance

experiments. Irradiation of the triplet at highest field led

to collapse of the doublet at 5«5 to a singlet» The hydrox:vlic

54

protons appear in t h i s spectrum as tv;o sharp s i n g l e t s in

the of f se t not l i ke the broad mul t ip le t s observed ear l ie r ' .

F ina l ly , the s t ruc tu re (32) , assigned to gnetol

on the bas i s of data presented above, vras confirmed through

26 synthesis. Condensation of 2,6-dimethoxybenzaldehyde with 5,5-dimethoxyphcnylacetic acid (prepared by Arndt-Eistert

on OQ

reaction) ' • in the presence of piperidine gave the stilbene

p-carboxylic acid (55) I'/hich on decarboxylation v/ith copper

bronze and quinoline afforded 2,6,11,13-tetramethoxystilbene

(5- ) identical in all respects v/ith gnetol tetramethyl ether.

Attempted demethylation of synthetic (3^) with BCl^ at 0° led

only to partially demethylated product 2,6,11,15~tetraraethoxy-

stilbene which on heating with pyridine hydrochloride at 200

for four hours gave a compound which was found to be identical

with the product obtained when gnetol (32) v;as subjected to

similar reaction conditions. The >mass soectrum of this compouD:

indicated it to be dincric in nature but its structure could

nob be established with certainty.

55

,OMe iCOOH

OMe

OMe

OMe OMe

33) 34!

Bej?ore s t ruc tu re (32) v/as conclusively es tabl i shed

for g;netol some spec t ra l evidence pointed tov;ards (35) as

a possible s t ruc tu re for gneto l . Compound (36) ' ^ as therefore

b j i L u n e S i S y u u j ^JuiiO tJij toc!.i/j-uii UJ. C: ^^^ J~ KyV^xaSifiiO.^j Duli/'cii-ueiij'u.c:

v/ith n~'aetboxy phenyl acoLic acid . The s t i lbene (36) thus

obtained along;vifch i t s corresponding s t i lbene B -carboxyl ic

acid (37) was shown to be d i f fe ren t froia gnetol tetracaeth^/l

e ther (34)„

56

HO

( 35 (36)

MeO,

MeO OMe

37 1

As noted at; the begining structure (27) assigned

to one stilbene had to be revised to (28) in the light oi

results obtained with gnetol» It is necessary in this

coiitort to go briefly into reasons for allotting initially

structure (27).

57

The 90 MHz spectrum of stilbene (27) in DM30-dg

is reproduced in (Fig~VI). The spectrum shows one methoxyl

and the signal in the offset integrates roughly also for

three protons. The number of OH groups in any case is

settled through methylation and acetylation. The spectrum

shows two clear doublets at 7* 0 and 6.78 without any sign

of further coupling. This requires that one of the tv;o

benzene rings must be tetrasubstituted and therefore the

other ring can have only one hydroxyl group. The other

hydroxyl could either be at 0-10, 11, 12 positions but if

it were to occupy position 12, the spectrum v/ould have an

AA'BB' pattern which is nowhere evident. The hydroxyl was

not put in the ortho T30sition to the double bond because

under strongly acidic condition or with sercuric acetate

there is no cyclisation to a dihydrobcnzofuran. This i?

also the reason for olacing the methoxyl ortho to the 0=0 in

the other benzene ring. This structure, however fails to

explain the triplet at 6.;5 v/nich '/as tentatively assigned

to 'che G-10 proton. The assigniient agrees vjith the observed

multiplicity but protons adjacent to only one hydroxyl

normally resonate at lower fields. It seeaed at that time

that either interaction with double bond or the solvent

i -

in

>

- ^ " / ^ S v ^ , "

•(]• •r'-^'i-

t> ^

.1-

X

> > „

X o.

