Asymmetric Diels–Alder reactions. Part 5. Influence of sugar substituents upon the diastereofacial reactivity of (E-)3-(t-butyldimethylsiloxy)-1-(D-glucopyranosyloxy)buta-1,3-dienes
The 1-[3′,4′,6′-tn-O-acetyl-2′-O-(t-butyldimethylsilyl)-α-D-glucopyranosyloxy], 1-[2,′,3′,4′-tri-O-acetyl-6′-O-(t-butyldimethylsily)-α-D-glucopyranosyloxy], 1-(3′,4′,6′-tri-O-acetyl-2′-deoxy-α-D-glucopyranosyloxy), and 1-(2′,3′,4′-tri-O-acetyl-α-D-xylopyranosyloxy) derivatives of (E-)3-(t-butyldimethylsiloxy)buta-1,3-diene, i.e.(6d-g), and their β-anomers, i.e.(11c-f), have been prepared and their diastereofacial reactivities towards N-phenylmaleimide assessed. Whereas the 2′-O-(t-butyldimethylsilyl)-α-diene (6d) gave a 20 : 80 mixture of thecycloadducts (9d) and (10d), its β-anomer, i.e.(11c), afforded a 66 : 34 mixture of the cycloadducts (12c) and (13c)[the major cycloadducts having the (1R,2R,3S)-configuration]. The 6′-O-(t-butyldimethylsilyl)-α-diene (6e) gave a 71:29 mixture of the cycloadducts (9e) and (10e)[the major cycloadduct having the (1S,2S,3R)-geometry]; its β-anomer, i.e.(11d), afforded an 89:11 mixture of the cycloadducts (12d) and (13d)[the major-product possessing the (1R,2R,3S)-stereochemistry]. Although no stereoselection was observed with, the 2′-deoxy-α-diene (6f), an 80:20 mixture of the cycloadducts (12e) and (13e)[the major product having the (1R,2R,3S)-geometry] was formed in the case of its β-anomer, i.e.(11e). The diastereofacial reactivities of the α- and β-D-xylopyranosyldienes, i.e.(6g) and (11f), were found to be equal and opposite; thus, the α-diene (6g) yielded an 81:19 mixture of the cycloadducts (9g) and (10g)[the major product having the (1S,2S,3R)-configuration]. The stereostructure of the cycloadduct (9g)[(1S,2S,3R)-5-(t-butyldimethylsiloxy)-N-phenyl-3-(2′,3′,4′-tri-O-acetyl-α-D-xylopyranosyloxy)-cyclohex-4-ene-1,2-dicarboximide] was confirmed by an X-ray crystallographic analysis.