Convenient synthesis of 1-alkoxy-1,2-dihydrophosphinine 1-oxides by ring enlargement

The addition of dichlorocarbene to the double bond of 1‐alkoxy‐dihydrophosphole oxides and subsequent thermolysis of the adduct so obtained affords mixtures of the two double bond isomers of alkoxy‐dihydrophosphinine oxides, if the latter step is carried out in the presence of triethylamine. Experimental data support the involvement of a cationic intermediate during the opening of the cyclopropane ring. A simplified procedure for the preparation of the starting dihydrophospholes is also presented.