chiral selenium compounds: versatile reagents in organic synthesis

MICROREVIEW

Chiral Selenium Compounds: Versatile Reagents in Organic Synthesis Thomas Wirth

Institut fur Organische Chemie der Universitat Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland Fax: (internat.) + 41(0)61 / 267-1 105 E-mail: [email protected]

Received July 7, 1997

Keywords: Asymmetric synthesis I Diselenides / Natural products / Selenium I Total synthesis

Chiral diselenides are easily accessible and can be used for various asymmetric reactions. Oxy- and aminoselenenyla- tions with selenium electrophiles generated from the diselenides are used to build new asymmetric centers in the addition products. Improvements in the synthesis of chiral diselenides are described as well as the generation of addition prod-

ucts suitable for natural product synthesis. Oxyselenenyl- ation-elimination sequences can be performed with catalytic amounts of chiral diselenides. In addition, such compounds can be used as ligands in other metal-catalyzed asymmetric reactions.

Contents

1. 2. 3 . 4. 4.1. 4.2. 4.3. 5. 6. 7 . 8.

Introduction Synthesis of Chiral Diselenides Optimization of Chiral Selenium Electrophiles Addition Reactions with Chiral Selenium Electrophiles Natural Product Synthesis: Furofuran Lignans Cyclization Reactions Aminoselenenylation: Tetrahydroisoquinoline Alkaloids Catalytic Selenenylation-Elimination Sequences Facial Selectivity in Selenenylation Reactions Diselenides as Procatalysts in Diethylzinc Additions Conclusions

1. Introduction

Selenium and its compounds have been known for a long time to be versatile reagents in organic chemistry. The con- siderable progress in the development of selenium-based methods started in the early seventies and is still under way. Today many of these methods have been developed into

standard procedures to introduce new functional groups under mild conditions. Organoselenium reagents have pro- vided useful methods for the generation of double bonds, for addition to double bonds, for the attachment of oxygen, as well as for the cyclization of unsaturated systems[']. Compared with organosulfur chemistry, the selenium based methodologies have not been used in asymmetric synthesis until recently. Because organoselenium compounds can be employed as nucleophilic and electrophilic reagents, as well as precursors for radicals and for rearrangements, they have a high potential for performing stereoselective reactions. Recently the syntheses of several optically active selenium derivatives were reported together with their first applications in the reactions mentioned above. Diselenides are common precursors for these reactions because they are stable and easy to handle. They can be transformed into electrophilic, radical, or nucleophilic species. In addition, some diselenides can serve as ligands in catalytic reactions. A large number of methods are therefore available for the synthesis of diselenides[2]. The most important routes in-

Thomas Wirth was horn in Leverkusen in 1964. He studied chemistry at the University of Bonn and stayed there to carry out his Diplomarbeit under the guidance of Prof Siegjiried Blechert. Then he moved along with his supervisor to the Technical University of Berlin, where he received his Ph. D. degree in 1992. For postdoctoral studies he joined then the group of Pro$ Kaoru Fuji at Kyoto UniversitylJapan as a JSPS fellow. A t the beginning of 1994 he started his independent research at the University of BaseNSwitwrland. His research interests are the development of reagents and methods for asymmetric oxidative junctionalizations as well as their use in stereoselective synthesis.

MICROREVIEWS: This feature introduces Annalen's readers to the authors' research through a concise overview of the selected topic. Reference to important work from others in the field is included.

Liebigs Ann./Recueil1997,2189-2196 0 WILEY-VCHVerlag GmbH, D-69451 Weinheim, 1997 0947~3440/97/1111~2189 $17.50+.50/0 2189

MICROREVIEW T. Wirth

volve either the reaction of metal diselenides with halides or the preparation of selenols which can be oxidized to the diselenides.

