Asymmetric Diels–Alder reactions. Part 2. A model to account for the diastereofacial reactivity of (E)-1-(2′,3′,4′,6′-tetra-O-acetyl-β-D-glucopyranosyloxy)-3-trimethylsiloxybuta-1,3-diene and its 2-methyl derivative. X-Ray molecular structure of (E)-1-(2′,3′,4′,6′-tetra-O-acetyl-β-D-glucopyranosyloxy)-3-trimethylsiloxybuta-1,3-diene and of (1R,6R,9R,10S)-9-methyl-10-(2′,3′,4′,6′-tetra-O-acetyl-β-D-glucopyranosyloxy)bicyclo[4.4.0]dec-3-ene-2,5,8-trione

Abstract

The geometry of the title diene (1a) has been probed by X-ray crystallography and nuclear Overhauser difference (n.O.e.d.) spectroscopy. In the crystal state, the sugar adopts the expected chair conformation and the equatorially disposed anomeric substituent is planar, with the s-trans-diene entity and the anomeric C–O bond bearing an anti-relationship, i.e. (13a). Although the aforecited conformer is the main rotamer in deuteriochloroform, there is also evidence for a conformer in which the s-trans-diene moiety and the anomeric C–O bond possess a syn-disposition, i.e. (14a). On the basis of n.O.e.d. spectroscopy, the 2-methyl derivative of the title diene, i.e. (1b), is present in deuteriochloroform as a single conformer in which the s-trans-diene entity and the anomeric C–O bond bear an anti-disposition, i.e. (13b). The geometries (13a and b) and (14a) are considered to be stabilised by the exo-anomeric effect.

Essentially single cycloadducts were produced in the reactions of the diene (1b) with p-benzoquinone, methoxycarbonyl-p-benzoquinone, acetyl-p-benzoquinone, and N-phenylmaleimide. The cycloadduct obtained from p-benzoquinone was shown to possess the stereostructure (6d) on the basis of an X-ray crystallographic study of its hydrolysis product (9d). An 80 : 20 mixture of cycloadducts, with the presumed stereostructures (24b) and (25b), arose in the reaction of the diene (1b) with tetracyanoethylene [the corresponding reaction of the diene (1a) led to a 65 : 35 mixture of the presumed cycloadducts (24a) and (25a)].

The improved diastereofacial reactivity of the methylated diene (1b) over its counterpart (1a) is attributed to it reacting only by way of a conformer in which the s-cis-diene entity and the anomeric C–O bond bear an anti-relationship, i.e. (19b). Dienophiles then undergo endo-additions preferentially to the least-hindered ‘top’ face of this conformer. In the case of the diene (1a), two reacting conformers are postulated. The major and minor cycloadducts of cyclic dienophiles are considered to arise by endo-additions to the last-hindered ‘top’ faces of the conformers (19a) and (20), in which the s-cis-diene moiety and the anomeric C–O bond bear respective anti- and syn-orientations.