Easy Access to a Cyclic Key Intermediate for the Synthesis of Trisporic Acids and Related Compounds

Article abstract of DOI:10.3390/molecules19021748

The synthesis of a cyclohexane skeleton possessing different oxygenated functional groups at C-3, C-8 and C-9, and a δ^1,6-double bond has been accomplished in 10 steps with an overall 17% yield. This compound is a key intermediate for access to a wide range of compounds of the bioactive trisporoid family. The synthetic sequence consists of the preparation of a properly functionalized epoxygeraniol derivative, and its subsequent stereoselective cyclization mediated by Ti(III). This last step implies a domino process that starts with a homolytic epoxide opening followed by a radical cyclization and regioselective elimination. This concerted process gives access to the cyclohexane moiety with stereochemical control of five of its six carbon atoms.

Full text of DOI:10.3390/molecules19021748

Molecules 2014, 19, 1748-1762; doi:10.3390/molecules19021748
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Easy Access to a Cyclic Key Intermediate for the Synthesis of
Trisporic Acids and Related Compounds
José A. González-Delgado 1,2, Gustavo Escobar 1,3, Jesús F. Arteaga 2,* and
Alejandro F. Barrero 1,*
1
Department of Organic Chemistry and Institute of Biotechnology, University of Granada,
Avda Fuentenueva s/n, 18071 Granada, Spain
2
CIQSO–Center for Research in Sustainable Chemistry and Department of Chemical Engineering,
Physical Chemistry and Organic Chemistry, University of Huelva, Avda 3 Marzo s/n, 21071
Huelva, Spain
3
University of Antioquia, Calle 70 N° 52-21, Medellín, Colombia
* Authors to whom correspondence should be addressed; E-Mails: jesus.fernandez@diq.uhu.es (J.F.A.);
afbarre@ugr.es (A.F.B.); Tel./Fax: +34-958-243-318.
Received: 19 December 2013; in revised form: 22 January 2014 / Accepted: 23 January 2014 /
Published: 3 February 2014
Abstract: The synthesis of a cyclohexane skeleton possessing different oxygenated
functional groups at C–3, C–8 and C–9, and a 1,6-double bond has been accomplished in
10 steps with an overall 17% yield. This compound is a key intermediate for access to a
wide range of compounds of the bioactive trisporoid family. The synthetic sequence
consists of the preparation of a properly functionalized epoxygeraniol derivative, and its
subsequent stereoselective cyclization mediated by Ti(III). This last step implies a domino
process that starts with a homolytic epoxide opening followed by a radical cyclization and
regioselective elimination. This concerted process gives access to the cyclohexane moiety
with stereochemical control of five of its six carbon atoms.
Keywords: trisporoids; apocarotenoid; domino reaction; stereoselective synthesis
1. Introduction
Trisporic acids (Figure 1), their precursors and derivatives are an interesting group of bioactive
natural products biosynthetically derived from the degradative oxidation of -carotene. They act as

