Enantiospecific total synthesis of both enantiomers of laurene by a chemical diastereoselection/lipase-catalyzed kinetic resolution sequence

Article abstract of DOI:10.1016/S0957-4166(99)00195-0

A short and efficient enantiospecific total synthesis of natural (+)- laurene and its enantiomer is described. The methodology was developed by employing a stereoselective H-ene reaction of an isocyclic allyltrimethylsilane with paraformaldehyde, followed by a lipase-mediated kinetic resolution of the racemic key intermediate.

Full text of DOI:10.1016/S0957-4166(99)00195-0

Pergamon
Tetrahedron: Asymmetry 10 (1999) 1927–1933
Enantiospecific total synthesis of both enantiomers of laurene by
a chemical diastereoselection/lipase-catalyzed kinetic resolution
sequence
Gilles Laval, Gérard Audran, Sylvie Sanchez and Honoré Monti ∗
Laboratoire de Réactivité Organique Sélective associé au CNRS, Faculté des Sciences et Techniques de St Jérôme,
Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France
Received 2 April 1999; accepted 17 May 1999
Abstract
A short and efficient enantiospecific total synthesis of natural (+)-laurene and its enantiomer is described. The
methodology was developed by employing a stereoselective H-ene reaction of an isocyclic allyltrimethylsilane
with paraformaldehyde, followed by a lipase-mediated kinetic resolution of the racemic key intermediate. © 1999
Elsevier Science Ltd. All rights reserved.
1. Introduction
The sesquiterpene hydrocarbon laurene was first isolated as the (+)-enantiomer 1 from Laurencia
glandulifera and subsequently found in several other Laurencia species (Fig. 1).1–3 Its carbon framework
is suitable for the construction of similar biologically important compounds.4,5 Despite the relatively
simple substitution pattern on the cyclopentane skeleton, the cis-1,2 relationship of the secondary methyl
group with the p-tolyl group has made both the stereoselective6,7 and enantioselective8,9 synthesis of this
product difficult.
Figure 1.
∗
Corresponding author. Tel/fax: +33 (0)4 91 28 88 62; e-mail: honore.monti@ros.u-3mrs.fr
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00195-0

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In this paper, we report the first enantiospecific synthesis of both enantiomers of laurene. Our
approach is based upon the stereoselective H-ene reaction between an isocyclic allyltrimethylsilane and
paraformaldehyde,10 followed by an enzymatic kinetic resolution of the key racemate. Our methodology
is described in Scheme 1.
Scheme 1. Reagents: (a) p-TolylMgBr, CuBr·DMS 25%, THF, −5°C then ClPO(OEt)2, HMPA, rt; (b) ClMgCH2SiMe3,
Ni(acac)2 10%, THF, reflux; (c) (CH2O)n, Me2AlCl, MS 4 Å, CH2Cl2, −10°C; (d) MeOH, 1 equiv. HCl (35%), rt; (e) MsCl, py,
rt; (f) LiAlH4, Et2O, reflux
2. Results and discussion
The commercially available 3-methyl-cyclopent-2-en-1-one was converted into the cyclic diethyl enol
phosphate 2 in 76% yield by the CuBr·DMS-catalyzed 1,4-addition of p-tolyl magnesium bromide,
followed by quenching with diethylchlorophosphate in hexamethylphosphoric triamide.11,12
The nickel acetylacetonate-catalyzed reaction of 2 with (trimethylsilyl)methylmagnesium chloride
gave trimethyl[(3-methyl-3-p-tolyl-cyclopent-1-enyl)methyl]silane 3 in 91% yield.12 The Me2AlCl-
induced H-ene reaction of 3 with paraformaldehyde in the presence of 4 Å molecular sieves (zeolite)
furnished, in 87% overall yield, a mixture of three main products (4+5+6) and small amounts (9:1) of a
fourth (7).
