A study of transesterification of chiral (-)-pinanediol methylboronic ester with various structurally modified diols

Article abstract of DOI:10.1007/s00706-007-0681-7

The transesterification of chiral (-)-pinanediol methylboronic ester was studied with various structurally modified diols by ¹H NMR to understand the factors influencing the unusual stability of this boronic ester as well as to find ways of recovering pinanediol from its methylboronic ester. In all the cases, reactions were allowed to proceed to equilibrium. The preliminary experiments indeed have shown some encouraging results (displacement of pinanediol up to 40-53%). Amongst cyclopentane-based cis-1,2-diols, endo-2-phenyl-exo,exo-2,3-norbornane-diol appeared to be the most effective diol in displacing pinanediol (38%). In the cases of pinane-based diols, the best result was obtained with 2-ethyl-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol (53%). It was interesting to observe that the transesterification with 2-phenyl-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol resulted in a 50% conversion after 4 days only, whereas the former diol took 24 days to reach equilibrium.

Full text of DOI:10.1007/s00706-007-0681-7

Monatshefte fu¨r Chemie 138, 747–753 (2007)
DOI 10.1007/s00706-007-0681-7
Printed in The Netherlands
A Study of Transesterification of Chiral (–)-Pinanediol Methylboronic Ester
with Various Structurally Modified Diolsy
Chandra D. Roy1;2;ꢀ and Herbert C. Brown1
1
H. C. Brown Center for Borane Research, Department of Chemistry, Purdue University, IN, USA
2
EMD Biosciences Inc., San Diego, CA, USA
Received February 18, 2007; accepted (revised) March 11, 2007; published online May 23, 2007
# Springer-Verlag 2007
Summary. The transesterification of chiral (ꢁ)-pinanediol
rectors have been used extensively by Brown and
methylboronic ester was studied with various structurally mod-
ified diols by H NMR to understand the factors influencing
metric homologation of boronic esters. The stability
the unusual stability of this boronic ester as well as to find
Zweifel [5] and Matteson [6], especially in the asym-
1
of boronic esters is a key factor both in the protec-
ways of recovering pinanediol from its methylboronic ester. In
tion of diols as well as in introducing and retrieving
the chiral auxiliaries in a chiral auxiliary directed
multistep organic synthesis. Pinanediol boronic es-
ters appeared to be remarkably resistant toward hy-
drolysis, transesterification, or ligand exchange,
which posed tremendous difficulties in introducing
new chiral director of interest or recovering the
chiral auxiliary. Transesterification is a convenient
and gentle procedure that not only can introduce
but also recover the chiral auxiliary to or from boron-
ic ester, provided the former boronic ester is thermo-
dynamically less stable than the latter. Although, we
and others developed few procedures for the recovery
of pinanediol from pinanediol boronic esters [7],
there is only one report on the successful removal of
pinanediol as pinanediol phenylboronic ester using
PhB(OH)2 in a biphasic system from Boc-protected
dipeptide of proline boronic acid utilizing mild trans-
esterification [8].
all the cases, reactions were allowed to proceed to equilibrium.
The preliminary experiments indeed have shown some encour-
aging results (displacement of pinanediol up to 40–53%).
Amongst cyclopentane-based cis-1,2-diols, endo-2-phenyl-
exo,exo-2,3-norbornane-diol appeared to be the most effective
diol in displacing pinanediol (38%). In the cases of pinane-
based diols, the best result was obtained with 2-ethyl-6,6-
dimethylbicyclo[3.1.1]heptane-cis-2,3-diol (53%). It was
interesting to observe that the transesterification with 2-
phenyl-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol result-
ed in a 50% conversion after 4 days only, whereas the former
diol took 24 days to reach equilibrium.
Keywords. Pinanediol; Boronic ester; Diol; Transesterification.
