A convenient resolution method for 1,1′-bi-2-naphthol and 4,4′-dibromo-1,1′-spirobiindane-7,7′-diol with menthyl chloroformate in the presence of TBAB

Article abstract of DOI:10.1016/j.tetasy.2003.12.041

A convenient resolution method for 1,1′-bi-2-naphthol and 4,4′-dibromo-1,1′-spirobiindane-7,7′-diol has been developed with crude (-)-menthyl chloroformate as the resolution reagent in the presence of tetrabutylammonium bromide acting as a phase transfer catalyst in an aqueous NaOH/CH₂Cl₂ two phase solution. The deprotection of the hydroxy group was carried out via an aqueous KOH/EtOH solution. Both enantiomers of 1,1′-bi-2-naphthol and 4,4′-dibromo-1,1′- spirobiindane-7,7′-diol were obtained in high yields. Both ee's of (S)-(+) and (R)-(-)-4,4′-dibromo-1,1′-spirobiindane-7,7′-diol were above 99%. Most of the (-)-menthol could be easily recovered in a deprotection procedure and thus be reused for the formation of (-)-menthyl chloroformate without further purification.

Full text of DOI:10.1016/j.tetasy.2003.12.041

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 665–669
A convenient resolution method for 1,10-bi-2-naphthol and
4,40-dibromo-1,10-spirobiindane-7,70-diol with menthyl
chloroformate in the presence of TBAB
Zhian Li, Xinmiao Liang, Fan Wu and Boshun Wan*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
Received 4 November 2003; accepted 12 December 2003
Abstract—A convenient resolution method for 1,10-bi-2-naphthol and 4,40-dibromo-1,10-spirobiindane-7,70-diol has been developed
with crude ())-menthyl chloroformate as the resolution reagent in the presence of tetrabutylammonium bromide acting as a phase
transfer catalyst in an aqueous NaOH/CH2Cl2 two phase solution. The deprotection of the hydroxy group was carried out via an
aqueous KOH/EtOH solution. Both enantiomers of 1,10-bi-2-naphthol and 4,40-dibromo-1,10-spirobiindane-7,70-diol were obtained
in high yields. Both eeÕs of (S)-(+) and (R)-())-4,40-dibromo-1,10-spirobiindane-7,70-diol were above 99%. Most of the ())-menthol
could be easily recovered in a deprotection procedure and thus be reused for the formation of ())-menthyl chloroformate without
further purification.
ꢀ 2004 Elsevier Ltd. All rights reserved.
1. Introduction
for the resolution of 4,40-dibromo-1,10-spirobiindane-
7,70-diol (DBSPINOL) and BINOL because of the
abundance of nonracemic menthol. We found that the
reaction of BINOL or DBSPINOL with crude ())-
menthyl chloroformate in aqueous NaOH/CH2Cl2
solution in the presence of tetrabutylammonium bro-
mide (TBAB) was very fast (the two diastereomers could
be easily separated by recrystallization) and that the
deprotection of the hydroxy group of the diastereomers
could be performed via an aqueous KOH/EtOH solu-
tion instead of with LiAlH4. Herein, we report a con-
venient procedure for the resolution of BINOL and
DBSPINOL.