In view of the elucidation of their different structures, the mixture of products was submitted to silica
gel column chromatography and the different components readily separated and analyzed. In the 13C
NMR spectra, regioisomers 4, 5 and 7 were identified by the chemical shifts of the quaternary sp2-

G. Laval et al. / Tetrahedron: Asymmetry 10 (1999) 1927–1933
1929
hybridized carbons which differed highly.13 Those of the allylic isomers 4 (δ=143.5) and 7 (δ=148.4)
gave more upfield signals than that of the vinylic isomer 5 (δ=161.5). The stereoselectivity was
1
assessed by comparing the chemical shift of the hydroxymethyl group in the H NMR spectra. When
the hydroxymethyl and p-tolyl substituents are oriented syn-like in 4, 5 or 6, an upfield shift for the
hydroxymethyl AB system is observed relative to the anti-orientation as in 7.14
NOESY experiments on the major compound 4 have corroborated this observation. The NOESY
plots were characterized by strong cross-peaks between H-2 and Me-3 as well as between the CH2OH
protons and the ortho-positioned protons of the p-tolyl group (Scheme 1). So 4, 5 and 6 had the carbon
stereostructure of laurene, and 7 (which could be conveniently discarded by column chromatography) had
that of epi-laurene. Remarkably, treatment of the mixture (4+5) with 1 equiv. of HCl (35%) in MeOH
provided, without any further isomerization of the exocyclic double bond, the alcohol 6 as the only
product in 78% overall yield based on 3. Treatment of 6 with vinyl acetate:hexane (1:5) in the presence
of Candida rugosa lipase for 5 days gave the results outlined in Scheme 2.15–17
Scheme 2.
Enantiomeric excess (ee) was determined by the ratio of the peak areas obtained by GC separation
using a chiral phase (see Section 3). The enantiomers of the alcohol were perfectly separated. However,
those of the acetate did not separate. Accordingly, the enantiomeric purity of the acetate was determined
after hydrolysis to the corresponding alcohol. So, acetate (−)-8, readily separated from unreacted
alcohol (+)-6 (ee >99%) by silica gel column chromatography, was quantitatively hydrolyzed with 3
M KOH/MeOH. The resulting alcohol, (−)-6 (ee 72%), was resubjected to enzymatic transesterification
to provide, after a new hydrolysis, (−)-6 in 74% yield and an enantiopurity enhancement to 96%. The
overall production of (+)-6 and (−)-6 from racemic 6 was 42% and 40.7%, respectively. Treatment of
alcohol (+)-6 with an excess of MsCl in pyridine afforded the corresponding mesylate (+)-9, which was
then reduced with LiAlH4 in refluxing Et2O to (+)-laurene 1 (96% overall yield from (+)-6). The known
absolute configuration of natural (+)-laurene8,9 correlated with the absolute configurations of alcohol
(+)-6 and mesylate (+)-9. The same sequence applied to alcohol (−)-6 provided unnatural (−)-laurene.
In summary, we have developed a short and efficient enantiospecific synthesis of both enantiomers
of laurene in very high enantiomeric excess via a stereoselective H-ene reaction and an asymmetric
enzymatic transesterification. Following this sequence, the natural enantiomer (+)-laurene is available in
an overall yield of 22% and in >99% ee, whereas (−)-laurene is available in an overall yield of 21% and
96% ee.

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G. Laval et al. / Tetrahedron: Asymmetry 10 (1999) 1927–1933
3. Experimental
3.1. General procedures
1H and 13C NMR spectra were recorded in CDCl3 solution on Bruker AM-400 or Bruker AM-200
spectrometers (Bruker AM-400 for NOESY experiments). Infrared spectra were obtained as films using
a Perkin–Elmer 257 infrared spectrophotometer. Routine monitoring of reactions was performed using
Merck 60F 254 silica gel, aluminum-supported TLC plates. Column chromatography was performed
using silica gel 60 (230–400 mesh) and pentane/ether (gradients) as eluent unless otherwise stated. GC
analyses were carried out on a Chrompack 9001 using a WCOT fused silica column (25 m×0.32 mm
i.d.; CP-Wax-52 CB stationary phase; N2 carrier gas: 50 kPa). Enantiomeric excess determinations were
carried out using a MEGADEX DETTBSβ fused silica column (30 m×0.25 mm i.d.; N2 carrier gas:
70 kPa). Optical rotations were measured on a Perkin–Elmer 341 polarimeter. CRL (Candida rugosa
lipase; EC 3.1.1.3, Type VII, lyophilized, 746 U/mg) was purchased from Sigma. Microanalyses were
performed on a Technicon CHN analyzer at our University. Solutions were dried over magnesium sulfate
and evaporated in a rotary evaporator under reduced pressure.
