| I. Syntheses of trans-, syn-cis-, and anti-cis-6,7-dimethyltricyclo{3.2.1.0('1,5)}octanes (52, 53, and 54), where the terms syn and anti refer to the stereochemical relationships of the methyl substituents and the cyclopropane ring, are presented. Upon flash vacuum pyrolysis at 520(DEGREES)C the propellanes undergo high yield unimolecular rearrangements to varying proportions of syn-syn-, syn-anti-, and anti-anti-1,3-bisethylidenecyclohexanes (74, 75, and 76), where syn and anti refer to an allylic methyl group pointing towards or away from the other ethylidene unit: propellane 52 yields 3.9, 41.4, and 55.0% of dienes 74, 75, and 76, respectively; propellane 53 yields 39.6, 47.8, and 12.6% of dienes 74, 75, and 76, respectively; and propellane 54 yields less than 0.1, 0.5 and 99.4% of dienes 74, 75, and 76, respectively. The stereochemical results show that the cycloreversions are not strictly orbital symmetry controlled, nor do they proceed by a pathway that eradicates the stereochemical information. Factors that appear to affect the stereochemical outcome are steric crowding and electron overlap in the transition state.;II. Photolysis of isotopically labeled (alpha)-diazoketones yields products in which the isotopic label is partially scrambled. This scrambling has been attributed to the intermediacy of oxirenes; however, the possibility that (alpha)-diazoketones might undergo a reversible photochemical transformation to a symmetric intermediate, which would also lead to isotope scrambling in the reaction products, had not been excluded. Accordingly, azibenzil-('13)CO was photolyzed in methanol and recovered after partial conversion. No scrambling of the isotope position in recovered azibenzil was found, a result which supports the intermediacy of oxirenes. |