Enantiopure 1,10-bi-2-naphthol (BINOL) is one of the
most important C2 symmetric chiral compounds,1 and
occupies a prominent position in catalytic asymmetric
synthesis.2 It is widely used for the resolution of racemic
compounds3 and as a chiral source.4 Recently, a new C2
symmetrical diol, 1,10-spirobiindane-7,70-diol (SPI-
NOL), has proven to be an excellent framework for
chiral ligands.5 Several methods have successfully been
developed for the resolution of BINOL and other diols,
such as the classical crystallization of diastereomeric
derivatives,6 enantioselective formation of inclusion
crystals with chiral host molecules,7 and enzymatic
hydrolysis of esters.8 In 1995, Lucchi and co-workers
reported a resolution method for BINOL, based on the
separation of a pair of diastereomers, which were
derived from BINOL and ())-menthyl chloroformate
under anhydrous conditions, then deprotection of the
diastereomers with LiAlH4 to afford enantiopure
BINOL.9 This method has also been used for the reso-
lution of other racemic compounds, such as binaphthyl,
biphenyl diol systems.9;10 In our efforts toward opti-
mizing the electronic and steric properties of the chiral
ligands, we were particularly interested in this method
2. Results and discussion
Although ())-menthyl chloroformate is commercially
available, it can be conveniently prepared with com-
mercially available ())-menthol and triphosgene. In our
experiment, we found that the purity of ())-menthyl
chloroformate had no effect on the formation of dia-
stereomers with BINOL. After workup, crude ())-
menthyl chloroformate could be directly used for the
next step. It was found that the formation of diastereo-
mers could be greatly accelerated in the presence of
TBAB in aqueous NaOH/CH2Cl2 (reaction 1 in Scheme
1). Compared to LucchiÕs procedure,9 the formation of
* Corresponding author. Tel.: +86-411-4379260; fax: +86-411-46847-
46; e-mail: bswan@dicp.ac.cn
0957-4166/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2003.12.041

666
Z. Li et al. / Tetrahedron: Asymmetry 15 (2004) 665–669
CH2Cl2/NaOH(aq)
OR*
OH
OH
OR*
OR*
1
+
+
OR*
TBAB
OCOCl
a
b
( )-BINOL
O
C
KOH(aq)/EtOH
KOH(aq)/EtOH
reflux
reflux
R*
=
O
(R)-(+)-BINOL
(S)-(-)-BINOL
Br
Br
Br
CH2Cl2/NaOH(aq)
TBAB
OH
OH
OR*
OR*
OR*
OR*
+
2
+
OCOCl
Br
( )-DBSPINOL
Br
Br
c
d
KOH(aq)/EtOH
KOH(aq)/EtOH
reflux
reflux
(S)-(+)-DBSPINOL
(R)-(-)-DBSPINOL
Scheme 1.
diastereomers in our experiment was quicker. We tried
to optimize the reaction in other solvents, such as
toluene, benzene, and n-hexane; however, the reaction
did not go to completion under similar conditions as
evidenced by TLC. We then increased the reaction
temperature: the reaction proved to be faster at high
temperatures but at the same time produced more
impurities as detected by TLC. The optimal reaction
conditions for the formation of the diastereomers was in
aqueous NaOH/CH2Cl2 with 0.2 mol equiv of TBAB
and 3 mol equiv of ())-menthyl chloroformate at 0 ꢁC.
After phase separation and removal of solvent, the
crude mixture of the diastereomers was obtained in high
yield. We then tried to purify the crude diastereomers
via recrystallization in n-hexane. Successfully, one of the
diastereomers a precipitated from this recrystallization
procedure in high yield. This allowed us to avoid
column separation of diastereomers. The other zdiaste-
reomers b in the mother liquid was then separated
through a short silica-gel column with n-hexane/EtOAc
as the eluent, to give the diastereomer b. After carefully
optimizing the separation procedure, diastereomer a was
obtained in 90% yield and diastereomer b in 86% yield.
than 4 equiv of KOH for 1 h. After the hydrolysis was
completed, most of the EtOH was removed by distilla-
tion under reduced pressure. The residue was extracted
with n-hexane to remove ())-menthol. The aqueous
layer was then acidified with HCl. The resulting pre-
cipitate was extracted with diethyl ether. The diethyl
ether layer was dried over anhydrous Na2SO4, filtered,
and the solvent evaporated to afford (R)-BINOL in 92%
yield with >99% ee. Using a similar procedure of
treatment of diastereomer b, (S)-BINOL was obtained
in 96% yield with 91% ee. These results were close to the
procedure with LiAlH4.9 Moreover, ())-menthol in n-
hexane extraction, which had been dried over Na2SO4,
can be recovered in high yield after evaporation of sol-
vent. The recovered ())-menthol can be used directly to
prepare ())-menthyl chloroformate. Therefore, this
method provides the possibility of a large-scale resolu-
tion of BINOL. To the best of our knowledge, no such
liquid/liquid phase-transfer catalytic reaction has been
shown in the resolution of the BINOL using related
ammonium salts as catalysts.
In fact, our studies on the resolution method were
applied to the recently developed chiral backbone
DBSPINOL, which was derived from chiral SPINOL.5e
Because chiral SPINOL was proven to provide an
excellent framework for chiral ligands,5 we were inter-
ested in developing new chiral ligands by optimizing the
electronic and steric property of SPINOL derivatives for
asymmetric catalysis with DBSPINOL as the chiral
backbone. When enough racemic DBSPINOL was
prepared according to the literature procedure,12 we
tried to resolve DBSPINOL (reaction 2 in Scheme 1)
with the above improved resolution method for BINOL.
