A straightforward and practical approach to chiral inducer: (2R,3R)-1,4-dimethoxy-1,1,4,4-tetraphenylbutane-2,3-diol

Article abstract of DOI:10.1071/CH14505

A straightforward and practical access to chiral inducer (2R,3R)-1,4- dimethoxyl-1,1,4,4-tetraphenyl-2,3-diol has been developed. It is based on highly regioselective 2,3-cyclosulfitation of (2R,3R)-1,1,4,4-tetraphenyl-butanetetraol, in which selective pr

Full text of DOI:10.1071/CH14505

CSIRO PUBLISHING
Aust. J. Chem.
Full Paper
http://dx.doi.org/10.1071/CH14505
A Straightforward and Practical Approach
to Chiral Inducer: (2R,3R)-1,4-Dimethoxy-
1,1,4,4-tetraphenylbutane-2,3-diol
A
A C
,
B C
,
Jin-Sheng Xie, Xiao-Yun Hu,
Zi-Xing Shan,
A
and Zhong-Qiang Zhou
A
College of Chemistry and Materials, South-Central University for Nationalities,
Wuhan, China.
B
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, China.
C
Corresponding authors. Email: xyhu@mail.scuec.edu.cn, zxshan@whu.edu.cn
A straightforward and practical access to chiral inducer (2R,3R)-1,4- dimethoxyl-1,1,4,4-tetraphenyl-2,3-diol has been
developed. It is based on highly regioselective 2,3-cyclosulfitation of (2R,3R)-1,1,4,4-tetraphenyl-butanetetraol, in which
selective protection of the secondary hydroxyls and chlorination of the tertiary hydroxyls of (2R,3R)-1,1,4,4-tetraphenyl-
butanetetraol are accomplished via one step. In the preparation, methanol was used as a methylating reagent and common
alkali liquor was used for cleavage of the protection group. It may be one of the most straightforward and practical
syntheses of the title compound.
Manuscript received: 14 August 2014.
Manuscript accepted: 8 October 2014.
Published online: 22 December 2014.
Introduction
spiroborate salt, then a methylation procedure with NaH/MeI
was employed, and deprotection of the spiroborate salt was
accomplished by hydrolysis with HF. In all the above prepara-
tions, NaH/MeI was employed in the methylating step, and the
so-called improvements are mainly focussed on the protection
and deprotection procedures. Though our approach avoids
tedious oxidation and reduction procedures with DDQ and
LiAlH4, unpopular HF is required. Furthermore, toxic and
expensive reagent MeI is employed in the both synthesis routes.
Herein, we report a more straightforward and practical
synthesis of (2R,3R)-1 based on highly selective 2,3-cyclosulfi-
tation of (2R,3R)-2 (Scheme 1). In this program, thionyl is used
as protecting group, methanol serves as methylating reagent,
and removal of the thionyl group is achieved by NaOH liquor.
For this preparative process, common reagents were used,
oxidation and reduction procedures were avoided, and both
protection and deprotection steps were easy to implement. The
current approach may be one of the most practical strategies to
prepare (2R,3R)-1 to date.
(2R,3R)-1,4-Dimethoxy-1,1,4,4-tetraphenyl-2,3-diol 1, a C2-
symmetrical sterically hindered chiral diol, has found diverse
applications in asymmetric organic synthesis.[1] Among these,
(2R,3R)-1 as a chiral auxiliary in chiral boronic acids chemistry
has been investigated extensively. It has been reported as a
protective group for boronic acid in cyclopropane reactions,[2]
epoxidation reactions,[3] [3.3]-Sigmatropic rearrangements,[4–6]
and carbonyl allylation reactions.[7,8] Recently, it has been
applied in the synthesis of the Solandelactones A–H,[9] rugu-
lactone,[10] analogues of combretastatin A4,[11] and dihydro-a-
pyrone-containing natural products.[12]
Though (2R,3R)-1 has been widely used in asymmetric
synthesis, studies on its synthesis are rarely reported. The first
synthesis of (2R,3R)-1 was put forward by Nakayama and
Rainier as early as in 1990,[13] in which the secondary hydroxyl
groups of (2R,3R)-tartrate ester were first protected by forming
benzylidene acetal, then phenyl groups were introduced via
Grignard reaction. Subsequently, the freshly formed tertiary
hydroxyls were methylated with methyl iodide (MeI) and NaH,
and finally the protection group was removed by oxidation
and reduction procedures with 2,3-dichloro-4,5-dicyano-1,4-
benzoquinone (DDQ) and LiAlH4 to afford (2R,3R)-1. After
several years, Srimurugan et al.[14] and later Pietruszka and
coworkers[15] reported an improved synthesis of (2R,3R)-1. In
their preparation, toxic and expensive DDQ was replaced by
inexpensive inorganic salts, although LiAlH4 was still required.
