On the additions of lithium methyl p-tolyl sulfoxide to N-(PMP)arylaldimines

Article abstract of DOI:10.3390/60500424

The results presented in this paper confirm that the stereochemical outcome of the reversible additions of lithium (R)-methyl p-tolyl sulfoxide to N-arylidene-p-anisidines (N-PMP imines) depends on: (a) the reaction conditions used and (b) the electronic properties of the arylidene moiety on the starting imine. In particular, we show that under kinetic control (-70 °C) the additions involving electron-rich N-arylidene groups occur with very high stereocontrol in favor of the (2S,R_S)-diastereomers, whereas an electron-deficient group favors the opposite stereochemical outcome. Based on the observations above, a mechanistic hypothesis is proposed.

Full text of DOI:10.3390/60500424

Molecules 2001, 6, 424–432
molecules
ISSN 1420-3049
http://www.mdpi.org
On the Additions of Lithium Methyl p-Tolyl Sulfoxide to N-
(PMP)Arylaldimines
Cristina Zucca,1 Pierfrancesco Bravo,1,2 Eleonora Corradi,1 Stefano V. Meille,1 Alessandro
Volonterio,2 and Matteo Zanda1,*
1 Dipartimento di Chimica del Politecnico di Milano, via Mancinelli 7, I-20131 Milano, Italy. Tel. +39
(02)-23993084, Fax +39 (02)-23993080. 2 C.N.R. - Centro di Studio sulle Sostanze Organiche
Naturali, via Mancinelli 7, I-20131 Milano, Italy
* Author to whom correspondence should be addressed; e-mail zanda@dept.chem.polimi.it
Received: 8 March 2001; in revised form 3 April 2001 / Accepted: 4 April 2001 / Published: 30 April
2001
Abstract: The results presented in this paper confirm that the stereochemical outcome of the
reversible additions of lithium (R)-methyl p-tolyl sulfoxide to N-arylidene-p-anisidines (N-
PMP imines) depends on: (a) the reaction conditions used and (b) the electronic properties
of the arylidene moiety on the starting imine. In particular, we show that under kinetic
control (-70 °C) the additions involving electron-rich N-arylidene groups occur with very
high stereocontrol in favor of the (2S,RS)-diastereomers, whereas an electron-deficient group
favors the opposite stereochemical outcome. Based on the observations above, a mechanistic
hypothesis is proposed.
Keywords: Sulfoxides, asymmetric synthesis, imines.
Introduction
Additions of chiral lithium sulfoxides to imines represent a powerful tool in asymmetric synthesis
[1]. We have recently reported that kinetically controlled additions (-70 °C) of lithium methyl p-tolyl
sulfoxide (2) (Scheme 1) to N-PMP(imines) (PMP = p-methoxyphenyl) derived from a series of
aromatic aldehydes having a variable degree of fluorination occur with a progressively higher
stereocontrol in favor of (2S,RS)-β-aminosulfoxides 3 with decreasing the number of fluorine atoms on
the N-arylidene groups [2]. This observation, supported by previous literature reports [3], led us to

