A useful enantioselective synthesis of chroman-2-ylmethanol

Article abstract of DOI:10.1039/b110578g

An enantioselective synthesis of chromanylmethanol is described. An allylic alcohol moiety is, first, introduced on the aromatic ring through a Claisen transposition. Chirality is then introduced through asymmetric Sharpless epoxidation on the allylic alcohol moiety. Cyclization into the benzopyran ring is achieved by an intramolecular coupling between the tertiary alcoholic hydroxy and the hydroquinone moiety with an excellent retention of configuration.

Full text of DOI:10.1039/b110578g

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A useful enantioselective synthesis of chroman-2-ylmethanol
Jean-Yves Goujon, Alexis Duval and Bernard Kirschleger*
Laboratoire de Synthèse Organique UMR 6513, Faculté des Sciences et des Techniques,
2, Rue de la Houssinière, BP 92208, , 44322 Nantes Cedex 3, France.
E-mail: kirschle@chimie.univ-nantes.fr; Fax: 0033-2 5112 5402; Tel: 0033-2 5112 5419
1
Received (in Cambridge, UK) 19th November 2001, Accepted 14th January 2002
First published as an Advance Article on the web 28th January 2002
An enantioselective synthesis of chromanylmethanol is described. An allylic alcohol moiety is, first, introduced
on the aromatic ring through a Claisen transposition. Chirality is then introduced through asymmetric Sharpless
epoxidation on the allylic alcohol moiety. Cyclization into the benzopyran ring is achieved by an intramolecular
coupling between the tertiary alcoholic hydroxy and the hydroquinone moiety with an excellent retention of
configuration.
Introduction
Chroman-2-ylmethanol, as an optically active moiety, is an
important synthetic intermediate, which can be used for the
synthesis of biologically active compounds such as α-toco-
pherol1 (vitamin E), levcromakalim,2 cannabichromene3 or
cordiachromene.4 All syntheses of the chromanylmethanol
intermediate of α-tocopherol are achieved by using optical reso-
lution5 or asymmetric synthesis.6 Recently, we have reported a
multi-step enantioselective synthesis of cordiachromene7 using
chromanylmethanol 14, which was formed with an overall yield
of 30%. However, further studies were needed for a more
efficient access to 14. In this paper, we describe this useful
approach by using a succession of high yielding short and
simple reactions.
Scheme 1 Reagents and conditions i) PdP(Ph3)4, DCM, rt, 4 h, 95%.
ii) Ac2O, Et3N, DMAP, AcOEt, rt, 4 h, 98%. iii) HCl(g), DCM, rt,
2 min, 98%. iv) ClCH2OCH3, iPr2NEt, DCM, 40 ЊC, 12 h, 97%.
v) K2CO3, MeOH, rt, 2 h, 97%.
-(ϩ)-diethyl tartrate as the chiral reagent, compound 8, with a
(S,S)-configuration, was isolated with 85% yield and 93% de
(determined by NMR analysis). Before the epoxide ring open-
ing, the alcoholic hydroxy was protected as a benzyl group
according to classical methods. The epoxide was then regio-
selectively opened with lithium aluminium hydride to obtain
10b in an almost quantitative yield (90% ee). It is noteworthy
that starting from 8 the same reaction gave directly diol 10a.7
The third part of this synthesis (Scheme 3) concerned the
cyclization into the benzopyran ring. Previous results showed
that direct cyclisation was achieved from the 3,5,6-trimethyl-
hydroquinone homologous compound of 12a by intra-
molecular coupling between tertiary alcoholic hydroxy and
semi-quinonic or quinonic form of the hydroquinone moiety.6a
An excellent retention of configuration was observed. Before
cyclisation, the hydroquinone dialkoxy ether moiety of 10
needed to be deprotected. Starting from 10a, oxidative cleavage
with ceric ammonium nitrate (CAN)11 failed. However, with
10b, the same reaction succeeded affording quinone 11 which
was, then, reduced with sodium dithionite to give hydroquinone
Results and discussion
The first part of the synthesis (Scheme 1) was devoted to the
coupling of an allylic alcohol moiety in the o-position to
p-dialkoxybenzene using a sequence of short, simple and high
yielding reactions. The synthesis started with a regiospecific
addition8 of 1-methyl-1-vinyloxirane9 2 on the phenolic
hydroxy of p-methoxyphenol 1 through a π-allylpalladium
complex intermediate obtained from vinyloxirane and Pd().