In this paper we would like to summarize our results on the development of efficient and easily accessible chiral selenium reagents and their use in asymmetric addition reactions to C = C bonds. The action of intramolecular nucleophiles, as well as external nucleophiles, bearing further functionalities in the field of natural product synthesis is described. Chiral diselenides have also been used in catalytic oxyselenenylation-P-hydride elimination reactions and as ligands in asymmetric catalysis. Some important results developed by other researchers are also referred.

2. Synthesis of Chiral Diselenides

Selenenylation of C=C bonds leads to addition products 1 viu the seleniranium intermediates and subsequent unti attack of nucleophiles. This reaction has been widely used in many areas, including the preparation of natural products, because the oxidative removal of the selenium moiety from the addition products 1 makes it possible to introduce C = C bonds into the products (path a). The generation of radicals by a homolytic cleavage of the carbon-selenium bond in 1 opens the door for subsequent radical reactions (path b). The selenium moiety can also be replaced by other functional groups depending on the nature of the substituents (path c).

Scheme I

r D i Nu \

I Nli' ?

Chiral binaphthyl diselenides of type 2 were one of the first types of optically active selenium reagents, reported by Tomoda et al., which were employed in asymmetric electrophilic selenenylations"].

After this discovery, other optically active diselenides ap- peared in literatureL4]. Diselenides with a ferrocenyl ske- leton, such as 3, were described by Uemura et al.L51, a C2- symmetrical diselenide (4) was reported by Deziel et a1.16], diselenides with various chiral pyrrolidine moieties (5) have been characterized by Tomoda et al.[7], and chiral diselenides derived from camphor, such as 6, have been employed by Back et al. in asymmetric electrophilic selenenylations.

The diselenides 2-6 were converted to the corresponding selenium electrophiles and used in addition reactions with double bonds. In all cases high diastereoselectivities were obtained in the addition products. However, the necessity to use stoichiometric amounts of these complex diselenides is a major drawback because they are only available by mul-

Scheme 2

2 3 4

5 6

tistep syntheses and are therefore inconvenient for syn- thetic applications.

A common feature of these dieselenides is the close prox- imity of the heteroatom to the selenium. It has been reported on numerous occasions that selenium can interact with nearby heteroat~msr~l.

For the efficient use of organoselenium compounds in synthesis, optically active diselenides as versatile precursors for the selenium electrophiles must be prepared by a short and convenient synthesis from readily available starting ma- terials. The success of the diselenides 2-6 led to the design and synthesis of more simple diselenides of type 9. These compounds are accessible from the bromo precursors, which are obtained by chiral reduction of the 2-bromo ke- tones 7 and alkylation of the hydroxy group. The diselenides with R = H can be synthesized in one step from the chiral alcohols 8 by ortho-deprotonation and treatment with ele- mental selenium.

Scheme 3

R' 1 (-)-(Ipc),BCI 2. NaH, RX

5'

sk+ X -

I Brz 2. AgX

5'

7

p R R' TMEDA, Se 9a: tBu Me

9b: Me Me

10 Ar*Se'X-

8 9c: H Me 9d: H Et 9e: H rBu

3. Optimization of Chiral Selenium Electrophiles

After the transformation of 9 into the electrophilic selenium species 10, we observed a good transfer of chirality in addition reactions with styrene. This observation can be understood in terms of an interaction between the selenium and the oxygen in the chiral side chain. The smaller the R group, the better the coordination to selenium and the inducing effect upon the newly created stereogenic center (entries 1-3, Table 1). Subsequent variation of R' showed that 9d, with R = H and R' = Et, yields the addition products with diastereoselectivities of up to 92% (entries 3, 4

2190 Liebigs Ann. IRecueil 1997, 2189-2196

Chiral Selenium Compounds MICROREVIEW and 7)[101. Several other diselenides of type 9 have also been prepared and these show similar results[''1. A further indi- cation of the interaction between selenium and oxygen is revealed by the 77Se-NMR shifts. Stronger coordination between selenium and oxygen leads to a downfield shift in the frequencies of the diselenides 9. A number of results obtained in the addition reaction to styrene using methanol as the nucleophile are summarized in Table 1. Further optimization of the reaction conditions were made with diselenide 9d. On using different silver salts (AgSbF6, AgPF6, AgBF4, AgOTf) to replace the nucleophilic bromine with a less nucleophilic counterion, we observed the highest stereoselectivities and yields by employing silver triflate. Various solvents were also tested and we found diethyl ether to be the most suitable for the addition reactions. On lowering the reaction temperature we observed a slight increase in the selectivity (entries 4-6).