X

J

y " " " • ' " **lftJL ?!?*• f

s ( l

i ' <w

rrva

X

• ' " 1 t

E a a

<iJ

t-.

tG}

o

o

c o

<5

o to a

Q

o

ft

•a"

59

induced shift was responsible for it but vjith the elucidation

of structure (32) for gnetol it became evident that this

proton was wrongly assigned and the structure had to be

revised,

60

The flavonoids form an important group of natural

products and their occurrence, structure, chemistry and

29

pharmacology has been the subject of many publications ^

since the initial work of Robinson on anthocyanins. A

rearrangement of the flavonoid carbon skeleton (38) leads

to isoflavonoids of \;hich by far the most abundant group is

that of isoflavones (39). Derivatives of isoflavanone (40)

i.Cc. compounds in uhich the double bond in the pyrone ring

is missing are far less common in nature but rotenoids (4-1)

which have an extra methylene group and a tetracyclic

system possess basically the isoflavanone nucleus. Another

( 39)

61

class of compounds akin to isoflavanones is represented by

pterocarpanes (42). If both the double bond and. the carbohyl

group of the pyrone ring are completely reduced, isoflavans

(45) result. Untill recently the only representative of this

group v/as isolated from mares urine and appropriately named

equol- vestitol'' , laxifloran , mucronulatol lonchocarpai

came to light after 1950 but the number of isoflavonoid

isolated to date does not exceed forty three «

(42 431

The only direct method for synthesis of isoflavanone 25/4.

v/as introduced by Wanslick v/hich makes use of the base

catalysed coupling of benzopyran -4—one (4- 0 with nascent

quinones (45) but this method, though of much theoretical

62

interest, is limited in scope and the usual procedure is to

BASE

44' (45)

prepare tlie isoflavono having bhe required substitution and

reduce it to the desi3?ed levcl»

The isolation in this laboratory of an isoflav^n

and isoflavanone- - directed attention towards the search for

a. more conv anient s/nthetic method for these two types and

in the follov/ing section the possible approaches and some

preliminai'y vjork is dibcubsed =

The startinr; ooint of -311 isofljvone snytheses is

a deoxybonzoin. Several roooents have been employed for

introduction of C~2 of the isoflavone nucleus, the latest to

63

make the grade 3,s dLniethyl amino formamide dimethyl acetal .

Reduction of isoflavones to isoflavanones and isoflavans is

usually effected over catalysts, Pd/C or PtO, but NaBH^ has

also been employed" . Since the isoflavanone carbonyl is

more easily reducec than the isoflavone carbonyl it is

difficult to stop the reduction at the isoflavanone stage

and yields are poor. Better yields have been reported for

isoflavans but in practice, using samples available in the

laboratory, variable results were obtained. li? therefore,

seemed v/orthwhile to try other ap-oroaches to isoflavanone

and isoflavone synthesis. One method, the reaction of 3-phenyl

4-hydroxycoumarin v/ith DHSO/AcpO (Scheme-Il), discussed

elsev/here'' , would have been ideal but it gave poor results

30 with substituted coumarins' '',

DMSO Ac 2 O. >

H^-S-CK

(I) MeL

(ID RefluxecJ over

K^CO,

in acetone

- >

4 0 )

SCHEME I J l

Si

It v/as sought to be extended bj employing chloro-

methyl, methyl sulphide (;dcheme-IIl) but the results were

ambiguous and are s'sil under investigation,

.fC£CH S~CH 2 3

->

o. 0 "S-CH

(i) Mel (ii) OH

^ -> 40

ISCHEWEIII ]

Attempts -pre a lso iiao'e ro reverse the r e a c t i v i t y

of ortho Mcthoxv bcnz^ldehyde carbonyl bv converting i t to

1,3 d i t ' l l an e (^6) Tiirt a l ] owing th i s to react '-libh

-bromoacctoDhenonD. The reac t ion could take t\/o courses

65

leading to products which could serve as starting material

for either flavan or isoflavan (Scheme-IV) but this approach

to proved sterile and the reaction gave rise to a mixture.

U-. H

SH SH I

CH a: ( 4 6 )

CH ^

S s

o li

PhC-CH2"Br >

S ^ ^

Reduction >

OR

: SCI lEWE I T •

66

Since much syntheijic work has been done on rotenoid;

the literature on rotcnoid syntheois v;as surveyed to check if

any of the methods employed can be modified for isoflavanone

or isoflavan synthesis. The search focussed attention on a

paper published by Ollis "" as a short communication in 1960

in which the synthesis of the rotenoid aunduserone (4-8)

through condensation of the enol acetate (4?) with salicylal-

dehyde is reported. Piperidine was used as the basic catalyst

•OH AcO

H + ilMzi

ii] Reduction iiOOxidation

(47) (48)

and the reaction was conducted in EtOH at room temperature.

