Glycosylated cardenolides. VI. Ribosides of strophanthidin and digitoxigenin
Partial syntheses of ribosides of strophanthidin (I) and digitoxigenin (II) have been carried out by the orthoester method [i] by analogy with previous work [2]. 3,4-Di-O-acetyl~-D-ribopyranose 1,2-(methyl orthoacetate) (III) was obtained by the method of Mazurek and Perlin [3]. The orthoester (III), Cz2H~,Os, had mp 82-84°C (from diethyl ether--petroleum ether), [~]~4 --7.7 _+ 2 ° (c 1.78; chloroform). NMR spectrum (CDCI3), ppm: 1.75 (3 H at CH3, s), 2.10 (6 H at 2 Ac, s), 3.28 (3 H at OCH~, s). The NMR spectrum shows the exo position of the OCH3 group in compound (III) [3, 4].
SYNTHESIS OF β-D-MANNOSE 1,2-ORTHOACETATES
Synthesis of 3,4,6-tri-O-acetyl-β-D-mannopyranose 1,2-orthoacetates (I) from 2,3,4,6-tetra-O-acetyl-α-D-mannopyranosyl bromide (II) and various alcohols in the presence of pyridine and substituted pyridines is described. 2,6-Lutidine is a particularly effective acid acceptor in these condensations, and the use of chloroform as a diluent is advantageous. The lutidine-buffered system has afforded a good yield of orthoacetate (I) with several alcohols that barely react with II in the presence of silver ion; this difference is attributed to the dominance of side reactions under the latter conditions. The N-bases also promote a more highly stereoselective orthoester synthesis than does the silver-catalyzed reaction, leading to formation of the OR-exo-diastereoisomer almost exclusively.
Synthesis of aminoacyl derivatives of sugars
Abstract A simple and convenient method for preparation of aminoacyl derivatives of simple sugars by the condensation of unprotected carbohydrate with N-carbobenzoxyaminoacid in the presence of carbodiimide has been developed. The condensation of glucose with N-carbobenzoxyaminoacid affords almost exclusively the 6-O (N-carbobenzoxyaminoacyl)-sugars. By this method 6-O (N-carbobenzoxyaminoacyl)- d -glucoses containing residues of glycine, d , l -alanine, d , l -valine, d , l -norleucine, β-alanine and ϵ-aminocaproic acid have been prepared. The unprotected aminosugars condense smoothly with N-carbobenzoxyaminoacid in the presence of carbodiimide in aqueous pyridine to form N-(N′-carbobenzoxyaminoacyl)-hexosamine. The N-carbobenzoxy derivatives were converted to O- and N-aminoacyl derivatives with free amino groups by hydrogenolysis in aqueous methanol in the presence of oxalic acid. Corresponding O- and N-aminoacyl derivatives have been isolated as oxalates.