Lupulin structures revisited

The crystal structures of lupulin A, C₃₀H₄₆O₁₁, and lupulin D, C₂₅H₃₈O₈, should both display the neoclerodane skeleton, but the deposited atomic coordinates of lupulin D correspond to the inverted enanti­omer.

1. Comment

The X-ray analysis reports for the isolates from Ajuga lupulina, viz. lupulin A (Chen et al., 1997) and lupulin D (Chen, Tan, Liu, Liu & Chen, 1996) (also called clerodinin B; Lin et al., 1989), show contradictory graphical information. The simpler structure of lupulin D, as shown in Chen, Tan, Liu, Liu & Chen (1996) (Fig. 1 of the paper = Fig. 1a here), could be described as 15α-methoxy­dihydro­clerodin (but is named 3-de­­oxy-14,15-dihydro-15-methoxy­caryoptinol). The formula shows the corresponding neoclerodane skeleton with a few stereochemistries undisclosed. The 15α-meth­oxy substitution (discussed as C17 in the Comment, according to the atom-numbering scheme in Fig. 1 of the paper) was `in agreement with the structural elucidation of Lin et al. (1989)'. However, this Fig. 1 displays the view with displacement ellipsoids of an ent-neoclerodane structure with a 15α-meth­oxy substitution. Thus, the deposited atomic coordinates must correspond to the inverted enanti­omer of lupulin D [Cambridge Structural Database (Allen, 2002) refcode TEHZOV, Fig. 1b] and, further, the correct stereoformulae should be revised to 15β-methoxy­dihydro­clerodin [see revised lupulin D in Fig. 1c]. As a consequence, the early C-15 stereochemical assignments for clerodinins A and B (Lin et al., 1989) must also be reversed. The reversal of the previously assigned stereochemistries at position C-15 for both clerodinins A and B (change from β to α and from α to β, respectively), was already proposed by Ben Jannet et al. (1999) from the results of NMR NOESY experiments reported for hativenes A–C.

The structures of lupulin A and lupulin B were first elucidated by spectroscopic means (Chen, Tan, Liu, Zhang & Yang, 1996) and reported as 15β-meth­oxy (Fig. 2a) and 2-deoxy-15α-methoxy­dihydro­ajugapitin, respectively, by comparison with NMR data of clerodinin A and clerodinin B. Lupulin A showed strong anti­bacterial activity, and accordingly, it should be the compound studied by means of X-ray crystallographic analysis (Chen et al., 1997) (confirmed by melting point and empirical formula). This X-ray report shows the neo­clerodane structure of 15α-methoxy­dihydro­ajugapitin (Fig. 2b) in the view with displacement ellipsoids. In this instance, quite unexpectedly, opposite graphical displays were again shown. There, the stereoformula displayed was the inverted enanti­omer ent-neoclerodane 15β-methoxy­dihydro­ajugapitin (Fig. 2c).

Furthermore, this X-ray structure already points out the reversal of the previously assigned stereochemistries at position C-15 for lupulins A and B (change from β to α and from α to β, respectively), based on NMR data. As already mentioned for clerodinins A and B, the reversal of the stereochemistries at C-15 for lupulins A and B had been already proposed from the results of NMR NOESY experiments (Ben Jannet et al., 1999, 2000). Unfortunately, the structure depicted in both papers for lupulin A (as shown in Fig. 3) was yet the uncorrected 15β-methoxy­dihydro­ajugapitin analogous to hativene A, rather than the revised proposal analogous to hativene B.

In summary, lupulin A should display a revised 15α-meth­oxyneoclerodane structure (Fig. 2b), whereas lupulin B and lupulin D (Fig. 1c) display a 15β-meth­oxy neoclerodane structure.

Note added in proof. The authors acknowledge Professor Hichem Ben Jannet for kindly providing a reprint (Ben Jannet et al., 2002) reporting the `Structure of a new neoclerodane diterpenoid from Ajuga pseudoiva'. The compound was named hativene D, being in fact elucidated as the 15-epimer (15α-MeO) of the uncorrected structure reported for lupulin A (Fig. 3d, 15β-MeO). However, the reported NMR data (an extended set showing a few corrections and assignment changes likely based on HMQC or HMBC spectroscopic data) match, not unexpectedly, quite well those of lupulin A, since the proposed structure revision points out the identical nature of hativene D and lupulin A.

