(4S,5R)-7-((R)-2-amino-4-phenoxybutanoyl)-2-methyl-4-phenyl-2,7-diazaspiro[4.5]decan-1-one (55 mg; 0.130 mmol) was solubilised in acetonitrile (0.5 ml). 2-(tert-butoxycarbonylamino)-2-methylpropanoic acid (27 mg; 0.130 mmol) was added followed by diisopropylethylamine (0.091 ml; 0.522 mmol). The solution was stirred at RT for 5 minutes before ®T3P (50% solution in ethyl acetate) (0.152 ml; 0.261 mmol) was added. The mixture was stirred overnight, then concentrated in vacuo and purified by chromatography on silica eluting with 0-10% MeOH in TBME with ammonia to yield tert-butyl 2-methyl-1-((R)-1-((4S,5R)-2-methyl-1-oxo-4-phenyl-2,7-diazaspiro[4.5]decan-7-yl)-1-oxo-4-phenoxybutan-2-ylamino)-1-oxopropan-2-ylcarbamate.
