Stereocontrolled allylation of 2-amino-2-deoxy sugar derivatives by a free-radical procedure

Preparative routes for anomerically specific 1-C-allylation of 2-amino-2-deoxy sugars have been evaluated in a comparative study of various N-substituents and aglycons as precursors for glycosyl radicals that effectively capture an allyl group from allyltributyltin. The crystalline triacetate 4 of 3-(2-acetamido-2-deoxy-α-d-glucopyranosyl)-1-propene (6) was obtained in 70% yield when 2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-α-d-glucopyranosyl chloride (1) was treated with allyltributyltin under free-radical conditions, whereas the corresponding bromide 3 led only to an oxazolidine derivative; the β-1-ethylxanthate analogue of 1 gave 4, but in only 25% yield. The 2-trifluoroacetamido 1-bromide analogue of 1 was also an effective radical source, giving the 2-trifluoroacetamido analogue 8 of 4 in 60% yield. The free amino analogue 7 of 4 was conveniently obtained via the 2-p-methoxybenzylideneamino 1-bromide analogue of 1. Use of 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl bromide as radical precursor allowed stereospecific access to β-1-C-allyl derivatives of the amino sugar. The crystalline galactosamine analogue 12 of 4 was obtained by using the galacto analogue of chloride 1, but the corresponding manno chloride gave only an oxazoline product. The 1-C-allylated amino sugar derivatives are conformationally more mobile than derivatives not having a 1-C-linked substituent.