Molecules 2014, 19
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inducers of carotenogenesis as well as pheromones involved in the regulation of different stages of
sexual development and stimulation of the production of zygophores in the fungal phylum
Zygomycota and other Mucorales. These facts are well reported for different species of fungi such as
Blakeslea trispora (Choaneforaceae family) [1–3], Phycomyces blakesleeanus (Phycomycetaceae
family) [4], Mucor mucedo (Mocuraceae family) [2,5–7], and the homothallic Zygorhynchus moelleri
(Mocuraceae family) [8]. Furthermore several species of Mortierella (Mortierallale order) produce
trisporoids that can induce sexual responses in Mucor mucedo and Phycomyces blakesleeanus [9].
Recently it has been demonstrated that the recognition between the parasite fungi Parasitella
parasitica (Mocuraceae family) and the host Absidia glauca is mediated by trisporoids which are
responsible for sexual phenomena [10,11]. Furthermore, evidence for the existence of genes of
trisporoids synthesis in Glomus-like fungi (Phylum Glomeromycota) has been reported [12]. In recent
years, a number of studies for implementing assays for the isolation and characterization of trisporoids
have been developed [13–16] because they occur in fungi in very low concentrations and are
chemically characterized for their relative instability. Sometimes this characteristic feature of these
hormones hinders their direct structural characterization. Hence, the availability of this important class
of compounds through chemical synthesis is a pre-requirement for their correct structural
characterization and for carrying out physiological studies.
Figure 1. Structural diversity in the family of trisporoids.
Position Oxidation degree
Subfamily
-H, -OH, =O
R2
C–2
C–3
C–4
C–13
C–17
C–17
C–17
C–17
–H, –OH
–H, –OH, =O
–H, –OH, =O
–H, –OH, =O
–H
-H, -OH, =O
R3
4
1
[ox]
2
6
R4
13
trisporins
trisporols
trisporic acids
methyl trisporates
-OH
17R1
R5
–CH2OH
–COOH
-CH3, -CH2OH,
-COOH, -COOMe
–COOMe
Several routes to trisporoids such as trisporins, methyl trisporates, trisporols, and free trisporic acids
have been developed and reported [17–34]. The disadvantage of most routes is the large number of
steps required for the synthesis of the final targets and the lack of flexibility to produce early and late
trisporoids along the same protocol using common intermediates. The approaches by Boland [31] and
Rodríguez-García [32] are examples of modern strategies that carry out the synthesis of compounds of
this family by using organometallic couplings. However, they lead only to early trisporoids without
oxygen functions in the gem-dimethyl group of the cyclohexane. The first total synthesis of a number
of apotrisporoids C15 (monocyclofarnesoids) has been accomplished for the first time in our
laboratories [35], enabling the corroboration of the relative stereochemistry and the assignation of the
absolute configuration for these compounds (Scheme 1). This approach is based on a bio-inspired
cyclization constituted by a domino process: homolytic oxyrane opening, radical cyclization,
alkyltitanium formation and final regioslective elimination and a HWE olefination to attach the dienic
side chain as key steps.

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Scheme 1. Ti(III)-mediated synthesis of apotrisporoids and trisporoids based on
bioinspired cyclizations.
Following this strategy, herein a significant advance in the development of a selective synthesis of
highly functionalized key intermediates such as II (Schemes 1 and 2) is described. These may be
considered as effective precursors for the synthesis of trisporic acids and their corresponding alcohol
derivatives (trisporols), mainly for their use as a standard either in subsequent identifications,
quantifications, and biological activity tests (as fungi pheromones). Its selective preparation supposes
an extension and generalization of the above outlined previous approach. For trisporic acids or
trisporols the methodology proposed constitutes a new synthetic route and involves the access to key
intermediate type II starting from commercial geranyl acetate.
Scheme 2. Retrosynthetic approach.
Having in consideration the objective to achieve straightforward functional modifications at C–2,
C–4, C–5, C–13 and C–17 (Figure 1), such approach would therefore open the door to the synthesis of
the majority of the known natural trisporoids C18; not only to early trisporoids such as trisporins but
also to late ones such as trisporols, trisporic acid and methyl trisporates.
2. Results and Discussion
The retrosynthetic approach towards natural trisporic acids and trisporols (as trisporic acid C in
Scheme 2) is based on a disconnection through a synthon of ten carbon atoms whose cyclohexane

