The synthesis of bicyclo[4.4.l]undec-1(ll)-ene (5) by intr,amolecular Wittig reaction is described. The Bredt olefin could not be isolated, but dimerized rapidly to a novel compound 15 containing a cyclopropane ring. The olefin 5 was trapped in situ by 2,5-diphenylbenzo [clfuran.Introduction. -The question of the limits of Bredt's rule is of considerable current interest [l] [2]. It is now generally accepted that the strain of a bicyclic or polycyclic bridgehead olefin (Bredt olefin) is comparable to the strain of the corresponding trans-cycloalkene from which it may be formally derived by bridging. This hypothesis, advocated by Wiseman in 1967 [3], has been supported by experimental evidence from a large number of synthetic approaches to different Bredt olefins and by two independent force-field calculations [4] [S].However, within a series of homologous bridged trans-cyclooctenes, truns-cycloheptenes, and trans-cyclohexenes, the principles which govern relative strain energies are not obvious. Kobrich [l] formulated a rule ('rule A'), according to which increasing the number of carbon atoms in the bridge should decrease the strain of the Bredt olefins. Indeed, whereas the ethylene-bridged truns-cyclo-