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.
THE IRON-ACETOCARMINE METHOD OF FIXING AND STAINING CHROMOSOMES
A short description of this procedure was given in Vol. 55 of the American Naturalist, pp. 573â€”574. The method has now been used almost daily for five years, and certain improvements have suggested themselves. The solution can be prepared by mixing about equal volumes of glacial acetic acid and water. To this is added powdered carmine in excess. The liquid is then â€ ? just brought to boil, cooled, and decanted. To the red solution there are then added a few drops of a solution of ferric hydrate in 50 per cent. acetic acid, to act as a mordant. The amount of iron varies with different objects. Too much produces a precipitate in a short time. The more