clopentadiene and 12a and 12c re~pectively."~~ We assign to the remaining products (22 and 11 %) the structures of the still missing cis-diastereomers 13c, d. No significant cis-trans isomerization 12c -+ 12a ( < S o / , ) takes place under the reaction conditions." It is demonstrated therefore, that the cycloaddition of bicyclo[2.1 .O]pentene 6 proceeds nonstereospecifically, as expected in the case of a two-step process. R' R' R' R' 12a,c 13a-d 14a,c,d a: R~= R ' + = C N , RZ=R3=C0,CH,; b: R1=R4=C0,CH,, R Z = R 3 = C N ; c: R' = R3 =CO,CH,, R2 = R4 =CN; d: R1 = R3 = CN, R2 =R4 =C02CH3 carried out reactions of the [S]cumulenes 5 with nickel cata-The [S]cumulenes 5 a-c were prepared according to the procedure outlined in Scheme 1. Ethynylation of the ketones lysts.

Selective Dimerization by a Radical Chain Reaction in γ-Irradiated Crystals of 1,3-Bis(3-acetoxyprop-1-ynyl)-4-methoxy-6-methylbenzene. -γ-Irradiation of (I) under vacuum forms the single dimer (II). Such an exclusive formation of a dimeric product by the γ-irradiation is observed neither for benzene solutions of (I) nor for crystals of other disubstituted bis(3-acetoxyprop-1-ynyl)benzenes. The crystal structure of (I) is responsible for the selective dimerization by the chain reaction. A mechanism for the chain reaction is proposed on the basis of the crystal structure. -(YAMAMOTO, Y.; UEDA, T.; KANESHISA, N.; MIYAKAWA, K.; TAKAI, Y.; TAGAWA, S.; SAWADA, M.; KAI, Y.; J. Chem. Soc.