Very recently, our group reported another approach[16] to
(2R,3R)-1 based on selective 2,3-spiroboration of (2R,3R)-
1,1,4,4-tetraphenyl-butanetetraol 2, in which the secondary
hydroxyl groups of (2R,3R)-2 were protected by formation of
Results and Discussion
During our ongoing research on the chemistry of chiral 1,1,4,4-
tetrasubstituted butanetetraol,[17–21] we found that highly
selective 2,3-cyclosulfitaton of (2R,3R)-2 preferentially
occurred with thionyl chloride to form (4R,5R)-4,5-bis
(diphenylhydroxymethyl)-1,3,2-dioxathiolane-2-oxide 5, and
sequentially, the resulting (4R,5R)-5 could be chlorinated step
by step to offer (4R,5R)-3. As shown in Scheme 2, (4R,5R)-3
also could be obtained via one step using excess thionyl chloride
in high yields.[17] So, selective 2,3-cyclosulfitation and
Journal compilation Ó CSIRO 2014
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B
J.-S. Xie et al.
Ph
Ph
Ph Ph
Ph
Cl
COOEt
COOEt
HO
HO
HO
C3
C4
1) PhMgBr, THF
O
O
SOCl2/Py
OH
OH
Ph Ph
C21
C19
O
S
2) NH Cl(aq)
Cl
Ph
4
C2
C1
HO
C22
C23
C20
O2
S1
2
3
C5
O4
Ph
Ph
Ph Ph
C15
HO
HO
C6
O
O
MeO
MeOH/Py
NaOH/THF
C13
OMe
OMe
C18
C24
O
O3
C17
S
C7
C8
MeO
Ph
Ph
Ph
C12
C11
C16
Ph
1
C14
4
O5
45 % over 4 steps
O1
C25
C29
Scheme 1. Synthesis of (2R,3R)-1 based on selective 2,3-cyclosulfitation
of (2R,3R)-2.
C9
C10
C28
C30
C26
C27
Ph
Ph
O
O
HO
Fig. 1. ORTEP of (4R,5R)-4.
S
O
HO
Ph
/Py
2
In the presence of organic bases, reaction of (4R,5R)-3 with
MeOH was examined. A methanol solution of (4R,5R)-3 and two
equivalents of pyridine (Py) were allowed to reflux for several
hours until (4R,5R)-3 was completely reacted (detected by TLC).
The resulting methanol solution was concentrated to give a pale
yellow crystal, which was pure enough for NMR analysis. The
1H NMR spectra show that there are one set of aromatic proton
resonances from 7.45 to 7.30 (m) ppm and four sets of non-
aromatic proton resonances at 5.87 (d), 5.54 (d), 2.64 (s), and
2.30(s)ppm,andtheintensityratioofthe signalsis20: 1 : 1 : 3 : 3.
Furthermore, the 13C NMR spectra exhibit four types of aliphatic
carbons at 90.8, 88.9, 57.9, and 56.5ppm. The spectral character-
ization indicated that C–Cl bonds of (4R,5R)-3 were broken and
C–O bonds formed as expected. This result was confirmed by
single-crystalX-raydiffraction.Thepaleyellowcrystalsobtained
above were dissolved in hot methanol and then cooled slowly
to room temperature; a colourless crystal suitable for X-ray
diffraction analysis was obtained. The crystallographic data
revealed that it was (4R,5R)-4,5-bis(methoxydiphenylmethyl)-
1,3,2-di-oxathiolane-2-oxide 4 (Fig. 1). This is the first time that
the crystal structure parameters of (4R,5R)-4 are obtained.
