Acetic anhydride formation mechanism from methyl

Heating ( )-methyl l-oxide-2-pyridyl ketone oxime (482) in acetic anhydride at temperatures above 100°C initiated an exothermic reaction from which 3-methyl-3a-(3 -methylisoxazolo[4,5-b]pyridin-5 -yl)-3a//-[l,2,5]-oxadiazolo[2,3-a]pyridine (484) was isolated in 18% yield (Scheme 57) <86CPB4984>. If the reaction is carried out at a temperature less than 100°C, only the acetylated oxime (483) is isolated. Heating the (Z)-isomer in acetic anhydride at similar temperatures resulted only in the formation of the acetylated oxime. The structure of the product (484) was determined on the basis of two-dimensional COSY, DEPT, and 2D H-I3C chemical shift correlation experiments. The structure was also confirmed chemically via bromination which affords a mixture of the bromine adducts (485) and (486). Attempts to hydrolyze (484) under either acidic or basic conditions afforded only unchanged starting material. The formation of compound (484) from the TV-oxide (483) proceeds via a radical mechanism. This mechanism was elucidated by a study of the reaction... 

This type of reaction was first used by Dilthey and Fischer69 in connection with the mechanism of the type in Section II, D, 2. By analogy with the well-known formation of benzopyrylium salts from methyl(ene) ketones and o-hydroxybenzaldehyde or o-hydroxy-acetophenone,340 benzoylacetone (84, R = Me, R = H, R" = Ph) and acetophenone (85, RIV = Ph, R" = H) afforded in the presence of acetic anhydride and sulfuric acid, 2-methyl-4,6-diphenylpyrylium sulfoacetate. The interesting feature of this reaction is that a unique product is obtained (i.e., only the carbonyl group of the diketone adjacent to the phenyl enters the condensation, although two such groups are available) R and R" could be reversed, but they are not.69-69 Similarly, dibenzoylmethane (84, R = R" = Ph, R =H) afforded 2,4,6-triphenylpyrylium with acetophenone in 80% yield.69... 

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