The condensation of 3-(tert butoxycarbonyl amino)-1-hydroxy-4-methylazetidin-2-one (I) with chloroacetic acid (II) by means of K2CO3 gives 2-(3-(tert butoxycarbonylamino)-4-methyl-2-oxoazetidin-1-yloxy]acetic acid (III), which is deprotected with trifluoroacetic acid yielding 2-(3-amino 4-methyl-2-oxoazetidin-1-yloxy)acetic acid (IV). Finally, this compound is condensed with 2-(2-amino-thiazol-4-yl)-2-(methoxyimino)acetyl chloride (V) by means of Na2CO3.