Synthesis and stability of new spiroaminoborate esters

Article abstract of DOI:10.1016/j.tetlet.2011.12.054

New spiroaminoborate esters derived from 1,1-diphenylprolinol, ephedrine, and dihydroquinine with different alkoxy substituents were prepared as stable crystalline compounds and characterized by spectroscopical analysis and specific rotation. The structure of the spiroborate 4 derived from 1,1-diphenylprolinol and dicyclohexyl-1,1′-diol was confirmed by X-ray analysis.

Full text of DOI:10.1016/j.tetlet.2011.12.054

Tetrahedron Letters 53 (2012) 910–913
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and stability of new spiroaminoborate esters
Viatcheslav Stepanenko a, Melvin de Jesús a, Carmelo Garcia a, Charles L. Barnes b,
Margarita Ortiz-Marciales a,
⇑
a Department of Chemistry, University of Puerto Rico – Humacao, Call Box 860, Humacao, PR 00792, USA
b Department of Chemistry, University of Missouri – Columbia, 125 Chemistry Building, Columbia, MO 65211, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
New spiroaminoborate esters derived from 1,1-diphenylprolinol, ephedrine, and dihydroquinine with
different alkoxy substituents were prepared as stable crystalline compounds and characterized by spec-
troscopical analysis and specific rotation. The structure of the spiroborate 4 derived from 1,1-diphenyl-
prolinol and dicyclohexyl-1,10-diol was confirmed by X-ray analysis.
Received 30 November 2011
Revised 8 December 2011
Accepted 13 December 2011
Available online 19 December 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Chiral spiroaminoborates
Spiroborates
Nonracemic aminoalcohols
Diols
H
Ph
Ph
Chiral aminoborate esters derived from nonracemic amino alco-
hols are highly effective and efficient catalysts for the asymmetric
H
Ph
Ph
H
Ph
H
Ph
Ph
O
H
Ph
Ph
Ph
O
N
O
O
borane-mediated reduction of prochiral ketones and oximes.1–5
A
N
O
O
N
O
N
O
H
B
O
N
O
H
B
H
B
H
B
H
B
O
O
O
O
series of spiroborate esters derived from nonracemic 1,2-amino
alcohols were previously prepared in our laboratory and their
enantioselectivity studied as a function of the amino alcohol struc-
ture. Spiroaminoborate ester 1 (Fig. 1), derived from (S)-1,1-diphe-
nyl prolinol and ethylene glycol, achieved outstanding
stereoselection in the reduction of a large variety of arylalkyl and
aromatic heterocyclic ketones, comparable in their enantioselec-
tivity to the CBS oxazaborolidine.6,7 Additionally, spiroborate 6,
prepared from (S)-1,1-diphenyl prolinol and trimethoxy borate,
was found to be an outstanding catalyst for the reduction of aryl
ketones with only 1 mol % loading.2a Furthermore, the spiroborate
ester derived from diphenyl valinol and ethylene glycol was the
first effective catalyst for the asymmetric reduction of arylalkyl
and heterocyclic benzyloxime ethers, therefore, it was successfully
applied for the synthesis of a diverse group of aryl and heterocyclic
enantiopure primary amines and amino ethers.3 Recently, similar
O
1
2
3
5
4
H
H
Ph
Ph
Me
Ph
O
N
O
O
N
O
H
O
N
O
H
B
O
O
N
O
H
B
B
Me
O
N
O
B
O
O
9
6
7
8
Figure 1. Spiroaminoborate esters.
synthetic templates for further asymmetric transformations.8b
Aminoborate esters, in contrast to other chiral reducing catalysts,
are stable to air and moisture and, consequently, they are easier
to prepare, purify, store, and handle. Their outstanding enantiose-
lectivity in the borane-mediated reductions may be attributed to
their high purity.