Molecules 2001, 6
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propose that the presence of electron-rich N-arylidene groups was responsible for the very high kinetic
stereocontrol in favor of the (2S,RS)-diastereomers, whereas an electron-deficient group should favor
the opposite stereochemical outcome. In this paper we present a complementary part of work, in which
the additions of (R)-2 are extended to a number of variously substituted N-arylidene-p-anisidines 1.
Results and Discussion
The starting N-(PMP)arylimines 1a-g were prepared in good yields through acid catalyzed
condensation of the corresponding aldehyde and p-anisidine (Scheme 1) [2]. All imines 1a-g were
obtained as geometrically (E)-homogeneous compounds, according to their 1H-NMR spectra. First, we
studied the stereochemical outcome of reactions between lithium sulfoxide (R)-2 and p-methoxy-imine
1a (Table 1).
O
S
H
H
Tol
(R)-2
H
H
O H NH-PMP
Ar
H2N-PMP, C6H6
O H NH-PMP
Ar
Li
PMP
S
S
+
Ar
N
Tol
DOWEX-50W
Dean-Stark
Ar
O
Tol
1
(2S,RS)-3
(2R,RS)-4
Scheme 1. Additions carried out at –70 °C under kinetic control.
We have already demonstrated [2] that the additions of (R)-2 to N-(PMP)arylimines are reversible
processes and that the diastereomeric products like 3 and 4 readily equilibrate at 0 °C. Thus, in order to
ensure kinetic control, all the reactions in Table 1 were carried out at –70 °C. The only exception can
be found in entry 1, because imine 1a did not undergo addition of (R)-2 at that temperature (entry 2).
However, the addition proceeded smoothly within –40 and –50 °C, providing the diastereomer 3a with
excellent stereoselectively, evidently under kinetic control. Next we examined the imine 1b derived
from 3,4-(methylenedioxy)benzaldehyde (entry 3). Due to the presence of an additional alkoxy
substituent in meta-position this imine was expected to be more electrophilic than 1a. In fact, addition
of (R)-2 took place even at –70 °C, although rather slowly. After 60 min, 34% yield was obtained, and
the d.r. 3:4 decreased in comparison to 1a. Imine 1c derived from p-CF3-benzaldehyde featured a
further decrease of stereoselectivity (entry 4) and, as expected, increased reactivity, the addition being
nearly complete after 30 min. With imine 1d (entry 5), obtained from 2-naphthaldehyde both isolated
yield and the observed diastereocontrol were slightly lower than that of 1c. A further decrease of
stereoselectivity was observed with the p-nitrobenzaldehyde-derived imine 1e (entry 6) and (2,6-
difluorophenyl)imine 1f, which provided the lowest 3:4 d.r. (ca. 3:1, entry 7). Finally, no reaction was
achieved with imine 1g derived from mesitaldehyde (entry 8), as a likely result of a strong steric
hindrance.

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426
Table 1
Entry
1c (a)
2 (a)
Ar
Time (min)
Yield (%)a
(2S,RS)-3 (%)b
95.0
(2R,RS)-4 (%)b
80
80
91
5.0
OMe
< 2
Not determined
Not determined
O
3 (b)
4 (c)
5 (d)
6 (e)
60
30
30
40
34
88
76
83
91.7
83.8
81.6
76.5
8.3
O
CF3
16.2
18.4
23.5
NO2
F
7 (f)
30
86
73.7
26.3
F
H3C
H3C
CH3
8 (g)
90
< 2
Not determined
Not determined
Key: a Isolated yields. b Determined by 1H-NMR and HPLC. c Carried out at –50/-40 °C.
The stereochemical assignments were carried out as follows. β-Sulfinyl-amines 3 and 4a,e are
known compounds [3b], whereas the stereochemistry of 3f was determined through X-ray diffraction
of a suitable single crystal (Figure 1) [4]. In the remaining three cases the stereochemistry was
confidentially assigned exploiting some unambiguous chemical-physical similarities existing among
1
the major diastereomers 3 in comparison with the minor ones 4. In particular, in the H-NMR spectra
of 4 the methylene signals invariably appeared as well defined ABX systems with eight lines and large
∆ν between the diastereotopic protons (see Spectral Data section). In contrast, the corresponding
signals of the major diastereomers 3 showed a contracted pattern with smaller ∆ν. In addition, all the
diastereomers 3 displayed lower tr and higher Rf than diastereomers 4 by HPLC and TLC analyses,
respectively.
The data summarized in Table 1 confirm [2] that an increase of electron-deficiency of the
benzylidene aromatic group of imines 1 brings about a decrease of kinetic selectivity (namely, the d.r.
3:4). This means that electron-releasing substituents, which enhance the ring electron density, produce
a much better stereocontrol than electron-withdrawing groups. Such a picture might be rationalized by
supposing two possible transition states A (leading to the diastereomers 3) and B (leading to 4) (Figure

