Asymmetric Diels–Alder reactions. Part 4. Influence of anomeric configuration and 2′- and 6′-O-benzyl substitution on the diastereofacial reactivity of (E)-3-(t-butyldimethylsiloxy)-1-(2′,3′,4′,6′-tetra-O-acetyl-β-D-glucopyranosyloxy)-buta-1,3-diene

Abstract

The 1-(2′,3′,4′,6′-tetra-O-acetyl-α-D-glucopyranosyloxy), 1-(6′-O-benzyl-2′,3′,4′-tri-O-acetyl-α-D-glucopyranosyloxy), 1 -(2′-O-benzyl-3′,4′,6′-tri-O-acetyl-α-D-glucopyranosyloxy), 1-(6′-O-benzyl-2′,3′,4′-tri-O-acetyl-β-D-glucopyranosyloxy), and 1-(2′-O-benzyl-3′,4′,6′-tri-O-acetyl-β-D-glucopyra-nosyloxy) derivatives of (E)-3-(t-butyldimethylsiloxy)buta-1,3-diene, i.e.(8b–d) and (34a,b) have been prepared and their diastereofacial reactivities towardsN-phenylmaleimide assessed. Whereas the α-diene (8b) gave a 55 : 45 mixture of the cycloadducts (11b) and (12b)[the major product having the (1S,2S,3R)-configuration], its β-anomer, i.e.(1b), afforded an 85 : 15 mixture of the cycloadducts (2b) and (3b)[the major product having the (1R,2R,3S)-geometry]. Improved diastereoselections were displayed by the 6′-O-benzyl-α-diene (8c), which gave a 69 : 31 mixture of the cycloadducts (11c) and (12c), and by the 6′-O-benzyl-β-diene (34a), which yielded only the cycloadduct (38a). By contrast, reduced diastereofacial reactivities were exhibited by the 2′-O-benzyl-α-diene (8d), which afforded a 36 : 64 mixture of the cycloadducts (11d) and (12d)[the major product now having the (1R,2R,3S)-stereochemistry], and by the 2′-O-benzyl-β-diene (34b), which gave a 74 : 26 mixture of the cycloadducts (38b) and (39b).