The crude reaction product 3 was then reacted with acetic
anhydride to protect the alcoholic hydroxy as an acetoxy group
and this gave 4. By using anhydrous hydrogen chloride, the
latter was transposed8 (Claisen transposition) into 5. This
sequence was achieved by the protection of the phenolic
hydroxy as a methoxymethoxy group (MOM) giving 6,
followed by liberation of the alcoholic hydroxy affording 7.
The second part of this synthesis (Scheme 2) was concerned
with the introduction of chirality on the allylic double bond
through Sharpless asymmetric epoxidation (SAE).10 Using
496
J. Chem. Soc., Perkin Trans. 1, 2002, 496–499
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b110578g

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Elemental analyses were carried out by the CNRS Analysis
Laboratory, Vernaison, France.
1-(1-Hydroxy-2-methylbut-3-en-2-yloxy)-4-methoxybenzene (3)
To a deoxygenated solution of p-methoxyphenol 1 (8.54 g,
68.9 mmol) and vinyloxirane 2 (8.68 g, 103.3 mmol) in DCM
(40 mL) was added PdP(Ph3)4 (1.2 g, 1 mmol). The mixture was
stirred at rt for 4 h. Then, the mixture was filtered on a Celite
pad to remove the catalyst and the solvent was evaporated
in vacuo. A colourless oily residue remained (14.22 g, 95%) and
it was used without further purification. δH 6.93–6.72 (m, 4H,
4CH ), 6.07 (dd, 1H, J 11.3, J 17.5, CH᎐), 5.27 (dd, 1H , J 11.3,
᎐
ar
cis
J 1, CH᎐), 5.18 (dd, 1H , J 17.5, J 1, CH᎐), 3.74 (s, 3H, OCH ),
᎐
᎐
trans
3
3.67 (dt, 1H, J 6.6, J 10.1, CH2O), 3.56 (dt, 1H, J 6.6, J 10.1,
CH2O), 2.34 (t, 1H, J 6.6, OH), 1.32 (s, 3H, CH3). δC 155.6,
148.3, 139.7, 123.8, 116.5, 113.9, 79.9, 69.3, 55.4, 20.4. MS, m/z
(relative intensity) 208 (15%, Mϩ), 124 (71), 85 (100), 15 (8).
1-(1-Acetyloxy-2-methylbut-3-en-2-yloxy)-4-methoxybenzene
(4)
To a solution of 3 (0.208 g, 1 mmol) and dimethylamino-
pyridine (catalytic amount) in ethyl acetate (30 mL) was added
a solution of acetic anhydride (0.306 g, 3 mmol) and triethyl-
amine (0.303 g, 3 mmol). The mixture was stirred at rt for 4 h,
then brine (30 mL) was added. The organic layer was separated
and dried over MgSO4. The solvent was evaporated in vacuo. A
yellow oily residue remained (0.250 g, 98%) and it was used
without further purification. δH 6.94–6.73 (m, 4H, 4CHar), 6.05
Scheme 2 Reagents and conditions i) tBuOOH, Ti(OiPr)4, -(ϩ)-DET,
DCM, Ϫ24 ЊC, 20 h, 85%. ii) BnBr, NaH, nBu4NI, THF, rt, 3h, 88%.
iii) LiAlH4, diethyl ether, rt, 5 h, 97%. iv) CAN, CH3CN, H2O, Ϫ5 ЊC,
90 min, 90%. v) Na2S2O4, acetone, H2O, rt, 30 min, 96%.
(dd, 1H, J 10.8, J 17.8, CH᎐), 5.27 (dd, 1H , J 10.8, J 1, CH᎐),
᎐
᎐
cis
5.26 (dd, 1Htrans, J 17.8, J 1, CH᎐), 4.16 (s, 2H, CH O), 3.76 (s,
᎐
2
3H, OCH3), 2.12 (s, 3H, CH3), 1.35 (s, 3H, CH3). δC 170.8,
155.6, 148.3, 139.7, 123.8, 116.5, 113.8, 79.8, 68.9, 55.4, 20.9,
20.4. MS, m/z (relative intensity) 250 (6%, Mϩ), 177 (10), 127
(100), 124 (65), 85 (16), 43 (70).
