The acid (32) (6.4 g, 0.019 mol) was dissolved in dry, ethanol free, chloroform (120 ml) at room temperature under nitrogen and sequentially treated with dimethylaminopyridine (2.55 g, 0.0209 mol), tert butanol (14.09 g, 0.19 mol), dimethylaminopyridine-trifluoroacetic acid complex (4.94 g, 0.0209 mol) and dicyclohexylcarbodiimide (4.31 g, 0.0209 mol). The resulting solution was stirred at room temperature overnight then filtered, the organic layer washed with 5% acetic acid in water, then dried (MgSO4), filtered and evaporated. The residue was purified by chromatography on silica gel (50 g). Elution with 10% ether in petrol gave the title compound (33) as white crystals (6.45 g, 86%) mp 44°-45° C.
