Cyclic phosphoranes conformation

The reaction of cyclic phosphine oxides bearing a 2,4,6-trialkylphenyl substituent on the phosphorus atom (87) with DMAD affording phosphorane/ylide 88 was assumed to involve an intermediate of oxaphosphete type (89) formed by the [2 + 2] cycloaddition of the P = O group and the acetylenic moiety. Ring opening of the strained oxaphosphete (89), that may adopt different conformations (see below), may give the phosphorane/ylide (88) (Scheme 42)[60, 61]. This kind of reaction has been observed for the first time by Keglevich et al. 

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