Acetylene Stacking: a New Concept for the Synthesis of Cyclic Polyenes. Application to the Preparation of a 1,6-Bridged Bicyclo[4.2.0]octa-2,4,7-triene and Some Novel Oxepins

Abstract

A new protocol for a general synthesis of cyclic polyenes is presented; in this protocol two new concepts are introduced. The first, acetylene stacking, is the generic term covering both the synthesis of a target molecule and the specific retrosynthesis upon which it is based. This retrosynthetic analysis involves conversion of the target molecule, through a series of transannular [ π2s+π2s] valence isomerizations , into a key polycyclobutanoid intermediate (24). This intermediate is cleaved, in turn, through a similar number of [ σ2s+ σ2s] electrocyclic fragmentations, to yield a set of acetylene synthons . The synthetic phase is based on the recombination of the corresponding acetylene synthetic equivalents in a stacking mode which leads to the target molecule by way of the same polycyclobutanoid intermediate (24). To achieve the ordered assembly of the acteylene synthons, in the required stacking mode, it has been necessary to develop the second concept, namely transfer technology. This involves the use of special transfer reagents, to act as synthetic equivalents and provide the molecular equivalent of acetylenes without ever producing acetylenes per se. Similar transfer reagents are described for cyclobutadienes which serves to demonstrate the wider implications of this new concept in organic synthesis. The bridged bicyclic polyene 8-oxatricyclo [4.3.2.01,6]undeca-2,4,10-triene (73), and the heterocycle 2,4,5,7,8-pentamethyl-1H-oxepino[4,5-c]pyrrole-1,3(2H)- dione (91) are used as the target molecules to illustrate this new synthesis protocol involving sets of stacked acetylenes (or cyclobutadienes ) as the synthetic base. The conversion of the oxepin (91) into isomeric oxepins, and the role of their tautomeric benzene oxides, present as equilibrium partners, is discussed.