Organobase catalysed one-pot exo-selective synthesis of meso-spiro[cyclohexanone-pyrandione] derivatives†
An exo-selective one-pot synthesis of novel spiro[cyclohexanone-pyrandione] derivatives through two sequential organobase-catalysed 1,4-conjugate additions of pyran-2,4-dione precursors on diarylideneacetone derivatives is described. The resulting 7,11-diaryl-2-oxaspiro[5.5]undec-3-ene-1,5,9-triones were obtained as a 2 : 1 mixture of exo- and endo-diastereomers and adopt a meso-form due to the bilateral symmetry existing in the scaffold. Chiral-HPLC, NMR and single-crystal X-ray diffraction (including synchrotron data) have been used to elucidate the spiro[cyclohexanone-pyrandione] structure, showing therefore the exo/endo-diastereomers obtained in a meso-form, the cis-7,11-diaryl configuration (equatorial position) and different conformational changes in the cyclohexanone skeleton both in liquid and solid states. DFT studies proved the exo-stereoselectivity of the reaction by determining the exo/cis equatorial–equatorial configuration as the most thermodynamically stable form of our spiro[cyclohexanone-pyrandione] skeleton, while the structural substitution has no effect.