3.2. Diethyl[3-methyl-3-p-tolyl-cyclopent-1-en-1-yl]phosphoric triester 2
p-Tolylmagnesium bromide was prepared in the standard fashion from Mg turnings and p-
bromotoluene in refluxing dry THF.
To a stirred suspension of cuprous bromide–dimethylsulfide complex (3.47 g, 16.9 mmol) and 3-
methyl-cyclopent-2-en-1-one (6.50 g, 67.7 mmol) in dry THF (150 ml) at −5°C was added dropwise
p-tolylMgBr (1.5 M in THF, 90 ml, 135 mmol) under an Ar atmosphere. The reaction mixture was
stirred for 2 h at this temperature, and then a mixture of ClPO(OEt)2 (20.2 ml, 135 mmol) and HMPA
(23.4 ml, 135 mmol) in THF (50 ml) was added dropwise. The solution temperature was allowed to
rise to rt, stirred for 15 h and then poured into a saturated aqueous NH4Cl/NH4OH solution (400 ml)
and extracted with ether (3×100 ml). The organic layers were combined, washed with 100 ml of brine,
dried, and evaporated. The oily residue was subjected to rapid column chromatography on silica gel
(pentane:ether, 30:70) to afford 16.7 g (76%) of 2. IR (neat): ν 3030, 1645, 1280, 1040, 960 cm−1; 1H
NMR (200 MHz, CDCl3): δ 7.22 (d, J=8.1 Hz, 2H), 7.11 (d, J=8.1 Hz, 2H), 5.45 (br s, 1H), 4.20 (q,
J=7.1 Hz, 4H), 2.72–2.48 (m, 2H), 2.32 (s, 3H), 2.20–1.98 (m, 2H), 1.45 (s, 3H), 1.37 (t, J=7.1 Hz,
6H); 13C NMR (50 MHz, CDCl3): δ 149.1 (d, J=−8.1 Hz), 146.6, 135.1, 128.7, 125.3, 117.8 (d, J=5.0
Hz), 64.2 (d, J=−6.0 Hz), 48.6, 38.7, 31.1 (d, J=5.0 Hz), 28.5, 20.7, 15.9 (d, J=6.7 Hz). Anal. calcd for
C17H25O4P: C, 62.95; H, 7.77. Found: C, 63.26; H, 7.80.
3.3. Trimethyl[(3-methyl-3-p-tolyl-cyclopent-1-en-1-yl)methyl]silane 3
To a stirred solution of enol phosphate 2 (4.00 g, 12.3 mmol) and Ni(acac)2 (474 mg, 1.84 mmol)
in THF (80 ml) was added dropwise a 1 M ether solution of (trimethylsilyl)methyl magnesium chloride
(36.9 ml, 36.9 mmol) and the mixture was refluxed for 2 h. The reaction was cooled and quenched with 70
ml of a 20% aqueous NH4Cl solution and stirred for 45 min. After extractive workup with ether (3×100
ml), the organic layers were combined, washed with 50 ml of brine, dried, and evaporated. Purification
by chromatography on silica gel (pentane) afforded 2.89 g (91%) of 3. IR (neat): ν 3020, 1640, 1250,
850 cm−1; 1H NMR (200 MHz, CDCl3): δ 7.25 (d, J=7.9 Hz, 2H), 7.11 (d, J=7.9 Hz, 2H), 5.28 (s, 1H),
2.32 (s, 3H), 2.51–2.17 (m partially overlapped, 2H), 2.15–1.91 (m, 2H), 1.62 (s, 2H), 1.39 (s, 3H), 0.06

G. Laval et al. / Tetrahedron: Asymmetry 10 (1999) 1927–1933
1931
(s, 9H); 13C NMR (50 MHz, CDCl3): δ 148.1, 140.7, 134.7, 131.1, 128.7, 125.8, 52.2, 41.8, 36.9, 28.3,
21.4, 20.9, −1.2. Anal. calcd for C17H26Si: C, 79.00; H, 10.14. Found: C, 78.76; H, 10.12.