Introduction
Boronic acids and their esters are highly versatile
building blocks in organic synthesis, which have
found a broad range of applications in the areas of
synthetic and pharmaceutical chemistry over the last
decade [1–4]. ꢀ-Pinene-based chiral reagents and di-
The main objective of this exercise is to under-
stand the unusual stability of pinanediol boronic
ester as well as to find ways of recovering pinanediol
from its methylboronic ester by transesterification
using various diols. During the study on the rela-
tive stability of various achiral and chiral boronic
esters utilizing transesterification methodology [9],
ꢀ
Corresponding author. E-mail: chandra0919@gmail.com
This paper is dedicated to the memory of my mentor,
y
the late Professor Herbert C. Brown (1912–2004). Professor
Herbert C. Brown deceased on December 19, 2004. The work
described herein was performed at Purdue University during
my stay as a post-doctoral research associate (1997–2001)

748
C. D. Roy and H. C. Brown
we observed that certain substituted cyclopentane-
and norbornane-based diols were comparatively
very effective, as reflected by the extent of trans-
esterification. Consequently, we designed and syn-
thesized several diols having cyclopentane and
norbornane skeleton as well as few structurally mod-
ified pinanediols and tested their effectiveness in
transesterification of pinanediol methylboronic ester
and the preliminary results of such study are pre-
sented in this short communication.
2-phenylbicyclo[3.1.1]heptan-2-ol (60%) [11]. A
similar result was obtained when the nopinone was
treated with PhLi at ꢁ25ꢂC to room temperature
for 20 h. The resulting alcohol was dehydrated to
6,6-dimethyl-2-phenylbicyclo[3.1.1]hept-2-ene (75%)
using POCl3 in pyridine at 0ꢂC to room tempera-
ture for 24 h. The product olefin was subjected to
OsO4-catalyzed cis-dihydroxylation to afford 2-phe-
nylapopinanediol 14 in 80% yield. A similar synthetic
strategy was followed to obtain other alkyl- or aryl-
substituted diols.
2-(ꢁ-Aminoethyl)apopinanediol 17 was prepared
from the readily available starting material, nopol
(Fig. 2). (R)-(ꢁ)-Nopol was converted to nopol tosy-
late by treating with p-toluenesulfonyl chloride in
pyridine at 0ꢂC for 40–45 h in 83% yield [12]. Under
OsO4-catalyzed cis-dihydroxylation conditions, the
nopol tosylate underwent solvolytic rearrangement.
Therefore, the nopol tosylate was converted to the
phthalimide derivative using potassium phthalimide
(prepared in situ by reacting phthalimide with freshly
powdered anhydrous K2CO3 in DMF at 130–140ꢂC
Results and Discussion
All the structurally modified diols were prepared
by OsO4-catalysed cis-dihydroxylation [10] of the
corresponding olefins (commercial or synthesized)
in >75% chemical yields (Figs. 1 and 2). 2-Phenyl-
apopinanediol 14 was synthesized starting from
ꢁ-pinene as illustrated in Fig. 1. The ozonolysis of
ꢁ-pinene provided nopinone, which was reacted
with the Grignard reagent, PhMgBr in EE at ꢁ40ꢂC
to room temperature for 20 h to get 6,6-dimethyl-
Fig. 1. Synthesis of 2-phenylapopinanediol 14
Fig. 2. Preparation of 2-(ꢁ-hydroxyethyl)apopinanediol 16 and 2-(ꢁ-aminoethyl)apopinanediol 17

Transesterification of Chiral (ꢁ)-Pinanediol Methylboronic Ester
749
for 2 h) in DMF at 130ꢂC for 15 h in 75% chemical
yield. This phthalimide derivative underwent cis-
dihydroxylation very smoothly to give correspond-
ing diol in good chemical yield (73%). Finally, the
desired compound, 2-(ꢁ-aminoethyl)apopinanediol
was obtained by the reduction of imido group with
hydrazine in methanol in excellent yield (85%).
