A crotofolane-type diterpenoid and a rearranged nor-crotofolane-type diterpenoid with a new skeleton from the stems of Croton cascarilloides

Article abstract of DOI:10.1016/j.tetlet.2010.06.033

From the stems of Croton cascarilloides collected in the Okinawa Islands, a structurally rare crotofolane-type diterpenoid (1) and a rearranged nor-crotofalane, a new skeletal diterpenoid (2) were isolated. The structures were determined by X-ray crystallographic analyses, establishing their absolute stereostructures for the first time. Compound 2 was probably biosynthesized from 1 through several steps, such as decarboxylation, oxidation, C-C bond migration.

Full text of DOI:10.1016/j.tetlet.2010.06.033

Tetrahedron Letters 51 (2010) 4320–4322
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A crotofolane-type diterpenoid and a rearranged nor-crotofolane-type
diterpenoid with a new skeleton from the stems of Croton cascarilloides
b
c
Susumu Kawakami a, Katsuyoshi Matsunami a, Hideaki Otsuka a, , Takakazu Shinzato , Yoshio Takeda ,
*
Masatoshi Kawahata d, Kentaro Yamaguchi d
a Department of Pharmacognosy, Graduate School of Biomedical Sciences, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8553, Japan
b Subtropical Field Science Center, Faculty of Agriculture, University of the Ryukyus, 1 Senbaru, Nishihara-cho, Nakagami-gun, Okinawa 903-0213, Japan
c Faculty of Pharmacy, Yasuda Women’s University, 8-13-1 Yasuhigashi, Asaminami-ku, Hiroshima 731-0153, Japan
d Department of Analytical Chemistry, Faculty of Pharmaceutical Sciences, Tokushima Bunri University, Kagawa, 1314-1 Shido, Sanuki City, Kagawa 769-2193, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
From the stems of Croton cascarilloides collected in the Okinawa Islands, a structurally rare crotofolane-
type diterpenoid (1) and a rearranged nor-crotofalane, a new skeletal diterpenoid (2) were isolated. The
structures were determined by X-ray crystallographic analyses, establishing their absolute stereostruc-
tures for the first time. Compound 2 was probably biosynthesized from 1 through several steps, such
as decarboxylation, oxidation, C–C bond migration.
Received 15 May 2010
Revised 2 June 2010
Accepted 7 June 2010
Available online 11 June 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Croton cascarilloides
Euphorbiaceae
Crotofolane
Diterpenoid
Crotofolane-type diterpenoids have fused 5-, 6-, and 7-mem-
bered rings and are expected to be biosynthesized from cembrane
via lathyrane through cross annular cyclization.1 These diterpe-
noids have been found in only three Croton species, Jamaican Cro-
ton corylifoius,1,2 Kenyan Croton dichogamus,3 and Congolese Croton
humanianus.4
Our phytochemical investigation of the stems (14.5 kg) of Cro-
ton cascarilloides Räuschel, collected in the Okinawa Islands, led
to the isolation of a crotofolane-type diterpenoid and a rearranged
nor-molecular species of it having a new skeleton. A MeOH extract
of the branches of C. cascarilloides was washed with n-hexane and
then evaporated to a gummy mass, which was then suspended in
H2O and extracted with CH2Cl2. The CH2Cl2-soluble fraction was
separated by normal and reversed-phase silica gel column chroma-
tographies, Sephadex LH-20 column chromatography, and then
HPLC to afford compounds 15 and
2 (30.0 mg and 3.5 mg,
respectively).6
Compound 15 was isolated as colorless plates and its elemental
composition was determined to be C25H32O7. The IR spectrum of 1
showed absorptions for ester carbonyl and lactone carbonyl
groups. 13C NMR of compound 1 revealed 25 resonances, five of
which were assignable to 2-methylbutanoic acid. The remaining
20 signals comprised those of three methyls, two methylenes,
* Corresponding author. Tel./fax: +81 82 257 5335.