Molecules 2001, 6
427
2). The former (chair-like) could be mainly operating in the case of imines having an electron-rich
benzylidene ring, whereas the latter (boat-like) becomes competitive with electron-poor rings.
Figure 1. ORTEP view of (2S,RS)-3f showing the atomic labeling scheme. Only non hydrogen
atoms are shown, using 30% thermal ellipsoids.
In fact, in the case of electron-rich Ar, TS-B experiences a repulsive steric flagpole interaction with
the sulfinyl oxygen, that is absent in TS-A, where in addition the unfavorable steric interactions are
minimized. However, in the case of electron-poor Ar an attractive π-p interaction with the sulfinyl
oxygen may be invoked to stabilize TS-B lowering its energy, as well as the 3:4 ratio [5].
Ar
PMP
L
H
H
O
Li
N
H
O
L
Li
L
H
Tol
N
H
Tol
L
S
S
PMP
Ar
H
TS-A
TS-B
to give (2R,RS)-4a-g
to give (2S,RS)-3a-g
Figure 2.

Molecules 2001, 6
428
Conclusions
The data coming out from this investigation confirm the validity of the previously proposed picture
[2], which appears to be valid beyond the realm of fluorinated compounds. Thus, a general and
predictive rationale for the highly synthetically useful additions of chiral lithium sulfoxides to
arylimines is now available.
Experimental
General.
For general experimental details see Ref. 2. Coupling constants (J) are reported in Hertz. HPLC
analyses were performed using a LiChrosorb Si60 prepacked column. Retention times (tr) are
expressed in minutes.
Representative procedure for the additions of (R)-2 to 1. To a solution of (R)-methyl-p-tolylsulfoxide
(1 mmol) in dry THF (2.7 mL) cooled to –70 °C, a 1.5 M solution of LDA THF in cyclohexane was
added dropwise. After 15 min at the same temperature, a suspension of imine 1a (1.2 mmol) in 2.3 mL
of dry THF was added dropwise. The temperature was allowed to raise up to –50 °C, then maintained
in the range –50/-40 °C, and the mixture was magnetically stirred for 80 min. The reaction was
quenched at the same temperature with a saturated NH4Cl solution, and the mixture was allowed to
warm up to r.t.. The phases were separated, and the aqueous phase was extracted three times with
AcOEt. The collected organic phases were dried over anhydrous Na2SO4, filtered, then the solvent was
removed in vacuo. Purification by flash chromatography (Hex/AcOEt 7:3) afforded the
diastereomerically pure products 3a and 4a in overall 91% yield.
Spectral Data
20
3a: Rf (Hex/AcOEt 6:4) 0.27; tr 11.49 (Hex/AcOEt 1:1, 1 mL/min); [α]D +179.7 (CHCl3, c 1.22);
m.p. (AcOEt) 180.5-181.5 °C; 1H-NMR (CDCl3) δ 7.49 (2H, d, J = 8.2), 7.27 (4H, m), 6.82 (2H, d, J =
8.6), 6.68 (2H, d, J = 9.0), 6.58 (2H, d, J = 9.0), 4.70 (1H, dd, J = 3.4 and 9.4), 3.76 (3H, s), 3.70 (3H,
s), 3.30 (1H, dd, J = 9.4 and 13.9), 3.11 (1H, dd, J = 3.4 and 13.9), 2.40 (3H, s); 13C-NMR (CDCl3) δ
159.1, 152.6, 141.7, 140.2, 139.9, 133.0, 130.0, 127.6, 124.0, 115.7, 114.6, 114.3, 63.9, 55.2, 55.0,
21.4; MS (EI, 70 eV) m/Z (%) 395 (M+, 63), 255 (88), 242 (39), 134 (92), 122 (100), 91 (81); FT IR
(cm-1) 3449 (br), 1510, 1241, 1035, 808.
20
4a: Rf (Hex/AcOEt 6:4) 0.21; tr 15.64 (Hex/AcOEt 1:1, 1 mL/min); [α]D +64.2 (CHCl3, c 0.47);
1H-NMR (CDCl3) δ 7.52 (2H, d, J = 7.9), 7.30 (4H, m), 6.84 (2H, d, J = 8.6), 6.68 (2H, d, J = 9.0),
6.54 (2H, d, J = 9.0), 5.28 (1H, s), 4.74 (1H, dd, J = 5.2 and 9.4), 3.76 (3H, s), 3.69 (3H, s), 3.32 (1H,
dd, J = 9.4 and 13.5), 2.94 (1H, dd, J = 5.2 and 13.5), 2.40 (3H, s).

