2- Vinylcyclopropyllithium

A slight modification of the cyclopropyl conjunctive reagent transforms a cyclopentannulation into a cycloheptannulation. Thus, the 2-vinylcyclopropyllithium reagent 3, converted to its cuprate 4, generates a 1,2-divinylcyclopropane. Heating to only 180 °C leads to smooth Cope type rearrangement, driven by the release of the cyclopropyl strain, to create a perhydroazulene ring systerh of many sesquiterpenoids (Eq. 19) 20>. 

Additions of 2-vinylcyclopropyllithium derivatives to /1-alkoxyenones such as 49 are more widely applicable. The cyclopropyllithium precursors, generated from either cis- or tran.v-l-bro-mo-2-vinylcyclopropanes (which can be separated by spinning band distillation), have proved to be configurationally stable, e.g., formation of 50/51852. 

Bicyclic 2-vinylcyclopropyllithium substrates allow tricyclic systems containing a cyclohepladi-ene moiety to be constructed. Thus, dienone 64, containing the backbone of the tigliane, daphnane. and ingenane families of natural products, can be prepared from 61 by the reaction sequence shown868. 

As an alternative to the addition of vinylcyclopropyllithium derivatives to /5-alkoxycy-cloalkenones, Cope substrates for 1,2 3,4 rearrangements can also be prepared by addition of phenylthio(2-vinylcyclopropyl)cuprate to readily available 3-halo-2-cycloalkenones871 875, e.g., 69, 72, 74, and 77 in tetrahydrofuran/diethyl ether/pentane at —78 to — 20 °C 876. 

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