An oxidative rearrangement of t-butyl (3RS,5SR)-2-ethoxycarbonylcarbapen-1-em-3-carboxylate to t-butyl (1RS,5SR,7RS,8SR)-8-ethoxycarbonyl-8-hydroxy-3-oxo-2-oxa-6-azabicyclo[3.3.0]octane-7-carboxylate

Abstract

The title reaction [(1a)→(10a)] is induced by the action of osmium(VIII) oxide. Presumably, the diol (12), formed by the addition of the elements of hydrogen peroxide to the endo-face of the 1,2-double bond of the precursor (1a), is an intermediate in the reorganisation. The rearrangement product is shown to possess structure (10a)[rather than (11a) as proposed in the preliminary communication of this work] by X-ray crystallography. Compound (10a) undergoes dehydrogenation in the presence of lead(IV) acetate or chromium(VI) oxide to give t-butyl (1RS,5SR,8SR)-8-ethoxycarbonyl-8-hydroxy-3-oxo-2-oxa-6-azabicyclo[3.3.0]oct-6-ene-7-carboxylate (13a). Unusual results are encountered when compound (10a) is subjected to methylsulphonylation conditions. Thus, with methanesulphonyl chloride and triethylamine in dichloromethane, it reacts to give the O-methanesulphonate (10d) as the major product and (depending upon the experimental conditions) either the N-methylsulphonylmethylsulphonyl-O-methylsulphonyl derivative (10f) or the N-methylsulphonylmethylsulphonyl derivative (10h). The last-cited compound is the major product when the oxazabicyclo-octane (10a) is added to an ‘aged’ mixture of methanesulphonyl chloride and triethylamine in acetonitrile. In the presence of methanesulphonyl chloride in pyridine, compound (10a) is transformed into the N-methylsulphonyl derivative (10c). A study of the 300 MHz 1H n.m.r. spectra of oxazabicyclo-octanes of type (10) and oxazabicyclo-octenes of type (13) reveals some unexpected downfield shifts of the 1-hydrogen atoms as a consequence of O-substitution.