Enantiomerically pure Diels-Alder cycloadducts from aldohexose-derived dihydropyranones

C. A. IRIARTE CAPACCIO; OSCAR VARELA

TETRAHEDRON-ASYMMETRY

Elsevier

Lugar: Amsterdam; Año: 2004 vol. 15 p. 3023 - 3028

0957-4166

The thermal and Et2O·BF3-catalyzed Diels-Alder additions of 2,3-dimethylbutadiene and cyclopentadiene to (2S,6S)-6-acetoxymethyl-2-(2-propyloxy)-2H-pyran-3(6H)-one (1), derived from D-galactose, afforded the corresponding optically pure tetrahydrobenzopyrones resulting from the addition of the diene to the a or b face of 1. The facial and endo-exo selectivities in the formation of the adducts, and their yields, are influenced by the diene and the reaction conditions employed. In particular, low overall yields were obtained in the Lewis acid-promoted reactions. This was attributed to the presence of a Lewis base (an acetoxymethyl group) in the pyranone 1. Therefore, (2R,6S)-6-methyl-2-(2-propyloxy)-2H-pyran-3(6H)-one (7), which possesses a C-6 methyl substituent instead of an acetoxymethyl, was prepared from L-fucose and subjected to the Et2O·BF3-catalyzed addition of cyclopentadiene. As expected, a good yield (74%) of the corresponding Diels-Alder product was obtained, with excellent b- and endo-diastereoselectivities