The stereochemistry of addition of allyl sulfone carbanions to aldehydes.Formation of dihydrofurans

Abstract

The reaction of the mono-anion of the bromoallyl sulfone 1 with aldehydes 2 was examined with the aim of obtaining selectively substituted tetrahydrofurans. At -100°C syn- and anti- open chain adducts 3 and 4 were isolated together with a low yield of 4-methylene-2,3-disubstituted tetrahydrofuran 5. In the presence of HMPA or at higher temperature the reaction led to formation of 2,5-dihydrofurans 6. The stereochemical results are consistent with initial addition of 1 to the aldehyde involving Li ion chelation.