Herein, we describe the synthesis and characterization of novel
nonracemic spiroaminoborate complexes, in particular, with a
variety of dialkoxy substituents. Furthermore, we are interested
in studying the role that the dialkoxy fragment plays in the prop-
erties and stability of these catalysts.
spiroborates derived from
a-pinene as the amino alcohol source
and different diols demonstrated to be excellent catalysts for the
reduction of aromatic ketones.5 Due to their Lewis acid-base nat-
ure, aminoborate esters can also be effective catalysts for other
types of reactions, as it was recently reported in the selective
reduction of ketones, esters, and amides using N,N-diethylani-
line-borane.8a Additionally, spiroborate esters have been used as
The new spiroaminoborates 2–5, and 7 (Fig. 1), derived from di-
phenyl prolinol and the corresponding diol, were successfully pre-
pared and isolated as colorless crystalline materials. Similarly,
⇑
Corresponding author.
E-mail
addresses:
margarita.ortiz1@upr.edu,
ortiz@quimica.uprh.edu
(M. Ortiz-Marciales).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.12.054

V. Stepanenko et al. / Tetrahedron Letters 53 (2012) 910–913
911
spiroborates
dihydroquinine,
8
and
9
derived from (R,S)(ꢀ)-ephedrine and
(9R)-10,11-dihydro-60-methoxycinchonan-9-ol,
respectively, were successfully prepared and characterized.9
The spiroborates 2, 5, 7, 8, and 9 were prepared as shown in
Scheme 1 by gently heating isopropyl borate with the correspond-
ing diol in toluene until
a clear solution of the 2-isopro-
poxy[1,3,2]dioxaborolane 10 was observed, followed by the slow
addition of the chiral b-amino alcohol in toluene.1b,9
The formation of intermediate 10 and the spiroborate was
determined by 11B NMR analysis. Initially, it was observed a strong
signal around 23 ppm corresponding to 10, together with a weak
peak at 17.7 ppm, due to a slight excess of triisopropyl borate. A
strong singlet between 5 and 10 ppm indicated the formation of
the aminoborate ester. Surprisingly, the syntheses of the spirobo-
rates 3 and 4 were not successfully achieved after several attempts
using triisopropyl borate or trimethyl borate for the formation of
intermediate 10. However, a modified approach employing boronic
acid under reflux in toluene with azeotropic removal of water, and
subsequent reaction of the boric ester with the b-amino alcohol
provided the desired compounds in quantitative yield.10 In some
cases, impurities of unreacted amino alcohols (ca. 10%) and/or bo-
rate species were removed by recrystallization, except for com-
pound 5, which was obtained after several attempts in a 90%
purity by 11B, 1H, and 13C NMR.9 Borate 6 was prepared as reported
Figure 2. X-ray structure of spiroaminoborate ester 4.
H
Ph
O
B
H
Ph
Ph
OH
Ph
A
O
O
N
O
HO
R1
NH
- HOiPr
B(OiPr)3
- HOiPr
OH
R3
R2
previously by transesterification with trimethyl borate and fully
characterized.2e,11
In general, the pure spiroaminoborate esters were isolated as
crystalline solids and fully characterized by mp, spectroscopic
methods, and specific rotation (Table 1).
H
B
B
R3
R1
O
C
O
R2
R
R
R3
R1
R2
10
R
11B NMR: +23 ppm
Scheme 1. General synthesis of spiroaminoborate esters.
Additionally, as indicated in Figure 2, the structure of 4 was cor-
roborated by X-ray analysis.12
Density functional theory calculations (DFT) were performed to
obtain the relative energies of the aminoborate esters 1–8, shown
in Figure 1. The geometry pre-optimizations were performed in
vacuum with the PM3 semi empirical method using the Polak–
Ribiere conjugated gradient protocol. The final optimizations were
performed with DFT [B3LYP/6-31G(d)OPT] using GAUSSIAN 03. All
conformational and thermodynamical parameters were obtained
with DFT single point calculations. The relative energy of com-
pounds compared to the most stable spiroborate 4 was calculated
by the Eq. (1).
