Diastereoselective Construction of Contiguous Chilal Centers by the Aldol Reaction of Acylsilane Silyl Enol Ethers.

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There has been considerable interest in recent years in the stereoselective formation of multiple chiral centers using aldol reaction between prochiral enolates and aldehydes. We disclosed here an convenient method of diastereoselective construction of the three or four contiguous chiral centers using the Lewis acid mediated reaction of acylsilane silyl enol ethers (I) with acetals and the subsequent nucleophilic addition to the carbonyl group of the resulting acylsilanes (II).Treatment of E-or Z-acylsilane silyl enol ethers (I) derived from acylsilanes having enolizable methylene proton with a mixture of aldehyde dimethylacetals and TiCl_4 in dichloromethane gave the corresponding 2,3-anti-3-methoxy-1-silyl-1-alkanones (II) in high diastereo excess,independent of the double bond stereochemistry of I used. The similar reaction of E-I with d,1-phenylpropionealdehyde afforded 2,3-syn-3,4-syn-3-methoxy-1-silyl-1-butanal (V) in high stereoselectivity.The resulting acylsilane 2,3-anti-II and 2,3-syn-3,4-syn-V were subjected to the nucleophilic reaction with alkyl or phenyllithium to yield the corresponding 3-methoxy-1-silyl-1-alkanols with three and four contiguous asymmetric centers (III and VI, respectively) stereoselectively. The protiodesilylation of III and VI with F^- reagent to afford the corresponding 1,2-anti-al- kanols with three and four contiguous asymmetric centers, IV and VII in 95-99% diastereo excess.