Synthesis of Highly Functionalized Spirobutenolides via a Nitroalkane-Mediated Ring Contraction of 2-Oxobenzo[h]chromenes through Denitration

A facile synthesis of highly functionalized spirobutenolides was carried out by nitroalkane carbanion induced ring opening and relactonization via denitration reaction of 2-oxo-5,6-dihydro-2H-benzo[h]chromene-3-carbonitriles and 2-oxo-2,5-dihydrothiochromeno[4,3-b]pyran-3-carbonitriles. However, when nitroethane was used as a nucleophile source in lieu of nitromethane, a mixture of (E)- and (Z)-isomer of corresponding spirobutenolides were obtained in different ratio. The structure and geometry of the product was confirmed by single crystal X-ray diffraction. The isolated (E)- and (Z)-butenolides on treatment with sodium ethoxide in DMF at room temperature provided highly substituted trienes via allylic ring opening followed by decarboxylation.