Evaluation of 2-Bromocyclohexenone Acetals as Vehicles for the Introduction of C-7 Oxygen Preliminary to the Synthesis of Taxane Diterpenes
作者:Leo A. Paquette、Zhuang Su、Simon Bailey、Francis J. Montgomery
DOI:10.1021/jo00109a021
日期:1995.2
The coupling of an optically pure, camphor-derived, beta,gamma-unsaturated, bicyclic ketone with a suitable vinyl organometallic is recognized to result in 1,2-addition from the endo face. Anionic oxy-Cope rearrangement of these carbinols proceeds via an ''endo-chair'' transition state to deliver a strained and reactive enolate that is formed regioselectively and is amenable to alpha-methylation from the top face. These steps, which are preliminary to a broad-based approach to the taxane diterpenes, had not yet accommodated suitable introduction of a C-7 oxygen substituent as required of taxol. Typically, an ether substituent at this site experiences beta-elimination once the enolate anion intermediate is accessed. Herein it is demonstrated that the parent 2-bromocyclohexenone acetal is well suited to resolving this complication. Halogen-metal exchange proceeds well to provide a suitably nucleophilic building block. Direct charge-accelerated sigmatropic rearrangement of the carbinol products does proceed with beta-elimination, but under the present circumstances the second C-O bond remains in the form of a vinylogous ester. Alternatively, the carbinols are amenable to chemoselective hydrolysis of the acetal, thereby unmasking a conjugated cyclohexenone part structure. These intermediates have been found to rearrange along completely analogous reaction trajectories to provide enolates of a beta-diketone subunit. The extent to which these anions undergo C- versus O-methylation under various conditions has been examined. When O-methylation does occur, it is the C-7 oxygen (and not the one at C-9) that is engaged in reaction.