Unexpected [2 + 2] cycloaddition between the PO group of P-(2,4,6-triisopropylphenyl) P-heterocycles and dimethyl acetylenedicarboxylate

Abstract

The reaction of 1-(2,4,6-triisopropylphenyl)-1,2-dihydrophosphinine 1-oxide 1d with dimethyl acetylenedicarboxylate (DMAD) affords, surprisingly, oxaphosphetene 3 instead of the expected Diels–Alder cycloadduct; the unusual reactivity of the trialkylphenylphosphine oxides towards DMAD seems to be of general value.