Chiral Synthesis of Erythrina Alkaloids. (2). Synthesis of enantio-Type Erythrinan Alkaloids Utilizing Asymmetric Acylation and Kinetic Resolution of Diastereomers.
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Oxalylation of the enamino-ester 13 derived from the L-dopa derivative 12 gave the dioxopyrroline of (6S)-configuration (15A) diastereoselectively (50-60% diastereomer excess). This was converted to a mixture of erythrinans of (5S, 6R, 7R, 10S) and (5R, 6S, 7S, 10S) configuration by cyclization with BF3·Et2O. The de of the major diastereomer (A) was elevated to 82% by application of a kinetic resolution of diastereomers (partial hydrolysis of the ethylene acetal group), where the minor diastereomer (B) was hydrolyzed more rapidly. The acetal 17A which remained unchanged was converted, in several steps, to the enantio-type erythrinan alkaloid, (-)-3-demethoxyerythratidinone (-)-7, and also to the 1, 7-cycloerythrinan (-)-9, a key intermediate to Erythrina alkaloids. The more easily hydrolyzable diastereomer (B) was similarly converted to the enantiomer (+)-9. The mechanism of partial racemization, sometimes observed in the product, is discussed.