The initial exchange reaction of the boronic ester
21 [7b, 13] with commercially available cis-1,2-
cyclopentanediol in CDCl3 showed 11% exchange
1
(as was observed by H NMR spectroscopy) which
was quite encouraging considering the fact that no
diol was reported to show any detectable amount of
pinanediol exchange from its boronic ester (Fig. 3).
Among the various substituted cis-1,2-cyclopentane-
diols [15], the best result was obtained with 1-iso-
propyl-cis-1,2-cyclopentanediol (4) which showed
31% transesterification (Fig. 4). Although the steric
bulk of the substituents present on the diols slow down
the transesterification, but they lead to the thermo-
dynamically more stable boronic esters. In the case
of 1,2-dimethyl-cis-1,2-cyclopentanediol (6) [16],
the ineffectiveness (<5%) was attributed to the steric
factors. To confirm this, the boronic ester derived from
the diol 6 was prepared and subjected to transester-
ification with (ꢁ)-pinanediol under similar conditions.
As expected, no significant transesterification (<5%)
occurred even after 15 days, which confirmed the
above conclusion. When boronic ester 21 was treat-
ed with exo,exo-2,3-norbornanediol (7) [17], 25%
exchange was observed. 2,3-Norbornanediol, being
a rigid 3,5-disubstituted analog of 1,2-cyclopentane-
diol, appeared to be much more effective. Substitu-
tion at C4-, C5- and C6-positions (9 and 10) [18–20]
had no appreciable effects on the equilibrium com-
positions whereas the substitution at C2-position by
phenyl group 8 had positive effect on equilibrium
Transesterification of (1S,2S,3R,5S)-(ꢁ)-2ꢀ,3ꢀ-
Pinanediol Methylboronic Ester (21)
In order to follow the exchange very accurately and
precisely, transesterification reactions were con-
ducted in CDCl3 in NMR tubes under an inert at-
mosphere and the progress of each reaction was
1
monitored by H NMR. The stoichiometric ratios
of both the boronic ester 21 and the diol were
0.05 mmol in 1 cm3 CDCl3 and all the exchange re-
actions were allowed to proceed to equilibrium.
Depending upon the fastness of the reactions (as re-
flected by the exchange values obtained soon after
mixing), the reactions were frequently monitored
(every 10 min to 1 h to 12–24 h) and the % transes-
terification were determined by integrating and com-
paring the distinct proton signals originated from the
reactant boronic ester and the product boronic ester.
The equilibrium proportions and the time after mix-
1
ing which H NMR used to determine those propor-
tions were recorded are used to indicate the efficacy
of the diols at displacing the pinanediol.
Fig. 3. Transesterification of chiral (ꢁ)-pinanediol methylboronic ester 21
Fig. 4. Cyclopentane- and norbornane-based structurally modified diols 1–11

750
C. D. Roy and H. C. Brown
(from 25 to 38%). The extent of transesterification
with various cyclopentane-based cis-1,2-diols are tab-
ulated in the Table 1.
After examining various cyclopentane- and nor-
bornane-based diols, we turned our attention to test
the effectiveness of few pinane-based diols (Fig. 5).
