Mechanism of the formation of the two epimeric diamino-hexose rings of neomycin B

Abstract

By using variously tritiated hexoses and Streptomyces fradiae(a neomycin-producer), the mechanism and stereochemistry of the formation of various subunits of neomycin B were investigated. The results of the incorporation of (6RS)-D-[6-3H] glucose into neomycin B followed by specific degradation showed that during the formation of the aminomethyl group of the neosamine B ring one of the 6-3H atoms of the precursor was removed. The 3H remaining at C-6 of the neosamine B ring was shown to be located in the Hsi orientation through the isolation of the chiral centre as glycine and the analysis of the latter by using an exchange reaction catalysed by serine hydroxymethyltransferase. These results suggested that, as has been previously shown for neosamine C, the aminomethyl group of neosamine B is formed by an oxidation-transamination process; –CH2OH →–CHO →–CH2NH2 D-[5-3H]Glucose was prepared by an unambiguous method and incorporated into neomycin B. The radiochemical data on the various subunits of the antibiotic could be interpreted to suggest that the L-idose configuration at C-5 of the neosamine B ring is produced via an enolisation process involving a carbonyl group at either C-4 or C-6. The involvement of C-4 was then eliminated by feeding experiments in which it was shown that the hydrogen atom at C-4 of D-glucose was undisturbed during the incorporation of this position into the C-4 of neosamine B or C.