Abstract
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.
Highlights
Spiro polycyclic compounds bearing pyran ring systems are found in bioactive molecules, and we recently reported the construction of spirooxindole all-carbon polycycles
We report stereoselective construction of pyran polycycles using annulation reactions via the formation of C–C and C–O bonds; these reactions provide products with stereoselectivities distinct from those demonstrated in the formation of all-carbon polycycles (Fig. 1b, c)
We have developed catalytic stereoselective annulation reactions that afford spirooxindole pyran polycycles
Summary
Our strategy for the construction of spirooxindole pyran polycycles uses oxindole-derived spiro[4,5]decanes[21] as starting a. Catalysts and conditions were evaluated to identify those that catalyzed the reactions of oxindole-derived spiro[4,5]decane 1a or its diastereomer 2a with phenylglyoxal to afford pyran-derived polycycles (Table 1). The use of DBU instead of DABCO under otherwise the same conditions led to the formation of 3a from 1a (Table 1, entry 3). With the use of DBU as catalyst under appropriate conditions, product 3a was obtained from both the reactions of 1a and of 2a (Table 1, entries 4 and 5). These results suggested that, with 1a as starting material, 1a first isomerized to 2a, and 2a reacted with phenylglyoxal to give 3a. When the reaction was performed in the presence of DBU (0.2 equiv) as catalyst with addition of
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