Summary2-Cyclopentenyl and 3-phenyl-2-cyclopentenyl methyl ketones (15-18, 30, 31) undergo a 1,3-acetyl shift on direct irradiation, and the oxa-di-7c-methane rearrangement to photochemically non-interconverting endo and exo bicyclo-[2.l.O]pentyl methyl ketones on triplet sensitization. Exceptions include the 2-methyl-3-phenyl-2-cyclopentenyl methyl ketone 32 and the I-phenyl-2-cyclopentenyl methyl ketone 44 which are unreactive on direct irradiation and on triplet sensitization, respectively, and the 2-phenyl-2-cyclopentenyl methyl ketones 42 and 43 which do not react under either condition. The reactive triplet of the 3-phenyl-2-cyclopentenyl methyl ketone 30 has been identified as the localized styrene 7c, 7c*-state of ET= 59 kcaVmol by comparison of its phosphorescence at 77 K in rigid glasses with that of l-phenyl-cyclopentene, and by the independence of the quantum yield on sensitizer energy in the range of 61-74 kcaVmol.

Bicyclo[2.1.0]pent-5-y1 methyl ketones undergo endo-exo stereomutation and cyclopropyl-allylic rearrangement to cyclopent-2-enyl methyl ketones at temperatures above ca. 130" in inert solvents. These isornerizations are not accompanied by any other detectable thermal processes. The stereomutation has been described first by Jorgenson & Thacher [2] for the (1,5-dimethylbicyclopent-5-y1) methyl ketones, and the rearrangement was encountered simultaneously by these authors with the same compounds and by ourselves [3] with the 1,4,5-triniethyl homologs. We now report tlie results of a study designed to elucidate the mechanistic paths followed by the two thermal reactions3). 1) 2) 3)Taken in part from the Doctoral Theses by Gonzenbach (ETH Zurich, 1973) and Grosclaude (UniversitB de GenBve, 1976).

Ausgehend von den Cyclopropanen (I) ((Ia) und (Ic) = E‐Isomere; (Ib) = E/Z‐Gemisch) werden die Titelverbindungen (III) dargestellt (die Ausbeute von (IIIb) bezieht sich auf (Ib)); analog und in ähnlichen Ausbeuten werden die Verbindungen (IVa)‐(IVe) bzw. (Vb) (aus (Va)) erhalten.

Durch Thermolyse (200°C) in Benzol entstehen sowohl aus dem Bicyclus (‐)‐(I) als auch aus dem Bicyclus (+)‐(II) Gemische, die (‐)‐(I) und (+)‐(II) (Ausgangsstoff und Produkt der endo‐exo‐Stereomutation) sowie die Ketone (‐)‐(III) und i (+)‐(IV) (Produkte der Cyclopropyl‐allyl‐Umlagerung) enthalten.

Es wird gezeigt, daß die Umwandlung zum Keton (III) spezifisch über das endo‐Keton (II) erfolgt und daß dieser Prozeß sowie die Isomerisierung des exo‐Ketons (I) zum ende‐Keton (II) kein gemeinsames Intermediärprodukt aufweisen.