(6R,8S,11S,14S)-ethyl 8-(tert-butoxycarbonyl(methyl)amino)-14-((R)-methoxy(phenyl)methyl)-2,2,6,11-tetramethyl-9,12-dioxo-3,3-diphenyl-4-oxa-10,13-diaza-3-silapentadecan-15-oate