Using this similar procedure, (S)-(+)-DBSPINOL in
98% yield with >99% ee and (R)-())-DBSPINOL in 98%
Deprotection to form homochiral BINOL was usually
accomplished with LiAlH4 as reported in the litera-
ture.9;10a–e Very recently, our colleagues, Hu and
co-workers reported a method using KOH/EtOH to
perform deacetylation with no racemization being
observed in the biphenyl backbone.11 We also found
that deprotection of the hydroxy group of diastereomer
a can be performed at reflux in aqueous KOH/EtOH
solution instead of LiAlH4. Parallel experiments showed
that the quantity of water in the system was very
important for a successful hydrolysis of the diastereomer
and excess KOH was necessary to complete the reaction.
The deprotection of the diastereomers was carried out
smoothly at reflux in 1:4 (v/v) aqueous EtOH with more
yield with >92% ee were successfully obtained.13
simple recrystallization of the latter isomer from
A

Z. Li et al. / Tetrahedron: Asymmetry 15 (2004) 665–669
667
n-hexane afforded enantiopure (R)-())-DBSPINOL in
74% yield with >99% ee. Moreover, ())-menthol in
n-hexane extraction was recovered in high yield after
evaporation of solvent. The recovered ())-menthol can
then be directly reused. This resolution procedure is the
only method so far reported for DBSPINOL.
was cooled to )10 ꢁC. ())-Menthol (5.00 g, 32 mmol)
was then added into the flask. After it completely dis-
solved, pyridine (4.13 g, 32 mmol) was added dropwise
over a period of 30 min. After stirring for 18 h at room
temperature, the mixture was filtered off, and the filtrate
washed with water (2 · 60 mL). The organic layer was
dried over Na2SO4. Removal of the solvent under
reduced pressure afforded the crude ())-menthyl chlo-
roformate, which could be used for the next step without
further purification. The content of menthyl chlorofor-
mate was determined by titration.14
3. Conclusion
In conclusion, we have developed a convenient resolu-
tion method for BINOL with crude ())-menthyl chlo-
roformate in the presence of TBAB in aqueous NaOH/
CH2Cl2 solution. The deprotection of the hydroxy
group of the diastereomers can be smoothly performed
at reflux in aqueous KOH/EtOH solution. Racemic
DBSPINOL was successfully resolved with the above
method for resolution BINOL to afford enantiopure (S)-
(+)-DBSPINOL and (R)-())-DBSPINOL in high yield.
Moreover, ())-menthol in the deprotection procedure
can easily be recovered and reused. This improved
method provides the possibility of a large-scale resolu-
tion of racemic diols. Further studies concerning other
phase-transfer catalyst and using chiral DBSPINOL as
the backbone for asymmetric chiral ligand are currently
underway.
4.3. Resolution of BINOL
An aqueous solution of racemic BINOL15 (1.0 g,
3.49 mmol) and NaOH (0.6 g, 15.0 mmol) in water was
made up to 10 mL. To it 0.2 g (0.62 mmol) of tetra-
butylammonium bromide dissolved in 10 mL of chlo-
roform was added. With rapid stirring, ())-menthyl
chloroformate 2.84 g (10.5 mmol, 80.9% w/w) was
added. After stirring at room temperature for 5 min, the
two phases were separated. The aqueous phase was
extracted with CH2Cl2 (2 · 10 mL). The combined
organic phases were dried over Na2SO4 and evaporation
of the solvent under reduced pressure afforded a crude
product, which was recrystallized from n-hexane to give
the diastereomer of (R)-BINOL a 1.03 g, 90% yield. Mp
25
1
195–198 ꢁC; ½aꢀ ¼ )134.9 (c 1.1, CHCl3). H NMR d
D
0.17 (d, J ¼ 6:8 Hz, 6H), 0.53 (d, J ¼ 6:8 Hz, 6H), 0.68–
1.21 (series of m, 20H), 1.53 (d, 4H), 1.75 (t, 2H), 4.21
(td, 4.0 Hz, 2H), 7.25 (m, 4H), 7.43 (t, J ¼ 7:2 Hz, 2H),
7.49 (d, J ¼ 8:8 Hz, 2H), 7.90 (d, J ¼ 8:8 Hz, 2H), 7.99
(d, J ¼ 8:8 Hz, 2H). 13C NMR (CDCl3) 16.08, 20.50,
22.14, 23.54, 26.01, 31.47, 34.14, 40.54, 46.86, 78.98,
121.51, 123.47, 125.80, 126.44, 126.84, 127.99, 129.89,
131.79, 133.21, 146.96, 152.91. IR (KBr pellet, cmꢁ1):
2958, 2935, 1756, 1223, 1251, 962.
