[CH3:1][O:2][C:3](=[O:23])[CH2:4][C:5]1([CH2:21][CH3:22])[C:10]2[NH:11][C:12]3[C:17]([C:9]=2[CH:8](N)[CH2:7][O:6]1)=[CH:16][CH:15]=[CH:14][C:13]=3[CH2:18][CH3:19].[OH2:24].C=O>C1COCC1>[CH3:1][O:2][C:3](=[O:23])[CH2:4][C:5]1([CH2:21][CH3:22])[C:10]2[NH:11][C:12]3[C:17]([C:9]=2[CH:8]([OH:24])[CH2:7][O:6]1)=[CH:16][CH:15]=[CH:14][C:13]=3[CH2:18][CH3:19]To a solution of 4-amino-1,8-diethyl-1,3,4,9-tetrahydropyrano[3,4-b]indole-1-acetic acid methyl ester (prepared in Example 1) (1.5 g, 0.0047 mol) in THF (5 mL) was added water (5 mL) and paraformaldehyde (0.5 g). The mixture was heated at reflux for 1 hour, then concentrated in vacuo and the residue extracted with ether. The ether extracts were washed with saturated NaHCO3 solution, brine, and dried and concentrated in vacuo. Elution from a silica gel column with a 7:3 hexane:ethyl acetate mixture gave the product (1.2 g, 80.5%), m.p. 117°-118° C. (ether/hexane). Analytical HPLC showed that the product consisted of two diastereomers in a ratio of 19:1.
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