The reaction of 1-(2,4,6-trimethylphenyl)-1,2-dihydrophosphinine 1-oxides with dimethyl acetylenedicarboxylate; a [4+2] or a [2+2] cycloaddition?

The reaction of dimethyl acetylenedicarboxylate (DMAD) with 3- and 5-methyl -1-aryl-1,2-dihydrophosphinine oxides (6a and 6b, respectively) obtained by the two-step ring enlargement of 2,5-dihydro-1H-phosphole oxide 4 followed different routes. Isomer 6a entered into a [4+2] cycloaddition with DMAD g iving, although in low yield, phosphabicyclooctadiene 7, while 6b reacted w ith the acetylene moiety according to a recently discovered [2+2] protocol to afford spirocyclic oxaphosphete 8. The reaction of isomers 6a and 6b wit h N-phenylmaleimide under forcing conditions furnished the expected Diels-A lder cycloadducts (10a and 10b, respectively). Hence, depending on the reac tant, isomer 6b displayed a dual reactivity.