Triphospha-Dewar-phosphinines

The 1,3,5-triphospha-Dewar-phosphinines (1,2,5-triphosphabicyclo [2.2.0]-hexa-2,5-diene)s 19 (see Scheme 4) readily participate in homo-Diels-Alder reactions with alkynes. Not only acetylene itself but also diphenylacetylene and acetylenedicarboxylates undergo smooth reactions to furnish the tetracyclic compounds 103 [20,29,78,88]. 

Compound 105 has been prepared by an independent protocol. Phosphaalkyne la undergoes quantitative cyclotrimerization under the influence of a Lewis acid such as AICI3 to furnish the spirocyclic diphosphirenium betaine 106 [91 ]. The Lewis acid can be removed by treatment with the weak Lewis base DMSO. The spirocycUc system 107 cannot be isolated since it rearranges rapidly to the 1,3,5-triphospha-Dewar-phosphinine 19 which - again - cannot be isolated under these reaction conditions but is, in turn, trapped by added phosphaalkyne in a homo-Diels-Alder reaction to afford the phosphaalkyne tetramer 105 [92]. 

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