Showing papers on "Glucal published in 1973"

A Rapid Route to Hex-1-enopyran-3-uloses

Abstract: Whereas the pseudo-axial 3-hydroxyl group of allal derivatives is known to be inert to manganese dioxide oxidation, the equatorial counterpart in glucal derivatives is shown to be readily oxidized. Thus glucal is converted to the 6-O-triphenylmethyl ether or 6-O-benzoyl ester to promote solubility in chloroform, and the derivatives are oxidized in that solvent in 12 h or less.Acetonation of glucal is readily accomplished in 45 min using 2,2-dimethoxypropane and p-toluenesulfonic acid in dimethylformamide; the product may be oxidized directly either with manganese dioxide or dipyridine chromium oxide. Thus it is possible to obtain 1–3-g quantities of the crystalline acetonated enone from triacetyl glucal (via deacetylation, acetonation, and oxidation) in 3–4 h.If the acetonation medium stands for 2 h methyl 2,3-dideoxy-4,6-O-isopropylidene-α-D-erythro-hex-2-enopyranoside is obtained. The mechanism of its formation in the reaction medium is discussed.

Phosphorhaltige Kohlenhydrate. VIII. Synthese von Glycopyranosylphosphonaten durch Dialkyl-phosphit-Addition an Glycale unter Allylverschiebung

Abstract: 3,4,6-Tri-O-acetyl-1,2-didesoxyhex-1-enopyranosen (Glycale) reagieren in Dimethylphosphit bei BF3-Katalyse unter Allylumlagerung zu Dimethyl-(4,6-di-O-acetyl-2,3-didesoxyhex-2-enopyranosyl)phosphonaten. In der Pentose-Reihe verlauft die Reaktion entsprechend. Hei Einsatz der arabino-Verbindung 2 (Glucal) und der ribo-Verbindung 5 (Allal) wird das gleiche Anomeren-Verhaltnis 3a zu 6a (1 : 2) erhalten. Bei Reaktion der lyxo-Verbindung 8 (Galactal) und der xylo-Verbindung 11 (Gulal) enthalt das gemeinsame Produkt α-Form 9a und β-Form 12a im gleichen Verhaltnis (1 :3). Die Befunde sprechen fur eine SN1-Reaktion mit Umlagerung. Die Phosphonsaure-dialkylester lassen sich durch Umesterung zu Bistri-methylsilylestern unter milden Bedingungen in freie Phosphonsauren uberfuhren. 1H-NMR-, 31P-NMR-und Massenspektren der 2-Enopyranosylphosphonate werden diskutiert. Die Hex-2-enopyranosylphosphonate bevorzugen die OH5(D)-Halbsesselkonformation 37. Phosphorus-containing Carbohydrates, VIII. Synthesis of Glycopyranosylphosphonates by Addition of Dialkyl Phosphites to Glycals with Allylic Shift The reaction of 3,4,6-tri-O-acetyl-1,2-dideoxyhex-1-enopyranoses (glycals) in dimethyl phosphite catalysed by boron trifluoride affords dimethyl (4,6-di-O-acetyl-2,3-didcoxyhex-2-enopyranosyl)phosphonates by allylic rearrangement. A corresponding reaction proceeds in the pentose series. On treatment of the arabino-compound 2 (glucal) or the ribo-compound 5 (allal) the same anomeric distribution 3a to 6a (1 :2) is obtained. In the corresponding reaction or the lyxo-compound 8 (galactal) or the xylo-compound 11 (gulal) the mutual product contains α-form 9a and β-form 12a again in the same ratio (1:3). These findings support a SNl reaction with rearrangement. Via transesterification of the dialkyl esters to bistrimethyl-silyl phosphonates the free phosphonic acids are obtained under very mild conditions. 1H n.m.r., 31P n.m.r., and mass spectra of the 2-enopyranosylphosphonates are discussed. The hex-2-enopyranosylphosphonates prefer the OH5(D) half chair conformation 37.