Molecules 2001, 6
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20
3b: Rf (Hex/AcOEt 6:4) 0.29; tr 9.98 (Hex/AcOEt 1:1, 1 mL/min); [α]D +168.2 (CHCl3, c 1.18);
m.p. (MeOH) 182-184 °C; 1H-NMR (CDCl3) δ 7.49 (2H, d, J = 8.1), 7.30 (2H, d, J = 8.1), 6.85 (1H, d,
J = 1.5), 6.77 (1H, d, J = 1.5), 6.70 (3H, m), 6.60 (2H, d, J = 8.9), 5.91 (2H, m), 4.64 (1H, dd, J = 2.7
and 8.9), 3.70 (3H, s), 3.31 (1H, m), 3.08 (1H, dd, J = 3.5 and 13.9), 2.41 (3H, s); 13C-NMR (CDCl3) δ
141.7, 139.7, 130.1, 124.0, 119.7, 115.6, 114.7, 108.6, 106.8, 101.1, 63.7, 55.7, 55.4, 21.4; MS (EI, 70
eV) m/Z (%) 409 (M+, 10), 269 (100), 256 (66), 148 (65), 122 (77), 91 (8); FT IR (cm-1) 3448 (br),
3307, 1510, 1239, 1035, 817.
1
4b: Rf (Hex/AcOEt 6:4) 0.23; tr 13.80 (Hex/AcOEt 1:1, 1 mL/min); H-NMR (CDCl3) δ 7.53 (2H,
m), 7.30 (2H, d, J = 78.1), 6.90 (2H, m), 6.71 (4H, m), 6.58 (2H, m), 5.91 (2H, m), 4.70 (1H, dd, J =
5.4 and 9.3), 3.70 (3H, s), 3.35 (1H, dd, J = 9.3 and 13.5), 2.95 (1H, dd, J = 5.4 and 13.5), 2.40 (3H, s).
20
3c: Rf (Hex/AcOEt 6:4) 0.39; tr 8.07 (Hex/AcOEt 1:1, 1 mL/min); [α]D +160.8 (CHCl3, c 1.04);
1
m.p. (MeOH) 184-185 °C; H-NMR (CDCl3) δ 7.48 (6H, m), 7.27 (2H, d, J = 8.1), 6.69 (2H, d, J =
8.5), 6.59 (2H, d, J = 8.5), 4.82 (1H, dd, J = 3.5 and 8.9), 3.70 (3H, s), 3.38 (1H, dd, J = 8.9 and 13.5),
13
3.14 (1H, dd, J = 3.5 and 13.5), 2.39 (3H, s); C NMR (CDCl3) δ 152.6, 145.7, 141.9, 140.1, 139.4,
19
136.6, 126.8, 125.9 (q, J = 3.7), 124.0, 115.3, 114.7, 63.4, 55.6, 55.1, 21.4; F NMR (CDCl3) δ -63.6
