Synthesis of New Chiral Auxiliares from Carbohydrates for Et2AlCl-Promoted Diels-Alder Reactions

Abstract

Acrylates 3-O-acryloyl-4,5-di-O-benzoyl-1,2-O-isopropylidene-β-D-fructopyranose (6) and 3-O-benzyl-5-O-acryloyl-1,2-O-isopropylidene-α-D-xylofuranose (8) were synthesized in few steps from carbohydrates to construct chiral dienophiles for asymmetric Diels-Alder reaction with cyclopentadiene under thermal and Et2AlCl-promoted conditions. Although from moderate to excellent endo/exo ratios were obtained, low p-facial diastereoselectivities were observed for the cycloadducts 3-O-[(1′R, 4′R)-bicyclo-[2.2.1]hept-2′-ene-5′-formyl]-4,5-di-O-benzoyl-1, 2-O-isopropylidene-β-D-fructopyranose (21a) and 3-O-benzyl-5-O-[(1′S, 4′S)-bicyclo-[2.2.1]hept-2′-ene-5′-formyl]-1,2-O- isopropylideno-α-D-yilofuranose (22b) indicating that π-stacked conformations in the Lewis acid-acrylates complex were not effective.