Benzoic acid (7S,8R,9S)-9-(tert-butyl-diphenyl-silanyloxymethyl)-8-[(Z)-4-(4-methyl-2,6,7-trioxa-bicyclo[2.2.2]oct-1-yl)-but-2-enyl]-1,4-dithia-spiro[4.4]non-7-yl ester