Styrene or 1-vinylnaphthalene react with Moore's ketene (t-butylcyanoketene) to give cyclobutanones1a and1b, respectively, withcis configuration of the vicinal bulkyt-butyl and aryl groups. Steric crowding is reduced by (i) nonplanarity distortion of the cyclobutanone ring (ca. 20°), (ii) elongation to ca. 1.6 A of the C2-C3 bond (which is flanked by the bulky groups), (iii) deviation of the vicinal bulky groups by ca. 22° from the eclipsed conformation, and (iv) increased bond angles of thet-Bu-C2-C3 and Ar-C3-C2 groups.p-Methoxystyrene reacts with Moore's ketene to give cyclobutanone2a with the vicinal bulkyt-butyl and aryl groups in thetrans configuration. In contrast to1a and1b, (i) the cyclobutanone ring is planar, (ii) the C2-C3 bond is shorter (1.59 A), and the bond angles (iii) and (iv) are each reduced by 12.5°. It is concluded that in the case of styrene and 1-vinylnaphthalene the cycloaddition reaction is seemingly concerted and occurs according to a2 π s+2π a reaction mode, whereasp-methoxystyrene reacts stepwise with Moore's ketene, giving the thermodynamically more stable cyclobutanone isomer.