4. Experimental
4.1. General procedures
Melting points were measured on a Yazawa micro-
melting point apparatus and are uncorrected. Optical
rotations were measured on a HORIBA SEPA-200
1
highly sensitive polarimeter. H and 13C NMR spectra
were recorded on a Bruker DRX-400 spectrometer
(400 MHz). Elemental analyses were performed using an
Elementar vario EL instrument. High resolution mass
spectra (HRMS) were recorded on a Mariner-TOF 5303
(Applied Biosystems, USA). Enantiomeric excess values
were analyzed by HPLC on a Chiralcel AD column
(4.6 mm · 250 mm) at room temperature with n-hexane/
i-propanol (4:1) as eluent. All reagents were of com-
mercial quality and used as purchased. Flash chroma-
tography was carried out with silica gel 230–400 mesh
eluting with appropriate solution in the stated v/v pro-
portions. Analytical thin-layer chromatography (TLC)
was performed with 0.25 mm thick silica-gel plates. ())-
Menthol, boron tribromide, and triphosgene were pur-
chased from Aldrich Chem. Co. Dichloromethane was
distilled from CaH2 and toluene was distilled from
sodium and benzophenone for the preparation of
menthyl chloroformate and DBSPINOL.
The solution was evaporated to dryness, and the residue
chromatographed through a short column of silica gel
with n-hexane/EtOAc as eluent, to afford the diastereo-
mer of (S)-BINOL b, 0.98 g, 86% yield. Mp 60–65 ꢁC;
25
½aꢀ ¼ )19.1 (c 0.95, CHCl3). 1H NMR d 0.64 (d,
D
J ¼ 6:8 Hz, 6H), 0.77 (d, J ¼ 6:8 Hz, 12H), 0.45–1.21
(series of m, 14H), 1.53 (d, 4H), 4.20 (td, 4.0 Hz, 2H),
7.29 (d, J ¼ 4 Hz, 4H), 7.43 (m, J ¼ 4 Hz, 2H), 7.49 (d,
2H), 7.89 (d, 2H), 7.98 (d, 2H). 13C NMR (CDCl3)
16.44, 20.79, 22.00, 23.40, 26.05, 31.23, 34.12, 39.74,
46.56, 79.07, 121.54, 123.49, 125.80, 126.46, 126.82,
127.98, 129.91, 131.77, 133.22, 147.08, 152.71. IR (KBr
pellet, cmꢁ1): 2958, 2935, 1756, 1223, 1251, 962.
To a solution of KOH (8.7 g, 155 mmol) in 4:1 ethanol/
degassed water (180 mL), 1.03 g (1.58 mmol) of a was
added and the mixture refluxed for 1 h after which none
of a remained (TLC). The mixture was then cooled and
rotoevaporated. To it was added 20 mL of water and
extracted with n-hexane (2 · 30 mL). Simple workup
gave enantiomerically pure ())-menthol (0.48 g, 97%
recovery yield). The aqueous layer was separated and
acidified with concentrated HCl resulting in a white
precipitate that was extracted with diethyl ether
4.2. Preparation of menthyl chloroformate
Triphosgene (3.16 g, 11 mmol) and toluene (60 mL) were
added into a 100 mL three-necked round-bottom flask
fitted with addition funnel and argon inlet. The solution

668
Z. Li et al. / Tetrahedron: Asymmetry 15 (2004) 665–669
(2 · 30 mL). The ether layer was dried over Na2SO4,
filtered off, and the solvent evaporated in vacuo to give
37.9, 40.5, 46.7, 61.8, 79.2, 116.3, 122.1, 131.2, 140.2,
145.4, 146.8, 152.3. Anal. Calcd for C39H50Br2O6: C,
60.47; H, 6.51. Found: C, 60.48; H, 6.66.
