A neo-clerodane glucoside and neo-clerodane diterpenoids from Teucrium flavum subsp. Glaucum

1H and13C NMR spectral assignments and conformational analysis of 14 19‐nor‐neoclerodane diterpenoids

“…Moreover, since 15 and 9 are epimeric structures at C-2, the previous assignment 15 of the signal at υ 5.31 to the H-2˛proton in the former must be changed to the H-2ˇproton in the latter. In the case of 13, the signal at υ 3.63 (1H, dt, J D 14.1 and 3.2 Hz) had been erroneously attributed 16 to the H-10˛proton in the wrong structure 16, and it must be assigned to the equatorial H-6p roton in the correct structure 13. (For a complete discussion justifying that structures 15 and 16 must be amended, respectively, to 9 and 13, see below).…”

“…The previous assignment 16 of a one proton signal at υ 3.36 (dt, J D 14.1 and 3.2 Hz) to the H-10˛proton of 16 is wrong, and that signal must be assigned to the H-6˛proton in 13. This conclusion was strongly supported by the gHSQC spectrum of this substance, because that proton and another proton resonating at υ 1.97 (ddd, J D 14.1, 13.1 and 2.8 Hz) both belong to the same methylene group, the carbon atom of which appears at υ 27.63, whereas the C-10 methine carbon (υ 47.63) is connected with a proton at υ 2.43 (H-10, dddd, J D 10.0, 3.3, 1.8 and 1.6 Hz).…”

“…1,5 -7 Moreover, some clerodane-type diterpenoids are highly hepatotoxic in mice, causing mid zonal hepatic necrosis, 8 and they have been claimed to be the cause of fulminant hepatitis, chronic hepatitis or cirrhosis in humans. 8 -11 Within the clerodane diterpenes, compounds 1-14 are representatives of the 19-nor-clerodane class 1 -4 and they were isolated from plants some years ago, except for 3, 5, 9, 13 and 14, which are semisynthetic derivatives previously obtained by acetylation of the naturally occurring diterpenes teucrin A, 6-epiteucrin A and 2-hydroxyteucvidin (derivatives 3, 12,13 5 14 and 9, 15 respectively), by acid hydrolysis of 18,2 0 -bisdeacetylteuflavoside (derivative 13) 16 and by alkaline treatment of 1 (derivative 14). 17,18 The diterpenoids teucvin 17,18 (1), 12-epiteucvin 19 (2), teuflin 20 -22 (4), teucrin H 4 23 (6), teucvidin 24 (7), 12-epiteucvidin 25 (8) (11) and montanin A 29,30 (12) are constituents of several Teucrium species 1 -4 (family Labiatae) and all of them, and also the synthetic derivatives 3, 5, 9, 13 and 14, possess a neoclerodane absolute stereochemistry.…”

“…It is of interest that only total syntheses of the racemic forms of 1 and 2 have been achieved 32 previous assignments and to provide several sets of data that might serve as models for establishing the structures of similar compounds in further research. Moreover, another aim of this work was to revise the structures previously attributed to the 2-O-acetyl derivative of 2-hydroxyteucvidin 15 (15) and to an aldehyde (16) obtained from teuflavoside. 16 In our opinion, these structures are not sufficiently supported by the reported data, 15,16 and, in the light of the results described here, structures 15 and 16 must be amended to 9 and 13, respectively.…”

“…Structure 16 had been erroneously assigned to one of the derivatives obtained by acid hydrolysis of 18,2 0 -bisdeacetylteuflavoside, 16 and the correct structure for this compound (13) is supported on the following facts. The previous assignment 16 of a one proton signal at υ 3.36 (dt, J D 14.1 and 3.2 Hz) to the H-10˛proton of 16 is wrong, and that signal must be assigned to the H-6˛proton in 13.…”