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structure will be provided by the intermediate I (Scheme 2). I could be formed from intermediate II by
using well known transformations such as epoxidation of the trisubstituted double bond, opening of the
oxirane in basic medium, selective deprotection and subsequent oxidation. II derives from a Ti(III)-
mediated radical cyclization of the acyclic precursor III. Finally the highly functionalized acyclic
epoxide III could be obtained from commercial geranyl acetate (IV).
To achieve the access towards intermediate II, a synthetic sequence of ten steps (Scheme 3) was
realized, in which the key step is a stereoselective cyclization mediated by Cp2TiIIICl [36] by means of
the homolytic opening of the highly functionalized epoxypolyprene (8, III in Scheme 2) [37–40]. In
this regard the preparation of 8 (or III) in good yield constitutes a useful synthetic novelty which could
facilitate the synthesis of either trisporoids or labdanes. Globally the synthetic sequence presents two
different regioselective allylic oxidations that can also be considered as critical steps in the entire
route. In the first one, we decided to use catalytic SeO2 [41] bearing in mind that the –CH3 at the end
of the chain is the object of the reaction, a process well known to occur properly. In the second one,
which aims to functionalize a –CH2 placed inside the carbon chain, the reaction occurs slowly and is
more difficult to process with respect to the above oxidation reaction. In this case excess SeO2 together
with aqueous EtOH has been used [42–44].
Scheme 3. Synthesis of polyfunctionalized cyclohexane 9, key structure towards trisporic
acids family.
Reagents and conditions: a. K2CO3, MeOH, 25 °C, 10 min, 99%; b. PivCl, Imidazole, Pyridine, rt, 1 h, 99%;
c. SeO2, t-BuOOH, DCM, 0 °C, 3 h, 84% based on recovered starting material; d. NaBH4, MeOH, rt, 30 min,
95%; e. Ac2O, Pyridine, DMAP, rt, 3 h, 85%; f. SeO2, aq. EtOH, reflux, 2 h, 72% based on recovered starting
material, g. K2CO3, MeOH, 0 °C, 2 h, 99%; h. TBSCl, Imidazole, DMF, rt, 3 h, 79%; i. Ac2O, Pyridine,
DMAP, rt, 1 h, 73%; j. m-CPBA, DCM, 0 °C, 2 h 30 min, 90%; k. Cp2TiCl2, Mn, THF, 40 °C, 6 h, 70%.

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Sequentially, the first stage involves the protective group exchange of the starting material to arrive
at compound 1 (98% yield). Allylic oxidation of 1 with catalytic amounts of SeO2 in the presence of
equimolar t-BuOOH led to a mixture of the hydroxyester 3 together with the over-oxidation product 2.
This aldehyde 2 can be reduced to the corresponding alcohol in an efficient manner by treatment with
NaBH4 in MeOH. The final yield of alcohol 3 obtained by this oxidation protocol is 84%, taking into
account the recycling of the unreacted starting material. Subsequent acetylation of the hydroxyester 3
with Ac2O in pyridine led to acetate 4 with 85% yield. Next we attempted to oxidize the –CH2 placed
inside the carbon chain (at C–4) of 4 by means of excess SeO2 in aqueous EtOH. Different experiments
modifying either the ratio of H2O–EtOH and/or the temperature were performed to increase the yield of
the reaction (Table 1). The best results were obtained when using 97.5% EtOH in water yielding 72%
of the secondary alcohol (5); taking into account the recycled unreacted starting material. Compound 5
is selectively deacetylated to give the diol 6 (99% yield) by using K2CO3 and MeOH.
Table 1. Optimization of the conditions of allylic oxidation of 4.
EtOH purity Temperature Time Ratio Yield
Entry
[%]
40
[°C]
Reflux
Reflux
Reflux
Reflux
60
[min] [5:4]
[%]
5
1
2
3
4
5
6
7
8
9
360
1.0:10
65
360 1.0:7.0
360 1.0:4.5
120 1.0:4.0
240 1.0:3.2
7
80
15
36
24
90
90
99.8
95.0
97.5
97.5
Reflux
60
100 1.0:1.7 51 a
35
1.0:1.0 49 a
60
120 1.0:1.0 67 a
Reflux
120 1.1:1.0 72 a
a Yield based on recovering the starting material 4.
At this point, we tested the selective protection of the primary alcohol of 6 by means of TBDMSCl.
This reagent should react chemoselectively due to steric constraints in the analogous process with
secondary alcohols. However, to avoid over-protection an equimolar proportion of TBDMSCl with
respect to the starting material was used. This, together with the strict temperature control (0 °C
throughout the process), enabled the preparation of the desired hydroxy-silyl derivative in 89% yield.
The secondary –OH was then acetylated to obtain 7 in a yield of 73%. The selective location of the
protective groups in different segments of the molecule promotes the electronic differentiation of the
monoterpenic derivative and this is of primordial importance for the subsequent epoxidation reaction.
An electron donating protective group (a silyl ether) has been located near the Δ6,7 double bond, while
electron withdrawing protective groups (such as pivaloyl and acetate) have been placed around the Δ2,3
double bond. The latter, moreover, has been specifically selected to act as a potential leaving group
when carrying out the Ti(III)-mediated radical cyclization of the acyclic precursor, which is the last
step of the herein performed synthetic sequence.
The selectively activated Δ6,7 double bond was epoxidized by means of m-CPBA to obtain
compound 8 with 90% yield. Finally, the acyclic epoxide 8, which also corresponds to the intermediate
III of Scheme 1, was cyclized following the Ti(III)-mediated methodology [39,45–50]. In this case