E-mail address: hotsuka@hirohisma-u.ac.jp (H. Otsuka).
Figure 1. Structures of Compounds 1 and 2.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.06.033

S. Kawakami et al. / Tetrahedron Letters 51 (2010) 4320–4322
4321
seven methines, one tetra and one disubstituted double bonds, and
three quaternary carbons. A precise inspection of two-dimensional
NMR spectra led to the conclusion that compound 1 was a
diterpenoid with an unusual carbon skeleton. Thus, X-ray crystal-
lographic analysis of 1 was performed and the relative stereostruc-
ture of 1 was established to be a derivative of crotofolane-type
diterpenoid (Figs. 1 and 2).7 The positive Cotton effect in the CD
spectrum empirically indicated that the absolute configuration at
the 9-position was S8 and chirality analysis of the 2-methylbuta-
noic acid moiety by HPLC established the absolute configuration
of 1, as shown in Figure 1.9 This is the first report of the absolute
structure of a crotofolane and the absolute configuration of the
pentanolide portion, presumably based on the empirical rule for
the CD spectrum, was proved to be correct.
ylbutanoic acid. Thus, the core skeleton was constituted of 19
carbons. X-ray crystallographic analysis revealed that compound
2 has a new skeleton, such as that of a rearranged mononor-cro-
tofolane, as shown in Figures 1 and 3.10 Compound 2 was probably
derived from some crotofolane, like compound 1, through several
steps, such as decarboxylation, oxidation, C–C bond migration.
Supplementary data
Supplementary X-ray crystallographic data for 1 (CCDC 761004)
and
2 (CCDC 761005) can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or deposit@ccdc.cam.ac.uk).
Compound 26 was isolated as colorless plates and its elemental
composition was determined to be C24H32O8. The 13C NMR spec-
trum displayed 24 signals, including five attributable to 2-meth-
Acknowledgments
The authors are grateful for access to the superconducting NMR
instrument at the Analytical Center of Molecular Medicine of the
Hiroshima University Faculty of Medicine and an Applied Biosys-
tem QSTAR XL system ESI (Nano Spray)-MS at the Analysis Center
of Life Science of the Graduate School of Biomedical Sciences, Hiro-
shima University.
References and notes
1. Chan, W. R.; Prince, E. C.; Manchand, P. S.; Springer, J. P.; Clardy, J. J. Am. Chem.
Soc. 1975, 97, 4439.
2. Burke, B. A.; Chan, W. R.; Pascoe, K. O.; Blout, J. F.; Manchand, P. S. Tetrahedron
Lett. 1979, 3345.
3. Jogia, M. K.; Andersen, R. A.; Párkány, L.; Clardy, J.; Dublin, H. T.; Sinclari, A. R. E.
J. Org. Chem. 1989, 54, 1654.
4. Tchissambou, L.; Chiaroni, A.; Riche, C.; Khung-Huu, F. Tetrahedron 1990, 46,
5199.
