Mass spectral cycloreversion reactions of 2-chloro-3,6-diaryl-3,4-dihydro-1,3,2-oxazaphosphorin-2-oxides

The mass spectral fragmentation modes of various 2-chloro-3,6-diaryl-3,4-dihydro-1,3,2-oxazaphosphorin-2-oxides—a novel ring system—reveal cycloreversion by two pathways. Retro Diels-Alder reaction by a stepwise mechanism is prominent in this system. The relative abundances of the enone and dienophile ions depend on the nature of the substituent attached to the double bond in this ring. Another retro Diels-Alder fragmentation process, involving the loss of PO₂Cl from the molecular ion, is preceded by a 1,3-allylic rearrangement and is the major fragmentation mode in the metastable time scale. Further fragmentation of the M? PO₂Cl]⁺ imine ion seems to occur from cyclic dihydroquinoline intermediates by substitution elimination steps.