Rhodium-catalyzed cycloaddition of carbonyl ylides for the synthesis of spiro[furo[2,3-a] xanthene-2,3 '-indolin]-2 '-one scaffolds

An intramolecular cycloaddition of oxonium ylides generated from 3-diazooxindole and a bifunctional substrate i.e. 2-methyl-2-(4-methylpent-3-en-1-yl)-2H-chromene-3-carbaldehyde has been achieved using 5 mol% of Rh-2(OAc)(4) to produce highly substituted 3,3,5a-trimethyl-3a,5,5a,11b-tetrahydro-3H,4H-spiro[ furo[2,3-a]xanthene-2,3'-indolin]-2'-ones in good yields with high diastereoselectivity. This is the first example of the synthesis of biologically relavant polycyclic frameworks from readily accessible precursors.