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‐oxabicyclo[3.3.1]nonadiene ( 1 ) with t BuLi/TMEDA occurs selectively in the allylic position. Whilst 1 has four allyl positions, only one bridgehead proton was removed, as demonstrated by quenching with Me 3 SnCl, Me 3 SiOTf, and Me 3 PbCl. With a threefold excess of deprotonating agent and subsequent treatment with Me 3 SnCl, 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 6 Li and 13 C NMR spectroscopy. After quenching of the anion with Me 3 SiOTf, and Me 3 PbCl, two silylated and plumbylated derivatives of 1 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.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)