Allylic alcohols, enantioselective oxirane

The growing interest in enantioselective isomerization of meso oxiranes to allylic alcohols arises from the ready availabihty of starting materials and the synthetic value of the homochiral products. First apphed to simple meso cycloalkene oxides, this methodology has been successfully exteuded to fuuctioualized meso oxiranes, and even to the kinetic resolution of racemic oxiranes, demonstrating its potential in accessing highly advanced synthons. 

Numerous HCLA have been developed and used for the enantioselective isomerization of oxiranes to allylic alcohols and, in most cases, their efficiency strongly depends on the structure of the oxirane. The HCLA species can be divided into two groups monohthiated vicinal diamines or ether-amines and dilithiated diamines or amido-alcoholates. 

It is interesting to note that no reaction is observed with oxirane 72a if the hydroxyl group is protected. Moreover, whereas deuterium labeling experiments indicate a clean /3-deprotonation process for both oxiranes 69 and 72a, the same enantiomer of base 71 furnishes the corresponding allylic alcohols 70 and 73a with the opposite absolute configurations (Scheme 30 vs. 31). The same studies on vicinal disubstituted analogues 72b,c showed that both the sense and the level of enantioselectivity are unchanged, which... 

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 excellent enantioselectivities generally achieved with titanium tartrate-catalyzed epoxidation of allylic alcohols with ferf-butylhydroperoxide was exploited, for example, for the preparation of (2S,3S)- and (2/ ,3f )-[2,3- H2]oxiranes (72), two valuable chiral building blocks (Figure 11.28). By using a slightly modified Sharpless procedure, ( )-3-triphenylsilyl-2-[2,3- H2]propenol (20) was converted into the (S,S)- and (/ ,/ )-forms, respectively, of 3-triphenylsilyl[2,3- H2]glycidol (71). The hydroxylmethyl substituent... 

---