Polycyclobutanoid compounds containing an end-fused 3-membered ring are central to the synthesis of the title target molecules: a new mode for their production is presented. Cyclopentadiene 1,3diester is isolated and Its isomerisation studied. Intermediates supporting the oxygen-walk lsomerisation of maleimido-oxepines are isolated.
In the accompanying papert we reported on a new algorithm for the retrosynthesis of unsaturated cyclic compounds. This involved transannular pairing of ring n-bonds to form a polycyclobutane which could be split into sets of acetylenic or cyclobutadiene synthons. Transfer reagents, also reported therein, are an important component of this protocol since they provide the reactive synthons in protected form. In this paper we show how this protocol can be applied to the synthesis of cyclic compounds having an odd number of ring atoms using cyclopentenes and oxepines as the illustrative examples and further discuss transfer reagents in this context.
Cyclopentadienes were selected as the carbocyclic example since they are important starting materials for the synthesis of natural products and new ways to achieve specific functionalisation in this ring-system should be useful. Application of the transannular pairing algorithm, as delineated in Scheme 1, reveals that 1,3disubstituted cyclopentadienes should be available from coupling acetylene, a disubstituted acetylene, and a carbene. As noted earlier,1 a transfer reagent can be used to deliver these two acetylenic synthons via the cyclobutadiene (3). The carbocyclic diester(7) was selected in this role since it is readily prepared in a reaction which accommodates several other electron-withdrawing substituents in place of the ester groups2 thus broadening its potential. Compound (7) was allowed to react with dimethyl diazomethane, acting as the carbene transfer reagent, thereby producing the 1:1-adduct ( 8) by attack at its under face.3 Efficient elimination of dinitrogen occurred upon ultraviolet irradtation to produce the cyclopropane (10). Heating (10) caused loss of cyclopentadiene (CPD) to occur in a retro Diels-Alder reaction and led to the production