ChemInform Abstract: Intramolecular Cycloaddition Reactions of Furo[3,4‐b]indoles for Alkaloid Synthesis.

Abstract

Model studies dealing with the Cu(II)- or Rh(II)-catalyzed carbenoid cyclization/cycloaddition cascade of several α-diazo indolo amido esters have been carried out as an approach to the alkaloid scandine. The Cu(II)-catalyzed reaction of an α-diazo indolo diester that contains a tethered oxa-pentenyl side chain was found to give rise to a reactive benzo[c]furan which undergoes a subsequent [4 + 2]-cycloaddition across the tethered π-bond. The reaction proceeds by the initial generation of a copper carbenoid intermediate which cyclizes onto the adjacent carbonyl group to give a reactive benzo[c]furan which in certain cases can be isolated. Disappointingly, the analogous reaction with the related amido indolo ester failed to take place, even when the tethered π-bond contained an electron-withdrawing carbomethoxy group. It would seem that the geometric requirements for the intramolecular cycloaddition of the furo[3,4-b]indole system with the tethered π-bond imposes distinct restrictions upon the bond angles ...