Straightforward synthesis of a derivative of purpurosamine C from d-galactose

Abstract

Purpurosamine C ( 1) is a component of the aminoglycoside antibiotic gentamicin C 1a. A derivative of 1 was synthesized from d-galactose via its 2-acetoxy-3,4,6-tri- O-acetyl glycal ( 3). Compound 3 undergoes glycosylation with 2-propanol in the presence of SnCl 4, with two succesive allylic rearrangements of the double bond to give isopropyl 6- O-acetyl-3,4-dideoxy-α- d- glycero-hex-3-enopyranosid-2-ulose ( 7). Compound 7 was hydrogenated, and O-deacetylated to afford 8. The free OH group of 8 was tosylated and substituted by azide, and the carbonyl function of the resulting ulose 10 reacted with hydroxylamine to give the E,Z-oximes ( 11, 12). Highly diastereoselective reduction of the oxime acetate ( 13) by borane, which also reduced the azide function, led to the purpurosamine C derivative 14 (∼40% yield from 3).