5. Compound 1: colorless plates (2-PrOH), mp 152–153 °C, ½a D26
ꢀ
+81.8 (c 1.52,
CHCl3). IR (KBr) mmax cmꢁ1: 3478, 2972, 2929, 2879, 1769, 1739, 1659, 1457,
1185, 1143, 1014, 804. UV (MeOH) kmax nm (log e
): 218 (4.00). 1H NMR (CDCl3,
400 MHz): d 5.38 (1H, d, J = 5 Hz, H-1), 5.20 (1H, s, H-18a), 5.17 (1H, s, H-18b),
5.14 (1H, dddd, J = 13, 4, 2, 2 Hz, H-9), 4.53 (1H, ddd, J = 4, 2, 2 Hz, H-13), 3.19
(1H, s, H-5), 3.15 (1H, d, J = 13 Hz, H-13), 3.06 (1H, br d, J = 12 Hz, H-7), 2.49
(1H, dd, J = 14, 8 Hz, H-3a), 2.73 (1H, ddd, J = 13, 4, 4 Hz, H-10a), 2.49 (1H, qdd,
J = 7, 7, 7 Hz, H-20), 2.44 (1H, dd, J = 2, 2 Hz, –OH), 2.18 (1H, dqdd, J = 8, 7, 7,
5 Hz, H-2), 1.90 (3H, br s, H3-17), 1.74 (1H, ddq, J = 14, 7, 7 Hz, H-30a), 1.70 (1H,
dd, J = 14, 10 Hz, H-2b), 1.50 (1H, ddq, J = 14, 7, 7 Hz H-30b), 1.27 (1H, dddd,
J = 13, 13, 4, 2 Hz, H-10b), 1.17 (3H, d, J = 7 Hz, H3-50), 1.06 (3H, s, H3-20), 0.98
(3H, d, J = 7 Hz, H3-19), 0.93 (3H, t, J = 7 Hz, H3-40). 13C NMR (CDCl3, 100 MHz):
d 178.0 (C-10), 173.4 (C-16), 162.0 (C-8), 148.9 (C-12), 128.2 (C-15), 115.2 (C-
18), 78.4 (C-9), 75.8 (C-1), 72.7 (C-11), 68.8 (C-14), 60.1 (C-4), 57.8 (C-5), 55.9
(C-6), 44.4 (C-7), 44.1 (C-10), 41.2 (C-20), 36.4 (C-3), 32.7 (C-2), 31.7 (C-13), 26.6
Figure 2. ORTEP drawing of compound 1.
(C-30), 19.3 (C-20), 16.2 (C-50), 12.3 (C-19), 11.4 (C-40), 9.7 (C-17). CD
D
e
(nm):
+1.36 (249), ꢁ1.27 (210) (c 4.31 ꢂ 10ꢁ5
, MeOH). HR-ESI-MS (positive-ion
mode) m/z: 467.2017 [M+Na]+ (C25H32O7Na requires 467.2040).
6. Compound 2: colorless plates (CHCl3), mp 202–203 °C, ½a D26
ꢀ
+78.7 (c 0.13,
CHCl3). IR (KBr) mmax cmꢁ1:3479, 2968, 2926, 2855, 1761, 1721, 1634, 1461,
1193, 884. 1H NMR (CDCl3, 400 MHz): d 5.78 (1H, d, J = 5 Hz, H-1), 5.39 (1H, br
s, H-18a), 5.23 (1H, s, H-18b), 4.39 (1H, d, J = 6 Hz, H-5), 4.17 (1H, br t-like,
J = 8 Hz, H-11), 3.11 (1H, br d, J = 13 Hz, H-13), 2.92 (1H, d, J = 13 Hz, H-7), 2.46
(1H, dd, J = 15, 7 Hz, H-10a), 2.39 (3H, s, H3-17), 2.38 (1H, overlapped, H-20),
2.34 (1H, dd, J = 14, 7 Hz, H-3a), 2.26 (1H, d, J = 6 Hz, –OH at C-5), 2.22 (1H, m,
H-2), 2.07 (1H, dd, J = 15, 10 Hz, H-10b), 1.68 (1H, ddq, J = 14, 7, 7 Hz, H-30a),
1.60 (1H, br d, J = 3 Hz, –OH at C-11), 1.56 (1H, dd, J = 14, 10 Hz, H-3b), 1.43
(1H, ddq, J = 14, 7, 7 Hz, H-30b), 1.27 (3H, s, H3-20), 1.12 (3H, d, J = 7 Hz, H3-50),
0.92 (3H, d, J = 7 Hz, H3-19), 0.90 (3H, t, J = 7 Hz, H3-40). 13C NMR (CDCl3,
100 MHz): d 202.0 (C-9), 175.5 (C-10), 174.7 (C-15), 146.1 (C-12), 113.8 (C-18),
88.0 (C-6), 75.5 (C-1), 75.4 (C-5), 67.9 (C-11), 66.0 (C-14), 64.8 (C-4), 60.1 (C-8),
48.0 (C-7), 41.2 (C-20), 35.8 (C-10), 34.4 (C-2), 34.3 (C-3), 32.4 (C-13), 26.6 (C-
30), 26.0 (C-17), 22.0 (C-20), 16.9 (C-50), 12.6 (C-19), 11.8 (C-40). HR-ESI-MS
(positive-ion mode) m/z: 471.1973 [M+Na]+ (C24H32O8Na requires 471.1989).