In Fig. 1, we can see that the two methoxyl groups, CH3O (O1)
and CH3O (O5), and the two C–H bonds, H–C (C15) and H–C
(C16), are located at both sides of the five-membered ring,
respectively, whereas the thionyl group, S1¼O4, is located at
one side of the five-membered ring. Obviously, this asymmetrical
distribution of the two methoxyl groups relative tothe sulfite group
results in non-equivalence for the proton and carbon resonances in
the two methoxyl groups and two C–H bonds in (4R,5R)-4.
It should be pointed out that, Srimurugan et al. reported that
cyclic sulfite (4R,5R)-4 was synthesized as a colourless oil via
an unusual methanesulfonylation reaction of (2R,3R)-1[14] in
2007. However, Srimurugan’s product is very different from our
product in many aspects such as state, melting point, and optical
rotation value, but both products exhibit the same optical
rotation direction.
Ph
2.3
SOCl
1.1
:
(4R,5R)-5
Ph
Ph
Ph
Ph
Ph
Ph
Cl
HO
HO
O
O
SOCl2/Py
3.2:6.5
OH
OH
S
O
Cl
Ph
Ph
(2R,3R)-2
(4R,5R)-3
Scheme 2. Synthesis of (4R,5R)-3.
Ph
Ph
Ph
Ph
O
O
O
Cl
O
O
Cl
Saturated K2CO3/THF
Reflux
S
O
S
Cl
Cl
O
Ph
Ph
Ph
Ph
Scheme 3. Examination of the behaviour of (4R,5R)-3.
chlorination of the two tertiary hydroxyl groups at 1,4-position
of (2R,3R)-2 could be achieved in one step by changing the ratio
of the reactants.
It is well known that alkyl halides are typical key intermedi-
ates for organic synthesis. The C–Cl bond is polarized and
readily broken upon attack of a nucleophile, such as methanol
(MeOH), so methoxyl groups could be introduced into (4R,5R)-
3 by a nucleophilic substitution reaction with MeOH, thereby
avoiding the traditional methylation procedure with NaH/MeI.
Nevertheless, preliminary examination on the behaviour of
(4R,5R)-3 showed that it was relatively stable compared with
its analogues, 2,3-ketone-protected chloro-substituted TAD-
DOLs (a,a,a,a-tetraaryl-1,3-dioxolane-4,5-dimethanols),[22]
which are highly reactive and readily undergo solvolysis even
on silica gel.[23,24] However, (4R,5R)-3 could be purified via
re-crystallization from ethanol and did not suffer solvolysis in
refluxing ethanol. Moreover, (4R,5R)-3 did not undergo ring-
opening and cleavage of the C–Cl bonds in 2 M NaOH at room
temperature, but was oxidized to the corresponding sulfate ester
in air.[17] Further studies revealed that it did not undergo
solvolysis even in refluxing solution of THF and saturated
K2CO3. In contrast, it was oxidized to the sulfate ester
(Scheme 3). These results revealed that (4R,5R)-3 had unusual
stability.
Hydrolysis of cyclic sulfite (4R,5R)-4 was also examined.
A THF solution of (4R,5R)-4 and 2 M NaOH were mixed to give
a homogenous solution and stirred overnight at room tempera-
ture. TLC detection indicated that the hydrolysis nearly did not
take place. The homogenous solution was heated to reflux for
several hours, (4R,5R)-4 was completely hydrolyzed (detected
by TLC). Diluted hydrochloric acid was added to adjust the pH
value to nearly neutral, and THF was removed. (2R,3R)-1 was

(2R,3R)-1,4-Dimethoxy-1,1,4,4-tetraphenylbutane-2,3-diol
C
Ph
Ph
Ph
Ph
Ph
Ph
Ph
OϪ
OH
MeO
OHϪ
O
MeO
O
O
MeO
O
O
S
O
S
S
O
MeO
Ph
OϪ
OH
MeO
Ph
MeO
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
MeO
OϪ
O
H2O
MeO
O
O
OHϪ
S
S
OH
MeO
Ph
OH OH
OH
MeO
Ph
OH
Ph
Ph
Ph
Ph
Ph
Ph
OϪ
OH
MeO
OH
OH
MeO
H2O
MeO
Ph
MeO
Ph
Ph
Scheme 4. Possible hydrolysis mechanism for (4R,5R)-4.
obtained as a white solid, which was pure enough for 1H NMR
analysis after washing with cold ethanol.
