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

The addition of l-lithio-l-methoxy-l,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-l-ones in good to excellent yields. Acid-catalyzed ring expansion of the addition products of l-lithio-l-methoxy-l,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-l-ones to the corresponding allylic acetates, palladium(II)-catalyzed [3,3]-sigmatropic rearrangement can be effected, furnishing 5-(2-acetoxyethylidene)-2-cyclopenten-l-ones with high kinetic selectivity favoring the isomer with the 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. © 1990, American Chemical Society. All rights reserved.