7. X-ray diffraction study on compound 1: C25H32O7ꢃ ꢃ ꢃC3H8O, M = 504.60, crystal
size: 0.50 ꢂ 0.30 ꢂ 0.15 mm3, space group: orthorhombic, P212121, T = 120 K,
a = 10.1775(10) Å, b = 10.4348(10) Å, c = 25.908(3) Å, V = 2751.5(5) Å3, Z = 4,
Dc = 1.218 Mg/m3, F(000) = 1088. The data were measured using a Bruker APEX
II CCD diffractometer, using MoK
a graphite-monochromated radiation
(k = 0.71073 Å) in the range of 3.14 < 2h < 53.4. Of 13,566 reflections
collected, 3212 were unique (Rint = 0.0224), data/restraints/parameters 3212/
0/334. The structure was solved by a direct method using the SHELXS-97.11 The
refinement and all further calculations were carried out using SHELXL-97.11 The
H atoms were included at calculated positions and treated as riding atoms
Figure 3. ORTEP drawing of compound 2.

4322
S. Kawakami et al. / Tetrahedron Letters 51 (2010) 4320–4322
using the SHELXL default parameters. The non-H atoms were refined
anisotropically using weighted full-matrix least-squares on F2. Final
goodness-of-fit on F2 = 1.048, R1 = 0.0344, wR2 = 0.0868 based on I > 2
(I) and
peak appeared at 16.4 min which showed positive chirality and was indentified
as that of authentic (S)-(+)-2-methylbutanoic acid.
r
10. X-ray diffraction study on compound 2: C24H32O8, M = 448.50, crystal size:
R1 = 0.0378, wR2 = 0.0891 based on all data. The largest difference peak and
0.30 ꢂ 0.15 ꢂ 0.15 mm3, space group: monoclinic, P21, T = 120 K, a = 9.9294(12)
Å, b = 9.1267(11) Å, c = 12.5443(15) Å, b = 98.650(1) °, V = 1123.9(2) Å3, Z = 2,
Dc = 1.325 Mg/m3, F(000) = 1088. Of 5560 reflections collected in the range of
3.28° < 2h < 54.1°, 2416 were unique (Rint = 0.0154), data/restraints/parameters
2416/1/296. The structure was solved in a similar manner as that for compound
hole were 0.335 and ꢁ0.228 eÅꢁ3, respectively.
8. Fragoso-Serrano, M.; Gibbons, S.; Pereda-Miranda, R. Planta Med. 2005, 71, 278.
9. Compound 1 (2 mg) was dissolved in 100
then 100 L of a 10% KOH solution was added. The reaction mixture was kept at
100 °C for 3 h and then the cooled solution was neutralized by the addition of
IR-120B (H+) ion-exchange resin. An aliquot (20
L) was analyzed by a HPLC
lL of 50% aqueous 1,4-dioxane and
l
1. Final goodness-of-fit on
F
2 = 1.056, R1 = 0.0315, wR2 = 0.0794 based on
l
I > 2r(I) and R1 = 0.0335, wR2 = 0.0809 based on all data. The largest
system equipped with an optical rotation detector on an ODS column with a
solvent system of 20% CH3CN in H2O, containing 0.5% trifluoroacetic acid. A
difference peak and hole were 0.285 and ꢁ0.208 eÅꢁ3, respectively.
11. Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.