Phenylsulfonyl-methylenation of aldonolactones

The low temperature addition of dilithiomethylphenyl sulfone to 2,3-O-isopropylidene-D-erythronolactone, and to 5-O-(tert-butyldimethylsilyl)-2,3-O-isopropylidene-D-ribofuranolactone gives the lactols 2 and 3a , respectively. Dehydration of lactol 2 in the presence of boron trifluoride etherate produces tetrahydrofuranylidene sulfones 5 and 6 in good yields. The reaction of lactol 3a under the same reaction conditions proceeds with formation of the 1′,5′-anhydro derivative 7 via competing intramolecular substitution.