product the crude product was purified by consecutive elution on silica gel with a solution of 60 etoac in hexane to yield the named compound a solution of the ester of example 1 556 mg 1 17 mmol in 0 5 n aqueous lithium hydroxide 3 5 ml 1 76 mmol and thf 7 0 ml was stirred at 23 u00b0 c for 24 h and acidified with 10 citric acid the mixture was extracted with etoac and the combined organics were dried mgso 4 filtered and concentrated in vacuo the crude residue was treated with 0 benzyl n n 39 diisopropylisourea 0 41g 1 76 mmol and heated to 65 u00b0 c in benzene 7 0 ml for 24 h the reaction was cooled to room temperature and stripped of the solvent fcc 2 1 hex etoac of the residue gave 553 mg 85 of the named compound 330 mg 0 591 mmol of the compound of example 4a were treated in accordance with the procedure of example 2 to yield 235 7 mg 68 yield of the named compound 60 mg 0 1027 mmol of the compound of example 5 in 1 0 ml of thf was treated with 0 2054 ml a 1 0 m solution of bu 4 nf and stirred at 23 u00b0 c for 16 hours the reaction mixture was diluted with etoac and washed consecutively with h 2 o and brine and dried.