2-(4-Acetyloxy-3-methylbut-2-en-1-yl)-4-methoxyphenol (5)
Through a solution of 4 (2.10 g, 15 mmol) in DCM (40 mL),
anhydrous HCl was bubbled for 2 min. The solvent was evapor-
ated in vacuo. A yellow oily residue remained (2.09 g, 98%)
which was used without further purification. δH 6.77– 659 (m,
Scheme 3 Reagents and conditions i) p-TSA, toluene, 45 ЊC, 1 h, 94%.
ii) H2, Pd/C, MeOH, rt, 4 h, 95%.
3H, 3CH ), 5.64 (t, 1H, J 7.4, CH᎐), 4.50 (s, 2H, CH O), 3.74
᎐
ar
2
(s, 3H, OCH3), 3.38 (d, 2H, J 7.4, CH2), 2.08 (s, 3H, CH3), 1.78
(s, 3H, CH3). δC 171.5, 153.5, 148.0, 131.5, 127.8, 127.0, 116,
112, 70.1, 55.7, 28.7, 20.9, 14.0. MS, m/z (relative intensity) 250
(4%, Mϩ), 190 (34), 175 (100), 91 (12), 77 (15), 43 (64).
derivative 12.12 In acidic medium (p-TSA), 12 was cyclized
affording 13 in high yield (94%) which was easily debenzylated
to produce chroman-2-ylmethanol 14. As expected cyclization
was achieved with an excellent retention of configuration of the
stereogenic carbon (ee = 85%).
(E )-2-(4-Acetyloxy-3-methylbut-2-en-1-yl)-4-methoxy-1-
methoxymethoxybenzene (6)
Conclusion
To a stirred solution of 5 (1.43 g, 5.7 mmol) and diisopropyl-
ethylamine (2.21 g, 1.7 mmol) in DCM (25 mL) was slowly
added methoxymethyl chloride (1.38 g, 1.7 mmol) at 0 ЊC. After
stirring at 40 ЊC for 12 h, sat. aq. NH4Cl (30 mL) was added.
The organic layer was extracted with diethyl ether (2 × 30 mL),
washed with water (20 mL) and brine (20 mL), dried over
MgSO4 and the solvent was evaporated in vacuo. The crude
product was purified by column chromatography (silica gel,
hexane–EtOAc, 8 : 2) to give a colourless oil (1.62 g, 97%).
Chroman-2-ylmethanol 14 was synthesised in 12 steps with an
overall yield of 48%. The method we have presented advan-
tageously complemented the synthesis previously reported,
with efficient reactions and short work-up.
Experimental
Every starting material was obtained from commercial
suppliers and used without further purification. Each solvent
(diethyl ether, THF, DMF, toluene) was dried by distillation
from sodium–benzophenone before use. 1H and 13C NMR
spectra were recorded at 200 and 50 MHz on a Bruker AC 200
with CDCl3 as solvent and TMS as internal standard; chemical
shifts (δ) were expressed in ppm and coupling constants (J) in
Hertz. Mass spectra (EI) were recorded on a Hewlett Packard
5989A (70 eV). Optical rotations were measured on a 341
δH 7.02–6.64 (m, 3H, 3CH ), 5.62 (t, 1H, J 7.4, CH᎐), 5.12 (s,
᎐
ar
2H, OCH2O), 4.49 (s, 2H, CH2O), 3.76 (s, 3H, OCH3), 3.47
(s, 3H, OCH3), 3.38 (d, 2H, J 7.4, CH2), 2.06 (s, 3H, CH3), 1.77
(s, 3H, CH3). δC 170.8, 154.4, 149.0, 130.9, 128.3, 127.2, 115.7,
111.3, 95.2, 70.0, 55.8, 55.5, 28.5, 20.9, 13.9. MS, m/z (relative
intensity) 294 (10%, Mϩ), 190 (53), 175 (100), 45 (50), 43 (25).
(E )-2-(4-Hydroxy-3-methylbut-2-en-1-yl)-4-methoxy-1-
methoxymethoxybenzene (7)
Perkin Elmer polarimeter and are given in 10Ϫ1 deg cm2 gϪ1
.
Enantiomeric excesses were determined by HPLC analysis
6 (1.3 g, 4.4 mmol) was added to a solution of methanol
using a column packed with Chiracel OD–H 150 × 4.6.
