Chloro-16-formyl-Δ1,3,5(10)-estratetraen-3-ol 2a (113 mg, 0.36 mmol) was added to a suspension of acetamidine hydrochloride (51 mg, 0.54 mmol) and potassium carbonate (120 mg, 0.89 mmol) in DMF (4 mL). The mixture was stirred at 80°C for 9 h until the complete conversion of the intermediates (TLC monitoring). The resulting mixture was cooled to room temperature and diluted with water (30 mL). The precipitate that formed was filtered, dried, and washed with water (3 × 5 mL). The product was isolated by column chromatography using petroleum ether/ethyl acetate, 1:1, to obtain colorless solid (34 mg, 30% yield). Rf 0.27 (petroleum ether:EtOAc, 1:2; visualized by UV light at 254 nm); m.p. 264–266°C. 1H NMR (600 MHz, DMSO-d6), δ: 0.92 (s, 3H, 18-CH3), 1.35–1.41 (m, 1H, 7-CH2), 1.50–1.56 (m, 1H, 11-CH2), 1.61–1.69 (m, 2H, 8-CH2, 12-CH2), 1.70–1.76 (m, 1H, 14-CH), 1.90–1.95 (m, 1H, 7-CH), 2.14–2.18 (m, 1H, 12-CH2), 2.21–2.27 (m, 1H, 9-CH), 2.37–2.42 (m, 1H, 11-CH2), 2.48–2.54 (m, 1H, 15-CH2), 2.56 (s, 3H, 2′-CH3), 2.73–2.85 (m, 3H, 6-CH2, 15-CH2), 6.47 (s, 1H, 4-CH), 6.53 (dd, J = 2.4, 8.4 Hz, 1H, 2-CH), 7.07 (d, 1H, J = 8.4 Hz, 1-CH), 8.46 (s, 1H, 4-CH), 9.00 (br.s, 1H, OH). 13C NMR (125 MHz, DMSO-d6), δ: 17.1 (18-CH3), 25.5 (2′-CH3), 25.8 (11-CH2), 26.9 (7-CH2), 27.0 (15-CH2), 28.9 (6-CH2), 32.7 (12-CH2), 37.2 (8-CH), 43.7 (9-CH), 45.8 (13-C), 54.3 (14-CH), 112.8 (2-CH), 115.0 (4-CH), 125.8 (1-CH), 130.1 (10-C), 130.5 (16-C), 137.0 (5-C), 151.8 (4′-CH), 155.0 (3-C), 165.1 (2′-C), 181.0 (17-C). IR (KBr), cm−1: 3179 (OH), 2986, 2929, 2859 (CH), 1607, 1585, 1555, 1501 (C = C, C = N). HRMS (ESI) for C21H25N2O ([M+H]+): calcd 321.1961, found 321.1951.