(R)-(+)-BINOL 0.42 g, 92% yield. Mp 210–211 ꢁC;
25
½aꢀ ¼ +34.6 (c 1.0, THF) with >99% ee. Similar treat-
25
D
ment of b (0.858 g, 1.32 mmol) gave (S)-())-BINOL
To solution of KOH (8.7 g, 155 mmol) in 10% degassed
water/ethanol (180 mL) 1.0 g of c (1.3 mmol) was added
and the mixture refluxed for 1 h after which none of c
remained (TLC). The mixture then cooled and roto-
evaporated. To it was added 20 mL of water and
extracted with n-hexane (2 · 30 mL). Simple workup
gave enantiomerically pure ())-menthol (0.40 g, 99%
recovery yield). The aqueous layer was separated and
acidified with 6 M HCl, producing a white precipitate
that was extracted with diethyl ether (2 · 30 mL). The
ether layer was dried over Na2SO4, filtered off and the
0.36 g, 96% yield. Mp 209–210 ꢁC; ½aꢀ ¼ )30.9 (c 1.0,
D
THF) of 91% ee and enantiomerically pure ())-menthol
(0.40 g, 97% recovery yield).
4.4. Preparation of DBSPINOL12
A solution of 4,40-dibromo-7,70-dimethoxy-1,10-spiro-
biindane16 (1.35 g, 2.71 mmol) in 15 mL of CH2Cl2 in a
flame-dried flask, under nitrogen, was cooled to )78 ꢁC,
treated with 1 M BBr3 in CH2Cl2 (9.0 mL, 2.9 equiv),
and allowed to warm to rt overnight. The reaction
mixture was diluted with CH2Cl2 and washed with water
until the washings had a neutral reaction. The solution
was dried over Na2SO4, evaporated, and the residue
recrystallized from n-hexane to give 1.20 g of the racemic
solvent evaporated in vacuo to give (S)-(+)-DBSPINOL
18
D
0.52 g, 98% yield. Mp 161–162 ꢁC; ½aꢀ ¼ +93.9 (c 0.5,
THF) with >99% ee. HRMS calcd for C17H14Br2O2
(M)1)ꢁ: 406.92768; found: 406.92588. The rest of the
physical data was identical in all respects to the race-
mate.
1
DBSPINOL (95% yield). Mp 148–149 ꢁC. H NMR d
2.24 (m, 2H), 3.00 (m, 4H), 4.63 (S, 2H), 6.55 (d,
J ¼ 8:4 Hz, 2H), 7.27 (d, J ¼ 8:4 Hz, 2H). 13C NMR
(CDCl3) 33.0, 37.0, 60.6, 111.1, 116.7, 132.4, 132.6,
145.5, 152.0. HRMS calcd for C17H14Br2O2 (M)1)ꢁ:
406.92768; found: 406.92779.
Similar hydrolysis treatment of d (1.0 g, 1.3 mmol) gave
(R)-())-DBSPINOL 0.52 g, 98% yield with >92% ee,
followed by recrystallization from n-hexane to give (R)-
())-DBSPINOL 0.40 g, 74% yield. Mp 160–161 ꢁC;
18
½aꢀ ¼ )93.5 (c 0.5, THF) with above 99% ee and
D
enantiomerically pure ())-menthol (0.40 g, 99% recovery
yield). HRMS calcd for C17H14Br2O2 (M)1)ꢁ:
406.92768; found: 406.92866. The rest of the physical
data was identical in all respects to the racemate.
4.5. Resolution of DBSPINOL
A solution of racemic DBSPINOL (1.388 g, 3.38 mmol)
and NaOH (0.6 g, 15 mmol) in water was made up to
10 mL. To it, 0.50 g (1.56 mmol) of tetrabutylammonium
bromide dissolved in 10 mL of chloroform were added.
With rapid stirring, ())-menthyl chloroformate 3.16 g
(10.1 mmol, 70.1% w/w) was added. After the solution
was stirred at room temperature for 7 min, the two
phases were separated. The aqueous phase was extracted
two times with CH2Cl2. The combined organic phases
were dried over Na2SO4 and evaporation of the solvent
under reduced pressure afforded a crude product, which
was recrystallized from n-hexane to give the diastereo-
Acknowledgements
We thank Prof. Xinquan Hu and Prof. Yonggui Zhou
for helpful discussions.
References and notes
mer of (S)-(+)-DBSPINOL c,13 1.175 g, 89% yield. Mp
25
D
220–222 ꢁC; ½aꢀ ¼ )91.0 (c 1.0, CHCl3). 1H NMR
1. For a review of C2-symmetry in asymmetric synthesis, see:
Whitesell, J. K. Chem. Rev. 1989, 89, 1581.
