Phosphaalkyne tetramers from the spirocyclotrimer

When the original Lewis acid in the spirocyclic species 71a is removed by treatment with dimethyl sulfoxide as a Lewis base, the X o -diphosphete 72 is liberated however, it cannot be isolated as such [61]. 

First of all it must be mentioned that the two compounds 75 and 76 resulting from the spirocyclotrimerization of phosphaalkyne 9 a with subsequent cyclotetramer formation establish a 1 1 photochemical equilibrium in benzene solution. When they are heated in- 

The thermolysis of 75 and 76 to 54a is thermodynamically acceptable the relative energies of the two cyclotetramers mentioned before (as calculated for the parent compounds with H in place of f-Bu) are higher than that of 54 with H in place of t-Bu [55], 

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