The nitrile oxide/isoxazoline approach to eleven-carbon monosaccharides: cycloaddition of d- and l-arabinonitrile oxides to 5,6-dideoxyhex-5-enofuranoses and characterisation of the resulting 2-isoxazolines

Cycloaddition of 2,3:4,5-di-O-isopropylidene-d-arabinonitrile oxide 5, generated by base-induced dehydrochlorination of hydroximoyl chloride 10, to d-Glc-derived alkene 7 afforded an 89:11 diastereomeric mixture of (5R)- and (5S)-3-(1,2:3,4-di-O-isopropylidene-d-arabino-tetritol-1-yl)-5-(3-O-benzyl-1,2-O-isopropylidene-α-d-xylo-tetrofuranos-4-yl)-4,5-dihydroisoxazoles 11 and 12. The corresponding reaction of nitrile oxide 5 with d-Man-derived alkene 8 proceeded similarly yielding 4,5-dihydroisoxazoles 17 and 18 (82:18). Comparable levels of π-facial selectivity (66–76% d.e.) in favour of 5R-adducts were observed for the reactions of l-Ara-derived nitrile oxide 6 with alkenes 7 and 8 to form dihydroisoxazoles 23/24 and 25/26, thus demonstrating that the configuration of the nitrile oxide component in the cycloaddition has little effect on the stereochemical outcome of the reaction. The structure of dihydroisoxazoline 23 was determined by X-ray crystallography, and cycloadducts 11, 12, 17, 18, 23–26 were identified from their physical and spectroscopic properties by comparison with those of known analogues prepared from the same alkenes.