Table 1
Spiroaminoborate esters characterization
11B NMR d
(ppm) (s)
Mp (°C)
½ ꢂ
a 2D3
Borate esters
Yield
(%)
2
3
4
5
6
7
8
98
99
92
99a
68b
99
99
9.4
9.8
9.8
6.1
6.7
169–174
188–192
234–236
120–123
132–136
192–196
136–138
ꢀ101 (c 1.3, CHCl3)
ꢀ95 (c 1.2, CHCl3)
ꢀ74 (c 2.3, CHCl3)
ꢀ59 (c 4.3, CHCl3)
ꢀ130 (c 1.3, CHCl3)
+13 (c 2.0, CHCl3)
ꢀ37.5 (c 5.6, DMSO)
ꢀ9 (c 2.9, CHCl3)
9.6
10.0
D
E ¼ ðComp ꢀ E1Þ ꢁ 627:503
ð1Þ
9
97
10.2
177–182
ꢀ137 (c 2.17, CHCl3)
In Table 2 are indicated the relative energy of the selected
aminoborates, the dipole moment, the B–N bond order, and the
conformational parameters: (a) the OA–B–OB–C torsion angle; (b)
a
Purity, 90%.
Recristalyzed in diisopropyl ether.
b
Table 2
Relative energy and geometrical parameters of compounds 1–8
Borate
D
E ꢃ10ꢀ4 (Kcal/mol)
l
(D)
Torsion angle (degree)
B–N bond order
a
b
c
1
2
3
4
5
6
7
8
24.52
14.65
4.93
4.39
3.91
4.26
3.92
3.53
3.61
4.34
4.44
120.7
110.3
113.9
108.6
91.7
59.1
113.6
142.2
108.2
113.4
112.9
113.0
156.4
103.1
112.69
131.8
72.0
69.9
71.2
66.6
69.7
66.7
76.1
79.4
0.4631
0.4591
0.4695
0.4579
0.5046
0.4442
0.4318
0.4201
—
22.05
24.44
33.93
43.87
a
b
c
Torsion angle for OA–B–OB–C.
Torsion angle for N–OC–C.
Torsion angle for N–OA–OC–OB.

912
V. Stepanenko et al. / Tetrahedron Letters 53 (2012) 910–913
was added to the warm stirring solution. The reaction mixture was then
gently heated to reflux until a homogeneous colorless solution was formed.
After cooling, the resulting solution with a white solid was concentrated in
the rotovaporator by heating at 80 °C/20 mm Hg for about 1 h. The white
crystalline solid was then dried overnight using high vacuum (0.5 mm Hg).
Generally, the desired compounds were obtained pure in quantitative yield.
The impure spiroborates were further recrystallized in toluene. Compound
(2): Yield 98%; mp 169–174 °C; 1H NMR (400 MHz, CDCl3): d 1.14 (s, 6H),
1.20 (s, 6H), 1.54–2.67 (m, 3H), 1.89 (m, 1H), 2.96 (m, 1H), 3.60 (m, 1H),
4.17 (s, 1H), 4.39 (m, 1H), 7.04–7.27 (m, 6H), 7.55 (d, J = 7.2 Hz, 2H), 7.67 (d,
J = 7.6 Hz, 2H); 13C NMR (100 MHz, CDCl3): d 24.9, 25.1, 25.4, 28.3, 46.6,
68.7, 79.3, 80.2, 125.7, 126.1, 126.5, 126.7, 127.6, 146.1, 146.8; 11B NMR
the N–B–OC–C torsion angle; and (c) the N–OA–OC–OB torsion an-
gle (Scheme 1). The thermal stability of compound 4 is, at least,
4.9 ꢃ 104 kcal/mol higher than the other derivatives. Systems with
no phenyl groups on the oxazaborolidine ring, such as 7, are less
stable. The same behavior is observed for systems without a cyclic
ring attached to the amino group, such as compound 8.
