Reaction of aromatic N-oxides with dipolarophiles. XI 1,3-Dipolar cycloaddition reaction of pyridine N-oxides with tosyl isocyanate and one-pot synthesis of 2-oxooxazolo(4,5-b)pyridine derivatives.

Abstract

The cycloaddition reactivity of tosyl isocyanate toward pyridine N-oxides was calculated by the CNDO/2 method using a perturbation equation, and the results indicated that the initial stage of the reaction might be controlled by the coulombic attraction. The 1, 3-dipolar cycloaddition reaction of tosyl isocyanate with two equivalents of 3, 5-dibromopyridine N-oxide in refluxing benzene gave 6-bromo-2-oxooxazolo[4, 5-b]pyridine, while the use of one equivalent of 3, 5-dichloropyridine N-oxide gave 6, 7a-dichloro-2-oxo-3-tosyl-3a, 7a-dihydrooxazolo[4-, 5-b]pyridine, formed from the 1, 5-sigmatropic rearrangement of the corresponding primary cycloadduct. When the reaction was carried out in the presence of triethylamine in benzene, 6-halogeno-2-oxo-3-tosyloxazolo[4, 5-b]pyridine was isolated.The observed activation parameters, as well as the small solvent effects, may be interpreted in termas of a concerted pathway.The reaction should be very valuable as a one-pot synthesis of 2-oxooxazolo[4, 5-b]pyridines, which are pyridine analogues of 5-chloro-2-benzoxazolinone.