Role of the C(5)-C(5a) Exomethylene Group in Bicyclomycin: Synthesis, Structure, and Biochemical and Biological Properties.
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.
Park Hg
Hyeung-geun
Department of Chemistry, University of Houston, Houston, Texas 77204-5641, and Department of Biochemical and Biophysical Sciences, University of Houston, Houston, Texas 77204-5934.
Zhang
Zhuming
Z
Zhang
Xiangdong
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Widger
William R.
WR
Kohn
Harold
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