Pentacovalent oxaphospholene

McClure, C.K., Grote, C.W., and Rheingold, A.L., Novel and efficient synthesis of uracil phosphonate derivatives via pentacovalent oxaphospholenes, Tetrahedron Lett., 34, 983, 1993. 

A related route to 3-oxoalkylphosphonates involves the conjugate addition of trialkyl phosphites to a,P-unsaturated ketones and hydrolysis of the intermediate oxaphospholenes. The utility of this method is enhanced by the nucleophilic character of the intermediate oxaphosphorane, which facilitates the stereoselective aldol reaction resulting in the formation of p-substituted y-ketophos-phonates. - The pentacovalent oxaphospholene reacts with dialkyl azidocarboxylates to give P-hydrazido-y-ketophosphonates in excellent yields. ... 

Pentacovalent 2,2,2-triethoxy-2,2-dihydro-5-methoxy-1,2-2 -oxapho-spholene (342) has been synthesized as a new type of enolate from a.jS-nn-saturated ketones (341) and trialkyl phosphites and then hydrolyzed to give a series of phosphonates (343) and (344) (Scheme 77). The aldol reaction of the oxaphospholene intermediate with several aldehydes as electrophiles under mild and neutral conditions produced phosphonate-containing aldol compounds. 

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