Oxiranes diastereoselective synthesis

The diastereoselective synthesis of spirofurooxindoles has been described based on the three-component reaction involving intermolecular carbonyl ylide dipole 23 [69]. The reaction of 3-diazoindol-2-one 21, benzaldehyde and DMAD in the presence of 0.5 mol % of rhodium(II) acetate catalyst furnished the spirofurooxindole 22 (Scheme 7). On the basis of the diastereoselectivity of these reactions, the conformation of the intermolecular carbonyl ylide 23 is provided. In the absence of dipolarophiles, the carbonyl yhde underwent electrocyclization to yield oxiranes 24 [70]. 

Figure 11.85 Diastereoselective synthesis of an e.p. (2R,3S)-p-hydroxy-a-amino[l,2- C2]-carboxylic acid from (2S)-f C2]BABS via a (2R,3R)-oxirane[l,2- C2lcarboxylic acid ester...

Reduction of propargyl oxiranes with copper hydride results in a diastereoselective synthesis of a-hydroxyallenes. DFT studies, supported by kinetic investigations, partially validate the previously proposed reaction mechanism and attribute the high... 

The remarkable stereospecificity of TBHP-transition metal epoxidations of allylic alcohols has been exploited by Sharpless group for the synthesis of chiral oxiranes from prochiral allylic alcohols (Scheme 76) (81JA464) and for diastereoselective oxirane synthesis from chiral allylic alcohols (Scheme 77) (81JA6237). It has been suggested that this latter reaction may enable the preparation of chiral compounds of complete enantiomeric purity cf. Scheme 78) ... 

Oxidation of cyclic phosphonoformaldehyde dithioacetal, using the Modena protocol, yields the trans disulfoxide 121 in excellent enantiomeric excess. Then 121, via HWE olefination and oxidation of the double bond has been used for the diastereoselective preparation of spirocyclic his-sulfinyl oxiranes (new versatile intermediates in asymmetric synthesis) [79] (Scheme 37). 

Another illustration of the use of such a biocatalytic approach was the synthesis of either enantiomer of a-bisabolol, one of these stereoisomers (out of four) which is of industrial value for the cosmetic industry. This approach was based on the diastereoselective hydrolysis of a mixture of oxirane-diastereoiso-mers obtained from (R)- or (S)-limonene [68]. Thus,starting from (S)-hmonene, the biohydrolysis of the mixture of (4S,81 S)-epoxides led to unreacted (4S,8S)-epoxide and (4S,8i )-diol. The former showed a diastereomeric purity (> 95%) and was chemically transformed into (4S,8S)-a-bisabolol. The formed diol... 

These lithiooxiranes can be trapped by various electrophiles with retention of the configuration. The addition to aldehydes occurs with a low diastereoselectivity [but this can be enhanced by adding ClTi(OPr-/)3]. The reaction with enones occurs in a 1,2 fashion only. Intramolecular 1,4-silicon shift has also been reported. The reaction of the enantiomerically pure TMS-stabilized lithiooxirane 189 (Scheme 80) with an aldehyde has been used in a total synthesis of (-l-)-cerulenine. It must be noted that protodesi-lylation using TBAF (tetrabutylammonium fluoride) occurs with conservation of the oxirane stereochemistry. 

A stereoselective total synthesis of dendrobatide toxin 251 D was developed by Overman et al.237) involving an epoxidation of the (S)-proline derivative (237) to furnish the oxirane (238) as major product. In their approach towards the total synthesis of the same natural product Thomas et al.238) investigated the stereoselectivity of the epoxide formation from (S)-5-acetylpyrrolidin-2-one and dimethyloxosulfonium methylide. A diastereoselectivity of d.s. 50-60% was achieved 238. ... 

Alcohols can be obtained from many other classes of compounds such as alkyl halides, amines, al-kenes, epoxides and carbonyl compounds. The addition of nucleophiles to carbonyl compounds is a versatile and convenient methc for the the preparation of alcohols. Regioselective oxirane ring opening of epoxides by nucleophiles is another important route for the synthesis of alcohols. However, stereospe-cific oxirane ring formation is prerequisite to the use of epoxides in organic synthesis. The chemistry of epoxides has been extensively studied in this decade and the development of the diastereoselective oxidations of alkenic alcohols makes epoxy alcohols with definite configurations readily available. Recently developed asymmetric epoxidation of prochiral allylic alcohols allows the enantioselective synthesis of 2,3-epoxy alcohols. 

The earliest general method for the reductive deoxygenation of oxiranes to alkenes involved multistep procedures in which halohydrins were intermediates. Comforth, for example, used an oxirane deoxygenation in an early synthesis of squalene (Scheme 11). Diastereoselective double addition of ho-mogeranyllithium (23) to the dichloro diketone (24) gave the bis(chlorohydrin) (25), which was first converted to the diepoxide (26). Subsequent treatment with Nal and Zn in HOAc buffered with NaOAc gave squalene (27). 

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