Role of the C(5)-C(5a) Exomethylene Group in Bicyclomycin: Synthesis, Structure, and Biochemical and Biological Properties.

Abstract

Thirty-two C(5)-C(5a) exomethylene-modified bicyclomycin derivatives were prepared to determine the effect of structural modification of this unit on bicyclomycin (1) function. The compounds were grouped into three categories: the C(5)-unsaturated bicyclomycins, the C(5a)-substituted C(5)-C(5a)-dihydrobicyclomycin derivatives, and the C(5)-modified norbicyclomycins. An efficient three-step procedure was developed to synthesize C(5a)-substituted C(5),C(5a)-dihydrobicyclomycins. Bicyclomycin was converted to bicyclomycin C(2'),C(3')-acetonide (36) and then treated with a nucleophile in 50% aqueous methanol ("pH" 10.5) to give the C(5a)-substituted C(5),C(5a)-dihydrobicyclomycin C(2'),C(3')-acetonide. Removal of the acetonide group (trifluoroacetic acid in 50% aqueous methanol) in the final step provided the desired bicyclomycin derivative. All the compounds were evaluated using the rho-dependent ATPase assay and their antimicrobial activities determined using a filter disc assay. Most of the compounds were also tested in the transcription termination assay. We observed that many of the C(5)-unsaturated bicyclomycins effectively inhibited ATP hydrolysis at 400 &mgr;M and inhibited the production of rho-dependent transcripts at 100 &mgr;M. The biochemical activities of C(5a)-bicyclomycincarboxylic acid (5), methyl C(5a)-bicyclomycincarboxylate (6), ethyl C(5a)-bicyclomycincarboxylate (7), and bicyclomycin C(5)-norketone O-methyloxime (11) were all similar to 1. Compounds 6, 7, and 11 exhibited diminished antibiotic activity compared to 1, and 5 displayed no detectable activity. Several C(5a)-substituted C(5),C(5a)-dihydrobicyclomycins showed significant inhibition of rho-dependent ATPase and transcription termination activities. The inhibitory properties of C(5),C(5a)-dihydrobicyclomycin C(5a)-methyl sulfide (18), C(5),C(5a)-dihydrobicyclomycin C(5a)-phenyl sulfide (23), and C(5)-C(5a)-dihydrobicyclomycin-5,5a-diol (31) approached those of 1. Compounds 18, 23, and 31 did not exhibit antibiotic activity. Two of the four C(5)-modified norbicyclomycin adducts showed moderate inhibitory activities in the ATPase assay, and none showed significant antibiotic activity. Our findings showed that the C(5)-C(5a) exomethylene unit retention in 1 was not essential for inhibition of in vitro rho activity. The structure-activity relationship data indicated that bicyclomycins that contained a small unsaturated C(5) unit or C(5),C(5a)-dihydrobicyclomycins that possessed a small, nonpolar C(5a) substituent effectively inhibited rho function in in vitro biochemical assays. We concluded that the C(5)-C(5a) unit in 1 was not a critical structural element necessary for drug binding to rho and that irreversible, inactivating units placed at this site would permit the bicyclomycin derivative to bind efficiently to rho.