Synthesis of (+)-totarol
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Totarol, a tricyclic diterpene, has been synthesised from zamoranic acid. The key step is the cyclisation of a 13,14-secototarane using SmI 2 .
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TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 8831–8835
Synthesis of (+)-totarol
I. S. Marcos,* M. A. Cubillo, R. F. Moro, D. Dı́ez, P. Basabe, F. Sanz† and J. G. Urones
Departamento de Quı́mica Orgánica, Facultad de Ciencias Quı́micas, Universidad de Salamanca, Plaza de los Caı́dos 1 -5,
E-37008 Salamanca, Spain
Received 28 July 2003; revised 18 September 2003; accepted 23 September 2003
Abstract—(+)-Totarol, a tricyclic diterpene, has been synthesised from zamoranic acid. The key step is the cyclisation of a
13,14-secototarane using SmI2.
© 2003 Elsevier Ltd. All rights reserved.
Totaranes, cleistantanes and cassanes are tricyclic diterpenes which are not very common in nature, but which
show important biological activities as antiinflammatories, analgesics, antitumour agents, growth regulators
and bacteriocides.1 All of them are characterized by
having an alkyl group on C-14 and/or C-13 (methyl,
ethyl or isopropyl).
Zamoranic acid2 1 is a labdane diterpene used by us for
the synthesis of biologically active natural products
such as sesquiterpene drimanes,3 labdenolides such as
limonidilactone,4 diterpenes with the isofregenedane
skeleton such as chrysolic acid5 and tetracyclic
diterpenes.6
Bearing in mind the following retrosynthetic scheme,
biologically active diterpenes with the totarane, cassane
or cleistantane skeleton could also be accessible from
zamoranic acid. (Scheme 1).
Thus, for the synthesis of tricyclic diterpenes II, secoderivatives I will be used as intermediates for cyclization via a pinacol coupling promoted by SmI2.
This strategy will be applied to the synthesis of (+)totarol, a tricyclic diterpene with an aromatic ring C
and a hydroxy group on C-13. (+)-Totarol is the main
component of Podocarpus totara 7 and Podocarpus nagi 8
(Podocarpaceae) and shows potent activity against
Gram-positive bacteria such as Propionibacterium
acnes, Streptococcus mutans, Bacillus subtilis, Mycobacterium tuberculosis, Brevibacterium ammoniagenes and
Staphylococcus aureus 9 (the last two being resistant to
penicillin). (+)-Totarol shows synergy with other natu-
Scheme 1.
Keywords: diterpenes; zamoranic acid; totarol; samarium iodide.
* Corresponding author. Tel.: +34-923-294474; fax: +34-923-294574; e-mail: ismarcos@usal.es
†
Servicio de rayos X, Universidad de Salamanca.
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.09.193
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I. S. Marcos et al. / Tetrahedron Letters 44 (2003) 8831–8835
ral products such as anacardic acid, which results in the
minimal antibacterial concentration (MBC) decreasing
from 15.6 to 0.2 mg/ml.10 SAR studies with totarol and
derivatives have shown totarol to be the most potent
agent against Staphylococcus aureus.7,11 To the best of
our knowledge there are only two syntheses of
totarol;12 a new approach to (+)-totarol from
zamoranic acid is shown in Schemes 2 and 3.
Scheme 2. Reagents and conditions: (i) Ref. 6; (ii)
rt, 40 min; (iv) (a) HMDSNa, TMSCl, −78°C, 2
MeOH, C6H6, rt, 12 h, (e) LiAlH4, Et2O, rt, 1 h;
rt, 2 h, (c) K2CO3, MeOH, rt, 4 h, (d) CrO3, py,
The transformation of the methyl ester of zamoranic
acid 2 into the aldehyde 3 was previously reported by
our group6 (Scheme 2). Addition of isopropylmagnesium chloride to aldehyde 3 proceeded with complete
stereoselectivity to give exclusively the 14R-hydroxy
derivative 4, in excellent yield. The structure of 4 was
confirmed by X-ray analysis of one of its derivatives
(compound 7, Scheme 2), showing that reaction of the
iPrMgCl, THF, 0°C, 1 h; (iii) (a) Ac2O, py, 70°C, 2 h, (b) p-TsOH, Me2CO,
h, (b) OsO4, NMO, rt, 48 h, (c) H5IO6, THF, H2O, rt, 1 h, (d) TMSCHN2,
(v) TPAP, NMO, rt, 30 min; (vi) (a) Ac2O, py, rt, 30 min, (b) TPAP, NMO,
rt, 1 h.
Scheme 3. Reagents and conditions: (i) SmI2, THF, 0.1 M, MeOH, rt, 2 h; (ii) TPAP, NMO, 0°C, 30 min; (iii) p-TsOH, C6H6,
60°C, 48 h; (iv) (a) LDA, PhSeCl, THF, HMPA, −78 to 0°C, 1 h, (b) m-CPBA, DCM, rt, 5 min; (v) CuBr2, MeCN, 72 h, 50°C;
(vi) Li2CO3, BrLi, DMF, 140°C, 16 h.
I. S. Marcos et al. / Tetrahedron Letters 44 (2003) 8831–8835
nucleophile had taken place from the less hindered si
face of the carbonyl group.
The transformation of 4 into secototarane 6 through
the methyl ketone 5 required acetylation at 70°C due to
the hindrance of the hydroxyl group followed by deprotection. Degradation of 5 by the haloform reaction13
did not work and it was therefore necessary to employ
a sequence of reactions. Thus, capture of the kinetic
enolate of 5 as a trimethylsilyl enol ether14 and cishydroxylation, followed by cleavage with H5IO6,15
esterification16 of the resulting acid and reduction with
LAH gave the diol 6 in excellent overall yield from 4.
