Acetolysis catalysts

Efforts to cause the carbon nucleophile available at C-2 (carbohydrate numbering) of the osulose derivative 66 to displace the methoxy group with allylic rearrangement and with consequent formation of a tricyclic product by use of Pd(0) catalysts [34] were unsuccessful, but the intended reaction proceeds "smoothly when tin(IV) chloride is used together with acetic anhydride in dichloromethane. Clearly, the Lewis acid activates the allylic ether group, and the C-2 nucleophile effects its displacement. Concurrently, acetolysis of the benzylidene ring occurs and the product isolated is the cu-decalin analogue 67 [33],... 

Deacetylation of 80 in methanolic sodium methoxide and successive O-isopropyli-denation with 2,2-dimethoxypropane gave 2-0-acetyl-3,4-O-isopropylidene-l,6-an-hydro-pseudo-p-DL-galactopyranose (81), after acetylation. Removal of the acetyl group of 81, followed by oxidation with ruthenium tetroxide and sodium metaperiodate afforded the 2-oxo derivative (82). Catalytic hydrogenation of 82 under the presence of platinum catalyst and acetolysis in a mixture of acetic acid, acetic anhydride and sulfuric acid gave pseudo-P-DL-talopyranose pentaacetate (83) [25] (Scheme 17). 

The method most likely to provide information concerning the nature of the reaction intermediates is that of detailed, kinetic analysis under a variety of carefully controlled conditions. It is evident that, in the application of this method, there is an extensive field of investigation to be surveyed, not only of the action of acyl trifiuoroacetates on hydroxylic compounds under the influence of different media and catalysts, but also of the peculiar differences between the ring-opening reactions of cyclic acetals with this type of reagent and with that which is employed in the Hudson acetolysis procedure. 

In 1934, Hudson and his group observed that treatment of a number of methyl D-aldohexopyranoside tetraacetates gave the a-D-aldohexopyranose pentaacetates on acetolysis with sulfuric acid as catalyst. Three years later, they observed that methyl jS-D-arabinopyranoside 2,3,4-triacetate gives 56% of alde%do-D-arabinose hexaacetate and 11% of /3-D-arabino-pyranose 1,2,3,4-tetraacetate the optical rotation of the mixture decreased steadily during the reaction. However, for the a-D-arabinoside, the rotation rose very sharply during the first minute and then fell more slowly. At the end of the reaction, the products were essentially those from the jS-D-glycoside at the point of maximum rotation, a 14% yield of methyl jS-D-arabinopyranoside was isolated, showing that anomerization can occur under these conditions, followed by subsequent acetolysis to the peracetate. 

Janson and Lindberg endeavored to distinguish between these two mechanisms by studying the acetolysis, with sulfuric acid as catalyst, of isopropyl 2,3,4,6-tetra-0-acetyl- 8-D-glucopyranoside (21) and comparing the reaction with that of the corresponding 2-methyl ether (22) and the 2-deoxy analog (23). 

Other reactions such as acetolysis of 1,6-anhydro-P-hexopyranoses and per-acety-lation of hexoses with acetic anhydride [95], condensation of indole and various aldehydes [96], Baeyer-Villiger oxidation with H2O2 [97, 98], Perrier rearrangement [71], and generation and reactions of a-organylselanyl carbenium ions with nucleophiles [99] also proceeded well over scandium catalysts. 

Acetolysis of methyl (methyl 4-0-methyl- -D-glucopyranosid)uronate with acetic acid and a Lewis acid (e.g., BF3.0Et2, AIQ3) gave the acylal (17) in almost quantitative yield. When sulphuric acid was used as catalyst, a number of intermediates such as the mixed acetals (18) could be isolated. The x-ray crystal structure of (f )-(18) is referred to in Chapter 22. 

The kinetics of acetolysis of cellulose acetate in mixtures of chloroform and acetic anhydride containing either perchloric acid or sulphuric acid have been examined.The rate of change of the molecular weight of cellulose acetate was followed by viscometry, and the dependences of the first-order rate constant on the concentrations of catalyst, acetic anhydride, and cellulose acetate and on the molecular weight and temperature were determined. A mechanism was proposed for the acetolysis. 

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