Acid-Catalyzed Ring Expansion of 1-(1-Methoxy-1,2-propadienyl)-2-cyclobuten-1-ols. Synthesis of 5-Hydroxy-5-vinyl-2-cyclopenten-1-ones and Their Stereoselective Transformation to 5-(2-Acetoxyethylidene)-2-cyclopenten-1-ones

Article abstract of DOI:10.1021/jo00302a027

The addition of 1-lithio-1-methoxy-1,2-propadiene to various cyclobutenones, cyclobutanones, and benzocyclobutenones produces sensitive 1,2-adducts that, in the presence of acid, rearrange to 5-hydroxy-5-vinyl-2-cyclopenten-1-ones in good to excellent yields.Acid-catalyzed ring expansion of the addition products of 1-lithio-1-methoxy-1,2-propadiene to cyclobutenones bearing a substituent at the 4-position occurs in a stereospecific fashion providing cyclopentenones with the 4-substituent and the 5-hydroxyl group in a cis relationship.After conversion of the 5-hydroxy-5-vinyl- 2-cyclopenten-1-ones to the corresponding allylic acetates, palladium(II)-catalyzed <3,3>-sigmatropic rearrangement can be effected, furnishing 5-(2-acetoxyethylidene)-2-cyclopenten-1-ones with high kinetic selectivity favoring the isomer with alkylidene substituent and the carbonyl group syn (Z-stereochemistry in most cases).On exposure to a trace of acid, equilibration occurs to the more stable isomer with the alkylidene substituent and carbonyl group anti.