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catalytic conditions [51] were employed, involving 0.3 equivalents of Ti(III) and 8.0 equivalents of Mn
in THF (0.1 M), with 2,4,6–collidine/TMSCl as regenerating system. To promote cyclization process,
the mixture was further heated to 40 °C. The catalytic cyclization reaction can be settled through the
intermediates described in Scheme 3. The transformation begins with the formation of a Lewis acid-
base complex between the oxirane group of 8 and Cp2TiCl [37,52], the reagent equivalent of Ti(III).
This complex promotes the homolytic opening of the epoxide via one-electron-transfer from the Ti(III)
atom, resulting in the formation of the carbonated acyclic β-titanoxy-radical V. V evolves by cyclizing
via radical attack at the Δ2,3 double bond, through a 6-endo-trig process favored by the electronic
structure of the tri-substituted double bond, leading to the more substituted radical VI. This radical
reacts with another Ti(III) forming the alkyl-Ti(IV) species VII, which through elimination of
Cp2TiCl(OAc) leads with complete regioselectivity to the compound possessing the tri-substituted
double bond Δ5,6 (VIII, Scheme 4). Cp2TiCl2 can be regenerated by means of the 2,4,6–collidine/TMSCl
system either from the Cp2Ti(IV)Cl(OAc) generated in the –OAc elimination step or from the titanium
alkoxide of VIII.
Scheme 4. Mechanistic proposal for Ti(III)-mediated catalytic domino process towards 9:
homolytic epoxide opening of 8, radical cyclization and selective elimination.
It should be pointed out that the Ti(III)-mediated radical cyclization reactions employing substrates
without secondary acetate groups at C–4 lead regioselectively to exocyclic double bonds
(Scheme 5) [45,47–50]. In contrast, in this study it has been proven that it is possible to change
selectively the direction of the formation of the olefin by using a suitable leaving group having in mind
the affinity for the oxygen of the Ti atom.

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Scheme 5. Usual cyclization pathway in non-functionalized at C–4 derivatives leading to
exocyclic double bonds.
The structure and stereochemistry of the cyclization product (9) is easily deduced from the
spectroscopic data, in particular NMR (Table 2).
Table 2. 1H-NMR and 13C-NMR data for 9.
Position
[NOE]
1H
13C
Representative NOE effects found in 9.
[ppm]
signal J [Hz] [ppm]
1
2
5.39
2.22
1H bs
2H m
121.1
30.8
8.4
5.6
3
3.85
1H dd
71.5
4
5
6
7
41.4
43.1
2.02
1H m
132.6
22.0
1.68
4.07
3H
11.8
4.4
1H dd
8
62.7
11.8
4.3
4.30
1H dd
3.53
3.68
0.88
1H
1H
1H
d
d
s
10.0
10.0
9
68.9
10
11
12.0
178.5
38.8
12
13
1.19
0.07
0.06
5.39
2.22
9H
3H
3H
s
s
s
27.2

14
–5.5
15
16
1H bs
2H m
18.2
25.9
The relative configuration is determined by means of NOE experiments. NOE effects can be
observed between the protons H–8 and H–10 as well as between the protons H–9 and H–3. This allows
us determining that each pair of H is placed in a syn disposition. Furthermore, considering the
proposed reaction mechanism, these evidences defined the stereochemistry of the various stereogenic

