Formation of unexpected heterocyclic products from pyrolysis of thiocarbonyl stabilised phosphonium ylides

Chiral phthalimido thioxo-stabilised phosphonium ylides, prepared starting from (S)-alanine and (S)-phenylalanine, undergo intramolecular Wittig reaction upon pyrolysis leading to the previously unknown pyrrolo[2,1-a]isoindol-5-one-2-thiones, rather than the expected P to S migration of a phenyl group. Pyrolysis of thioxo-stabilised ylides with three different groups on phosphorus gave some evidence of the required migration, but led in one case to unexpected formation of a 1,4-benzoxaphosphininium salt