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An Unusual Lewis Acid Promoted Isomerization of trans-3-Halo-3-phenylthio-b-lactams

2006; Elsevier BV; Volume: 68; Issue: 4 Linguagem: Inglês

10.3987/com-04-10299

1881-0942

Shamsher S. Bari, P. Venugopalan, Renu Arora, Garima Modi, Sachin Madan,

Pneumocystis jirovecii pneumonia detection and treatment

A method for C-3 epimerization of 3-halo-3-phenylthio-β-lactams, mediated by Lewis acids, is described.TiCl 4 promotes isomerization of trans-3chloro-3-phenylthioazetidin-2-ones (2) to cis-3-chloro-3-phenylthioazetidin-2ones (3).TiBr 4 promotes isomerization as well as substitution of chlorine with bromine affording isomeric cisand trans-3-bromo-3-phenylthioazetidin-2-ones (5) and ( 6) respectively, while TiI 4 is ineffective.A variety of Lewis acid promoted reactions in organic chemistry are becoming valuable synthetic processes.Moreover, the processes which convert readily available chemical entities to their less readily accessible isomers are of a synthetic value besides being useful in mechanistic studies in organic synthesis. 1 Over the years, structural diversity of β-lactam antibiotics has led to the development of various synthetic approaches with complete control of stereochemistry 2 at C-3 and C-4 of β-lactams.[5][6] In the most often employed methods [7][8][9][10][11] for β-lactam synthesis, the formation of kinetically controlled cisβ-lactam 12 is highly favoured.Leaving aside thermally induced isomerization 13 , all the reported procedures for isomerization 14 of cis-β-lactams to more stable trans-β-lactams require the presence of an acid or base sensitive functionality at the site of epimerization.