A novel synthesis of bicyclo[3.3.0]oct-1-en-3-ones from cyclobutanones through [chloro(p-tolylsulfinyl)methylidene]cyclobutanes with ring expansion

Treatment of [chloro(p-tolylsulfinyl)methylidene]cyclobutanes, which were synthesized from cyclobutanones and chloromethyl p-tolyl sulfoxide in three steps in high overall yields, with excess cyanomethyllithium gave enaminonitriles in high yields. Heating of these enaminonitriles with H3PO4 in acetic acid gave 2-cyanobicyclo[3.3.0]oct-1-en-3-ones in good yield. On the other hand, treatment of the [chloro(p-tolylsulfinyl)methylidene]cyclobutanes with cyanomethyllithium followed by lithium carbanion of the homologues of acetonitrile afforded enaminonitriles having a substituent at the 3-position. Heating of the enaminonitriles with H3PO4 in acetic acid gave 2-substituted bicyclo[3.3.0]oct-1-en-3-ones in good to high yields. This method offers a novel and versatile procedure for synthesis of 2-substituted bicyclo[3.3.0]oct-1-en-3-ones from cyclobutanones in good overall yields.