These structurally modified pinanediols [10, 21],
some of them having suitably substituted coordinat-
ing sites (ꢁOH, ꢁNH2, OMe, ꢁN(C¼O)2), which
were hoped to provide an additional stability to the
product boronic esters possibly by chelation. It is
clear from the data (Table 2) that the nature of the
Table 1. Transesterification of the boronic ester 21 with various cyclopentane-based diols 1–11
Entry
Diol
Time=d
Equilibrium compositions %a
RBE . PBE
1
2
cis-1,2-Cyclopentanediol (1)
1-Methyl-cis-1,2-cyclopentanediol (2)
4
7
89 . 11
87 . 13
3
4
5
6
7
8
9
10
11
1-Ethyl-cis-1,2-cyclopentanediol (3)
1-Isopropyl-cis-1,2-cyclopentanediol (4)
1-Phenyl-cis-1,2-cyclopentanediol (5)
1,2-Dimethyl-cis-1,2-cyclopentanediol (6)
exo,exo-2,3-Norbornanediol (7)
endo-2-Phenyl-exo,exo-2,3-norbornanediol (8)
Octahydro-4,7-methanoindene-exo,exo-5,6-diol (9)
exo,exo-4-Methyloctahydro-4,7-methanoindene-5,6-diol (10)
1,7,7-Trimethylbicyclo[2.2.1]heptane-exo,exo-2,3-diol (11)
14
45
16
7
10
28
6
78 . 22
69 . 31
86 . 14
No equilibrium
75 . 25
62 . 38
74 . 26
72 . 28
7
20
88 . 12
a
The exchange reaction was carried out in an NMR tube under nitrogen (boronic ester:diol ¼ 1:1; 0.05 mmol each in 1 cm3
CDCl3). RBE (reactant boronic ester) and PBE (product boronic ester)
Fig. 5. Pinane-based structurally modified diols 12–20
Table 2. Transesterification of (ꢁ)-pinanediol methylboronic ester 21 with various pinane-based diols 12–20
Entry Diol
Time=d Equilibrium compositions=%a
RBE . PBE
1
2
3
4
5
6
7
8
6,6-Dimethylbicyclo[3.1.1]heptane-cis-2,3-diol (12)
8
24
4
60
2
1
40
92
67 . 33
47 . 53
50 . 50
57 . 43
67 . 33
82 . 18
90 . 10
56 . 44
2-Ethyl-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol (13)
2-Phenyl-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol (14)
2-(2-Chloroethyl)-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol (15)
2-(2-Hydroxyethyl)-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol (16)
2-(2-Aminoethyl)-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol (17)
2-(2-Methoxyethyl)-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol (18)
2-[2-(2,3-Dihydroxy-6,6-dimethylbicyclo[3.1.1]hept-2-yl)-
ethyl]isoindole-1,3-dione (19)
9
2-Hydroxymethyl-6,6-dimethylbicyclo[3.1.1]heptan-2-ol (20)
22
No equilibrium reached
a
The exchange reaction was carried out in an NMR tube under nitrogen (boronic ester:diol ¼ 1:1; 0.05 mmol each in 1 cm3
CDCl3). RBE (reactant boronic ester) and PBE (product boronic ester)

Transesterification of Chiral (ꢁ)-Pinanediol Methylboronic Ester
751
side chain at C2-position has a significant effect
on stability. The apopinanediol 12, which lacks the
C2-Me group, could exchange only up to 33%. Both
apopinanediols 16 and 17 bearing ꢁOH and ꢁNH2
as potential chelating groups did not demonstrate
any superiority over the ethyl-substituted apopinane-
diol 13. The substitution of C2-Me by C2-Ph group
14 also did not show any advantage. On the contrary,
the diols 15 and 19 having ꢁ-chloroethyl and ꢁ-iso-
indoledione structures showed relatively higher per-
centages of transesterification (43–44%). The reason
for the lower exchange with diols 16 and 17 is not
clear at this stage. The unusual hydrolytic resistance
of pinanediol boronic esters is believed to be due to
the orientation of hydroxyl groups and rigidity of the
free diol [6c]. The influence of the rigid structural
framework, 6,6-dimethylbicyclo[3.1.1]heptane can’t
be underestimated. Other factors that influence the
thermodynamics of ligand exchange include the en-
tropies of internal rotations of free diols, the steric
repulsion on enthalpy and the chelation. A summary
of the best results obtained from diols of diverse
structural types is presented in Fig. 6.