Scheme 4

10 Ph,SnH, OMe

ph- Ph Ph

MeOH 11 12

Table 1. Reactions of diselenides 9 with styrene and methanol to give addition products 11

Entry Diselenide R' R T Yield de la] ["CI 11 ["/o] 11 [oh]

1 9a 2 9b 3 9c 4 9d 5 9d 6 9d 7 9e Lb1 8 9f 9 9g 10 9h

Me tBu - 78 68 Me Me - 78 63 Me H - 78 70 Et H - 78 31 Et H - 100 81 Et H - 114 46 ~ B u H - 100 33

- 114 37 ~ 100 63 ~ 100 28

36 74 17 86 89 92 35 95 60 93

[a] The de values of 11 (determined by 'H NMR) differ only slightly + 3% from the ee values of the corresponding cleavage product b. ~ ibI Diselenide 9e was employed as the racemate.

Based on these results, variations in the electronic properties of diselenide 9d were also made. The attachment of substituents at the aromatic ring change the electrophilicity of the selenium cations and has an influence on the reactivity as well as on the stereoselectivity of the addition reaction. A methyl- or trifluoromethyl-group in position 6, and even a tert-butyl group in position 5 , decreases the stereoselectivity of the addition reaction. Electron withdrawing substituents, such as a nitro-group in position 4, as shown in compound 9f, enhance the electrophilicity of the cationic species and diastereomeric excesses of up to 95%) are obtained (entry 8). The selenium electrophile generated from diselenide 9g, which contains an additional methylene-group, shows greater conformational flexibility and decreased stereoselectivity (entry 9) because the interaction between selenium and oxygen does not lead to a five-membered ring but to a six-membered ring. In order to increase the rigidity of the

system, diselenide 9h was synthesized in only one step from commercially available enantiomerically pure tetralol. The reduced conformational flexibility of the chiral moiety leads to an increase in stereoselectivity up to 93% de (entry 10) 11 11 [I 21,

Scheme 5

9f 9g 9h

4. Addition Reactions with Chiral Selenium Electrophiles

Various alkenes can be used in the oxyselenenylation reaction with chiral selenium electrophiles. We found that high stereoselectivities are obtained with aromatic alkenes. Selected results with substituted styrene derivatives are summarized in Table 2. Because of the good selectivities obtained with aromatic alkenes, the question of x,n-interactions between reagent and substrate was addressed. How- ever, on using an electron rich alkene such as 4-methoxystyrene, and an electron poor diselenide like 9f, no en- hanced stereoselectivities were observed (entry 4) compared with the unsubstituted compounds (entries 1 and 3). There- fore, n,n-interactions do not seem to play a determining role in the stereoselection. Other aromatic alkenes were used to generate addition products with good to excellent diastereoselectivities (entries 5 - 8). Oxyselenenylation can even be performed with cis-disubstituted alkenes. Compound 14 is an allylic selenide and is generated by the addition of a selenium electrophile to 1,3-~yclohexadiene (entry 10). This class of selenide is especially interesting because it has the potential for 2,3-sigmatropic rearrangements["]. After oxi-

Table 2. Reaction of diselenides 9 with various alkenes and methanol as the nucleophile["l

Entry Alkene Diselenide Yield ddhl 13 ['Yo] 13 ['Yo]

1 2 3 4 5 6 7 8 9 10

styrene styrene 4-methoxystyrene 4-methoxyst yrene 4-fluorostyrene 1 -vinylnaphthalene 2-vinylnaphthalene E-I-phenyl-prop-I-ene cyclo hexene 1,3-cyclohexadiene

9d 9f 9d 9f 9d 9d 9d 9d 9d 9d

81 89 72 91 41 74 78 79 49 64 35 85 29 95 45 80 31 80 41 71

The reactions were performed at - 100 "C. ~ ['I Determined by 'H NMR.

