Revisions of the diterpenoid alkaloids reported in a JNP paper (2012, 75, 1145-1159)

Eleven C19-diterpenoid alkaloids reported in Shi's paper were isolated or synthesized. Their 13C NMR and DEPT spectra were recorded using (CD3)2CO+TFA as solvents. Comparison of the acquired NMR data with those reported in Shi's paper, in combination with the fact that all alkaloids, expect for 8, 27, and 28, reported in Shi's paper were isolated using TFA as part of solvent system, led to the following revisions of Shi's paper: (1) compounds 1-4, 6, 7a, 9-26 should be their respective trifluoroacetates. Their structures were revised as 1a-4a, 6a, 7a′, 9a-26a; (2) compounds 7, 27a, and 28a are trifluoroacetates of free bases 8, 27, and 28, respectively; compound 7 should be corrected as 7b; (3) ring A of free bases (such as 8, 27, and 28) exists as chair conformation; while ring A of the protonated versions (such as 7, 27a, 28a, 6a, 9a-26a) exists as boat conformation. Consequently, compounds 7 and 8 are apparently not a pair of conformational isomers, instead, they are indeed a free base and its protonated version existing as a trifluoroacetate, respectively; (4) 'new' compounds 6, 13, and 15 are the protonated versions 6a, 13a, and 15a, respectively, of the corresponding known alkaloids, aconifine, deoxyaconitine, 14-benzoylaconine. In addition, all 1H (13C) NMR data for the free bases of new compounds 8, 9, 10, 17, and 24 (Tables 3 and 4) were also assigned based on their 2D NMR spectra.