Aromatic plants of tropical Central Africa. Part XXXI. Tricyclic sesquiterpenes from the root essential oil ofEchinops giganteus var.lelyi C. D. Adams

Total Synthesis of (±)-Cameroonan-7α-ol and Biomimetic Rearrangements to Related Nopsane Sesquiterpenes

“…The small, consistent discrepancies (Δδ ≈ 0.06) in the 1 H NMR data for nopsanol are likely attributable to differences in referencing the spectrum. It is noteworthy that (±)- 7 exhibited a woody fragrance consistent with that reported for the enantiomerically pure sesquiterpene alcohol 5a…”

“…Reduction of (±)-cameroonanone 17a with LiAlH 4 (Et 2 O, 0 °C) 5a afforded a 1.4:1 mixture of (±)-cameroonan-7α and 7β-ols ( 6 -OH and 18 -OH, 86%) (eq 1), samples of which were obtained in pure form by flash chromatography. The less polar (±)- 6 -OH (mp 48−49 °C) gave 1 H NMR (500 MHz, CDCl 3 ), 13 C NMR (126 MHz, CDCl 3 and C 6 D 6 ), IR (CCl 4 ), and mass spectral (EI, 70 eV) data that agree precisely with those given in the literature for (−)- 1 -OH 5a. Similar comparisons of the more polar isomer (±)- 18 -OH (mp 62−63 °C) with the unnatural cameroonan-7β-ol confirmed its identity as well.…”

“…The consistent isomer ratios and spectral data observed for the bromo ketones and the cyclization products ensured that 14a was converted into 17a having the natural configuration at the secondary methyl group. The epimeric ketones were separated by silica gel chromatography, and the 1 H NMR (500 MHz, CDCl 3 ), 13 C NMR (126 MHz, CDCl 3 and C 6 D 6 ), IR (CCl 4 ), and MS (EI, 70 eV) data for the less polar 17a agreed with those presented in the literature for (−)-cameroonanone obtained by H 2 CrO 4 oxidation of (−)- 6 -OH 5a. The relative stereochemistry of 17a was also confirmed by NOE experiments (750 MHz, C 5 D 5 N) as shown in 17A .…”

“…GC-HRMS (EI, 70 eV) calcd for C 15 H 26 O 222.1984, found 222.1989 (Δ = −2.4). The tabulated physical data listed in the Supporting Information for silphiperfolan-7β-ol and prenopsanol agreed with those reported in the literature. 5a, …”

“…HPLC purification (Method A) gave an analytical sample of 17b (68 mg, GC purity: 100%). The NMR spectral data were identical with those previously reported for 17a 5a. (±)- 17b : t R = 12.67 min (Method B); TLC R f 0.74 (15:85 Et 2 O−hexane); 1 H NMR (500 MHz, CDCl 3 ) δ 0.93 (d, 3H, J = 6.8 Hz), 1.06 (s, 3H), 1.09 (s, 3H), 1.14 (m, 1H), 1.15 (s, 3H), 1.46 (m, 1H), 1.56−1.82 (m, 6H), 1.65 and 1.76 (ABq, 2H, J AB = 13.6 Hz), 2.05 (m, 1H), 2.40 (m, 1H); 13 C NMR (126 MHz, CDCl 3 ) δ 15.2, 24.5, 24.6, 27.4, 28.6, 29.9, 34.8, 37.9, 43.1, 45.2, 48.5, 49.2, 57.1, 71.5, 228.1; IR (CCl 4 ) 1728 (CO); MS (EI, 70 eV) m / z 220.…”