Total synthesis of (-)-xyloketal D and its enantiomer - Confirmation of absolute stereochemistry

The total synthesis of (-)-xyloketal D and its enantiomer have been achieved by the reaction of an ortho-quinone methide with (4R)- and (4S)-4,5-dihydro-2,4-dimethylfuran via a diastereoselective inverse electron demand Diels-Alder reaction. This total synthesis confirmed the absolute stereochemistry of the natural product. The ortho-quinone methide was generated by reaction of an appropriately functionalized Mannich base with methyl iodide. The Mannich base was prepared in one step from 2,4-dihydroxyacetophenone, formaldehyde, and morpholine. The enantiomeric dihydrofurans were prepared from (2R)- and (2S)-2-methylpent-4-ynoic acid via a three-step reaction sequence. These chiral nonracemic synthetic precursors were prepared from the corresponding (R)-phenylglycinol-derived diastereomeric amides of the readily available racemic carboxylic acid. The absolute stereochemistry of these carboxylic acids was firmly established by conversion to a known compound that had been previously prepared from a chiral pool starting material.