(20 mL) and sat. aq. K2CO3 (2 mL). The mixture was stirred at
J. Chem. Soc., Perkin Trans. 1, 2002, 496–499
497

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rt for 2 h. Then water (20 mL) and diethyl ether (20 mL) were
added. The two layers were separated, the aqueous layer
was extracted with diethyl ether (20 mL). The combined
organic layer was dried over MgSO4. The solvent was evapor-
ated in vacuo. A colourless oily residue remained (1.09 g, 97%)
and it was used without further purification. Spectroscopic data
were in accordance with ref. 7.
used in the next reaction. δH 7.41–7.23 (m, 5H, 5CHar), 6.78–
6.56 (m, 3H, 3CHar), 4.56 (s, 2H, OCH2Ar), 3.37 (d, 1H, J 9.0,
CHOBn), 3.32 (d, 1H, J 9.0, CHOBn), 2.56–2.46 (m, 2H, CH2),
1.83–1.56 (m, 2H, CH2), 1.23 (s, 3H, CH3).
3R)-(؊)-(4-Benzyloxy-3-hydroxy-3-methylbutyl)-2-hydro-
quinone (12)
To a solution of 11 (0.12 g, 0.42 mmol) in acetone (40 mL) was
added Na2S2O4 (20 mL, 0.57 M aqueous solution). The mixture
was stirred at rt for 30 min. Then DCM (40 mL) was added.
The two layers were separated. The aqueous layer was extracted
with DCM (2 × 40 mL). The combined organic layer was dried
over MgSO4 and the solvent was evaporated in vacuo. The crude
product was purified by column chromatography (silica gel,
hexane–EtOAc, 5 : 5) to give hydroquinone 12 (0.116 g, 96%) as
a yellow oil. δH 7.41–7.23 (m, 5H, 5CHar), 6.71–6.53 (m, 3H,
3CHar), 4.56 (s, 2H, OCH2Ar), 3.37 (d, 1H, J 9.0, CHOBn),
3.31 (d, 1H, J 9.0, CHOBn), 2.62 (t, 2H, J 7.5, CH2Ar), 1.83–
1.73 (m, 2H, CH2), 1.22 (s, 3H, CH3). δC 149.7, 148.3, 137.5,
128.5, 128.4, 127.6, 116.8, 113.9, 77.8, 73.5, 72.1, 41.2, 24.1,
21.2 (Found: C, 71.30; H, 7.11%; C18H22O4 requires C, 71.50; H,
7.33%).
(2S,3S)-(؊)-2-(2,3-Epoxy-4-hydroxy-3-methylbutyl)-4-
methoxy-1-methoxymethoxybenzene (8)
See ref. 7.
(2S,3S)-(؊)-2-(4-Benzyloxy-2,3-epoxy-3-methylbutyl)-4-
methoxy-1-methoxymethoxybenzene (9)
To a solution of 8 (0.459 mg, 1.71 mmol) in THF (50 mL) was
slowly added NaH (0.072 g, 1.8 mmol) at 0 ЊC. Then, nBu4NI
(catalytic amount) and benzyl bromide (0.224 mL, 1.8 mmol)
were added. The mixture was stirred at rt for 3 h. Water (50 mL)
was added. The organic layer was extracted with diethyl ether
(2 × 30 mL). The combined organic layer was dried over
MgSO4 and the solvent was evaporated in vacuo. The crude
product was purified by column chromatography (silica gel,
hexane–EtOAc, 7 : 3) to give a colourless oil (0.54 g, 88%).
δH 7.41–7.23 (m, 5H, 5CHar), 7.07–6.68 (m, 3H, 3CHar), 5.12 (s,
2H, OCH2O), 4.55 (d, 1H, J 12, OCH2Ar), 4.48 (d, 2H, J 12,
OCH2Ar), 3.74 (s, 3H, OCH3), 3.52 (d, 1H, J 10, CH2OBn),
3.45 (s, 3H, OCH3), 3.44 (d, 1H, J 10, CH2OBn), 3.15 (t, 1H,
J 6.2, CHO), 2.91 (d, 2H, J 6.2, CH2), 1.46 (s, 3H, CH3).
δC 154.5, 149.3, 138.1, 128.3, 128.1, 127.8, 127.6, 116.0, 112.3,
95.3, 74.7, 73.0, 60.4, 60.1, 55.9, 55.6, 29.3, 14.7. MS, m/z
(relative intensity) 358 (22%, Mϩ), 205 (42), 91 (100), 45 (96),
43 (12). [α]2D2 Ϫ0.2 (c 1 in CHCl3).