3.4. Reaction of 3 with paraformaldehyde
To a slurry of isocyclic allylsilane 3 (2.20 g, 8.51 mmol), powdered 4 Å molecular sieves and
paraformaldehyde (280 mg, 9.36 mmol) in 25 ml of CH2Cl2 at −10°C was added dropwise 17.0 ml
of Me2AlCl (1 M in hexanes, 17.0 mmol). The reaction was monitored by TLC. After stirring for 1 h, the
reaction reached completion. It was then poured into 50 ml of NaHCO3 saturated ice-water and extracted
with 2×50 ml of Et2O. The collected organic extracts were washed with brine, dried and concentrated.
The mixture was then subjected to careful silica gel column chromatography which allowed successive
separation of pure 4 (1.15 g, 47%), 5 (383 mg, 15.6%), 7 (213 mg, 8.7%) and 6 (287 mg, 15.6%).
4: IR (neat): ν 3380, 3020, 1640, 1240, 850 cm−1; 1H NMR (200 MHz, CDCl3): δ 7.28 (d, J=8.0 Hz,
2H), 7.15 (d, J=8.0 Hz, 2H), 5.42 (s, 1H), 3.42 (m, 2H), 3.08 (br d, J=15.3 Hz, 1H), 2.43 (br s, 1H),
2.34 (s, 3H), 2.15 (dd, J=15.3 and 1.9 Hz, 1H), 1.70 and 1.46 (AB, J=−13.6 Hz, 2H), 1.43 (s, 3H), 0.09
(s, 9H); 13C NMR (50 MHz, CDCl3): δ 143.5, 140.6, 135.2, 129.0, 126.7, 122.7, 62.3, 61.7, 48.6, 43.4,
32.4, 20.8, 19.9, −1.1. Anal. calcd for C18H28OSi: C, 74.94; H, 9.78. Found: C, 75.16; H, 9.81.
1
5: IR (neat): ν 3360, 3020, 1620, 1250, 840 cm−1; H NMR (200 MHz, CDCl3): δ 7.17 (m, 4H),
5.61 (s, 1H), 3.45–3.29 (m, 1H), 3.14–2.98 (m, 1H), 2.96–2.82 (m, 1H), 2.70–2.53 (m, 2H), 2.50–2.22
(m partially overlapped, 1H), 2.33 (s, 3H), 1.84–1.67 (m, 1H), 1.22 (s, 3H), 0.17 (s, 9H); 13C NMR (50
MHz, CDCl3): δ 161.5, 143.4, 135.4, 129.1, 126.5, 124.8, 62.6, 57.1, 48.3, 32.3, 32.0, 30.7, 20.9, 0.4.
Anal. calcd for C18H28OSi: C, 74.94; H, 9.78. Found: C, 74.64; H, 9.75.
7: IR (neat): ν 3350, 3020, 1645, 1245, 840 cm−1; 1H NMR (200 MHz, CDCl3): δ 7.26 (d, J=8.1 Hz,
2H), 7.10 (d, J=8.1 Hz, 2H), 5.45 (s, 1H), 3.85 (m, 2H), 2.78 and 2.40 (AB, J=−16.4 Hz, 2H), 2.61 (br s,
1H), 2.31 (s, 3H), 1.64 and 1.39 (AB, J=−13.7 Hz, 2H), 1.50 (s, 3H), −0.14 (s, 9H); 13C NMR (50 MHz,
CDCl3): δ 148.4, 139.7, 134.8, 128.7, 125.8, 123.5, 62.1, 61.1, 47.6, 47.0, 23.3, 20.7, 19.6, −1.4. Anal.
calcd for C18H28OSi: C, 74.94; H, 9.78. Found: C, 75.29; H, 9.74.
