(1aS,3aR,6bS)-5-(carbamoyloxymethyl)-1-oxo-1,1a,2,3,3a,6b-hexahydro-4-oxa-2,6a-diazacyclobut[cd]indene-6-carboxylic acid 、
4-hydroxy-phenylcarbamic acid 2,5-dioxo-pyrrolidin-1-yl ester 以50%的产率得到(1aS,3aR,6bS)-5-(Carbamoyloxymethyl)-2-(4-hydroxyphenylcarbamoyl)-1-oxo-1,1a,2,3,3a,6b-hexahydro-4-oxa-2,6a-diazacyclobut[cd]indene-6-carboxylic acid sodium salt