(rac.)-(1R*,5S*)-7-{4-[3-(tert-Butyl-dimethyl-silanyloxy)-propyl]-phenyl}-9-methyl-3,9-diaza-bicyclo[3.3.1]non-6-ene-3,6-dicarboxylic acid 3-tert-butyl ester 6-ethyl ester