[GRAPHICS]
The Diels-Alder reaction of activated dihydropyrones with electron-rich die
nes proceeds with synthetically useful levels of diastereoselectivity Produ
ct ratios vary with the nature and location of substituents on diene and di
enophile, as well as with reaction conditions. Lewis acid catalyzed reactio
ns of related pyrone and benzopyrone dienophiles are also diastereoselectiv
e.