In conclusion, we have described the behaviour of
(ꢁ)-pinanediol methylboronic ester 21 in transester-
ification reactions with a variety of cyclopentene- and
1
pinane-based diols by H NMR. The displacement
of the pinanediol from its boronic ester has been
achieved to a considerable extent, if not quanti-
tatively. With cyclopentene-based-cis-1,2-diols, the
pinanediol was only poorly displaced (best result of
38% with diol 8). In the cases of pinane-based diols,
the displacements of pinanediol up to 43–53% could
be attained instead, and transesterification with diol
14 resulted in a 50% conversion after 4 days only,
though diols 13 and 15 showed similar values after
24 and 60 days, respectively. This study reveals that
the equilibrium can be driven towards favoured di-
rection by increasing the steric bulk on one of the
carbons bearing an OH group. 1,2-Disubstituted diols
were found to be ineffective, possibly due to steric
reasons (hard to approach the boron center). Among
the pinane-based diols, the nature of the side chain
at C2-position has significant effects on transesteri-
fication. The absence of methyl group in the cis-
apopinanediol 12 makes this boronic ester relatively
Fig. 6. A brief result summary of transesterification of chiral (ꢁ)-pinanediol methylboronic ester with structurally modified
diols

752
C. D. Roy and H. C. Brown
heptane-exo,exo-2,3-diol (8) [18], endo,endo-octahydro-
4,7-methanoindene-exo,exo-5,6-diol (9) [19], 1,7,7-tri-
methylbicyclo[2.2.1]heptane-exo,exo-2,3-diol (11) [20],
2-methoxyethyl-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-
diol (18) [10], 2-hydroxymethyl-6,6-dimethylbicyclo[3.1.1]-
heptan-2-ol (20) [21], and (ꢁ)-pinanediol methylboronic ester
(21) [7b] have been well characterized in literature.
less stable, as reflected by the equilibrium composi-
tions (67 . 33). It was interesting to note that both
diols 16 and 17 despite having an additional coordi-
nating or chelating sites (ꢁOH and ꢁNH2 groups)
did not show any improvements in comparison with
2-ethylapopinanediol 13. It would be interesting to
synthesize other structurally modified diols and test
their effectiveness in transesterification.
6,6-Dimethylbicyclo[3.1.1]heptane-cis-2,3-diol (12, C9H16O2)
1H NMR (CDCl3): ꢂ ¼ 4.15 (m, CHOH), 2.80 (br s, CHOH),
2.50–1.50 (m, CHOH, CH and CH2), 1.26 (s, CH3), 0.79
(s, CH3) ppm.
Experimental
All operations involving organoboranes were carried out under
an inert atmosphere. Detailed procedures and techniques for
handling air- and moisture-sensitive compounds have been
2-Ethyl-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol
(13, C11H20O2)
1H NMR (CDCl3): ꢂ ¼ 3.96 (m, CHOH), 2.60 (d, CHOH),
2.50–1.30 (m, CHOH, CH and CH2), 1.26 (s, CH3), 0.95
(t, CH2CH3), 0.92 (s, CH3) ppm.
1
reported elsewhere [14]. The 11B (96 MHz), H (300 MHz),
and 13C (75 Mhz) NMR spectra were recorded on a Varian
Gemini NMR instrument, and the chemical shifts (ꢂ) are given
in ppm relative to external standard BF3-Et2O and internal
standards TMS and CDCl3. THF was freshly distilled from
sodium benzophenone ketyl. Octahydro-4,7-methanoindene-
exo,exo-5,6-diol (9) and exo,exo-4-methyl-octahydro-4,7-
methanoindene-5,6-diol (10) were obtained as gift. ꢁ-Pinene,
(R)-(ꢁ)-nopol, camphor, N2H4-H2O, PhLi, PhMgBr, OsO4,
NMO, p-TsCl, phthalimide, cis-1,2-cyclopentanediol, (ꢁ)-ꢀ-
pinanediol, and methylboronic acid were purchased from the
Aldrich Chemical Co.
