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.
A COMPARISON OF THE PROPERTIES OF PENTAACETATES AND METHYL 1,2-ORTHOACETATES OF GLUCOSE AND MANNOSE
The rates of exchange of acetate—concurrent with anomerization—between the C1-acetoxy groups of the pentaacetates (0.05 M) of D-glucose and D-mannose and stannic trichloride acetate (0.05 M) in chloroform containing stannic chloride (0.05 M) were determined at 40 °C. using isotopically labelled acetate. 1,2-trans-α-D-Mannose pentaacetate underwent exchange seven times more rapidly than the β-1,2-cis-anomer but eight times less rapidly than 1,2-trans-β-D-glucose pentaacetate. The latter compound was 450 times more reactive than the α-1,2-cis-anomer. In accordance with these results, the D-mannose pentaacetate underwent mercaptolysis in ethyl mercaptan containing zinc chloride at rates intermediate to those found for the D-glucose pentaacetates. The main product from the mannose pentaacetates was in each case ethyl 1,2-trans-α-D-1-thiomannopyranoside tetraacetate (60–70% yield). Tetra-O-acetyl-β-D-glucopyranosyl chloride with silver acetate gave β-D-glucose pentaacetate when the reaction was carried out in ...