| abstract |
The invention relates to a new process for the preparation of (3R, 1'R) -4-acyloxy-3- (1'-hydroxyethyl) -2-azetidinones of the formula wherein R¹ is lower alkyl or aryl, by enantioselective reduction of the carbonyl group in a suitable α-acylaminomethylacetoacetic acid ester, cyclization of the available α-acylaminomethyl-β-hydroxybutyric acid ester to a 5,6-dihydro-1,3,4H-oxazine with inversion of the hydroxyl group C-atom, equilibration to the preferred trans-substituted dihydrooxazine, cleavage back to configuratively uniform α-aminomethyl-β-hydroxybutyric acid, ring closure to the β-lactam and oxidative acylation at C (4) of the β-lactam. Compounds of the formula I can be used as starting materials for the preparation of β-lactam antibiotics. New intermediates are also the subject of the invention. |