Scheme 6

10d OMe

14 1s

Liehigs Ann.lRecuei1 1997, 2189-2196 2191


dation of 14 to the selenoxide, a highly stereospecific re- arrangement took place yielding the allylic alcohol 15 with 70'% d ~ .

4.1. Natural Product Synthesis: Furofuran Lignans

We investigated the addition of nucleophiles other than methanol in order to expand the scope of the stereoselective addition of chiral selenium electrophiles to alkenes. Differ- ent alcohols or carboxylic acids can be used to add to the selenonium intermediates. Even alcohols bearing double or triple bonds can be added using slightly modified reaction condition^^'^]. These addition products can by cyclized by an intramolecular radical cyclization to afford substituted tetrahydrofuran derivatives. We applied this strategy of oxyselenenylation and subsequent radical cyclization to the synthesis of furofuran lignans. The addition reactions to the TBDMS-protected allylic alcohols, using 2,3-butadien-l-ol a s the nucleophile, were optimized and yielded the addition products 16 in about 55% yield with diastereomeric ratios of 15: 1 . The subsequent radical cyclization of the main diastereomers via the favored 5-exo-pathway led to the tetrahydrofuran derivatives 17. The stereochemistry of the carbon atom bearing the aromatic substituent (C-2) controls the stereochemistry at the neighboring carbon atom C-3. This can be explained in terms of a transition state in which the aromatic unit and the bulky silyloxymethyl substituent are arranged in pseudo-equatorial positions. At C-4 a 1:l mixture of stereoisomers was observed because the reaction proceeds viu a boat-like and a chair-like transition state which are similar in After transformation of the vinylic double bond to the aldehyde by diol formation and oxidative cleavage, epimerization occurred under the conditions of deprotection and the hemiacetals 18 were formed spontaneously. Compound 18a already represents a natural product, (+)-samin, a component of sesame The furofuran moiety of 18 is a precursor for several other lignans. Treatment of 18b with 4-methoxyphenylmagnesium bromide leads to addition and dehydration to complete the first total synthesis of (+)-membrine (19)[171.

Scheme 7 Ar*sefr;y PbSnH, OTBDMS 1. oso, */ ___) AIBN R,*ssr

R 111 R "' 0 0

17 IS

19

organoselenium reagents for this reaction was originally described 20 years ago[18]. In 1995 nearly every research group in the field of chiral selenium electrophiles reported the use of these reagents in asymmetric selenoetherification and se- lenolactonization reactions [5a1 [6a] [7c1 IX h] [ I O]. From comparison of the various results, it seems that the nature of the oxygen nucleophile (alcohols, carboxylic acids) has little influence on the facial selectivity observed. As a representa- tive example of various cases, the cyclization of 20 to the five-membered heterocycles 21 is described['"]. The reaction proceeds with a good asymmetric induction to give tetra- substituted carbon atoms which are not easy accessible by other Scheme 8

X 10d

MeOH

20% X = 0 Zla. 62 %, 88 % de 20b: X = Hz 21b:45%,78%de

Interestingly, we found that a small amount of methanol must be present in the cyclization reactions. Without methanol, the intramolecular cyclizations are very slow and the products are obtained with low diastereoselectivities. At- tempts to use chiral alcohols to stabilize the cation generated from diphenyl diselenide and to perform asymmetric cyclizations with 22 (R = Ph) failed, although asymmetric reactions with selenium compounds stabilized by chiral amines are known[20].

