Asymmetric Diels–Alder reactions. Part 1. Diastereofacial reactivity of (E)-3-trimethylsilyloxybuta-1,3-dienyl 2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside towards cyclic dienophiles

Abstract

The title diene (5a) reacted with p-benzoquinone in benzene at ambient temperature to give an 89 : 11 mixture of (1R,6R,10S)-10-[(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl)oxy]-8-trimethylsilyoxybicyclo[4.4.0]deca-3,8-diene-2,5-dione (9a) and its (1S,6S,10R)-diastereoisomer (10a). The stereostructure of the major cycloadduct was established by its conversion into (1R,6R,10S)-10[(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl)oxy]bicyclo[4.4.0]decane-2,5,8-trione (12), the structure of which was determined by X-ray crystallography. The diene (5a) showed a similar diastereofacial selectivity in its reactions with 2-methoxycarbonyl-p-benzoquinone [to give an 88 : 12 mixture of the cycloadducts (9b) and (10b)], 2-acetyl-p-benzoquinone [to yield a 75 : 25 mixture of the cycloadducts (9c) and (10c)], N-phenylmaleimide [to afford an 86 : 14 mixture of the cycloadducts (15a) and (16a)], maleimide [to produce an 85 : 15 mixture of the cycloadducts (15b) and (16b)], and malefic anhydride [to furnish mainly the cycloadduct (15c)]. In every instance, the major cycloadduct could be isolated in a pure state by crystallisation. Under mildly acidic conditions, it underwent hydrolysis resulting in the conversion of its O-silyl enol moiety into the ketone function. On the basis of c.d. spectroscopy, the ketones (11a–c)[derived from the respective cycloadducts (9a–c)] possessed the same absolute stereochemistry at positions 1, 6, and 10. The ketone (17c), formed by hydrolysis of the cycloadduct (15c), was transformed by the action of aniline and acetic anhydride into the ketone (17a), which was also obtained from the cycloadduct (15a) by hydrolysis. The conformational properties of the cycloadducts and their hydrolysis products were assessed by 300 MHz ¹H n.m.r. spectroscopy.