2-Phenyl-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol
(14, C15H20O2)
1H NMR (CDCl3): ꢂ ¼ 7.49–7.25 (m, ArH), 4.55 (m, CHOH),
2.80 (br s, CHOH), 2.50–1.50 (m, C(Ph)OH, CH and CH2),
1.27 (s, CH3), 0.80 (s, CH3) ppm.
2-Chloroethyl-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol
(15, C11H19ClO2)
1H NMR (CDCl3): ꢂ ¼ 4.15 (m, CHOH), 3.75 (m, CH2Cl),
3.04 (s, COH), 2.60 (d, CHOH), 2.60–1.30 (m, CH and CH2),
1.26 (s, CH3), 0.96 (s, CH3) ppm.
Experimental Techniques Used for Transesterification
In order to monitor the progress of the exchange reaction
precisely and conveniently, reactions were carried out in
CDCl3 solvent in a tightly closed NMR tubes under N2 atmo-
2-Hydroxyethyl-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol
(16, C11H20O3)
1H NMR (CDCl3): ꢂ ¼ 4.15 (m, CHOH), 4.05–3.80 (m and s
CH2OH and COH), 3.40 (d, CHOH), 2.55 (t, CH2OH), 2.50–
1.30 (m, CH and CH2), 1.27 (s, CH3), 0.93 (s, CH3) ppm.
1
sphere and the progress of the reaction was monitored by H
NMR spectroscopy. Both (ꢁ)-pinanediol methylboronic ester
and the diol (0.05 mmol of each) were dissolved in 1 cm3
CDCl3 and the NMR tube was flushed with N2. For fast reac-
tions, the progress of the reaction was followed quite frequent-
ly (every 5–15 min) whereas the reactions were monitored
every 12–24h for slow reactions. The starting and product
boronic esters always had certain distinguishable protons,
which could be followed, compared and integrated to obtain
the quantitative values of the transesterification. The 1H NMR
spectra were recorded for extended periods of time even after
the exchange had ceased (for slow reactions) to get accurate
equilibrium compositions. In some cases, product boronic
esters were prepared and subjected to transesterification
with the corresponding diols to confirm the unfavorable equi-
librium due to steric factors or to verify the obtained equi-
librium composition values by conducting the reactions in
reverse directions.
2-Aminoethyl-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol
(17, C11H21NO2)
1H NMR (CDCl3): ꢂ ¼ 4.00 (m, CHOH), 3.06 (m, CH2NH2),
2.50–1.40 (m, NH2, COH, CH and CH2), 1.26 (s, CH3), 0.93
(s, CH3) ppm.
2-[2-(cis-2,3-Dihydroxy-6,6-dimethylbicyclo[3.1.1]hept-2-yl)
ethyl]isoindole-1,3-dione (19, C19H23NO4)
1H NMR (CDCl3): ꢂ ¼ 7.82 (m, ArH), 7.70 (m, ArH), 4.10
(m, CHOH), 3.87 (m, CH2N), 3.30–3.00 (br s, CHOH and
COH), 2.60–1.30 (m, CH and CH2), 1.25 (s, CH3), 0.93 (s,
CH3) ppm.
1H NMR Spectroscopic Data for Diols
1-Methylcyclopentane-cis-1,2-diol (2) [15], 1-ethylcyclopen-
tane-cis-1,2-diol (3) [15], 1-isopropylcyclopentane-cis-1,2-
diol (4) [15], 1-phenylcyclopentane-cis-1,2-diol (5) [15],
1,2-dimethylcyclopentane-cis-1,2-diol (6) [16], bicyclo[2.2.1]
heptane-exo,exo-2,3-diol (7) [17], 2-phenylbicyclo[2.2.1]
Acknowledgements
Financial support from the Purdue Borane Research Fund is
gratefully acknowledged. We would also like to thank Profes-
sor Donald S. Matteson for his interest and encouragement.

Transesterification of Chiral (ꢁ)-Pinanediol Methylboronic Ester
753
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