In order to gain a better insight into the course of the attack of double bonds with chiral selenium electrophiles we investigated the facial selectivity in the cyclization of a series of differently substituted homoallylic alcohols 22, which are easily Depending on the nature of R in 22, we observed a different facial selectivity, as shown in Table 3[221. When R = Et, no selectivity was observed and 23a and 23b were formed in a 1:l ratio (entry 1 ) . How- ever, compounds 22, in which R = Ph and R = tBu,

Scheme 9

IOd

MeOH SeAr* SeAr* 22 23a 23b

P h 2 M & i G o H 24 Ar*Se -0" 25

OMe

Table 3. Selenoetherifications of compounds 22 with electrophile 1 Od

Entry Alkene 22

Yield 23 P/o]

Ratio 23a : 23b

4.2. Cyclization Reactions

Unsaturated alcohols or carboxylic acids can be cyclized by an electrophilic attack of the double bond. The use of

2192

1 R = Et 60 1:l 2 R = Ph 87 12:l 3 R = tBu 68 1:2.5 4 R cyclohexyl 49 3:l

Liehigs AnnlRecueil 1997, 2189-2196

Chiral Selenium Compounds MICROREVIEW showed different facial selectivity (entries 2 and 3). The absolute configurations of the main diastereomers were determined by radical cleavage of the selenium moiety and comparison with optically active tetrahydrofuran derivatives, which were synthesized independently. The cyclohexyl derivative 22 (R = cyclohexyl) was found to cyclize with the same facial selectivity as 22 in which R = Ph. Com- pound 22 with R = SiMePh2 did not cyclize. Because of the P-silyl effect, methanol attacks at the P-position and compound 24 is formed with 79% de. However, addition products of type 24, bearing a selenium and a silicon on the same carbon atom, are interesting compounds because this carbon atom can easily be converted to an aldehyde moiety[”]. Alkene 22, bearing a tin substituent (R =

Bu,Sn), did not cyclize, but a tin-selenium exchange took place and compound 25 was obtainedL2’1.

reaction, catalytic processes of addition and subsequent elimination may allow the use of the selenium species several times. The selenenic acid can add to double bonds to form the hydroxyselenenylated products, but dispro- portionation to the diselenide and the seleninic acid regen- erates the di~elenidel~~]. This process is represented in Scheme 11, where f3-methylstyrene 29 is involved in a methoxyselenenylation-elimination sequence.

Scheme 11

32 catalytic version

Ph ph- 29

- Ar*SeOH / Ar‘Se’X-

4.3. Arninoselenenylation: Tetrahydroisoquinoline Alkaloids

Only two examples of asymmetric cyclizations with nitrogen nucleophiles have been des~r ibed[~~l [~“] . Because the products of intramolecular aminoselenenylation are nitrogen containing heterocycles, they are interesting building blocks for alkaloid synthesis. The selenium functionality is still present in the addition products and can be used for further transformations. We applied this reaction to the synthesis of tetrahydroisoquinoline alkaloids. The styrene precursor 26 was synthesized by methylenation of the corresponding aldehyde. The poor nucleophilicity of the acet- amido group in 26a is probably the reason that this compound did not cyclize. However, cyclization did occur with the carbamate 26b to yield the isoquinoline derivative 27 with 90% de. Radical cleavage of the selenium moiety and deprotection afforded (-)-salsolidine 28[121.

Scheme 10 MeoqmR G=OTO 10h Mcoq MeoqNH M e 0 M e 0 M e 0

- NBoc -- 26s. R = Ac lbb: R = Boc

SeAr* \

27 28

5. Catalytic Selenenylation-Elimination Sequences

As shown in Scheme 1, asymmetric selenenylation reactions can be followed by a P-hydride elimination (path a). The selenenylated product has to be oxidized to the selenoxide and, after elimination, a double bond is obtained in the product. Uemura et al. have recently investigated the asymmetric selenoxide elimination with chiral selenium compounds. By generating chiral selenoxides they were able to prepare optically enriched allenecarboxylic esters[”]. In the elimination reaction of selenoxides, the selenium species is liberated in the form of the selenenic acid (R-Se-OH). A one-pot procedure should be possible if selenenylation and elimination are performed without isolating the initially formcd selenenylated product. Furthermore, if the liberated selenium compound can be used again for a selenenylation

