Stability of 3-glycosyloxyprolines in an alkaline medium. Synthesis of model compounds

Abstract

Glycosylation of the hindered secondary alcohol groups in substituted 3- and 4-hydroxyproline by reaction with 3,4,6-tri- O-acetyl- d-glucal and boron trifluoride etherate afforded 40–80% yields of 3- and 4-glycosyloxypyrrolidine-2-carboxylic acids. Modification of the 2,3-unsaturated glycosides of 4-hydroxyproline was achieved by hydrogenation, by addition of acetyl hypobromite, or by epoxidation with hydrogen peroxide in benzonitrile and ring-opening of the resultant 2,3-anhydro derivatives. From these model compounds was selected a 3-glycosyloxyproline that was suitable for assessment of the stability of the glycosyl linkage to base-catalyzed elimination. The model compound was N-acetyl-3- O-(4,6-di- O-acetyl-2,3-dideoxy-α- d- erythro-hexopyranosyl)- cis- d, l-hydroxyproline ethyl ester ( 12). Homogeneity of the compounds was ascertained by high-resolution n.m.r. spectral and chromatographic methods. The glycoside ( 12) of 3-hydroxyproline is stable even in warm base at 50°. The 3- and 4-hydroxyproline glycosides are both more stable to acid hydrolysis than the analogous compounds of serine and threonine. A reliable synthesis of crystalline cis- d, l-3-hydroxyproline ethyl ester hydrochloride is reported that affords quantities of 250 mg or more in overall yield of 15% from inexpensive starting materials.