The paper gave a yield of SOfs in this reaction but the

details were never published. The reaction must proceed

through initial base catalysed hydrolysis of the enol acetate,

67

the enolate anion attacking the aldehyde group follovjed by

addition of the phenolic hydroxyl across the carbonyl

generated in the reaction. Since reduction involves hydro-

genolysis and saturation of double bond it should proceed in

good yield. This route appeared extermely interesting and

it was hoped that a similar condensation of the enol acetate

of phenyl acetaldehyde with salicylaldehyde can be affected.

Since details v./ere not available, efforts were made initially

to work oub the appropriate reaction conditions by condensing

the enol acetate of cyclohexanone, v/hich resembles (4-7) more

closely, with o-hydroxybonzaldehyde. It night be noted here

that if the cnamine replaces the enol acetate in this

41 reaction, the condensation has been used by Paquette m

the synthesis of xanthones. All possible variation were

tried but only traces of some products could be detected in

uAu xeauuxuii luixouLi:; axL-cr wvxiv up anu. mosuxy SLaruxng

materials v;ere recovered. As against this the corresponding

renction of enamine of cyclohexanone gave very good yields

A'-? _ of tne xanfchone , i-'aouetre nas also used enamines of

aldehydes and obtained good yields of isoflavone by this

method but this does not offer any marked advantage over

existing methods for isoflavone synthesis and has not been

68

explored from thin view point. Recently a novel s.ynthesis

of isoflavylium salt from pbenylacetaldehyde dimethyl acetal

and o-hydroxy benzaldehyde has been reported ^. The conden­

sation v/as brought about v;ith anhydrous HOlO^ obtained

through adding 609i aq. HCIO^ to acetic anhydride. The reactioi

for which the adduced mechanism must be assumed was tried and

the reported yield was found to be reproducable. If the

Ph-CH -CH

^0CH3 H

\ ) CH3

Ph-CH~ CH z O-CH 2 3

,0H

\

O

/]

H CH-0 = CH-CH-Ph

3 2

f

5^

\.r \ H 19

Pn V-

•f

.0

H 0 -€ H

Vh

C£Q

69

reduction of isoriavjlium salt to isol'lavan could be afiected

in good yields this method could be developed as a general

isoflavan synthesis. The reduction was attempted in various

solvents e.g» methanol, diglyrae, acetic acid but led to

mixtures v/hich could not be satisfactorily resolved. In

tertiary butanol, hov/ever, the reduction gave only one major

product which had the same melting point as that of isoflavan.

Phenylacetaldehyde dimethylacetals are not readily accessible

compounds and if the procedure v/as to have general utility

an equivalent starting material would have to be employed.

It appeared of intrest to check if styrene oxide could be

substituted with phen3/lacetaldehyde dimethyl acebal. Ring

opening under acidic conditions should give the enol acetate

which can be expected to react in the same way as the enol

methyl ether interm.ediate in the original reaction. I'ais

turnedout to be the case and exposure of the styrene oxide

to the samic reaction conditions also afforded the isoflavyliun

salt in comparable ajiiour.ts. The poor result in hydrogenation

stop, however, did not encourage further investigation of

this approach for the desired G;,~nthe3is has to be superior

to method involving reduction of isoflavones.

70

The melting points were taken on Kofler block and

are uncorrected. Infrared spectra ;vere generally recorded on

'a Perkin Elmer 521 spectrometer. Ultraviolet spectra in 95/'

methanol solutions were measured on a Beckmann DK~2 spectro­

meter. The solvent used for NMH spectra is indicated. The

spectra v/ere recorded on different instranents and the field

strength is indicated, THS was employed as internal reference.

•^Q> spectra were measured on Brucker \'ffl-90 spectrometer at

22.63 WHz in the pulsed mode. Mass spectra were taken on an

Atlas instrument.

Solvents used for chromatography v/ere purified by

standard methods wherever necessary.