It is well known that energy results by the DFT methods do not
have a good correlation with the free energy, which is the best
indicator for the thermal stability of a molecule.13 Therefore, in this
study, the stability trend in the series 1–8 was also rationalized in
terms of the B–N bond order. For these compounds it is established
that the smaller the bond order the weaker the B–N bond, there-
fore the lower the thermal stability of the compound. In this case,
it is well known that calculated bond order is in good agreement
with the experimental values for the method used in this work,
B3LYP/6_31G(d).13 Based on this principle, compounds 7 and 8
should be the less stable, while compound 5, should have the high-
est thermal stability.
In summary, new stable spiroaminoborate esters were success-
fully prepared and fully characterized. Their structural parameters
and thermal stability were studied by molecular calculations.
Interestingly, structure 4 was the most thermally stable by the
DFT calculations, however spiroborate 5 has higher stability using
the B–N bond order method, although the difference in their bond
order is small. The effect of the aminoborate structure and the role
that the dialkoxy fragment plays on their reactivity toward borane
and the mechanism in the enantioselective ketone reduction, will
be addressed in future studies.
(128 MHz, CDCl3): d 9.40; IR (m
, cmꢀ1): 3258 (NH), 2970, 2928, 1448, 1376,
1274, 1105, 1018, 973. ½a D23
ꢀ101 (c 1.3, CHCl3). HRMS m/z: 380.2396 found
ꢂ
(calcd for C23H30NO3BH, (M+H)+ requires 380.2397). Compound (5): Yield
99% with 90% purity; mp 120–123 °C, 1H NMR (400 MHz, CDCl3): d 1.51–
1.85 (m, 6H), 2.96 (m, 1H), 3.33 (m, 1H), 3.82 (m, 4H), 4.27 (m, 1H), 4.63 (s,
1H), 7.06–7.28 (m, 6H), 7.50 (d, J = 8.0 Hz, 2H), 7.60 (d, J = 6 Hz, 2H); 13C
NMR (100 MHz, CDCl3): d 24.59, 28.07, 28.50, 45.69, 61.32, 68.85, 81.16,
125.89, 126.05, 126.19, 126.54, 127.66, 127.78; 11B NMR (128 MHz, CDCl3): d
6.09; IR (
ꢀ59.4 (c 6.3, CHCl3); HRMS m/z: 338.1871 found (calcd for
m
, cmꢀ1): 3221, 2915, 2836, 1490, 1382, 1242, 1151, 1022, 935;
½ ꢂ
a 2D3
C
20H25NO3BH, (M+H)+ requires 338.1927). Compound (7): Yield 99%; mp
192–196 °C, 1H NMR (400 MHz, CDCl3): d 1.52–2.00 (m, 20H), 2.84 (s, 1H),
3.40 (m, 2H), 3.87 (t, J = 8.0 Hz, 1H), 4.01 (s, 1H), 6.29 (s, 1H); 13C NMR
(100 MHz, CDCl3): d 24.3, 26.1, 29.4, 36.6, 46.8, 60.6, 65.6, 90.3; 11B NMR
(128 MHz, CDCl3): d 9.59; IR (m
, cmꢀ1): 3054, 2957, 2849, 1446, 1323, 1236,
1125, 1042, 1006, 950; ½a D23
ꢂ
+25.0 (c 2.3, CHCl3); HRMS m/z: 280.2090 found
(calcd for C15H26NO3BH, (M+H)+ requires 280.2079). Compound (8): Yield
99%; mp 136–138 °C, 1H NMR (400 MHz, CDCl3): d 0.89 (d, J = 7.2 Hz, 3H),
2.54 (s, 3H, NMe), 3.40–3.47 (m, 1H, NCH), 3.60–3.90 (m, 4H, OCH2), 5.17 (d,
J = 6 Hz, OCH), 6.10 (br s, 1H, NH), 7.22–7.25 (m, 1H), 7.26–7.33 (m, 2H),
