8-Formyl-labdanolic Acid (-)-(3S)-5-((1R,2R,4aS,8aS)-2-Formyloxy-2,5,5,8a-tetramethyldecahydro-1-naphthalenyl)-3-methylpentanoic Acid

A sample (0.85 mL, 6.5 mmol) of formic acid-acetic anhydride mixture (FAM), prepared from Ac₂O and formic acid as described in the literature [1], was added to the alcohol 1 [2](255 mg, 0.79 mmol) at 10 °C. After stirring for 48 h at room temperature, water (10 mL) was added and the mixture extracted with Et₂O (3×25 mL). The combined organic layers were washed with 2N HCl (25 mL), saturated aq. Na₂CO₃ (25 mL) and brine (25 mL). The organic phase was dried over anhydrous Na₂SO₄ and the solvent evaporated under reduced pressure to yield a residue (258 mg) which was purified by flash chromatography on silica gel, using a 3:2 hexane/EtOAc mixture as eluent, to give the pure title compound 2 (180 mg, 0.51 mmol, 65%).

Mp: 102.4-105.5 °C (pale yellow crystals, from hexane).

[a]_D = -33.7° (c 1.2 cg·mL^-1, CHCl₃).

IR (neat, n, cm^-1): 3600-2500, 1719 (COOH), 1719, 1200, 1180 (OOCH).

¹H NMR (300 MHz, CDCl₃, d, ppm): 0.79 (3H, s, Me_b-4), 0.84 (3H, s, Me-10), 0.87 (3H, s, Me_a-4), 0.98 (3H, d, J=6.6 Hz, Me-13), 1.49 (3H, s, Me-8), 0.99-2.00 (16H, m, H-1,2,3,5,6,7a,9,11,12,13), 2.15 (1H, dd, J=15.0 Hz, 8.0 Hz, H-14), 2.36 (1H, dd, J=15.0 Hz, 5.9 Hz, H'-14), 2.57 (1H, dt, J=12.3 Hz, 3.2 Hz, H_b-7), 8.00 (1H, s, OOCH).

¹³C NMR (75 MHz, CDCl₃, d, ppm): 39.46 (C-1), 18.23 (C-2), 41.78 (C-3), 33.05 (C-4), 55.51 (C-5), 19.98 (C-6), 39.36 (C-7), 89.17 (C-8), 58.91 (C-9), 39.42 (C-10), 22.99 (C-11), 39.63 (C-12), 30.88 (C-13), 41.44 (C-14), 179.48 (C-15), 19.63 (C-16), 21.09 (C-17), 33.24 (C-18), 21.37 (C-19), 15.65 (C-20), 160.61 (OOCH).