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-beta-D-fructopyranose (6) and 3-O-benzyl-5-O-acryloyl-1,2-O -isopropylidene-alpha-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-beta-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-alpha-D-yilofuranose (22b) indicating that pi-stacked conformations in the Lewis acid-acrylates complex were not effective.

Keywords

• asymmetric Diels-Alder reactions
• chiral dienophiles
• carbohydrates