(2R)-(؊)-6-Hydroxy-2-benzyloxy-2-methylchromane (13)
To a solution of 12 (0.069 g, 0.228 mmol) in toluene (15 mL),
p-TSA (catalytic amount) was added. The mixture was stirred
at 45 ЊC for 1 h. The solvent was evaporated in vacuo. The crude
product was purified by column chromatography (silica gel,
hexane–EtOAc, 8 : 2) to give 13 (0.063 g, 94%) as a yellow oil.
δH 7.41–7.23 (m, 5H, 5CHar), 6.70–6.51 (m, 3H, 3CHar), 4.62 (d,
1H, J 12.2, OCHAr), 4.55 (d, 1H, J 12.2, OCHAr), 3.50 (d, 1H,
J 9.6, CHOBn), 3.43 (d, 1H, J 9.6, CHOBn), 2.68 (t, 2H, J 6.6,
CH2Ar), 2.00 (dt, 1H, J 6.6, J 6.6, CHCq), 1.74 (dt, 1H, J 6.6,
J 6.6, CHCq), 1.64 (br s, 1H, OH), 1.22 (s, 3H, CH3). δC 148.7,
147.4, 138.3, 128.3, 127.6, 122.0, 117.7, 114.5, 75.7, 75.1, 73.5,
28.5, 22.4, 22.0. MS, m/z (relative intensity) 284 (65%, Mϩ), 163
(93), 123 (30), 107 (21), 91 (100), 65 (24). [α]2D2 Ϫ8.0 (c 1 in
CHCl3). ee = 85% (Found: C, 76.30; H, 7.01%; C18H20O3
requires C, 76.03; H, 7.09%).
(3R)-(؊)-2-(3,4-Dihydroxy-3-methylbutyl)-4-methoxy-1-
methoxymethoxybenzene (10a)
See ref. 7
(3R)-(؊)-2-(4-Benzyloxy-3-hydroxy-3-methylbutyl)-4-methoxy-
1-methoxymethoxybenzene (10b)
(2R)-(؊)-6-Hydroxy-2-hydroxymethyl-2-methylchromane (14)
Epoxide 9 (0.42 g, 0.74 mmol) was slowly added to a LiAlH4
solution (0.059 g 1.48 mmol) in diethyl ether (10 ml) at rt. The
mixture was stirred for 5 h. Water (25 ml) was added. The
organic layer was extracted with diethyl ether, dried over
MgSO4 and the solvent was evaporated in vacuo. The crude
product was purified by column chromatography (silica gel,
hexane–EtOAc, 7 : 3) to give a colourless oil (0.41 g, 97%).
δH 7.41–7.23 (m, 5H, 5CHar), 7.00–6.62 (m, 3H, 3CHar), 5.09 (s,
2H, OCH2O), 4.56 (s, 2H, OCH2Ar), 3.74 (s, 3H, OCH3), 3.45
(s, 3H, OCH3), 3.41 (d, 1H, J 8.8, CHOBn), 3.32 (d, 1H, J 8.8,
CHOBn), 2.72–2.63 (m, 2H, CH2), 1.86–1.74 (m, 2H, CH2),
1.66 (s, 3H, CH2). δC 154.5, 149.3, 138.2, 133.2, 128.4, 127.6,
115.8, 111.4, 95.4, 77.4, 73.5, 72.2, 56.0, 55.6, 39.5, 24.9, 23.7.
MS, m/z (relative intensity) 360 (11%, Mϩ), 328 (59), 298 (23),
137 (43), 91 (100), 65 (11), 45 (58). [α]2D2 Ϫ4.0 (c 1 in CHCl3).
ee = 90%.
To a solution of 13 (0.050 g, 0.17 mmol) in MeOH (10 mL),
10% palladium on carbon (catalytic amount) was added. The
mixture was stirred under a hydrogen atmosphere for 4 h. The
mixture was filtered and the solvent was evaporated in vacuo.
The crude product was purified by column chromatography
(silica gel, hexane–EtOAc, 5 : 5) to give 14 (0.028 g, 95%) as a
colourless oil. Spectroscopic data are in accordance with ref. 7.
[α]2D2 Ϫ8.1 (c 1.15 in acetone). ee = 85%.
Acknowledgements
The authors are indebted to the French Ministry of Education
and CNRS for financial support.
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