ChemInform Abstract: THE SYNTHESIS OF STRAINED METHYLENE‐BRIDGED BICYCLIC OLEFINS BY THE INTRAMOLECULAR WITTIG REACTION

Abstract

SummaryA convenient synthesis of the strained methylene-bridged trans-cyclooctenes bicyclo[3.3.l]-1(2)-nonene (l), bicyclo[4.2.1]-1(8)-nonene (2), and bicyclo[4.2.1]-l(2)-nonene (3) by the intramolecular Wittig reaction is described (see Schemes 1-4). The (3-oxocycloalkyl)alkyl-phosphonium bromides 20, 27 and 38 undergoing cyclization to the bridgehead olefins are formed by simple reaction sequences. The spectral properties (IR., lH-NMR., W-NMR., and UV.) of the olefins are discussed with regard to their strain.Introduction. -Considerable interest has been shown recently in defining the limits of Bredt's rule and in predicting the degree of strain in bridgehead double bonds [2] [3]. Bredt himself had limited his qualitative statement, that a double bond at the bridgehead is not possible, to bicyclic olefins of the pinane and camphane series and related compounds [4]. Later Prelog demonstrated that larger bicyclic ring systems indeed can accommodate a bridgehead double bond [5]. Based on this research, Fawcett drew the conclusion that compounds with a double bond at the bridgehead cannot be isolated if the total number of bridge atoms S2) is less than nine [6].In 1967, both Wiseman [7] and Marshall [8] reported the successful synthesis of bicyclo[3.3. I]-l(2)-nonene (l), a reactive but stable bridgehead olefin with seven bridge carbon atoms only. Wiseman emphasized that the stability of a bridgehead olefin is related to the stability of the largest trans-cycloalkene from which it can be derived formally by bridging. Since trans-cyclooctene is a stable, isolable compound [I 11, and the increasing number of synthetic approaches to even more strained bridgehead olefins [3].