An efficient synthesis of a spirocyclic oxindole analogue

An efficient, scaleable synthesis approach towards the spirocyclic oxindole analogue 1'-(tert-butoxycarbonyl)-2-oxospiroindoline-3,4'-piperidine]-5-carboxylic acid (1) is described. The key steps are dianion alkylation and cyclization of ethyl 2- oxindoline-5-carboxylate (4) and demethylation of the resulting spirocyclic oxindole ethyl 1'-methyl-2-oxospiroindoline-3,4'-piperidine]-5-carboxylate (5). The target compound was obtained in an overall yield of 35 % over eight steps without resorting to chromatographic purification.