A diastereoselective ring contraction of 1,3-dioxepins to 2,3,4-trisubstituted and tetrasubstituted tetrahydrofurans.

A modular and diastereoselective approach to 2,3,4-trisubstituted and tetrasubstituted tetrahydrofurans is reported. The use of dioxepins containing an embedded vinyl acetal functionality leads to a Lewis acid-mediated [1,3] ring contraction to afford tetrahydrofurans in good yield and excellent diastereoselectivity. The use of TMSOTf in MeCN leads to the 2,3-cis/3,4-trans diastereomer while SnCl4 in CH2Cl2 provides the 2,3-trans/3,4-cis diastereomer. A variety of substituents are tolerated at each position. The presence of Lewis basic functionality under the SnCl4 conditions alters the reaction favoring the diastereomer formed under the TMSOTf conditions. We present conclusive evidence that the products of each of these reactions are formed under kinetic control. We further provide stereochemical models consistent with each of these rearrangement reactions that account for the formation of the major diastereomer in each case.