(s); MS (EI, 70 eV) m/Z (%) 433 (M+, 12), 293 (100), 280 (30), 172 (11), 122 (51); FT IR (cm-1) 3629,
3020, 2977, 2401, 1514, 1421, 1326, 1217 (br), 1046, 929.
1
4c: Rf (Hex/AcOEt 6:4) 0.32; tr 10.31 (Hex/AcOEt 1:1, 1 mL/min); H-NMR (CDCl3) δ 7.50 (6H,
m), 7.31 (2H, m), 6.68 (2H, d, J = 8.5), 6.47 (2H, d, J = 8.5),4.83 (1H, m), 3.70 (3H, s), 3.41 (1H, m),
3.12 (1H, m), 2.40 (3H, s).
20
3d: Rf (Hex/AcOEt 6:4) 0.39; tr 8.49 (Hex/AcOEt 1:1, 1 mL/min); [α]D +111.1 (CHCl3, c 1.04);
m.p. (i-PrOH) 220-225 °C; 1H-NMR (CDCl3) δ 7.78 (5H, m), 7.44 (5H, m), 7.32 (1H, d, J = 8.1), 7.21
(2H, d, J = 7.3), 6.65 (2H, d, J = 8.9), 4.89 (1H, dd, J = 4.2 and 7.3), 3.67 (3H, s), 3.28 (2H, m), 2.33
(3H, s); MS (EI, 70 eV) m/Z (%) 415 (M+, 3), 275 (75), 260 (25), 154 (42), 122 (100), 91 (33); FT IR
(cm-1) 3304, 3052, 2939, 2829, 1542, 1511, 1396, 1243, 1014, 825.
1
4d: Rf (Hex/AcOEt 6:4) 0.33; tr 10.94 (Hex/AcOEt 1:1, 1 mL/min); H-NMR (CDCl3) δ 7.76 (5H,
m), 7.51 (5H, m), 7.29 (2H, d, J = 8.1), 6.63 (3H, m), 4.94 (1H, m), 3.69 (3H, s), 3.52 (1H, m), 3.08
(1H, m), 2.41 (3H, s).
20
3e: Rf (Hex/AcOEt 6:4) 0.25; tr 12.25 (Hex/AcOEt 1:1, 1 mL/min); [α]D +133.4 (CHCl3, c 0.68);
m.p. (MeOH): 191-193 °C; 1H-NMR (CDCl3) δ 8.11 (2H, d, J = 8.3), 7.53 (2H, d, J = 8.3), 7.44 (2H,
d, J = 7.9), 7.30 (2H, d, J = 9.4), 6.68 (4H, m), 4.86 (1H, br m), 3.71 (3H, s), 3.48 (1H, br m), 3.15
(1H, br m), 2.40 (3H, s); 13C-NMR (CDCl3) δ 149.3, 142.1, 139.9, 139.2, 130.2, 127.4, 124.3, 115.3,

