Asymmetric Diels–Alder reactions. Part 6. Regio- and stereo-selective cycloadditions of 5-(2′,3′,4′,6′-tetra-O-acetyl-β-D-glucopyranosyloxy)-1,4-naphthoquinone

Abstract

The title naphthoquinone 1d underwent reaction with cyclopentadiene to give a cycloadduct, established as (1R,4S,4aR,9aS)-1,4,4a,9a-tetrahydro-5-(2′,3′,4′,6′-tetra-O-acetyl-β-D-glucopyranosyloxy)-1,4-methano-9,10-anthraquinone 9a by X-ray crystallographic analysis. Single cycloadducts, assigned the structures 4d, 4e and 17 were also isolated from the reactions of the naphthoquinone 1d with (E)-1 -methoxy-3-trimethylsiloxybuta-1,3-diene 2b, (E)-3-methyl-1-trimethylsiloxybuta-1,3-diene 2f and (E)-2-methyl-1-trimethylsiloxybuta-1,3-diene 16. A 2:1 mixture of cycloadducts, formulated as the regioisomers 4g and 3f, arose in the reaction of the dienophile 1d with (E)-1-acetoxy-3-methylbuta-1,3-diene 2g.

The sugar auxiliary was readily detached from the oxidation product of compound 9a, i.e.(1R,4S)-1,4-dihydro-5-(2′,3′,4′,6′-tetra-O-acetyl-β-D-glucopyranosyloxy)-1,4-methano-9,10-anthraquinone 18b, by acidic hydrolysis to give the aglycone 18a.

An X-ray crystallographic analysis of compound 1d revealed that the quinone ring adopts a boat-like geometry in which the carbonyl oxygen atoms shield the syn-face of the C(2)–C(3) double bond. endo-Addition of dienes to the anti-face provides an explanation for the high diastereofacial reactivity of the naphthoquinone 1d.