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The procedure of S. Thaisrivong (J. Med. Chem. 1987, 30, 976) was employed. A solution of 40 g of the resultant compound of Example 17 and 102 g of 2-methoxypropene in 250 ml of dichloromethane was stirred at room temperature. Solid pyridinium p-toluenesulfonate (PPTS) (177 g) was added slowly to the reaction mixture. After addition was complete, the reaction was stirred for 1 h and neutralized by addition of solid sodium bicarbonate. The solids were filtered and the filtrate was concentrated. Flash chromatography on silica gel gave 57 g of the desired compound. IR (CDCl3) 1690 (C=O carbamate) cm-1 ; 1H NMR (CDCl3) δ5.95 (m,1H), 5.32 (m,1H), 5.20 (dt,1H), 4.27 (dd,1H), 1.47 (s,9H).

1. additionAfter addition
2. filtrationThe solids were filtered
3. concentrationthe filtrate was concentrated