Theoretical and experimental analysis of stable montréalones reveals overlapping azomethine ylide and Wittig-type moieties within the unsaturated 5-membered organophosphorus ring system.

Depending on the phosphorus substituents, ring-chain valence tautomerism allows montréalones to display variable degrees of equilibrium and structural blending with N-acyl amino phosphonium ylide forms.

Reaction of organophosphorus(III) compounds with N-acyliminium ions affords phosphonium salt intermediates which are deprotonated using non-nucleophilic bases (e.g. DBU, LiHMDS).

The most efficient phosphorus(III) precursor for dipole synthesis is 2-phenylbenzo[d][1,3,2]dioxaphosphole [PhP(catechyl)] owing to the balance of nucleophilicity and electrophilicity that it affords.

[11][12][13] In contrast to related Diels-Alder reactions, rationalization of regioisomeric bias using conventional frontier molecular orbital (FMO) theory fails.