6: IR (neat): ν 3350, 3040, 1650, 1040, 890, 815 cm−1; 1H NMR (200 MHz, CDCl3): δ 7.16 (s, 4H),
5.12 (br s, 2H), 3.33 and 2.97 (ABX, J=−10.5, 10.5 and 5.2 Hz, 2H), 2.75–2.51 (m, 3H), 2.44–2.20 (m
partially overlapped, 1H), 2.33 (s, 3H), 1.81 (dt, J=11.8 and 5.9 Hz, 1H), 1.29 (s, 3H); 13C NMR (50
MHz, CDCl3): δ 153.0, 143.2, 135.3, 129.0, 126.7, 109.4, 61.9, 58.2, 48.0, 33.7, 30.5, 28.6, 20.8. Anal.
calcd for C15H20O: C, 83.28; H, 9.32. Found: C, 82.98; H, 9.35.
3.5. 2-Hydroxymethyl-3-methyl-1-methylidene-3-p-tolyl-cyclopentane 6
To a stirred mixture of 4 and 5 (1.53 g, 5.33 mmol) in 30 ml of MeOH was added dropwise 0.5 ml
of 35% HCl (∼1 equiv.) at rt. TLC monitoring showed the total disappearance of 4 and 5 after 1 h. To
this mixture was then added 30 ml of CH2Cl2, the reaction mixture was poured into a saturated aqueous
NaHCO3 solution (40 ml) and extracted with CH2Cl2 (3×30 ml). The organic layers were combined,
dried and evaporated to afford a residue from which 6 (1.15 g, quant.) was obtained pure after rapid
chromatography (gradient pentane/ether).
3.6. General procedure for lipase-catalyzed acylation of (ꢀ)-6
A mixture of (ꢀ)-6 (1.00 g, 4.63 mmol), vinyl acetate (5 ml) and lipase CR (2.0 g) in 25 ml of hexane
was magnetically stirred at rt and the reaction progress monitored by GC on a chiral column. After 3 days

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the reaction became very slow. So the mixture was filtered, 2.0 g of lipase CR was added and the slurry
stirred until the desired conversion was achieved. After two more days, GC chromatography showed that
the remaining alcohol was enantiopure. The reaction was stopped by filtration. Removal of the solvent
followed by separation on a silica gel column yielded 420 mg (42%) of pure (+)-(2S,3R)-6, [α]D20=+66.1
1
(c 1.0, CHCl3), and 662 mg (55%) of acetate (−)-8; (+)-6: H and 13C NMR spectra were identical to
those of (ꢀ)-6.
1
(−)-8: IR (neat): ν 3030, 1730, 1650, 1250, 1040, 890, 810 cm−1; H NMR (200 MHz, CDCl3): δ
7.15 (m, 4H), 5.03 (br s, 2H), 3.86 (m, 2H), 3.28 (br t, J=6.8 Hz, 1H), 2.70–2.51 (m, 2H), 2.48–2.24
(m partially overlapped, 1H), 2.33 (s, 3H), 1.96 (s, 3H), 2.00–1.70 (m partially overlapped, 1H), 1.32 (s,
3H); 13C NMR (50 MHz, CDCl3): δ 170.8, 151.9, 143.0, 135.3, 128.9, 126.5, 109.1, 64.7, 54.3, 48.1,
34.4, 30.2, 28.8, 20.8. Anal. calcd for C17H22O2: C, 79.03; H, 8.58. Found: C, 79.34; H, 8.62.
Compound (−)-8 was treated with 3 M KOH (1 ml) in 10 ml of MeOH for 2 h at rt. The solution
was diluted with 20 ml of cold water, 20 ml of CH2Cl2 and neutralized with 1 M HCl. The aqueous
layer was extracted with CH2Cl2 (2×20 ml), and the recombined organic phases were dried, filtered and
concentrated to afford (−)-6 (550 mg, quant.) with 72% ee.
Alcohol (−)-6 was resubjected to lipase-catalyzed acylation as in the previously described conditions.
After 2 days, GC analysis showed that the remaining alcohol was near racemic. The reaction was stopped
by filtration. Removal of the solvent followed by separation on a silica gel column yielded 490 mg (74%)
of acetate (−)-8. Treatment of (−)-8 as previously described gave (−)-(2R,3S)-6 (407 mg, quant.) with
96% ee and 40.7% overall yield from (ꢀ)-6; [α]D20=−63.4 (c 1.0, CHCl3); (−)-6: H and 13C NMR
1
spectra were identical to those of (ꢀ)-6.
