Periodate oxidation products derived from methylated alpha-amanitin: evidence for distinct aldehydic and non-aldehydic forms.

Amatoxins are cyclic octapeptides which can be purified from various mushroom species. They have found widespread use due to their potent inhibition of eukaryotic RNA polymerase II. In the course of our efforts to prepare additional semisynthetic derivatives of the amatoxin, alpha-amanitin, we examined the products formed through the periodate oxidation of 6'-O-methyl-alpha-amanitin. Periodate oxidation under conditions of near-neutral pH yielded two chemically similar, yet chromatographically separable, products. On the basis of their proton NMR spectra and their lack of reactivity with sodium chlorite these products did not contain a free aldehydic group. However, both forms were interconvertible in aqueous neutral solution and could be converted to the same product, 6'-O-methyldemethyl-gamma-amanitin, through reduction with sodium borohydride. Periodate oxidation under mildly acidic conditions generated a single product which has properties of a free aldehyde. It exhibited a proton NMR spectrum with signals characteristic of an aliphatic aldehyde and was readily oxidized to the carboxylic acid with sodium chlorite. Conditions have been defined to synthesize the free aldehyde derivative of 6'-O-methyl-alpha-amanitin in generous yield to provide a precursor for oxidation and further derivatization.