Macrolide resistance from the ribosome perspective.

Macrolides are important antibiotics used in treatment of respiratory tract infections in humans. Although some of these compounds have been in use for 50 years, it has not been until the last few years that their mechanism of action and the nature of ribosomal-based resistance could be more fully understood. With the advent of robust crystals of ribosomal 50S subunits, and structural resolution of macrolides and ketolides complexed to either Haloarcula marismortui or Deinococcus radiodurans 50S, the ability to dissect the binding modes and understand resistance at the level of the ribosome became possible. This review article compares the binding features of 14-, 15-, and 16-membered macrolides to that of ketolides telithromycin and ABT-773 as revealed at the atomistic level. Attempts to understand how modifications to 23S rRNA and/or mutations in ribosomal proteins L4 and L22 that have been found to confer resistance in Streptococcus pneumoniae, Streptococcus pyogenes, and Haemophilus influenzae are told from the perspective of the ribosome.

Biradical and zwitterionic cyclizations of oxy-substituted enyne-allenes.

[see reaction]. Oxyanion substitution of enyne-allenes causes both Myers-Saito and Schmittel cyclizations to switch their product formation preferences from diradicals to polar, closed-shell singlets. The oxyanion stabilization is larger for the Schmittel products than the Myers-Saito products because the latter must sacrifice aromaticity to maximize interaction. The changing character of the different reaction paths is reflected in their activation energies.

Structural studies on bioactive compounds. Part 29: palladium catalysed arylations and alkynylations of sterically hindered immunomodulatory 2-amino-5-halo-4,6-(disubstituted)pyrimidines.

The immunological agent bropirimine 5 is a tetra-substituted pyrimidine with anticancer and interferon-inducing properties. Synthetic routes to novel 5-aryl analogues of bropirimine have been developed and their potential molecular recognition properties analysed by molecular modelling methods. Sterically challenged 2-amino-5-halo-6-phenylpyrimidin-4-ones (halo = Br or I) are poor substrates for palladium catalysed Suzuki cross-coupling reactions with benzeneboronic acid because the basic conditions of the reaction converts the amphoteric pyrimidinones to their unreactive enolic forms. Palladium-mediated reductive dehalogenation of the pyrimidinone substrates effectively competes with cross-coupling. 2-Amino-5-halo-4-methoxy-6-phenylpyrimidines can be converted to a range of 5-aryl derivatives with the 5-iodopyrimidines being the most efficient substrates. Hydrolysis of the 2-amino-5-aryl-4-methoxy-6-phenylpyrimidines affords the required pyrimidin-4-ones in high yields. Semi-empirical quantum mechanical calculations show how the nature of the 5-substituent influences the equilibrium between the 1H- and 3H-tautomeric forms, and the rotational freedom about the bond connecting the 6-phenyl group and the pyrimidine ring. Both of these factors may influence the biological properties of these compounds.