Base-induced cycloreversion of nitrile oxide cycloadducts: conversion of imines into secondary and tertiary amides and aromatic aldehydes into acids without a conventional oxidising agent

Abstract

A series of substituted Δ²-1,2,4-oxadiazolines (4,5-dihydro-1,2,4-oxadiazoles) 12 have been prepared by 1,3-dipolar cycloaddition of nitrile oxides to imines and are found, upon treatment with KOBu^t, to undergo cycloreversion to give nitriles and amide anions. These can be protonated to give secondary amides or treated in situ with alkyl halides to give tertiary amides in moderate to good overall yield, although the reaction is restricted to examples with an aromatic substituent at the 5-position. The 1,4,2-dioxazoles 15, formed by cycloaddition of benzonitrile oxide to aromatic aldehydes, similarly undergo cycloreversion allowing direct conversion either into substituted benzoic acids or their methyl esters.