Molecules 2014, 19
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centers of 9 (Table 2). In this way, it becomes clear that this cyclization process occurs with an
excellent regioselectivity control, both in the ring closure and in the formation of the tri-substituted
double bond. A good stereoselectivity in the formation of three stereogenic centers (located
consecutively in the cyclohexane structure of 9) is obtained. This is consistent with the precedents in
this kind of cyclizations [53].
Regarding the consequences of this approach for investigations at the biological level, the
development of a short synthetic process towards the intermediate 9 will allow to access different
compounds of both families of trisporoidss C18 and monocyclofarnesyl derivatives C15 (or
apotrisporoids). Thus, the hydroxyl located at C–3 will provide access to trisporoids of the D series, or
by means of a deoxygenation to those of C, B or A series. Furthermore the 1,6 double bond enables
the integration of either a carbonyl or hydroxyl group at C–6, as present in the trisporic acids and
dihydrotrisporols, respectively. The availability of these molecules will facilitate the testing of their
biological activity. These studies will help to find the true sex pheromones of both (+) and (–) genus in
these fungi, and also the specific molecules responsible of carotenogenesis.
3. Experimental
3.1. General
All NMR spectra (δ values, ppm) were recorded with Varian Direct-Drive 600 (1H 600 MHz/13C
150 MHz), Varian Direct-Drive 500 (1H 500 MHz/13C 125 MHz), Varian Direct-Drive 400 (1H
400 MHz/13C 100 MHz) and Varian Inova Unity 300 (1H 300 MHz/13C 75 MHz) spectrometers.
1
Tetramethylsilane (TMS) was used as an external reference for recording H (of residual proton;
13
δ = 7.26 ppm) and C (δ = 77.0 ppm) spectra in CDCl3. Chemical shift multiplicities are reported as
s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet and br = broad. The accurate mass
determination was carried out with a mass spectrometer equipped with a TOF, system Triwave®
WATERS model SYNAPG2 and an AutoSpec-Q mass spectrometer arranged in an EBE geometry
(Micromass Instrument, Manchester, UK) and equipped with a FAB (LSIMS) source. The instrument
was operated at 8 KV of accelerating voltage and Cs+ were used as primary ions. Optical rotations
were measured on a Perkin-Elmer 141 polarimeter, using CH2Cl2 as the solvent. Reactions were monitored
by thin layer chromatography (TLC) carried out on 0.25 mmE. Merck silica gel plates (60F-254) using
UV light as the visualizing agent and a solution of phosphomolybdic acid in ethanol and heat as
developing agent. Silica gel SDS 60 (35–70 mm) was used for flash column chromatography. HPLC
with UV and RI detection was used. Semi-preparative HPLC separations were carried out on a column
(5 µm Silica, 10 × 250 mm) at a flow rate of 2.0 mL/min in an Agilent Series 1100 instrument. All air-
and water-sensitive reactions were performed in flaks flame-dried under a positive flow of argon and
conducted under an argon atmosphere. The solvents used were purified according to standard literature
techniques and stored under argon. THF and toluene were freshly distilled immediately prior to use
from sodium/benzophenone and strictly deoxygenated for 30 min under argon for each of the
Cp2TiCl2/Mn or Zn reactions. Reagents were purchased at the higher commercial quality and used
without further purification, unless otherwise stated.

