Metalation of 9-oxabicyclo[3.3.1]nonadiene -: the deprotonation path to bridgehead olefins

A new route to bridgehead olefins based on a deprotonation reaction has been established. The deprotonation of 9-oxa-bicyclo[3.3.1]nonadiene (1) with tBuLi/TMEDA occurs selectively in the allylic position. Whilst I has four allyl positions, only one bridgehead proton was removed, as demonstrated by quenching with Me3SnCl, Me3SiOTf, and Me3PbCl. With a threefold excess of deprotonating agent and subsequent treatment with Me3SnCl, three stannylated derivatives - 3, 4a, and 4b - were obtained. Compound 3 is the starting diene 1 substituted at the bridgehead position 1 in the skeleton, and when smaller quantities of deprotonating agent were used it was the only stannylated derivative of 1 formed. Compounds 4a and 4b had each undergone a shift of one double bond to give bridgehead olefins exo- and endo-stannylated at position 3 in the skeleton. The common intermediate precursor of compounds 3, 4a, and 4b was a bridgehead allyl anion, which can also be generated from 3 and could be observed directly by Li-6 and C-13 NMR spectroscopy. After quenching of the anion with Me3SiOTf, and Me3PbCl, two silylated and plumbylated derivatives of I were obtained in each case: the bridgehead-substituted compounds and the 3-exo-substituted bridgehead olefins. The stannylated bridgehead olefins 4a and 4b rearranged, slowly to give 3 in what is formally a 1,3-stannatropic shift reaction. The kinetics of the reactions are zero-order in 4a and 4b, respectively, pointing to intermediate light-promoted homolytic cleavage of the Sn-C bond and the formation of a tight radical pair. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)