| Bis beta-lactams: 2,7-bis [3'-acetaniido-4 '-akoxyazetidin-2'-on-1'-yl]-4,5dithiasuberic acids have been researched as new beta-lactam antibiotic candidates. Significances of 2,7-bis beta-lactams are explained as (1) they have a symmetric bifunctional structural feature, therefore, their molecules have stronger affinity to enzyme substrates than other beta-lactams: (2) they may attack bacterial targets with a cross-linking mechanism [1], as, their antibacterial activities are proved much higher than monocyclic beta-lactams:;Two strategies were designed to accomplish the goal of obtaining pure bis beta-lactam diastereoisomers. The first strategy was the direct stereoselective synthetic methodology. Starting with L-L-, D-D- and L-D-meso cystines, four step reactions were performed, then, pure diastereoisomers of the 2,7-bis beta-lactam were crystallized and characterized with MALDI TOF-MS, NMR, FTIR and UV etc. Since stereo-selective mechanism favored 2,7-bis beta-lactam diastereoisomers with a trans-configuration at C3' and C4', computer simulations were carried in order to define configurations of crystalline 2,7-bis beta-lactam diastereoisomers. Simulations implemented with results of that crystalline 2(L-), 7(L-)-bis beta-lactam product favored C3·(S) and C4·(S) trans-configuration. Biological activities of crystalline 2,7-bis beta-lactams were evaluated. MIC assays demonstrated that both L-L- and D-D2,7-bis [3'-acetamido-4 '-ethoxyazetidin-2'-on-1' -yl]-4,5-dithiasuberic acids have very high antibacterial activities against both Mycobacterium and Bacillus subtilis . The second strategy was starting with S-trityl-L-cysteine to synthesize monocyclic beta-lactams: alkoxyazetidin-2'-on-1 '-yl]-propanoic 3-thiotrityl-2-[3'-acetamido-4 'acids. Since product of the monocyclic: beta-lactam was a mixture of diastereoisomers, multi-mode chromatographic methods were applied to separate these diastereoisomers. Separated diastereoisomeric components were collected, identified with the secondary ion mass spectrometer (SIMS) and distinguished with circular dichroism (CD). Then, purified monocyclic: beta-lactam precursors reacted each other by sulfur-sulfur bond linking to form bis beta-lactams: The generated 2,7-bis [3'-acetamido-4'-n-butoxyazetidin-2 '-on-1'-yl]-4,5dithiasuberic acid was also identified with the secondary ion mass spectrometer (SIMS) and UV. Based on results, both strategies successfully accomplished the goal of producing pure stereoisomers of 2,7-bis beta-lactams. |