(200 mL). The pressure-equalizing dropping funnel was charged
with a solution of phenyl bromide (52.3 mL, 0.5 mol) in dry THF
(100 mL). A quarter of the solution was added to the two-neck
round-bottom flask and stirred vigorously. After initiation of the
Grignard reaction, the solution was added dropwise so that THF
was gently refluxing. After complete addition, the mixture was
stirred vigorously for 2 h and refluxed for an additional 1 h,
then cooled to 08C, followed by careful dropwise addition of a
solution of (2R,3R)-diethyl tartrate (12.9 g, 62.5 mmol) in 50 mL
dry THF under vigorous stirring. After addition, the mixture was
allowed to stir for another 1 h, and then warmed up to reflux for
1.5 h. After cooling to room temperature, 380 mL saturated
aqueous NH4Cl was added with stirring. The organic layer was
separated and the aqueous layer was extracted with diethyl ether
(Et2O; 100 mL ꢁ 3). The combined extracts were dried over
Na2SO4 followed by nearly complete removal of the solvent.
Then, steam distillation was carried out and the residue was
stirred and washed with 50 mL 80 % ethanol to yield 13.8 g of
the crude product (2R,3R)-2 as a white solid, which was used in
the next step without further purification.
Measurement of optical rotation for (2R,3R)-1 revealed
that (4R,5R)-4 did not undergo racemization even under heating
in a strong alkali solution, thereby indicating that the chiral
centres of (4R,5R)-4 were not involved during the hydrolysis
process. ApossiblehydrolysismechanismisshowninScheme4,
where sulfur rather than the carbon centre was attacked by
nucleophile OHꢀ.
In summary, we report a straightforward and practical
approach to synthesize chiral inducer (2R,3R)-1 based on highly
selective 2,3-cyclosulfitation of (2R,3R)-2. In this synthesis,
the preparation procedure of (2R,3R)-1 was greatly simplified:
protection of the secondary hydroxyl groups and chlorination of
the tertiary hydroxyl groups of (2R,3R)-2 were accomplished via
one step; the traditional methylation procedure with MeI
and NaH was replaced by nucleophilic substitution with MeOH;
and the tedious deprotection procedure via oxidation and reduc-
tion steps was replaced by a simple hydrolysis with common
alkaline liquor. Moreover, toxic, unpopular, and expensive
reagents MeI, HF, DDQ, and LiAlH4 were all avoided. So, the
current preparation may be considered as the most practical and
concise approach to preparing (2R,3R)-1 by far.
Preparation of (4R,5R)-3
In a 250 mL dry round-bottom flask, (2R,3R)-2 (13.8 g,
32.4 mmol), pyridine (18 mL, 226.8 mmol), and dried THF
(150 mL) were added. The flask was sealed with a rubber septum
and stirred in an ice-bath for 0.5 h. Then, thionyl chloride
(7.5 mL, 103.7 mmol) was added slowly with a syringe. After
complete addition, the mixture was stirred for additional 3 h in
an ice-bath. The resultant was treated with water (100 mL), and
the organic phase was separated. The aqueous phase was
extracted with Et2O (50 mL ꢁ 3). The above organic solution
was combined and dried with Na2SO4, then the solvents were
removed under reduced pressure to afford 15.2 g of the crude
product (4R,5R)-3 as a white solid, which was used in the next
step without further purification.
Experimental
General
Diethyl (2R,3R)-tartrate was prepared from (2R,3R)-tartaric
acid and ethanol. Commercially available starting materials
were used without further purification if not specified.
NMR spectra were recorded at 300 MHz for 1H, and 75 MHz
for 13C on a Varian Mercury VS 300 (ppm, relative to TMS).
Optical rotations were measured on a Perkin–Elmer 341 Mc
polarimeter. Melting points were determined on a VEB Wage-
technik Rapio PHMK 05 and are uncorrected.
