Isolation, Synthesis, and Semi-synthesis of Amaryllidaceae Constituents from Narcissus and Galanthus sp.: De novo Total Synthesis of 2-epi-Narciclasine.
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An efficient protocol for the isolation of narciclasine from common Amaryllidaceae bulbs, separation from haemanthamine, and the occurrence of a trace alkaloid, 2-epi-narciclasine are reported. Attempts to convert natural narciclasine to its C-2 epimer by Mitsunobu inversion or oxidation/reduction sequences were compromised by rearrangement and aromatization processes, through which a synthesis of the alkaloid narciprimine was achieved. The methylation of the 7-hydroxy group of natural narciclasine followed by protection of the 3,4-diol function and oxidation/reduction sequence provided the target C-2 epimer. A de novo chemoenzymatic synthesis of 2-¬epi-narciclasine from m-dibromobenzene is also described. Haemanthamine and narciprimine were readily detected in the crude extracts of Narcissus and Galanthus bulbs containing narciclasine and the occurrence of 2-epi-narciclasine as a trace natural product in Galanthus sp. is reported for the first time.