TPAP/NMO17 oxidation of 6 gave the lactone 7, whose
structure was confirmed by single crystal X-ray structure determination (Fig. 1),18 corroborating the 14R
configuration in 4. Due to this unwanted lactonization,
the oxidation of 6 to the dicarbonyl system 8 needed for
the cyclization was carried out in an indirect way:
chemoselective acetylation of the primary hydroxyl
group of 6 with Ac2O in pyridine over 30 min, oxidation of the secondary alcohol of the resulting
monoacetyl derivative, hydrolysis of the primary acetoxy group, and subsequent oxidation which gave 8
(Scheme 2).
Treatment of 8 with SmI219 (0.1 M, prepared in situ)
gave a mixture of the totarane diastereomers 9 (80%)
and 10 (13%), which were separated by column chromatography (Scheme 3). The structures of 9 and 10
were established by NMR, including HMQC, HMBC
and nOe experiments. The signal for H-13 of 9 corresponded to an axial hydrogen (l 3.55, dd, J=11.1 and
4.1 Hz) and so the hydroxyl groups are both a. The
signal for H-13 in 10 corresponds to an equatorial
hydrogen (3.83 ppm, t, J=2.0 Hz). The structure for
the major diol 9 is that expected from a pinacol coupling with SmI2.6
Transformation of 9 into totarol requires the dehydration of the tertiary hydroxy group and ring C aromatization. Oxidation of 9 with TPAP/NMO gave 11 in
8833
good yield, but all attempts to dehydrate and aromatize
11 using different conditions and reagents (DDQ,20
TsCl/py, POCl3/py, SOCl2/py, HClO4, CF3COOH,
HCOOH, HI) failed to give the desired results.
The aromatization of ring C was therefore attempted
from ketone 12, obtained by pinacol rearrangement
from 9. Treatment of 9 with p-TsOH, and subsequent
column chromatography, gave 12 (61%) and the minor
compounds 13 and 14. The formation of the cyclohexanone 12 can be explained by the migration of hydride
from C-13 to C-14, although under the acidic conditions of the reaction, epimerization at C-14 takes place
to provide the more stable b-isopropyl group. The
aldehyde 13 is formed by ring C contraction in the
pinacol reaction and compound 14 is a dimer (M+ m/e:
580) formed by reaction of the starting material with
the aldehyde 13.
The aromatization of 12 was attempted via enone 15,
obtained by reaction of 12 with PhSeCl/LDA21 followed by oxidation of the selenide with m-CPBA at
0°C. All attempts at aromatization of 15 with DDQ,
SeO2 at 0°C or even Pd/C22 in refluxing m-xylene failed
to give totarol.
Aromatization of ring C was finally achieved by a
halogenation–dehydrohalogenation sequence via intermediate 16. Treatment of 12 with CuBr2/MeCN23 gave
the dibromide 16 in moderate yield, and subsequent
elimination with Li2CO3/LiBr24 gave a good yield of
1725 [mp 129–130°C; [h]25
D +41 (c 0.4, CHCl3)], whose
spectroscopic properties were coincident with those of
(+)-totarol; (lit.7b) mp 131–132°C, [h]25
D +42 (c 0.8,
CHCl3).
With the synthesis of (+)-totarol from zamoranic acid,
we have opened a new synthetic route to tricyclic
diterpenes with alkyl groups at C-14 (totaranes, cassanes or cleistantanes) using the cyclization of their
13,14-seco derivatives.
Acknowledgements
The authors thank the Spanish DGI for financial support (BQU2002-02049) for a doctoral fellowship to
M.A.C. and Junta de Castilla y León for financial
support (SA075/03).
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Dcalcd=1.120 Mg/m3, m=(Cu Ka)=0.070 mm−1,
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Mp: 129–130°C (n-hexane); [h]25
D +41 (c 0.4, CHCl3); IR
wmax (film) cm−1: 3482, 2932, 1458, 1387, 1263, 1103, 665.
The assignments for the spectra, 1H and 13C NMR, for
I. S. Marcos et al. / Tetrahedron Letters 44 (2003) 8831–8835
17 were carried out using two-dimensional HMQC and
HMBC experiments. 1H NMR (200 MHz, CDCl3, l
ppm): 7.00 (1H, d, J=8.4 Hz, H-12), 6.51 (1H, d, J=
8.4 Hz, H-11), 3.41 (1H, m, H-15), 3.00–2.80 (2H, m,
H-7), 2.30–1.20 (9H, m), 1.33 (3H, d, J=7.0 Hz, H-16),
1.32 (3H, d, J=7.0 Hz, H-17), 1.17 (3H, s, Me-20),
0.94 (3H, s, Me-18) and 0.90 (3H, s, Me-19); 13C NMR
8835
(50 MHz, CDCl3, l ppm): 152.1 (C-13), 143.3 (C-9),
133.9 (C-8), 131.1 (C-14), 123.1 (C-11), 114.5 (C-12),
49.7 (C-5), 42.0 (C-3), 39.8 (C-1), 38.0 (C-10), 33.5 (C4), 33.4 (C-18), 28.9 (C-7), 27.3 (C-15), 25.4 (C-20),
21.8 (C-19), 20.6 (C-16 and C-17), 19.8 (C-2), 19.6 (C6); EIHRMS: calcd for C20H30O (M+): 286.2297, found
286.2292.
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