The Boekelheide Rearrangement of Pyrimidine N‐oxides as a Case Study of Closed or Open Shell Reactions ‐ Experimental and Computational Evidence for the Participation of Radical Intermediates

Abstract

In a case study, the acetic anhydride-promoted reaction of a model pyrimidine N-oxide to the corresponding 4-acetoxymethyl-substituted pyrimidine derivative (Boekelheide rearrangement) was investigated in detail by experiment and quantum chemical calculations. The reaction conditions were varied and several side products formed in low to moderate yields were identified. These experiments indicate that a (pyrimidin-4-yl)methyl radical is one of the key species of the rearrangement. This interpretation was supported by the fact that rearrangements performed in solvents which can easily lose hydrogen atoms, afford considerable quantities of products incorporating the solvent. With TEMPO the key radical could be trapped. Other carboxylic acid anhydrides confirm the conclusion that the Boekelheide rearrangement of the model pyrimidine N-oxide proceeds, at least in part, via radical intermediates. The high level closed and open shell quantum chemical calculations show that concerted [3,3]-sigmatropic rearrangements or stepwise processes, either via ion pairs or via radicals, are energetically feasible.

Synthesis of Heterocyclic Compounds with Pyrimidine-4-carbaldehydes as Key Intermediates

“…6,7 The reaction of these versatile intermediates with hydroxylamine hydrochloride smoothly provides 4-methyl-substituted pyrimidine N-oxides B under mild conditions, 8 whereas the heating of A with ammonium salts furnishes the corresponding pyrimidine derivatives C in good yields (Scheme 1). 9 The subsequent Boekelheide rearrangement of B with carboxylic acid anhydrides converts these compounds into 4-carbonyloxysubstituted pyrimidine derivatives, 10 and, after saponification, hydroxymethyl-substituted pyrimidines D are available with good efficiency (Scheme 1). The selective oxidation of the methyl group of C into a formyl group constitutes an alternative route to pyrimidine derivatives E with a functionalized substituent at C-4.…”