Me3Al/AlCl3-mediated hetero-Diels-Alder (HDA) additions of 2-silyloxy-
1,3-dienes to formylated butenolide 6 containing a protected hydroxy f
unction afford, in one step, a variety of pyranofuranones including ma
noalide precursor 16, which is a stable monoprotected seco-manoalide.