Synthesis of angular triquinanes from 1-alkynylbicyclo[3.2.0]hept-2-en-7-ones. A tandem alkoxy-cope ring expansion/transannular ring closure reaction

Addition of ethenyllithium reagents to the carbonyl group of dialkyl squara te-derived 1-alkynylbicyclo-[3,2.0]hept-2-ene-7-anes (15),followed by a TBA F workup, results in a low-temperature anion-accelerated alkoxy-Cope rearra ngement-which proceeds by way of a strained cyclic allene intermediates (e. g.,17). This leads to the formation;of angularly fused triquinanes (e.g., 2 0) in which each of the rings is functionally differentiated. Bicyclo[6.3.0 ]undecadienones (e.g., 36) are the major products when the reactions are qu enched with aqueous bicarbonate rather than TBAF. Under analogous condition s 2-alkylidene-1-alkynylbicyclo[3.2.0]heptan-7-ones also give bicyclo[6.3.0 ]undecadienones by a mechanism that was established to involve a 1,5-hydrge n shift in a strained allene intermediate. The synthetic scope and mechanis m of these and related transformations are discussed.