Preparation of (2R,3R)-2
Preparation and Single-Crystal X-Ray Diffraction
Analysis of (4R,5R)-4
Under Ar, a freshly dried two-neck round-bottom 1 L flask
equipped with a magnetic bar, a 250 mL pressure-equalizing
dropping funnel, and a reflux condenser with oil seal was
charged with Mg turnings (12.24 g, 0.51 mol) and dry THF
To a 250 mL round-bottom flask fitted with a stirring magnetic
bar, 15.2 g (29.9 mmol) (4R,5R)-3, 4.9 mL pyridine (60 mmol),

D
J.-S. Xie et al.
and 150 mL MeOH were added, and the mixture was refluxed
for 5 h. After removal of the solvent under reduced pressure,
50 mL H2O was added and stirred vigorously at room temper-
ature to give 14.3 g of the crude product (4R,5R)-4 as a pale
yellow solid, which could be directly used in the next step
without further purification.
The crude product obtained above was re-crystallized with
MeOH to afford colourless crystal, mp 126–1298C, [a]2D5 ꢀ2108
(c 1.0 in CH2Cl2). dH (CDCl3) 7.46–7.25 (20H, m, ArH), 5.86
(1H, d, J 1.8, CH), 5.54 (1H, d, J 1.5, CH), 3.00 (3H, s, OCH3),
2.64 (3H, s, OCH3). dC (CDCl3) 145.7, 144.9, 144.7, 143.7,
135.4, 134.7, 134.5, 134.2, 133.1, 132.9, 132.8, 132.4, 131.7,
91.4, 89.5, 88.6, 58.5, 57.1.
of anisotropic thermal parameters, hydrogen coordinates, bond
lengths, and bond angles have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary publi-
cation no. CCDC-979502. Data can be obtained free of charge,
on request, from the Director, Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge, CB2 1EZ, UK; fax: (þ44)
1223-336-033; e-mail: deposit @ccdc.cam.ac.uk or at http://
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work was financially supported by the National Nature Science
Foundation of China (No. 21302233) and the Nature Science Foundation of
Hubei Province of China (No. 2012FFB07410).
A single crystal suitable for X-ray structural analysis was
obtained by slowly cooling a hot methanol solution of (4R,5R)-4
to room temperature. A colourless crystal wasmounted on a glass
fibre. X-Ray diffraction intensity data collection and cell refine-
ment were performed on a Bruker P4 four-circle diffractometer
equipped with a graphite monochromator. A total of 5263 unique
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ꢀ3
difference Fourier map correspond to 0.551 and ꢀ0.249 e A
.
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diffraction data were collected orthorhombic, space group
˚
P2(1)2(1)2(1), a 9.171(5), b 16.692(10), c 17.498(10) A, a 90,
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b 90, g 908, V 2679(3) A , Z 4, Dc 1.241 g cmꢀ3, m 0.158 mmꢀ1
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Preparation of (2R,3R)-1
In a round-bottom flask fitted with a stirring magnetic bar, a THF
(150 mL) solution of (4R,5R)-4 (14. 3 g, 28.6 mmol) was mixed
with 2 M NaOH (60 mL) to give a homogenous solution. The
solution was refluxed for ,5 h, cooled to room temperature, and
the solution pH adjusted to neutral with diluted HCl. Then, the
organic layer was separated and the aqueous layer was extracted
with Et2O (50 mL ꢁ 3). The combined extracts were dried over
Na2SO4 and the solvents were removed under reduced pressure
to yield 12.7 g of(2R,3R)-1 asa white solid (45 %over 4 steps). It
was pure enough for 1H NMR analysis after washing with cold
ethanol, mp 78–808C (lit.[15] 76–798C), [a]1D5 ¼ þ59.6 (c 1.0 in
CHCl3). dH (CDCl3) 7.25–7.44 (20H, m, PhH), 4.71 (2H, d,
J 3.3, CH), 3.16 (6H, s, OCH3), 2.74 (2H, br, OH). dC (CDCl3)
142.8, 141.5, 129.0, 128.3, 128.1, 128.0, 127.5, 127.7, 85.4,
71.3, 53.7. m/z (electrospray ionization) 477 [Mþ23].
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Supplementary Material
1H NMR and 13C NMR spectra can be found on the Journal’s
website. Final atomic coordinates of the crystal, along with lists