Structure-activity relationships in a series of (5i?)-6-triazolylmethylene penems with potent /Mactamase inhibitory activity are described. In most cases, their in vitro synergistic activity with amoxycillin is superior to that of clavulanic acid, sulbactam and tazobactam (YTR830). Against an Escherichia coli TEM-1infection in mice, the compoundsshowed a broad range of potencies; an optimum polarity was found, however, which gave maximumpotency.Earlier papers in this series1~4) have described the synthesis and biological properties of a series of racemic 6-(substituted methylene)penems. Of particular interest was the triazolylmethylene penem (5b), and this paper outlines some further work on a series of triazolyl derivatives with a chiral centre at C-5. ChemistryThe (5i?,6Z)penems (5) were prepared using two routes. Route A (Scheme 1) has been described for the preparation of 5b5) and is shown schematically in full elsewhere6): it requires a 1,2,3-triazolecarboxylic Scheme 1.

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Sodium (5i?S)-Z-6-(heterocyclylmethylene)penem-3-carboxylates(2) are a series of extremely potent inhibitors of bacterial /Mactamases. A variety of 5-membered heteroaromatic derivatives have been prepared and structure-activity studies reveal a preferred substituent orientation. One of these derivatives, the 1-methyl-1,2,3-triazolyl compound (5m) is a more potent synergist of amoxycillin than clavulanic acid, sulbactamor tazobactam. 331 Earlier papers1>2) have described the synthesis and biological properties of a series of racemic 6-(substituted methylene)penems. Of particular interest was the 2-furyl derivative (1) which had the Z configuration about the C-6-C-8 double bond and was unsubstituted at the 2-position2). This paper outlines some of the further work on a series of penems (2) in which we sought to investigate the effect of varying aromatic heterocyclic substituents on the biological properties of the series. A number of 5-membered heterocycles have been synthesised and someconclusions on structure-activity relationships within this group of derivatives are presented. ChemistryThe Z-isomers (4a~4q) were prepared in excellent yield by treating the 2-unsubstituted penem acetates (3a~3q)3) with l ,8-diazabicyclo[5.4.0]undec-7-ene (DBU) at low temperature. Removal of the/>-nitrobenzyl (PNB) protecting group was readily achieved by hydrogenolysis over palladium on carbon followed by treatment with sodium hydrogen carbonate. The resulting sodium salts (5a~5q) were obtained as homogeneousfreeze-dried solids after chromatography on Biogel P2.The UV spectra of the penems (5a~5q) implied a degree of n orbital overlap between the aromatic heterocyclic ring and the exocyclic double bond consistent with a coplanar conformation. Molecular models suggested that in compounds bearing either a substituent or a proton at position 4' (see Fig. 1) conformation A could sshow fewer non-bonded interactions between the heterocycle and the penem nucleus than conformation B. Support for this view was also obtained from *HNMR data on someof these compounds (4g, 4j and 5m). NOE's were observed between the heterocyclic ring proton (4'-H) and both 8-H and 5-H. The results of the experiments are shown in Table 1 and may be interpreted as indicating a fast equilibrium between conformers A and B. Furthermore the differences in internuclear distances and the ratio of A :B from the NOEstudies imply that A is the major conformer4).

Five-membered ring systems are ubiquitous throughout natural products and synthetic therapeutics, and thus, efficient methods to access this essential scaffold are required. Herein, we report the thioacid-mediated, 5-exo-trig cyclization of various 1,6-dienes, with high yields of up to 98%. The labile thioester functionality can be exploited to generate a free thiol residue which can be used as a functional handle or removed entirely to provide the traceless cyclized product.

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