The reduction of cyclic Mannich ketones having antibacterial activity showed poor stereoselectivity with L-Selectride ® , sodium trimethoxyborohydride and diisobutylaluminium hydride, while lithium aluminium hydride and, in particular, sodium borohydride often yielded a single stereoisomer. The size of the ring strongly influenced the stereocomposition of the reaction mixtures. An increased preference for the trans isomer was attributed to a weak intramolecular hydrogen bond between the OH and

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... he N, as demonstrated by X-ray crystallography. After reduction, the antibacterial activity of the product decreased dramatically ("S-form") but was not completely abolished in deep rough mutants of Gram-negative bacteria. agent for preparing allylic alcohols than lithium aluminium hydride. 4 The hydride reduction of α,β-unsaturated Mannich ketones may, however, afford 1,2-or 1,4-addition products. We also aimed at studying the stereocomposition of reduction products depending on the structure of the substrate (size of the ketone ring and the nature of the amine substituent). The starting materials for the reductions were selected as both effective antibacterial agents and less effective ones (cf. 1, 3, 10 and 6-9) . We also planned to analyse the effect of reduction on the antibacterial activity of our Mannich ketones. The antibacterial activity and the thiol depletion were examined in two E. coli strains, in an "S" type (ATCC 25292) and in a Re mutant one which has a defective cell wall biosynthesis. The Re mutants are therefore more permeable to some externally added chemicals.