Chemistry Letters, Vol.30, No.9, 906-907, 2001
Highly stereoselective 7-membered ring synthesis based upon the oxidopyrylium-alkene [5+2]cycloaddition reaction
The [5+2]cycloaddition reaction of 2-substituted-3-oxidopyrylium betaine with dialkyl fumarates was found to proceed with stereoselectivity up to 15.5:1. The stereoselectivity became higher as either the 2-substituent of the 3-oxidopyrylium or the alkoxy group of the fumarate became larger. The stereoselectivity could be rationalized by considering these steric effects.