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.