Computer-assisted carbon-13 nuclear magnetic resonance spectrum analysis and structure prediction for the C19-diterpenoid alkaloids

Abstract

mixture was cooled and poured into 150 mL of saturated NaHC03. The organic layer was separated, washed twice with 100-mL portions of saturated NaHC03 and once with brine, and dried over anhydrous Na¡S04, and the solvent was removed under reduced pressure to yield 3.03 g of crude product. Chromatography on silica gel (1:1 ether-hexane) afforded 644 mg (20%) of diketal 55, 1.23 g (44%) of 30, and 791 mg (33%) of recovered 29. The required monoketal was crystallized from ether: mp 139-141 °C; IR (CHC13) 1660 cm"1; NMR (CDC13) b 0.93 (d,

The Structures of Forestine and Foresticine, Two New C19-Diterpenoid Alkaloids from Aconitum forrestii Stapf

“…The modulated off-resonance decoupled spectrum showed three singlets at 39.1, 50.6, and 74.0 ppm, which can be attributed to the quaternary carbon atoms C-4, C-l 1, and C-8 (bearing an oxygen function), respectively. The methoxyls whose signals appear at 56.1, 56.4, and 59.2 ppm can be assigned to carbons C-l, C-16 and C-18, respectively, on the basis of the known substitution pattern in this class of alkaloids (11). Two of the remaining three hydroxyls should be placed at C-8 and C-14 on biogenetic considerations, consistent with the 13C-chemical shift assignments.…”

“…OH (cf. 1,2,4,9 in Table 1) and the normal oxygenation of this class of alkaloids indicate that C-l and C-6 are substiuted by methoxyl groups (11). The C-6 methoxyl is aoriented, as seen from the pmr signal at 4.06, which appears as a broad doublet (/=6Hz, */2 w 1.5 Hz) showing maximum coupling with the C-5 proton and a negligible coupling with the C-7 proton (-92°d ihedral angle).…”