ASYMMETRIC-SYNTHESIS OF FUNCTIONALIZED SECONDARY ALCOHOLS BY CATALYTIC RING-CLEAVAGE REACTIONS OF CYCLIC ACETALS DERIVED FROM (R)-1,3-BUTANEDIOL

In the presence of a catalytic amount (0.1-0.2 molar amount) of a 2-ph enyl-1,3,2-oxazaborolidin-5-one, prepared by the reaction of dichlorop henylborane and N-(trifluoromethylsulfonyl)-L-phenylalanine, and enol silyl ethers ((R(2))(2)C=C(R(3))OTMS; R(3) = t-BuS, EtS, EtO, Ph) (1.1 -1.5 molar amount), chiral cyclic acetals 6 derived from (R)-1,3-butan ediol and aldehydes undergo an efficient ring-cleavage reaction with t he inversion of the stereochemistry at the acetal carbon to give the a nti isomer of the corresponding products with high stereoselectivity. The reaction is applicable to acetals derived from aromatic, aliphatic , and alpha,beta-unsaturated aldehydes. Not only enol silyl ethers, bu t also methallyltrimethylsilane and allyltributyltin, can be employed as nucleophiles, leading to the selective formation of the anti isomer of the corresponding allylated ring-cleavage products. Removal of the chiral auxiliary from these ring-cleavage products by a two-step sequ ence ((i) PCC (ii) Bn(2)NH(2)(CF3CO2)) furnishes enantiomerically enri ched beta-hydroxy carbonyl compounds and homoallyl alcohols. A modest level of kinetic resolution is observed in the ring cleavage of a race mic acetal catalyzed by a phenylboron compound derived from N-mesyl-L- phenylalanine.