No abstract

epi-Progoitrin, the major thioglucoside of Crambe abyssinica seed, is cleaved by ferrous salts to yield (9-I-cyano-2-hydroxy-3-butene and (S)-3-hydroxypent-4-enethionamide. The products are the same whether obtained from the purified thioglucoside or from the seed meal. Identical behavior was observed for seed meal of Brassica napus, which gave corresponding derivatives of the opposite configuration. Optical rotatory dispersion and proton magnetic resonance studies suggest the existence of solvent-dependent differences in rotamer composition for the 3-hydroxypent-4-enethionamides.Canadian Journal of Chemistry, 46, 1507Chemistry, 46, (1968 Earlier publications from this laboratory have detailed the chemistry of epi-progoitrin, the major thioglucoside in seed meal of Crambe abyssinica Hochst ex R. E. Fries and of its enzymic conversion products (1-3). The enzymic reaction may lead to two distinct types of products. One, 5-vinyl-oxazolidinethione or (R)-goitrin, is formed by spontaneous cyclization of an intermediate P-hydroxy isothiocyanate (4); the other consists of open-chain nitriles derived from the aglucon of epi-progoitrin. Three of this group have been recognized: (S)-1-cyano-2-hydroxy-3-butene and a diastereoisomeric pair of episulfides related to this unsaturated nitrile.Several species of Brassica contain a thioglucoside with a similar structure to that found in Crambe. The two thioglucosides differ only in configuration about the asymmetric carbon atom in the aglucon portion of the molecule (I). The configurations persist without racemization during enzymic conversion, so that two series of enantiomorphic compounds result, depending on the source material (1, 2). A scheme showing these relationships, together with the absolute configurational assignments, was recently given (5).The occurrence of enantiomers of a compound in related plant families is unusual but not unique for Crambe and Brassica. The situation with respect to the thioglucosides of these species has been discussed by Daxenbichler et al. (1) who also cited other substances, mainly terpenes, which occur in nature as enantiomers. Analogous observations were noted for an alkaloid (20) and for the butanol derivative, betuligenol (21).Although enzymic transformation of the thioglucosides present in members of the Cruciferae and in a limited number of other plant families frequently proceeds via the isothiocyanate pathway, the route leading to nitriles may occur almost exclusively or the two types of products can be formed concurrently. Schwimmer (6) noted that the decomposition of sinigrin by myrosinase (thioglucosidase, EC3.2.3.1) in the presence of ascorbate and protein yielded nitrile at pH 3.0 but isothiocyanate at higher pH values. Further studies of this system by Ettlinger et al. (7) supported pH dependence of the observed ratio of nitrile to isothiocyanate produced. The results were explained in terms of the nonenzymatic breakdown of a labile intermediate whose rate was determined by pH. The intermediate was identified as the sinig...

No abstract