Dynamic stereoselective annulation via aldol-oxa-cyclization cascade reaction to afford spirooxindole pyran polycycles

Spiro polycyclic compounds bearing pyran ring systems are found in bioactive molecules, and we recently reported the construction of spirooxindole all-carbon polycycles. Here we show the development of catalytic stereoselective annulation reactions that afford spirooxindole pyran polycycles. Oxindole-derived spiro[4,5]decanes are reacted with arylglyoxal to construct a pyran ring via the formation of carbon-carbon and carbon-oxygen bonds through dynamic aldol-oxa-cyclization cascade reactions, leading to the formation of spirooxindole pyran polycycles bearing six stereogenic centers as single diastereomers. During the reaction, the starting material is isomerized to the diastereomer, and this is key to afford the product. Taking advantage of this isomerization, highly enantiomerically enriched single diastereomers of spirooxindole pyran polycycles are obtained. The reactions generating the spiro pyran polycycles show stereoselectivities distinct from those previously observed in the construction of all-carbon polycycles. Spirooxindoles are synthetic targets of wide interest. Here a diastereoselective aldol-oxa-cyclisation cascade affords spirocyclic pyrans with high stereoselectivity.