Ozonolyses of enantiopure 4-alkoxy-3,6-dihydro-2H-1,2-oxazines:: An expedient route to functionalized α-amino-β-hydroxy esters

Ozonolysis and subsequent in situ acylation/base treatment converted the enantiopure carbohydrate-derived 1,2-oxazines 1, 5, 8 (either syn- or anti-configured) and anti-11 into the highly functionalized alpha-amino-beta-hydroxy esters 2, 6, 9 and 12 in moderate to good yields. Ketals of 5-hydroxylated 1,2-oxazines (e.g., syn-3) were formed as side products in most cases; the reactions of precursors syn- and anti-8 even provided the ketals syn- or anti-10 as major components. The synthesized functionalized amino esters are valuable intermediates, as demonstrated by the subsequent transformation of anti-2 into the enantiopure building blocks 13-15. Since precursor 1,2-oxazines were prepared via lithiated alkoxyallenes, the latter species were serving as formyl ester anion equivalents. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).