合成路线1

1) The reaction of 2-methoxycyclohexanone (I) with dialkyl carbonate by means of NaH/KH in THF gives 3-methoxy-2-oxocyclohexanecarboxylic acid allyl ester (II), which is condensed with (3R,4R)-4-acetoxy-3-[1(R)-tert-butyldimethylsilyloxy)ethyl]azetidin-2-one (III) by means of NaH in THF yielding the condensation product (IV) as a mixture of the alpha- and beta-stereoisomers at the methoxy group. The protection of the NH group of (IV) with tert-butyldimethylsilyl (TBS) trifluoromethanesulfonate affords the bis-TBS-protected compound (V), which is stereospecifically decarboxylated with palladium acetate, formic acid and triphenylphosphine to the protected azetidinone (VI) also as alpha- and beta-isomers. The selective desilylation of (VI) with tetrabutylammonium fluoride (TBAF) in acetic acid gives the O-protected azetidinone (VII) also as a mixture, which is epimerized by sequential treatments with lithium diisopropylamide (LDA), with ZnBr2 and finally with diethyl malonate to afford the alpha-(S)-epimer (VIII). The acylation of (VIII) with benzyloxalyl chloride and pyridine in dichloromethane gives the N-benzyloxazyl derivative (IX), which is cyclized by means of triethyl phosphite in refluxing xylene yielding (4S,8S,9R,10S)-10-[1(R)-(tert-butyldimethylsilyloxy)ethyl]-4-methoxy-11-oxo-1-azatricyclo[7.2.0.0(3,8)]undec-2-ene-2-carboxylic acid benzyl ester (X). The desilylation of (X) with TBAF in THF/acetic acid affords (XI) with a 1-hydroxyethyl group, which is hydrogenated with H2 over Pd/C in dioxane in the presence of 3,3,6,9,9-pentamethyl-2,10-diazabicyclo[4.4.0]dec-1-ene (XII) giving the addition salt (XIII). The treatment of (XIII) with potassium 2-ethylhexanoate yields the corresponding potassium salt (XIV) (1), which is finally esterified with 1-chloroethyl cyclohexyl carbonate (XV) by means of benzyltrimethylammonium chloride in DMF.