7.36–7.40 (m, 2H); 13C NMR (100 MHz, CDCl3): d 11.7, 30.6, 60.2, 64.5, 76.6,
126.4, 127.4, 128.0, 140.0; 11B NMR (128 MHz, CDCl3): d 10.0; IR (
m
, cmꢀ1):
3099, 2870, 1452, 1131, 1114,1078, 965, 933; ½a D23
ꢂ
ꢀ37.5 (c 5.6, DMSO),
ꢀ9.0 (c 2.9, CHCl3). HRMS m/z: 236.1455 found (calcd for C12H18NO3BH,
(M+H)+ requires 236.1453). Compound (9): Yield 97%; mp 177–182 °C; 1H
NMR (400 MHz, CDCl3): d 0.79 (m, 1H), 0.95 (t, J = 7.4 Hz, 3H), 1.34–1.53 (m,
4H), 1.70 (m, 1H), 1.81 (m, 2H), 3.04 (m, 4H), 3.91 (s, 3H), 4.11 (m, 5H), 5.92
(d, J = 9.2 Hz, 1H), 6.82 (d, J = 2.8 Hz, 1H), 7.40 (dd, J1 = 2.8 Hz, J2 = 9.2 Hz,
1H), 7.92 (d, J = 4.4 Hz, 1H), 8.09 (d, J = 9.2 Hz, 1H), 8.82 (d, J = 4.4 Hz, 1H);
13C NMR (100 MHz, CDCl3): d 12.1, 23.4, 25.2, 25.8, 27.5, 36.8, 44.2, 54.2,
55.6, 59.0, 64.9, 70.6, 101.5, 118.8, 121.1, 127.2, 132.0, 143.9, 145.2, 148.4,
Acknowledgments
We thank NIH for the financial support through their MBRS (GM
08216) and INBRE (NC P20 RR-016470) grants.
157.5; 11B NMR (128 MHz, CDCl3): d 10.2; IR (
m
, cmꢀ1): 2934, 2872, 1620,
1507, 1469, 1229, 1146, 1056, 856; ½a D23
ꢂ
ꢀ137 (c 2.17, CHCl3); HRMS m/z:
Supplementary data
397.2411 found (calcd for
Supplementary data.
C
22H29BN2O4H, (M+H)+ requires 397.2299). See
10. Typical procedure for the synthesis of spiroborate ester 3: A mixture of boric
acid (618 mg, 10 mmol) and dicyclopentyl -1,10-diol (1.70 g, 10 mmol) in dry
toluene (40 mL) was gently heated with stirring up to 80 °C over 1 h using a
Dean–Stark distillation trap. A homogeneous sample was analyzed by 11B
NMR, which exhibited the corresponding signal for the boronic acid
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.12.054.
References and notes
intermediate, 11B NMR (128 MHz, CDCl3), d: 22.7 ppm.
A solution of (S)-
diphenyl(pyrrolidin-2-yl)methanol (2.53 g, 10 mmol) in dry toluene (20 mL)
was added to the reaction mixture and the resulting solution was refluxed in
a Dean–Stark water removing system over 2 h. The reaction mixture was
concentrated at 90 °C in a rotor evaporator under vacuum (10 mm Hg) and
the white solid was further dried under high vacuum (0.5 mm Hg) over 12 h
to give the final product in quantitative yield (99%); mp 188–192 °C, 1H NMR
(400 MHz, CDCl3): d 1.35–2.00 (m, 20H), 2.92–3.00 (m, 1H), 3.49–3.59 (m,
1H), 4.18–4.30 (m, 1H), 4.42–4.50 (m, 1H), 7.04–7.25 (m, 6H), 7.54 (d,
J = 7.6 Hz, 2H), 7.62 (d, J = 7.6 Hz, 2H); 13C NMR (100 MHz, CDCl3): d 23.1,
23.6, 25.3, 28.3, 34.4, 35.4, 47.0, 68.2, 80.4, 90.1, 125.8, 126.1, 126.5, 126.6,
1. (a) Stepanenko, V.; Ortíz-Marciales, M.; Correa, W.; de Jesús, M.; Espinosa, S.;
Ortíz, L. Tetrahedron: Asymmetry 2006, 17, 112; (b) Stepanenko, V.; Huang, K.;
Ortiz-Marciales, M. Org. Synth. 2010, 87, 26. and references cited therein; (c)
Huang, K.; Wang, H.; Stepanenko, V.; de Jesús, M.; Torruellas, C.; Correa, W.;
Ortiz-Marciales, M. J. Org. Chem. 2011, 76, 1883; (d) Cho, B. T. Chem. Soc. Rev.
