Aldol condensation of Evans chiral enolates with acetophenones. Its application to the stereoselective synthesis of homochiral antifungal agents

Article abstract of DOI:10.1021/jo00115a014

The results of the aldol condensation of Evans chiral imide enolates with a series of acetophenones are reported. Activated acetophenones, such as 2,4-difluoroacetophenone, α-chloroacetophenone, and α-chloro- and α-bromo-2,4-difluoroacetophenone, reacted with the lithium enolate of 5 with good levels of enolate facial diastereoselectivity toward the (2R)-isomers (> 10:1) but with low anti: syn selectivity (ca. 3:2). Sodium and potassium enolates of 5 were also tested. The nature of the solvent influenced the degree of diastereofacial biases. Less activated ketones, such as acetophenone, reacted only to a ca. 50% extent without facial or anti:syn stereoselectivities. Chairlike pericyclic transition states are believed to govern the reaction. When α-bromoacetophenones were used, longer reaction times and higher temperatures resulted in the selective formation of the S₂ epoxide (syn(2R,3R), 11) with good levels of selectivity. Equilibration studies performed in THF with the corresponding metal aldolates generated in situ by deprotonation of the aldol adducts indicated that an aldol/retroaldol process was first established followed by a slower formation of the epoxide. Stereoselection is thought to originate by a faster oxirane formation of the syn bromohydrins as compared to the anti due to steric interactions between the α-group and the leaving bromide. Optimum retroaldol-epoxide formation rates were obtained using the sodium enolate in ether at -78°C. Under these conditions the S₁:S₂:A₁:A₂ ratio of epoxides was 6:83:10:0.3 and the major isomer was isolated by recrystallization in 79% yield. An improved synthesis of amino alcohol 3, an advanced intermediate in the preparation of orally active antifungal agents, using a tandem of this new ketone-aldol technology and a Curtius rearrangement, is reported. The new sequence proceeds with an overall yield of 53% and does not require chromatographic purifications.