Sodium Formate → 4-Chloro-2-formylamino-6-methyl-10,10-dioxo-10,11-dihydro-5-oxa-10lambda*6*-thia-dibenzo[a,d]cycloheptene-8-carboxylic acid methyl ester

Sodium formate (7.29 g, 0.108 mol) was added to a suspension of compound of Example 1 (20 g, 0.054 mol) in formic acid (200 mL) and refluxed for 1.5 h. The reaction mixture was cooled and poured into ice water (800 mL). The solid precipitated was filtered, washed with water and dried. The crude product was purified by column chromatography (silica gel, 1% methanol in chloroform) to obtain the title compound. Yield: 18 g, (83%); mp 258-262° C. (decomposed); 1H NMR (DMSO-d6, 300 MHz): δ 2.64 (s, 3H, CH3), 3.86 (s, 3H, OCH3), 5.25 (s, 2H, CH2, trans minor isomer), 5.33 (s, 2H, CH2, cis major isomer), 7.33 (d, 1H, Ar, trans minor isomer), 7.50 (d, 1H, Ar, trans minor isomer), 7.69 (d, 1H, Ar, cis major isomer), 7.86 (d, 1H, Ar, cis major isomer), 8.10 (s, 1H, Ar), 8.15 (s, 1H, Ar), 8.30 (s, 1H, CHO, cis major isomer), 8.80 (d, 1H, CHO, trans minor isomer), 10.40 (d, 1H, NH, trans minor isomer), 10.57 (s, 1H, NH, cis major isomer); MS: m/e (ES−) 394 (M−1); analysis: C17H14ClNO6S requires C, 51.54; H, 3.56; N, 3.54; Cl, 8.96; S, 8.10. found: C, 51.96; H, 3.63; N, 3.60; Cl, 9.43; S, 8.50%.