Molecules 2014, 19
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3.2. Experimental Procedures
Synthesis of (E)-3,7-dimethylocta-2,6-dien-1-yl pivalate (1). To a stirred solution of geraniol (7.0 g,
45.75 mmol) in pyridine (26 mL), imidazole (0.622 g, 9.15 mmol) and PivCl (7.3 mL, 59.48 mmol)
were added at room temperature. After stirring for 1 h (TLC monitoring), the mixture was diluted with
CH2Cl2 and water and extracted with CH2Cl2. The combined organic layer was washed with 2 N HCl,
brine, dried over anhydrous Na2SO4 and concentrated under reduced pressure. The resulting crude was
purified by flash chromatography (hexane/t-BuOMe, 1:1) on silica gel to afford 1 (10.8 g, 45.29 mmol,
99% yield). 1H-NMR (400 MHz, CDCl3): δ 5.32 (1H, t), 5.08 (1H, t), 4.56 (2H, d), 2.08 (2H, q), 2.04
(2H, t), 1.69 (3H, s), 1.67 (3H, s), 1.60 (3H, s) and 1.18 (9H, s) ppm.
Synthesis of (2E,6E)-8-hydroxy-3,7-dimethylocta-2,6-dien-1-yl pivalate (3). A mixture of SeO2 (1,012 mg,
9.12 mmol), tert-butylhydroperoxide 5.0–6.0 M in decane (3.4 mL, 18.52 mmol) and DCM (49 mL)
was stirred at 0 °C for 20 min. Then, 1 was added (6.588 g, 27.64 mmol). The mixture was stirred for
3 h, diluted with 25 mL of DCM, washed with water for three times and brine, dried over anhydrous
Na2SO4 and concentrated under reduced pressure. The resulting crude was purified by flash
chromatography (hexane/t-BuOMe, 2:1) on silica gel to afford 3 (5.905 g, 23.22 mmol, 84% yield after
1
recovering starting material). H-NMR (400 MHz, CDCl3): δ 5.36 (1H, t), 5.34 (1H, t), 4.56 (2H, d),
3.98 (2H, s), 2.16 (2H, q), 2.08 (2H, t), 1.70 (3H, s), 1.68 (3H, s) and 1.12 (9H, s) ppm. HRFABMS:
calcd for C15H26O3Na [M+Na]+ 277.1780, found: 277.1769.
Synthesis of (2E,6E)-8-acetoxy-3,7-dimethylocta-2,6-dien-1-yl pivalate (4). Acetic anhydride (2.34 ml,
24.79 mmol) and 4-dimethylaminopyridine (5.0 mg) were added to solution of 3 (3.153 g, 12.4 mmol)
in pyridine (33 mL). The mixture was kept at room temperature for 3 h, diluted with 50 mL of ice-
water, extracted with t-BuOMe and washed with 1 N HCl for three times and brine. Consequently, the
organic layer was dried over anhydrous Na2SO4 and concentrated under reduced pressure. The
resulting crude was purified by flash chromatography (hexane/t-BuOMe, 4:1) on silica gel to afford 4
(3.124 g, 10.54 mmol, 85% yield). 1H-NMR (400 MHz, CDCl3): δ 5.38 (1H, t), 5.26 (1H, t), 4.50 (2H,
d), 4.38 (2H, s), 2.10 (2H, t), 2.05 (m, 2H), 2.00 (3H, s), 1.63 (3H, s), 1.59 (3H, s) and 1.12 (9H, s)
ppm. 13C-NMR (100 MHz, CDCl3): δ 178.5, 170.9, 141.0, 130.4, 128.8, 119.1, 70.1, 61.2, 38.8, 38.7,
27.2, 25.9, 20.9, 16.4 and 13.9 ppm. HRFABMS: calcd for C17H28O4Na [M+Na]+ 319.1885, found:
319.1890.
Synthesis of (2E,6E)-8-acetoxy-4-hydroxy-3,7-dimethylocta-2,6-dien-1-yl pivalate (5). A solution of 4
(3.122 g, 10.53 mmol) in DCM (49 mL) was added EtOH 97.5% (1.035 mL) and SeO2 (4.675 g,
42.14 mmol) under Ar atmosphere and refluxing. The mixture was stirred for 2 h, diluted with EtOAc,
washed with brine for three times and dried over anhydrous Na2SO4. Finally, the organic phase was
concentrated under reduced pressure and the resulting crude was purified by flash chromatography
(hexane/t-BuOMe, 2:1) on silica gel to afford 5 (2.37 g, 7.58 mmol, 72% yield after recovering starting
1
material). H-NMR (400 MHz, CDCl3): δ 5.52 (1H, t), 5.39 (1H, t), 4.55 (2H, d), 4.40 (2H, s), 4.02
13
(1H, t), 2.26 (2H, t), 2.00 (3H, s), 1.65 (3H, s), 1.62 (3H, s) and 1.12 (9H, s) ppm. C-NMR (100
MHz, CDCl3): δ 178.5, 170.9, 142.0, 133.0, 124.7, 120.2, 75.9, 69.9, 60.9, 49.4, 38.7, 33.6, 27.2, 27.0,
21.0, 14.2 and 12.3 ppm. HRFABMS: calcd for C17H28O5Na [M+Na]+ 335.1834, found: 335.1824.

