The structures of the pyrrolizidine alkaloids neoplatyphylline and hastacine

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Abstract

Neoplatyphylline, C18H27N05, was isolated from mother liquors resulting from the preparation of platyphylline (I) from Selzecio rhombifolius (Willd.)Soh.Bip.(formerly regarded as S. plutyphyllus (Bieb.)DC.) on an industrial scale.1 Alkaline and acid hydrolyses were said to give the same products as from platyphylline, which is an ester of platynecine with seneoio aoid.Hastacine, also C18H27N05, was obtained from Cacalia hastata,2 another species of the tribe Senecioneae, family Compositae.It gave on hydrolysis a saturated diol, hastanecine, isomeric with platynecine, and a dicarboxylic aoid, C10H1605, with properties similar to those later observed for integerrinecic acid.The structures of these two alkaloids have now been clarified in conjunction with a parallel study on the stereochemistry of the saturated pyrrolizidine diols.3In regard to signals due to the protons of the esterifying acids, the nuclear magnetic resonance spectra of neoplatyphylline and hastacine are closely similar to the spectrum of platyphylline, except for the chemical shift of the CH3-CH=C(-CO-) proton, 6 6.58 for neoplatyphylline, 6 6.72 for hastacine, and 6 5.83 for platyphylline.These shifts indicate that the first two bases have a trans-ethylidene group in the esterifying acid.This indication was confirmed by the alkaline hydrolysis of the alkaloids whioh led in each instance to integerrinecic acid.It follows that neoplatyphylline is (11), an ester of platynecine with integerrinecic acid, but elucidation of the complete structure of hastanecine requires consideration also of the nature of hastanecine.The saturated pyrrolizidine diols, platynecine (111) and dihydroxyheliotridane (IV), are known to be the two possible 7,9-diols with 1/3-hydroxymethyl-8a configuration.Apart from hastanecine, there are three other saturated diols which might be the unknown 7,9-diols of la-hydroxymethyl-8a configuration: turneforcidine, macronecine, and the amino alcohol from retusine.4Untch and Martin5 have recently reported a synthesis of the unknown 7,9-diols in their enantiomeric form (1S-hydroxymethyl-8/3 configuration) and stated that their properties agree with those reported

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