Oxidative C-Ring Opening of Abietane Diterpenes with Ammonium ­Cerium(IV) Nitrate

Abstract

The oxidation reaction of 11,12-di-O-methylcarnosol with ammonium cerium(IV) nitrate (CAN) gave an abietane diterpene with the C-ring opened and a diester function. The structure of this diester was established by spectroscopic analysis and by chemical means. The reaction of 11,12-di-O-dimethylgaldosol with CAN gave a similar result, yielding a δ-lactone which structure was established as 7-hydroxy-11,12-seco-8,13-abietadien-12,7:20,6β-dilacton-11-oic acid methyl ester by spectroscopic analysis and chemical means. These results could be useful to accede to other natural ­products like rosmic acid.

Key words

oxidation - abietane diterpenes - ring opening - ammonium cerium(IV) nitrate

References and Notes

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10

7-Hydroxy-11,12- seco -8,13-abietadien-12,7:20,6β-dilacton-11-oic Acid Methyl Ester ( 14) To a solution of 11,12-di-O-methylgaldosol (13, 19.5 mg, 0.0524 mmol) in MeCN (2 mL), was added a solution of CAN (163.2 mg, 0.298 mmol) in H₂O (1 mL). The reaction mixture was stirred at r.t. for 72 h and then H₂O (10 mL) was added, extracted with EtOAc (3 × 5 mL), washed with brine (2 × 5mL), dried over anhyd Na₂SO₄, and filtered. The sol-vent was eliminated under reduced pressure, and the crude product was purified by preparative TLC using n-hexane-EtOAc (6:4) as eluent to obtain 7-hydroxy-11,12-seco-8,13-abietadien-12,7:20,6β-dilacton-11-oic acid methyl ester (14, 9.2 mg, 44.8%). ¹H NMR (300 MHz, CDCl₃): δ = 0.91 (3 H, s, Me-19), 1.04 (3 H, s, Me-18), 1.12 (3 H, d, J = 7.0 Hz, Me-16), 1.13 (3 H, d, J = 7.0 Hz, Me-17), 2.42 (1 H, br d, J = 9.8 Hz, H-1β), 2.61 (1 H, s, H-5), 2.92 (1 H, hept, J = 7.0 Hz, H-15), 3.89 (3 H, s, OCH₃), 4.79 (1 H, s, H-6), 5.15 (1 H, br s, OH), 6.91 (1 H, s, H-14). ¹³C NMR (75 MHz, CDCl₃): δ = 17.7 (t, C-2), 21.1 (q, C-16), 21.1 (q, C-17), 21.3 (q, C-18), 23.8 (t, C-1), 28.7 (d, C-15), 30.9 (s, C-4), 31.1 (q, C-19), 37.6 (t, C-3), 46.5 (s, C-10), 52.3 (q, OCH₃), 53.2 (d, C-5), 77.2 (d, C-6), 96.7 (s, C-7), 129.1 (d, C-14), 130.0 (s, C-8), 139.8 (s, C-9), 140.0 (s, C-13), 164.6 (s, C-12), 165.1 (s, C-11), 174.5 (s, C-20). MS (EI): m/z (%) = 390 (16) [M]⁺, 345 (4), 313 (100), 303 (27), 281 (33), 272 (32), 259 (55), 231 (20), 217 (24), 197 (61), 181 (13), 166 (19), 137 (18), 69 (54). HRMS (EI): m/z calcd for C₂₁H₂₆O₇: 390.1679; found: 390.1678.