12 J. CHEM. SOC., CHEM. COMMUN., 1987 The Involvement of C-4 of D-Glucose in the Biosynthesis of the 2-Deoxystreptamine Ring of Neomycin Sayed K. Goda and Muhammad Akhtar* Department of Biochemistry, University of Southampton, Southampton SO9 3TU, U. K. A mechanism, rationalising the loss of the C-4 hydrogen atom of o-glucose in the biosynthesis of the deoxystrep- tamine ring (2) of neomycin (3), is considered. A large group of clinically useful antibiotics, to which streptomycin and neomycin belong, are classified as aminoglycoside-aminocyclitol antibiotics. The name high- lights the fact that these antibiotics, in addition to an array of variously modified sugars, contain an unusual cyclohexane derivative which can either be hexasubstituted as the strep- tamine unit (1) of streptomycin or pentasubstituted as the 2-deoxystreptamine ring (2) that is present in neomycin (3) and more than fifty other related antibiotics. It is now known that the carbon skeletons of both types of aminocyclitols (1) and (2) arise from D-glucose,lJ and the elegant studies of Walker and Walker3 have established that, in the formation of the streptamine ring system of streptomycin, the key event is the inositol synthase-catalysed cyclisation of glucose 6-phos- phate (4) into inositol 1-phosphate (5) (Scheme 1). The latter, through a multistage process, is then converted into strepti- HO NH2 HO (1); X = OH (2); X = H HZN\ 7 H : q Neosamine C o& I dine [this is a diguanidinated derivative of (l)]. The know- ledge of this biosynthesis provided the stimulus to consider the possibility that a cyclisation process similar to that shown in Scheme 1, followed by the reductive removal of the unwanted hydroxy group, may be involved in the formation of 2-deoxy- streptamine (see citations in ref. 4). Gradually however observations were made which questioned this hypothesis and suggested that the two aminocyclitols (1) and (2) may be synthesised by two entirely different mechanisms. For exam- ple, the patterns for the incorporation of the C-skeleton of D-glucose into the two aminocyclitols were found to be different; the C-6 of glucose occupied position 2 in deoxystrep- tamine4 but position 6 in the streptamine ring.5.6 Further- more, our work on neomycin7 and the studies of Kakinuma et al. on ribostamycins revealed that the 2-deoxystreptamine ring of these antibiotics was formed from D-glucose with the retention of both of its C-6 hydrogen atoms. Since the mechanism of the type shown in Scheme 1 requires the loss of one of the C-6 hydrogen atoms of glucose,g the results7?8 suggested that the pathways for streptamine and 2-deoxy- streptamine biosynthesis diverged prior to the cyclisation stage, and led to the proposal of two closely related mechanisms7.8 (paths A and B, Scheme 2) for the construction of the deoxy centre of the aminocyclitol (2). The common feature in both the mechanisms is the involvement of enolic intermediates [(7) and (lo), Scheme 21 which participate in nucleophilic attack at the C-1 carbonyl producing a deoxy carbocyclic ring. The difference in the two mechanisms, however, is that whereas in path A (Scheme 2) the enolic A HO OH HO (5) Scheme 1 ~~ ~ Table 1. 3H : 14C and atomic ratiosa of the various subunits of neomycin biosynthesised from double-labelled glucose. Subunits Neosaminol C 2-Deoxystreptamine Dialdehyde Formic acid [C-5 of (2)] Precursor (13) (2) (14) [U-*4C; 3-3H, 4-3HIglucose 7.7 (2 : 6) 7.5 (1.94 : 6) 3.7 (0.97 : 6) 0 (0 : 5) 20.4 (0.88 : 1) [U-"C; 3-3HIglucose 7.8 (1 : 6) 7.9 (1.01 : 6) 7.9 (1.01 : 6) 0 (0 : 5) 37 (0.80 : 1) a In parentheses.