Chiral Synthesis of Natural Products Employing Enantiofacial Discriminating Reaction

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Enantioselective reaction employing intramolecular hetero-Diels-Alder reaction with reverse electron demand has been examined. Reaction of the two isomeric glyceraldehydes, (2) and (3), prepared from diethyl L-tartrate (1), with Meldrum's acid underwent smooth concurrent condensation and intramolecular Diels-Alder reaction with reverse electron demand to give the corresponding cycloadducts in highly diastereoselective manners. It was found that the E-dienophile (2) afforded the cis-5/6-adduct (4) and the Z-dienophile (3) afforded the trans-5/6-adduct (5) each as a single epimer. Using two adducts a formal route to all possible stereoisomers of the heteroyohimbine alkaloids has been established. Furthermore, a new route to the secoiridoid monoterpene, (-)-methyl elenolate(6), and both cis- and trans-D/E ring heteroyohimbine alkaloids, (-)-tetrahydro-alstonine (7) and (-)-ajmalicine (8), has been developed using the cis-5/6-adduct (4) as a single precursor.