Supporting information

S1. Comment

The X-ray analysis reports for the isolates from Ajuga lupulina, viz. lupulin A, (I) (Chen et al., 1997), and lupulin D, (II) (Chen, Tan, Liu, Liu & Chen, 1996) (also called clerodinin B; Lin et al., 1989), show contradictory graphical information. The simpler structure of (II), as shown in Chen, Tan, Liu, Liu & Chen (1996) (Fig. 1 of the paper = Fig. 1a here), could be described as 15α-methoxydihydroclerodin (but is named 3-deoxy-14,15-dihydro-15-methoxycaryoptinol). The formula shows the corresponding neo-clerodane skeleton with a few stereochemistries undisclosed. The 15α-methoxy substitution (discussed as C17 in the Comment, according to the atom numbering scheme in Fig. 1 of the paper) was `in agreement with the structural elucidation of Lin et al. (1989)'. However, this Fig. 1 displays the view with displacement ellipsoids of an ent-neoclerodane structure with a 15α-methoxy substitution. Thus, the deposited atomic coordinates must correspond to the inverted enantiomer of lupulin D [Cambridge Structural Database (Allen, 2002) refcode TEHZOV, Fig. 1 b] and, further, the correct stereoformulae should be revised to 15β-methoxydihydroclerodin [see revised (II) in Fig. 1c]. As a consequence, the early C-15 stereochemical assignments for clerodinins A and B (Lin et al. 1989) must be reversed also. The reversal of the previously assigned stereochemistries at position C-15 for both clerodinins A and B (change from β to α and from α to β, respectively), was already proposed by Ben Jannet et al. (1999) from the results of NMR NOESY experiments reported for hativenes A—C.

The structures of lupulin A (I) and lupulin B were first elucidated by spectral means (Chen, Tan, Liu, Zhang & Yang, 1996) and reported as 15β-methoxy (Fig. 2a) and 15α-methoxydihydroajugapitin, respectively, by comparison with NMR data of clerodinin A and clerodinin B. Lupulin A showed strong antibacterial activity, and accordingly, it should be the compound studied by means of X-ray crystallographic analysis (Chen et al. 1997). This X-ray report shows the neo-clerodane structure of 15α-methoxydihydroajugapitin (Fig. 2 b) in the view with displacement ellipsoids. In this instance, quite unexpectedly, again opposite graphical displays were shown. There, the stereoformula displayed was the inverted enantiomer ent-neo-clerodane 15β-methoxydihydroajugapitin (Fig. 2c).

Furthermore, this X-ray structure already points out the reversal of the previously assigned stereochemistries at position C-15 for lupulins A and B (change from β to α and from α to β, respectively), based on NMR data. As already mentioned for clerodinins A and B, the reversal of the stereochemistries at C-15 for lupulins A and B had been already proposed from the results of NMR NOESY experiments (Ben Jannet et al., 1999, 2000). Unfortunately, the structure depicted in both papers for lupulin A was yet the uncorrected 15β-methoxydihydroajugapitin analogous to hativene A, rather than the revised proposal analogous to hativene B, as shown in Fig 3.

In summary, lupulin A should display a revised 15α-methoxy neo-clerodane structure (Fig. 2 b) whereas lupulin B and lupulin D (Fig, 1c), a 15β-methoxy neo-clerodane one.

Computing details

Figures

	Fig. 1. Lupulin D, (II), in Chen, Tan, Liu, Liu & Chen (1996), showing (a) the formula shown in the original paper, (b) the structure according to deposited data (TEHZOV) showing an ent-neoclerodane skeleton, and (c) lupulin D/clerodinin B (revised) showing the neo-clerodane skeleton and the 15β-methoxy substitution.
	Fig. 2. Lupulin A, (I), according to (a) Chen, Tan, Liu, Zhang & Yang (1996) and to deposited data RAVWUG, (b) (revised) showing the neo-clerodane skeleton and the C-15 α-substituent, and (c) the stereoformula shown in Chen et al. (1997).
	Fig. 3. Structures of (a) hativene A, (b) hativene B, (c) hativene C and (d) uncorrected lupulin A in Ben Jannet et al. (2000).

15α-methoxydihydroclerodin

Crystal data

C25H38O8	V = 2409.48 Å^3
Mr = ?	Z = 4
Orthorhombic, P212121	Dx = 1.29 Mg m^−3
a = 10.114 (3) Å	? radiation, λ = ? Å
b = 11.289 (4) Å	, white
c = 21.103 (9) Å	× × mm

Data collection

h = ?→?	l = ?→?
k = ?→?

