| Treatment of (E) ;The effects of alkene substituents on intramolecular type 1 cycloadditions are explored. Type 1 intramolecular cycloaddition of vinylketenes to a (E)-1,2-disubstituted alkene and a terminally disubstituted alkene gives 1-ethenyl-7-methylbicyclo (3.1.1) heptan-6-one (12) and 1-ethenyl-7,7-dimethylbicyclo (3.1.1) heptan-6-one (26) in good yields.;Treatment of cyclobutanones 12 and 26 with vinyllithium gives trans-divinyl-alkoxides which undergo oxy-Cope rearrangement to give (E)-11-methyl and (E)-11,11-dimethyl-bicyclo (5.3.1) undec-6-en-2-ones 62a and 62b. Intramolecular cycloaddition of the divinylketene derived from 3,7-dimethyl-2-ethenyl-2,6-octadienoic acid (77) gives 7,7-dimethyl-1-ethenyl-2-methylenebicyclo (3.1.1) heptan-6-one (48) in 30% yield. Treatment of 77 with vinyllithium gives the oxy-Cope rearrangement product (E)-11,11-dimethyl-8-methylenebicyclo (5.3.1) undec-6-en-2-one (88a) containing the bicyclo (5.3.1) undecane moiety of the taxane skeleton. These oxy-Cope rearrangements are constrained to produce (E)-cyclooctenones through a chair-like transition state. Introduction of alkyl substituents on the reacting alkenes prevents the alkenes from adopting the conformation required for an oxy-Cope rearrangement.;Ketoketenes (dialkyl substituted ketenes), which are easily generated from 2-alkyl acid chlorides, undergo intramolecular cycloaddition with alkenes to give 5-alkylbicyclo (3.2.0) heptanones and 1-alkylbicyclo (3.1.1.) heptanones in good yield. Intramolecular cycloaddition of a ketoketene in which both substituents contain double bonds, establish that (E)-alkenes are approximately 34 times more reactive than (Z)-alkenes in intramolecular cycloadditions with ketenes. |
