Trityl derivatives of cellobiose. IV. Studies on the relative reactivities of the secondary hydroxyl groups in 6,6'-di-O-tritylcellobiose and methyl 6,6'-di-O-trityl-.BETA.-cellobioside by selective acetylation.

Abstract

The selective acetylation of 6, 6'-di-O-tritylcellobiose (1) and its methyl β-glycoside (5) with 5 molar equivalents of acetyl chloride in pyridine-toluene at 1-4°C has been studied. Acetylation of 1 gave a complex mixture from which α- and β-anomers of three main products, the 1, 2, 2', 3', 4'-pentaacetate (A), the 1, 2, 2', 3'-tetraacetate (B), and the 1, 2, 3', 4'-tetraacetate (C) were isolated by column chromatography. From 5, the 2, 2', 3', 4'-tetraacetate (D, 22.9%), the 2, 2', 3'-triacetate (E, 15.7%), the 2, 3', 4'-triacetate (F, 15.7%), the 2, 3'-diacetate (G, 10.8%), the 2', 3', 4'-triacetate (H, 14.2%), the 2', 3'-diacetate (I, 10.8%), the 3, 3', 4'-triacetate (J, 3.4%), and the 3, 3'-diacetate (K, 5.6%) were obtained. The locations of the unacetylated hydroxyl groups in the partially acetylated compounds were determined by gas chromatography-mass spectrometry after the sequence of methylation (for labeling free hydroxyl groups) with methyl trifluoromethanesulfonate, detritylation, deacetylation, (reduction), methanolysis, and trimethylsilylation and by comparison of acetoxyl signals in 1H-NMR spectra after trideuterioacetylation of free hydroxyl groups in the partially acetylated compounds. From the yields of partially acetylated methyl 6, 6'-di-O-trityl-β-cellobiosides, it was deduced that the order of reactivity of the five secondary hydroxyl groups in 5 is HO-3'>HO-2>HO-2'>HO-4'»HO-3.