Refinement

R[F^2 > 2σ(F^2)] = 0.037

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O1	0.153100	−0.382100	0.141200	0.05000
O2	0.108600	−0.470700	0.049100	0.05000
O3	0.335900	−0.262900	0.211900	0.05000
O4	0.471100	−0.138500	0.263300	0.05000
O5	0.369500	−0.283100	0.073400	0.05000
O6	−0.341700	0.423900	0.057800	0.05000
O7	−0.052100	0.185400	0.137700	0.05000
O8	−0.236300	0.306800	0.132900	0.05000
C1	0.113900	−0.050400	0.117700	0.05000
C2	−0.025200	−0.033000	0.149400	0.05000
C3	−0.109400	−0.145300	0.136000	0.05000
C4	−0.037100	−0.258700	0.154900	0.05000
C5	0.093700	−0.271600	0.120800	0.05000
C6	0.189200	−0.169600	0.133800	0.05000
C7	0.309700	−0.169800	0.088800	0.05000
C8	0.405100	−0.069500	0.098300	0.05000
C9	0.331400	0.046400	0.085400	0.05000
C10	0.206000	0.057700	0.124600	0.05000
C11	−0.102800	0.071000	0.119600	0.05000
C12	−0.119600	0.269200	0.100400	0.05000
C13	−0.166200	0.206900	0.039400	0.05000
C14	−0.114400	0.079900	0.047900	0.05000
C15	−0.341700	0.315300	0.089700	0.05000
C16	−0.317600	0.217800	0.041600	0.05000
C17	−0.368100	0.522400	0.097900	0.05000
C18	−0.017100	−0.006300	0.221600	0.05000
C19	−0.247600	−0.146600	0.166200	0.05000
C20	0.158400	−0.472100	0.100800	0.05000
C21	0.234200	−0.574100	0.127600	0.05000
C22	0.237300	−0.172000	0.203200	0.05000
C23	0.442600	−0.235700	0.245700	0.05000
C24	0.520800	−0.346100	0.259900	0.05000
C25	0.301900	−0.217800	0.024300	0.05000
H1	0.092000	−0.056000	0.074000	0.05000
H2	−0.122000	−0.142000	0.091000	0.05000
H3	−0.021000	−0.256000	0.199000	0.05000
H4	−0.092000	−0.325000	0.145000	0.05000
H5	0.076000	−0.271000	0.077000	0.05000
H6	0.438000	−0.070000	0.140000	0.05000
H7	0.477000	−0.078000	0.069000	0.05000
H8	0.388000	0.111000	0.095000	0.05000
H9	0.307000	0.049000	0.042000	0.05000
H10	0.160000	0.127000	0.112000	0.05000
H11	0.231000	0.064000	0.168000	0.05000
H12	−0.187000	0.053000	0.137000	0.05000
H13	−0.064000	0.336000	0.094000	0.05000
H14	−0.136000	0.236000	0.000000	0.05000
H15	−0.031000	0.068000	0.028000	0.05000
H16	−0.176000	0.024000	0.032000	0.05000
H17	−0.423000	0.310000	0.112000	0.05000
H18	−0.353000	0.240000	0.002000	0.05000
H19	−0.356000	0.145000	0.055000	0.05000
H20	−0.419000	0.487000	0.130000	0.05000
H21	−0.292000	0.557000	0.117000	0.05000
H22	−0.420000	0.583000	0.079000	0.05000
H23	0.051000	0.050000	0.230000	0.05000
H24	−0.100000	0.025000	0.235000	0.05000
H25	0.001000	−0.078000	0.244000	0.05000
H26	−0.243000	−0.140000	0.211000	0.05000
H27	−0.297000	−0.080000	0.151000	0.05000
H28	−0.290000	−0.219000	0.155000	0.05000
H29	0.229000	−0.566000	0.172000	0.05000
H30	0.196000	−0.648000	0.116000	0.05000
H31	0.325000	−0.573000	0.116000	0.05000
H32	0.164000	−0.187000	0.230000	0.05000
H33	0.275000	−0.097000	0.213000	0.05000
H34	0.476000	−0.389000	0.292000	0.05000
H35	0.528000	−0.394000	0.223000	0.05000
H36	0.607000	−0.326000	0.275000	0.05000
H37	0.350000	−0.183000	−0.009000	0.05000
H38	0.219000	−0.246000	0.009000	0.05000