A thermally-induced, tandem [3,3]-sigmatropic rearrangement/[2 + 2] cycloaddition approach to carbocyclic spirooxindoles

Abstract

SummaryThe synthesis of C3-carbocyclic spirooxindoles was realized by way of an intramolecular [2 + 2] cycloaddition reaction between a vinylidene indolin-2-one and an alkyne. The cycloaddition reaction occurs selectively with the distal double bond of the allene, is tolerant of a phenyl and trimethylsilyl group on the terminus of the alkyne, and can be used to access bicyclo[4.2.0]octadienes and bicyclo[5.2.0]nonadienes. The allene precursors are not observed, but are likely intermediates of an infrequently encountered thermal [3,3]-sigmatropic rearrangement of a propargylic acetate.

Complete transfer of chirality in an intramolecular, thermal [2 + 2] cycloaddition of allene-ynes to form non-racemic spirooxindoles

“…This thermally forbidden process is believed to proceed via a biradical intermediate mechanism, a conclusion supported by both computational and experimental studies [ 3 ]. Recently, the scope of this method has been expanded to the synthesis of spirooxindole-containing skeletons 2 in a two-step one-pot process from propargyl acetates 1 [ 4 ] ( Scheme 1 ). Inspired by this rapid entry into the molecularly complex substructure 2 , and the structural similarity to welwitindolinone A isonitrile ( 3 ), we became interested in the synthesis of chiral non-racemic spirooxindoles for application to natural product synthesis [ 5 – 7 ].…”