30 31

Electrochemical oxyselenenylation-deselenenylation reactions have been reported with diphenyl diselenider261, and diselenides containing amine moieties have been described which perform the catalytic conversion of alkenes to allylic ethersL2’1. Catalytic stereoselective oxidation reactions should be possible if optically active selenium compounds can be used for this reaction. In this case, the generation of selenium electrophiles from diselenides can no longer be performed viu the selenenyl bromides because these reaction conditions are incompatible with a subsequent oxidative elimination. Tiecco et al. described the use of peroxodisulf- ate ions for the transformation of diselenides into the corresponding selenenyl sulfates. The advantage of this method is the generation of the selenium electrophiles in the absence of nucleophilic counter ions[28]. We employed this method in the methoxyselenenylation reaction of alkene 29 and found that the diselenides 9 containing an oxygen in the chiral moiety are less efficient than chiral diselenides that have a nitrogen as the heteroatom in the chiral moiety. Ni- trogen-containing diselenides of type 34 can be prepared in two steps from commercially available chiral phenylethyl- amine (33) by alkylation and subsequent diselenide formation through ortho-deprotonation, as shown in Scheme 1 2 ~ 9 d 1 .

Scheme 12

1 alkylation or R 34a NMez 34b NHMe 34c NEt, 34d pyrrolidin-I-yl 34e NH-CO-NMe2

$NH2 2 carbamoylation 1-BuLi,Se ~

33

Different reaction conditions and solvents were investigated in the catalytic methoxyselenenylation of 29. A screening of different metal salts showed that copper nitrate enhances the reaction rate in the formation of product 32. The different diselenides used for the conversion of 29 to

Liehigs Ann.lRecuei1 1997, 21 89-2196 2193


Table 4. Addition-elimination sequence with catalytic amounts (10 mol%) of diselenidesr”]

stituent has a remarkable influence on the course of the electrophilic addition reaction.

Entry Diselenide Yield ee 32 [‘Yo] 32 [%I (Conf.)

[“I 1 mmol 29, 1 mmol Na2S208, 0.1 mmol Cu(N02)? ml MeOH, RT, 7 d. - rb1 0 “C, 28 d. - rCl 15 ml MeOH

3 H20, 3

32 are listed in Table 4 together with the reaction conditions employed. Nitrogen-containing diselenides of type 34 (entries 2-5) are much more eficient than diselenide 9d (entry 1). which ha5 an oxygen in the chiral moiety. Lowering the reaction temperature requires very long reaction times (entry 6), and with higher dilutions lower yields are obtained (entry 7). In the experiments to date an enantiomeric excess of up to 42% has been reached for compound 32[””l. Other electronically modified nitrogen-containing chiral diselenides must be synthesized in order to improve the turnover number as well as the yield in the catalytic addition-elimination sequence.

6. Facial Selectivity in Selenenylation Reactions

The addition reactions of chiral selenium electrophiles to substituted alkenes show a strong facial selectivity. Most of the selenium electrophiles investigated do have a heteroatom which is connected to an asymmetric center close to the selenium. This heteroatom is able to coordinate to the divalent selenium and it has been shown that this coordination leads to the formation of pseudo-high-valent selenium species both in solution and in the solid stateI9l. From these studies it is also known that the non-bonding selenium-heteroatom interaction is predominantely an n-a*- type orbital interaction between the heteroatom and the selenenyl moiety. The selenium cations generated from the diselenides should therefore have a trigonal-bipyramidal ge- ometry with the two selenium lone pairs occupying the equatorial positions as shown in the structure of 10d.