2009, 38, 443. and references cited therein.
2. (a) Stepanenko, V.; Ortiz-Marciales, M.; Barnes, C. L.; Garcia, C. Tetrahedron Lett.
2006, 47, 7603; (b) Ortiz-Marciales, M.; de Jesús, M.; Gonzalez, E.; Raptis, R. G.;
Baran, P. Acta Crystallogr., Sect. C 2004, 60, 173; (c) Stepanenko, V.; Ortiz-
Marciales, M.; Barnes, C. L.; Garcia, C. Tetrahedron Lett. 2009, 50, 995.
3. (a) Huang, K.; Ortiz-Marciales, M.; Merced, F. G.; Meléndez, H. J.; Correa, W.; de
Jesús, M. J. Org. Chem. 2008, 73, 4017; (b) Huang, K.; Ortiz-Marciales, M. Org.
Synth. 2010, 87, 36. and references cited therein.
4. (a) Xu, J.; Wei, T.; Zhang, Q. J. Org. Chem. 2004, 69, 6860; (b) Liu, H.; Xu, J. X. J.
Mol. Catal. A: Chem. 2006, 244, 68; (c) Chu, Y.; Shan, Z.; Liu, D.; Sun, N. J. Org.
Chem. 2006, 71, 3998.
5. Krzeminski, M. P.; Cwiklinska, M. Tetrahedron Lett. 2011, 52, 3919.
6. (a) Corey, E. J.; Helal, C. J. Angew. Chem., Int. Ed. 1998, 37, 1986; (b) Cho, B. T.
Tetrahedron 2006, 62, 7621; (c) Glushkov, V. A.; Tolstikov, A. G. Russ. Chem. Rev.
2004, 73, 581; (d) Gnanadesikan, V.; Corey, E. J. Org. Lett. 2006, 8, 4943; (e)
Corey, E. J.; Bakshi, R. K.; Shibata, S. J. Am. Chem. Soc. 1987, 109, 5551.
7. (a) Krzeminski, M. P.; Wojtczak, A. Tetrahedron Lett. 2005, 46, 8299; (b) Santhi,
V.; Rao, J. M. Tetrahedron: Asymmetry 2000, 11, 3553.
127.7, 127.7, 146.2, 146.7; 11B NMR (128 MHz, CDCl3): d 9.8; IR (
m
, cmꢀ1):
3258, 2956, 2869, 1447, 1381, 1228, 1108, 1022, 1062, 945; ½a D23
ꢂ
ꢀ95 (c 1.2,
CHCl3); HRMS m/z: 432.2709 found (calcd for C27H34NO3BH, (M+H)+ requires
432.2705). Compound (4): Yield 92%; mp 234–236 °C; 1H NMR (400 MHz,
CDCl3): d 1.11–1.27 (m, 6H), 1.56–1.81 (m, 17H), 2.07 (m, 1H), 3.04 (m, 1H),
3.62 (m, 1H), 4.36 (s, 1H), 4.52 (m, 1H), 7.11–7.37 (m, 6H), 7.62 (d, J = 7.6 Hz,
2H), 7.70 (d, J = 7.2 Hz, 2H); 13C NMR (100 MHz, CDCl3): d 22.8, 23.2, 24.9,
26.5, 28.0, 32.7, 33.1, 46.6, 67.96, 80.4, 126.0, 126.2, 126.5, 126.8, 127.7,
127.7, 146.4, 147.1; 11B NMR (128 MHz, CDCl3): d 9.8; IR ( , cmꢀ1): 3055,
m
2932, 2853, 1488, 1444, 1308, 1226, 1153, 1034, 937; ½ ꢂ ꢀ74 (c 2.3,
a 2D3
CHCl3); HRMS m/z: 460.3023 found (calcd for C29H38NO3BH, (M+H)+ requires
460.3018). See Supplementary data.