Molecules 2014, 19
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Synthesis of (2E,6E)-4,8-dihydroxy-3,7-dimethylocta-2,6-dien-1-yl pivalate (6). To a stirred solution of
5 (1.198 g, 3.83 mmol) in MeOH (27 mL), K2CO3 (529 mg, 3.83 mmol) was added at 0 °C. After
stirring for 2 h (TLC monitoring), the mixture was diluted with brine and extracted with EtOAc. The
combined organic layer was dried over anhydrous Na2SO4 and concentrated under reduced pressure.
The resulting crude was purified by flash chromatography (hexane/t-BuOMe, 1:1) on silica gel to
1
afford 6 (1.025 g, 3.79 mmol, 99% yield). H-NMR (400 MHz, CDCl3): δ 5.52 (1H, t), 5.38 (1H, t),
4.58 (2H, d), 4.04 (1H, t), 3.94 (2H, s), 2.52 (2H, s), 1.68 (3H, s), 1.62 (3H, s) and 1.18 (9H, s) ppm.
13C-NMR (100 MHz, CDCl3): δ 178.7, 142.0, 137.8, 121.0, 120.1, 76.1, 68.5, 61.1, 38.7, 33.3, 27.2 (×
3), 14.0 and 12.4 ppm. HRFABMS: calcd for C15H26O4Na [M+Na]+ 293.1729, found: 293.1738.
Synthesis of (2E,6E)-4-acetoxy-8-((tert-butyldimethylsilyl)oxy)-3,7-dimethylocta-2,6-dien-1-yl pivalate
(7). To a stirred solution of 6 (839.0 mg, 3.1 mmol) in DMF (33 mL), imidazole (316.6 mg, 4.65 mmol)
and TBSCl (561.3 mg, 3.72 mmol) were added at room temperature. After stirring for 3 h (TLC
monitoring), the mixture was diluted with t-BuOMe and water and extracted with t-BuOMe. The
combined organic layer was washed with 2 N HCl, brine, dried over anhydrous Na2SO4 and
concentrated under reduced pressure. The resulting crude was purified by flash chromatography
(hexane/t-BuOMe, 1:1) on silica gel to afford 7 (1.045 g, 2.45 mmol, 79% yiedl). 1H-NMR (400 MHz,
CDCl3): δ 5.51 (1H, t), 5.32 (1H, t), 4.55 (2H, d), 4.02 (1H, t), 3.94 (2H, s), 2.25 (2H, t), 1.72 (1H, s),
13
1.64 (3H, s), 1.55 (3H, s), 1.12 (9H, s), 0.85 (9H, s) and 0.00 (6H, s) ppm. C-NMR (100 MHz,
CDCl3): δ 178.5, 170.2, 138.2, 137.4, 122.0, 118.5, 77.7, 68.2, 60.8, 38.8, 31.1, 27.2 (× 3), 26.0 (× 3),
21.3, 18.4, 13.7, 12.9, 1.1, −5.3 ppm. HRFABMS: calcd for C23H42O5NaSi [M+Na]+ 426.2802,
found: 426.2804.
Synthesis of (E)-4-acetoxy-5-(3-(((tert-butyldimethylsilyl)oxy)methyl)-3-methyloxiran-2-yl)-3-methyl-
pent-2-en-1-yl pivalate (8, mixture of diastereoisomers). 3-Chloroperoxybenzoic acid (348.65 mg,
2.02 mmol) in anhydrous CH2Cl2 (19 mL) was added to a solution of acetylated derivate of 7 (431 mg,
1.01 mmol) in anhydrous CH2Cl2 (20 mL) under an argon atmosphere at 0 °C, and the mixture was
stirred for 2.5 h. Then, CH2Cl2 (25 mL) was added and the mixture successively washed with saturated
aq. NaHCO3 solution and brine, dried with anhydrous Na2SO4, filtered, and evaporated to give a crude
residue which yielded 402 mg (0.91 mmol, 90% yield) of 8 after chromatography purification (hexane/t-
1
BuOMe, 4:1). H-NMR (400 MHz, CDCl3): δ 5.56 (1H, bt), 5.26 (1H, m), 4.54 (2H, d), 3.50 (2H, s),
2.84 (1H, m), 2.05 (3H, s), 1.85 (2H, m), 1.66 (3H, s), 1.22 (3H, s), 1.16 (9H, s), 0.82 (9H, s) and 0.00
13
(6H, s) ppm. C-NMR (100 MHz, CDCl3): δ 178.3, 169.9, 169.8, 138.1, 137.8, 122.1, 122.0, 75.7,
75.6, 67.5, 67.4, 60.8. 60.6, 60.4, 57.3, 57.1, 38.7, 32.3, 32.2, 29.7, 27.1 (× 3), 25.8 (× 3), 21.1, 18.3,
12.8, 1.0 and −5.4 ppm. HRFABMS: calcd for C23H42O6NaSi [M+Na]+ 465.2648, found: 465.2645.
Synthesis of ((1R,5S,6R)-6-(((tert-butyldimethylsilyl)oxy)methyl)-5-hydroxy-2,6-dimethylcyclohex-2-
en-1-yl)methyl pivalate (9). A mixture of Cp2TiCl2 (60.5 mg, 0.243 mmol) and Mn dust (356 mg,
6.48 mmol) in strictly deoxygenated THF (5.0 mL) under argon, was stirred at room temperature until
the red solution became green. A solution of the corresponding epoxide 8 (360 mg, 0.81 mmol), 2,4,6-
collidine (0.74 mL, 5.67 mmol), and TMSCl (0.41 mL, 3.24 mmol) in strictly deoxygenated THF (2.0 mL)
was then added, and the mixture was stirred until disappearance of the starting material (6 h) was