Scheme 13

Et.. 6’”s \ / Se

0 x

I Od

However, a detailed explanation for the preference of the re-attack of styrene observed with selenium electrophile 10d and similar compounds cannot be given at the moment. We are currently in the process of synthesizing diselenides with additional substituents at the second ovtho-position with re- spect to selenium. Preliminary results indicate that this sub-

7. Diselenides as Procatalysts in Diethylzinc Additions

The use of chiral diselenides and their derivatives in other catalytic reactions has also been described. These compounds are able to act as ligands in metal-catalyzed transformations. Nitrogen-containing diferrocenyl diselenides have been used as chiral ligands in rhodium- and iridium- catalyzed asymmetric hydrosilylation and transfer hydro- genation reactions[”]. We found that nitrogen-containing diselenides of type 34 can be used as very eficient procatalysts for the addition of organozinc reagents to alde- h y d e ~ [ ~ ~ ] . Diselenide 34d was the most efficient in the addition of diethylzinc to a variety of aldehydes 35. Only I mol% of 34d is necessary in the catalytic additions yielding the secondary alcohols 36 in high enantiomeric purities, as shown in Table 5.

Scheme 14

1.25 eq. Et,Zn 1 mol % 34 *

R-CHO R 35 36

Table 5. Addition of diethylzinc to aldehydes 35 [l mol% (R,R)- 34d as catalyst]

Entry Aldehyde 35 36 ee [“/ill

Yield [%I] (Configuration)

1 benzaldehyde 91 98 is) 2 3-(trifluoromethyl)benzaldehyde 98 91 [*I 3 4-(tert-butyl)benzaldehyde 61 98 1‘1 4 2-bromocyclopent- 1 -ene- 1 - 97 98 Id]

5rb1 pentanal 91 76 ( R ) carbaldehyde

Id] Absolute configuration not determined. - Ib] 1 mol% (S,S)-34a used as catalyst.

A more detailed investigation of this reaction revealed that the diselenides act as procatalysts in the addition reactions. After the addition of diethylzinc, the selenium- se- lenium bond is cleaved rapidly and catalytically active zinc selenolates are formed. Detailed NMR analysis showed an aggregation of the selenolates, which are in dynamic exchange with other species. In analogy to the work of Noyori et al. we assigned these as catalytically inactive meso dimeric species and chiral dimeric species which are in equi- librium with the catalytically active monomers. We observed an “asymmetric amplification”, i.e. a positive nonlin- ear relationship between the optical purities of the catalyst and the product, which supports these findings. With re- spect to the addition of diethylzinc to aldehydes, we found that spectroscopic, chemical, and stereochemical properties are in accordance with the well investigated properties of amino alcohols[’3].

2194 Liebigs Ann.lRecuei1 1997, 2189-21 96

Chiral Selenium Compounds

8. Conclusions

Although organoselenium compounds have been utilized in many selective reactions under mild conditions, the appli- cation of chiral selenium-containing reagents in asymmetric synthesis has only developed recently. The synthesis of easily accessible and efficient chiral diselenides highlighted in this microreview leads to several applications. The total synthesis of different natural products was achieved by asymmetric addition of selenium electrophiles to suitable alkene precursors. Catalytic amounts of chiral diselenides were employed in oxyselenenylation-elimination sequences. Further- more, these diselenides can also be used as procatalysts in the addition of diethylzinc to various aldehydes. Further in- vestigations into the scope and mechanisms of both stoichiometric and catalytic reactions of chiral selenium compounds are in progress.

I would very much like to thank my co-worker Dipl.-Chem. G Fragale for his excellent contributions to the work described in this review, and Professor B. Giese for his constant and generous support. Support by the Fords der ChemiJchen Industrie, the Deutschen Forschungsgemeinsclia~t, the Schweizer NationaEfonds, the Treubel- Fonds, BASF AG, Bayer AG, and Novartis AG are gratefully acknowledged.

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chiral selenium compounds: versatile reagents in organic synthesis

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