11. Preparation
of
(ꢀ)-(3aS)-1,1-dimethoxy-3,3-diphenyl-hexahydro-1H-
pyrrolo[1,2-c][1,3,2]oxazaborol-7-ium-1-uide (6): To
a
solution of (S)-
8. (a) Coleridge, B. M.; Angert, T. P.; Marks, L. R.; Hamilton, P. N.; Sutton, C. P.;
Matos, K.; Burkhardt, E. R. Tetrahedron Lett. 2010, 51, 5973; (b) Zhou, Y.; Shan, Z.
Tetrahedron Lett. 2007, 48, 3531.
9. General procedure for the synthesis spiroborate esters 1, 2, 5, 7, 8 and 9: To
a solution of the corresponding diol (5.05 mmol) in 15 mL of dry toluene
under a nitrogen flow was added via syringe triisopropyl borate (1.17 mL,
5.1 mmol). The reaction mixture was gently heated to reflux until an
diphenyl(pyrrolidin-2-yl)methanol (2.53 g, 10 mmol) in dry isopropyl ether
(25 mL) was added drop-wise freshly redistilled trimethyl borate (3 mL,
28.9 mmol) at room temperature under nitrogen atmosphere. The resulting
mixture was left without stirring over 24 h (after 2 h white crystals started
to grow). The precipitate was filtered, washed with isopropyl ether
(4 ꢃ 10 mL) under nitrogen atmosphere, and dried under vacuum at 60 °C
over 12 h to give the final product as a white solid. Yield 68%; mp 132–
136 °C. 1H NMR (400 MHz, CDCl3): d (ppm) 1.42–1.66 (m, 4H, 2CH2), 2.0–3.2
(m, 8H, B(OMe)2 and NCH2), 4.26–4.31 (m, 1H, NCH), 6.37 (br d, J = 6.0 Hz,
homogeneous colorless solution was observed.
diphenyl-2-pyrrolidinemethanol (1.267 g, 5.0 mmol) in dry toluene (10 mL)
A
solution of (S)-(ꢀ)-a,a-

V. Stepanenko et al. / Tetrahedron Letters 53 (2012) 910–913
913
1H, NH), 7.1–7.3 (m, 6H, CHAr), 7.46–7.50 (m, 4H, CHAr); 13CNMR (100 MHz,
12. The crystal structure has been deposited at the Cambridge Crystallographic
Data Centre and allocated the deposition number CCDC 844542. Formula:
CDCl3): d 24.8, 29.0, 46.7, 49.6 (br, 2MeO), 81.4, 126.1, 126.2, 126.3, 126.5,
127.7, 127.8, 146.2, 147.8; 11B NMR (128 MHz, CDCl3):
d
ꢂ
6.7 (s); IR
ꢀ130 (c 1.3,
(
m
,
C31H44B1N1O4S1 Unit cell parameters: a = 8.6764(4) Å, b = 18.0726(8) Å,
cmꢀ1): 3300 (NH), 3059, 2964, 1598, 1390, 1103, 1022; ½a 2D3
c = 9.2445(4) Å, alpha = 90°, beta = 95.5980(10)°, gamma = 90°. Crystal
system, space group Monoclinic, P21. Crystal size 0.35 ꢃ 0.35 ꢃ 0.25 mm.
13. Bastosa, E. L.; Baaderb, W. J. ARKIVOC 2007, 8, 257.
CHCl3); HRMS m/z: 326.1937 found (calcd for C19H24BNO3, (MꢀH)+ requires
326.1927.