3.7. (+)-(2S,3R)-2-[(Methanesulfonyl)-oxy]methyl-3-methyl-1-methylidene-3-p-tolyl-cyclopentane
(+)-9
To a solution of (+)-6 (216 mg, 1 mmol) in 10 ml of dry pyridine was added dropwise methanesulfonyl
chloride (126 mg, 1.1 mmol) under an Ar atmosphere. The solution was stirred at rt for 1 h, then poured
onto 100 ml of cold water and extracted with CH2Cl2 (3×20 ml). The combined organic extracts were
washed with water (5×20 ml), dried, filtered, and concentrated in vacuo (0.02 mmHg). Rapid silica gel
column chromatography afforded (+)-(2S,3R)-9 (288 mg, 98%); [α]D20=+22.8 (c 1.0, CHCl3). IR (neat):
ν 3020, 1655, 1350, 1175, 950, 900, 820, 730 cm−1; 1H NMR (200 MHz, CDCl3): δ 7.16 (s, 4H), 5.14
(br s, 2H), 3.96 and 3.76 (ABX, J=−9.7, 8.2 and 4.7 Hz, 2H), 2.90–2.78 (m, 1H), 2.70 (s, 3H), 2.68–2.52
(m, 2H), 2.32 (s, 3H), 2.43–2.22 (m partially overlapped, 1H), 1.86 (dt, J=12.2 and 6.1 Hz, 1H), 1.32
(s, 3H); 13C NMR (50 MHz, CDCl3): δ 150.6, 142.5, 135.7, 129.2, 126.5, 110.4, 70.9, 54.3, 48.3, 36.7,
34.6, 30.4, 28.9, 20.8. Anal. calcd for C16H22O3S: C, 65.28; H, 7.53. Found: C, 65.65; H, 7.50.
3.8. (−)-(2R,3S)-2-[(Methanesulfonyl)-oxy]methyl-3-methyl-1-methylidene-3-p-tolyl-cyclopentane
(−)-9
1
The same procedure applied to (−)-6 afforded (−)-(2R,3S)-9, [α]D20=−21.8 (c 1.0, CHCl3); H and
13C NMR spectra were identical to those of (+)-9.
3.9. (+)-(2S,3R)-2,3-Dimethyl-1-methylidene-3-p-tolyl-cyclopentane: (+)-laurene (+)-1
A mixture of mesylate (+)-9 (200 mg, 0.68 mmol) and LiAlH4 (25 mg, 0.68 mmol) in 10 ml of dry
ether was refluxed for 0.5 h. After cooling in an ice bath, Celite (0.7 g), Na2SO4·10H2O (0.7 g), and 0.5

G. Laval et al. / Tetrahedron: Asymmetry 10 (1999) 1927–1933
1933
ml of 10% NaOH were added. After 30 min the reaction mixture was filtered through a pad of MgSO4
and concentrated to give 133 mg (98%) of pure laurene (+)-(2S,3R)-1; [α]D20=+37.9 (c 1.0, EtOH). IR
(neat): ν 3060, 1380, 1280, 1020, 880, 815 cm−1; 1H NMR (200 MHz, CDCl3): δ 7.13 (s, 4H), 4.90 (br
s, 2H), 2.78–2.47 (m, 3H), 2.34 (s, 3H), 2.35–2.16 (m, 1H), 1.82 (dt, J=12.6 and 6.3 Hz, 1H), 1.30 (s,
3H), 0.73 (d, J=7.0 Hz, 3H); 13C NMR (50 MHz, CDCl3): δ 157.5, 144.6, 135.0, 128.6, 126.9, 105.6,
50.4, 49.0, 34.6, 29.6, 29.1, 20.9, 17.2. Anal. calcd for C15H20: C, 89.94; H, 10.06. Found: C, 89.69; H,
10.09.
3.10. (−)-(2R,3S)-2,3-Dimethyl-1-methylidene-3-p-tolyl-cyclopentane: (−)-laurene (−)-1
The same procedure applied to (−)-9 afforded (−)-(2R,3S)-1, [α]D20=−36.6 (c 1.0, EtOH); 1H and 13
NMR spectra were identical to those of (+)-1.
C
Acknowledgements
We are grateful to Dr. E. Ferré for friendly and helpful discussions about biocatalysts.
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