Base-catalysed Neber-type rearrangement of glucosinolates [1-(β-D-glucosylthio)-N-(sulphonato-oxy)alkylideneamines]

Abstract

Allylglucosinolate in aqueous base produces vinylglycine (2-aminobut-3-enoic acid), thioglucose, and sulphate by a reaction of the first order with regard to both glucosinolate and hydroxide ion. No deuterium is introduced at C-2 of the amino-acid when the reaction is performed in D₂O. The reaction presumably takes place through a Neber-type rearrangement, starting with concerted loss of a hydrogen atom at C-2 in the aglucone, bond formation between C-2 and the nitrogen atom, and loss of sulphate, followed by fast addition of water to the resulting azirine, ring opening of the hydroxyaziridine produced, and finally hydrolysis of the thioester thus obtained. Benzylglucosinolate correspondingly yields phenylglycine, thioglucose, and sulphate in aqueous base but in this case other products, including benzaldehyde, benzyl cyanide, cyanide, and thiocyanate, are observed. Both vinylglycine and phenylglycine are produced in partly racemised forms but with an excess of the L-isomer. The reaction of the glucosinolates is facilitated by a high kinetic acidity of the hydrogens at C-2 in the aglucone, but glucosinolates other than the allyl and benzyl compounds can also give α-amino-acids with aqueous base, although only at elevated temperature.