ASYMMETRIC DIELS-ALDER REACTIONS .4. INFLUENCE OF ANOMERIC CONFIGURATION AND 2'-O-BENZYL AND 6'-O-BENZYL SUBSTITUTION ON THE DIASTEREOFACIAL REACTIVITY OF (E)-3-(TERT-BUTYLDIMETHYLSILOXY)-1-(2',3',4',6'-TETRA-O-ACETYL-BETA-D-GLUCOPYRANOSYLOXY)-BUTA-1,3-DIENE

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- glucopyranosyloxy) derivatives of (E)-3-(t-butyldimethylsiloxy)buta-1,3-diene, i.e. (8b-d) and (34a,b) have been prepared and their diastereofacial reactivities towards N-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).