Studies of Strained Ring Systems: Cyclopropa[1]Phenanthrenes

Abstract

The aim of the present study has been the synthesis of 1H-cyclo-Propa[1]phenanthrene (16a) and its derivatives, the sole remaining unknown structural type of the cycloproparenes. Established procedures for cycloproparene synthesis are not readily adaptable to this ring system, and routes based upon new bridge-head-substituted 1a,9b-dihydrocyclopropa[1]phenanthrenes are examined. 1, 1-Dichloro-1a-phenylseleno-1a, 9b-dihydrocyclopropa [1] phenanthrene (73) is prepared by the addition of dichlorocarbene to the corresponding phenanthrenyl selenide (72). syn-Selenoxide elimination of PhSeOH from the derived selenoxide (74) gives 1,1-dichloro-1H-cyclopropa[1]phenanthrene (76) which is intercepted by methanolysis. Labelling studies provide convincing evidence for the intermediacy of the 1H-cycloproparene. The viability of an oxidative decarboxylation route to 1,1-dialkyl-1H-cyclopropa[1]phenanthrenes is investigated for the model compound 7,7-dimethylbicyclo[4.1.0]hept-3-ene-1-carboxylic acid (122). A product of formal cyclopropyl-allyl cation rearrangement, is isolated. 1a-Methylseleno-1a,9b-dihydrocyclopropa[1]phenanthrenes (174) is prepared by the unprecedented addition of methylselenide anion to 1aH-cyclopropa[1]phenanthrene (63) (generated by a new route involving the fluoride ion-promoted elimination of the elements of chlorotrimethylsilane from the isomeric 1-chloro-1a-trimethylsilyl-1a, 9b-dihydrocyclopropa[1]phenanthrenes (170) and (171)). Treatment of the drived dimethylselenonium tetra-fluoroborate (179) with base in the presence of furan gives the endo- and exo-furan cycloadducts (180) and (181) of 1H-cyclopropa[1]phenanthrene (16a). The results presented herein provide the first conclusive evidence for the existence of the 1H-cyclopropa[1]phenanthrene ring system, both as the parent hydrocarbon (16a) and the 1,1-dichloro-derivative(76).