(CDCl3) d 0.71 (d, 6H), 0.81–0.98 (m, 18H), 1.30–1.40
(m, 4H), 1.63 (d, 6H), 1.85 (d, 2H), 2.24–2.28 (m, 4H),
2.97–3.07 (m, 4H), 4.36 (d, 2H), 6.87 (d, J ¼ 8:8, 2H),
7.35 (d, J ¼ 8:8, 2H). 13C NMR (CDCl3) 16.0, 20.6,
21.9, 23.2, 26.0, 31.2, 32.8, 34.0, 38.4, 40.1, 46.6, 61.6,
61.8, 116.4, 122.5, 131.2, 140.4, 145.4, 146.7, 152.6.
Anal. Calcd for C39H50Br2O6: C, 60.47; H, 6.51. Found:
C, 60.35; H, 6.53.
2. For reviews of BINOL on asymmetric catalysis, see: (a)
Rosini, C.; Franzini, L.; Raffaeli, A.; Salvadori, P.
Synthesis 1992, 503; (b) Noyori, R. Asymmetric Catalysis
in Organic Synthesis; Wiley: New York, 1993; (c) Ojima, I.
Asymmetric Catalysis; VCH: New York, 2000; (d) Pu, L.
Chem. Rev. 1998, 98, 2405; (e) Shibasaki, M.; Sasai, H.;
Arai, T.. Angew. Chem., Int. Ed. Engl. 1997, 36, 1236.
3. For enantiomer separation, see examples (a) Kyba, E. P.;
Gokel, G. W.; De Jong, F.; Koga, K.; Sousa, L. R.; Siegel,
M. G.; Kaplan, L.; Sogah, G. D. Y.; Cram, D. J. J. Org.
Chem. 1977, 42, 4173; (b) Cram, D. J.; Helgeson, R. C.;
Peacock, S. C.; Kaplan, L. J.; Domeier, L. A.; Moreau, P.;
Koka, K.; Mayer, J. M.; Chao, Y.; Siegel, M. G.;
Hoffman, D. H.; Sogah, G. D. Y. J. J. Org. Chem. 1978,
43, 1930; (c) Judice, J. K.; Keipert, S. J.; Cram, D. J. J.
Chem. Soc., Chem. Commun. 1993, 1323.
The solution was rotoevaporated to dryness, and the
residue chromatographed through a short column of
silica gel with n-hexane/EtOAc to afford the diastereo-
mer of (R)-())-DBSPINOL d, 1.171 g, 89% yield. Mp
1
49–53 ꢁC. H NMR (CDCl3) d 0.68 (d, 6H), 0.81–0.98
(m, 18H), 1.24 (t, 2H), 1.41 (s, 2H), 1.64 (d, 6H), 1.88 (d,
2H), 2.25–2.34 (m, 4H), 2.99–3.06 (m, 4H), 4.32 (d, 2H),
6.92 (d, J ¼ 8:4, 2H), 7.35 (d, J ¼ 8:4, 2H). 13C NMR
(CDCl3) 16.4, 21.0, 22.3, 23.3, 25.9, 31.5, 33.0, 34.2,
4. For the application of the synthesis of materials, see Ref.
1; for other examples, see: (a) Brunner, H.; Schiessling, H.
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13. The absolute configuration of (S)-(+)-DBSPINOL was
assigned as follows. Method A: We double-checked the
specific rotation data of our (+)-DBSPINOL and com-
pared the rotation sign of the literature.5e Method B:
Debromination of diastereomer c with n-BuLi followed by
hydrolysis afforded (S)-())-SPINOL.16 Thus we can con-
firm the absolute configuration of (+)-DBSPINOL to be
S.
14. Weigh accurately the crude ())-menthyl chloroformate
samples (0.3563 g) dissolved in a solution of 2.0 mL of
water and 0.3 mL of pyridine. The mixture was stirred at
room temperature for 10 min. Then, nitric acid (20 mL,
6.0 mol Lꢁ1), 3.0 mL of NH4Fe(SO4)2 indicator (saturated,
3.0 mL), and AgNO3 (20.00 mL, 0.1 mol Lꢁ1) were added.
The contents were then titrated by adding NH4SCN
(0.1022 mol Lꢁ1) to a pink end point, 5.15 mL of NH4SCN
be added. The blank value was 18.05 mL of
NH4SCN through the whole procedure without pyridine.
The ())-menthyl chloroformate was calculated from
%menthyl chloroformate
M
ðVblank ꢁ Vpyridine
0:3563
Þ
NH4SCN
¼
ꢂ 0:2187 ꢂ 100%
¼ 80:9%
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