Molecules 2014, 19
1758
observed. The reaction was quenched with HCl (2 N, dropwise addition of 10 mL), extracted with
t-BuOMe (3 × 20 mL), washed with brine, dried with anhydrous Na2SO4, and concentrated under
reduced pressure. Product was purified by flash chromatography on silica gel (hexane/t-BuOMe, 7:1)
to yield the monocycle 9 (219.2 mg, 0.57 mmol, 70% yield) as a colorless solid. 1H-NMR (500 MHz,
CDCl3): δ 5.39 (1H, bs), 4.30 (1H, dd, J = 11.8, 4.4 Hz), 4.07 (1H, dd, J = 11.8, 4.3 Hz), 3.85 (1H, dd,
J = 8.4, 5.6 Hz), 3.68 (1H, d, J = 10.0 Hz), 3.53 (1H, d, J = 10.0 Hz), 2.22 (2H, m), 2.02 (1H, m), 1.68
13
(3H, s), 1.19 (9H, s), 0.89 (9H, s), 0.88 (3H, s), 0.07 (3H, s) and 0.06 (3H, s) ppm. C-NMR (125
MHz, CDCl3): δ 178.5, 132.6, 121.1, 71.5, 68.9, 62.7, 43.1, 41.4, 38.8, 30.8, 27.2, 25.9, 22.0, 18.2, 12.0
and −5.5 ppm. HRFABMS: calcd for C21H40O4SiNa [M+Na]+ 407.2594, found: 407.2593.
4. Conclusions
As a conclusion, the stereoselective synthesis of a highly functionalized cyclohexane key
intermediate has been accomplished. This enables potential access to both the labdane and trisporoid
families. The main objective is to facilitate the approach to the widest structural diversity of these
compounds for their use as a standard either in subsequent identifications, quantifications, and
biological activity tests. The final and most important step of the sequence consists of a domino
process including homolytic epoxide opening, Ti(III)-mediated stereoselective catalytic cyclization,
and regioselective elimination of an –OAc group of a conveniently functionalized epoxypolyprene.
The proper functionalization of the oxiranic intermediate allows one to obtain the final product with
complete stereocontrol in up to five of the six carbons of the final cyclohexane structure.
Supplementary Materials
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/19/2/1748/s1.
Acknowledgments
This project was supported by the Spanish Ministry of Economy and Competitiveness (project
CTQ2010-16818-BQ).
Author Contributions
The authors are contributed equally to this work.
Conflicts of Interest
The authors declare no conflict of interest.
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