Molecules 2001, 6
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114.8, 62.8, 55.6, 55.2, 29.7; MS (EI, 70 eV) m/Z (%) 410 (M+, 12), 270 (100), 255 (41), 209 (20), 139
(30), 122 (54), 91 (39); FT IR (cm-1) 3307, 2923, 1520, 1348, 1238, 1036, 1014, 809.
20
4e: Rf (Hex/AcOEt 6:4) 0.19; tr 16.11 (Hex/AcOEt 1:1, 1 mL/min); [α]D +44.0 (CHCl3, c 0.47);
1H-NMR (CDCl3) δ 8.17 (2H, d, J = 8.3), 7.64 (2H, d, J = 8.7), 7.53 (2H, d, J = 8.3), 7.32 (2H, d, J =
7.9), 6.68 (2H, d, J = 8.7), 6.54 (2H, d, J = 8.7), ), 4.91 (1H, m), 3.70 (3H, s), 3.38 (1H, m), 3.04 (1H,
m), 2.41 (3H, s).
20
3f: Rf (Hex/AcOEt 6:4) 0.25; tr 7.53 (Hex/AcOEt 1:1, 1 mL/min); [α]D +285.0 (CHCl3, c 0.77);
1
m.p. (MeOH) 210-212 °C; H-NMR (CDCl3) δ 7.56 (2H, d, J = 8.3), 7.29 (2H, d, J = 7.9), 7.14 (1H,
m), 6.76 (6H, m), 4.42 (1H, dd, J = 4.8 and 9.9), 3.70 (3H, s), 3.41 (1H, dd, J = 9.9 and 13.0), 3.19
(1H, dd, J = 4.8 and 13.0), 2.39 (3H, s); 13C-NMR (CDCl3) δ 163.1, 159.2, 154.0, 141.6, 140.7, 130.0,
19
124.1, 117.1, 114.7, 112.0, 111.7, 62.1, 55.6, 46.0, 21.4; F NMR (CDCl3) δ -115.5 (s); MS (EI, 70
eV) m/Z (%) 401 (M+, 6), 261 (100), 248 (19), 139 (82), 122 (99); FT IR (cm-1) 3446 (br), 1510, 1241,
1035, 808.
1
4f: Rf (Hex/AcOEt 6:4) 0.15; tr 14.07 (Hex/AcOEt 1:1, 1 mL/min); H-NMR (CDCl3) δ 7.55 (2H,
m), 7.29 (2H, m), 7.15 (1H, m), 6.71 (6H, m), 5.43 (1H, m), 3.68 (3H, s), 3.36 (1H, m), 3.13 (1H, m),
2.41 (3H, s); 19F NMR (CDCl3) δ -116.1 (s).
Crystal data for 3f. C44H42F4N2O4S2, f.w. 802.92, Triclinic, space group P1, a = 5.641(1) Å, b =
11.685(2) Å, c = 15.701(3) Å, α = 92.94(1)°, β = 98.34(1)°, γ = 101.75(1)°, V = 999.0(3) Å3, Z = 1, Dc
= 1.335 g/cm3, µ = 1.749 mm-1, F(000) = 420.
Data collection. X-ray diffraction data were collected from a colourless prismatic crystal of (2S,RS)-
3f (size 0.2 x 0.2 x 0.1), with graphite monochromated Cu-Kα radiation (λ = 1.5418 Å) on a Siemens
P4 diffractometer (θ-2θ scan technique). 6624 Reflections were collected (2.85<θ<67.99; +h,+k,+l and
–h,-k,-l), 6624 unique; 3 standard reflections, measured every 100 reflections, showed no significant
decay. Data were corrected for Lorentz and polarization effects and an empirical absorption correction
was applied.
Structure analysis and refinement. The structure was solved by direct methods using SIR92 [6] and
refined by full-matrix least squares on F2 using SHELXL97 [7]. Non hydrogen atoms were refined
anisotropically. All hydrogen atoms have been included at calculated positions and refined with group
temperature factors. Final values of the residual R1 for reflections with I>2σ and for all reflections
were respectively 0.050 and 0.061. The highest peak and hole in the final difference-Fourier map were
0.263 and -0.291 eÅ3. The refined value of Flack’s parameter [8] was –0.004(19).

Molecules 2001, 6
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References and Notes
1. (a) Walker, A. J. Tetrahedron: Asymmetry 1992, 3, 961-998. (b) Carreño, M. C. Chem. Rev. 1995,
95, 1717-1760.
2. Bravo, P.; Capelli, S.; Crucianelli, M.; Guidetti, M.; Markovsky, A. L.; Meille, S. V.; Soloshonok,
V. A.; Sorochinsky, A. E.; Viani, F.; Zanda, M. Tetrahedron 1999, 55, 3025-3040.
3. (a) Tsuchihashi, G.; Iriuchijima, S.; Maniwa, K. Tetrahedron Lett. 1973, 14, 3389-3392. (b)
Ronan, B.; Marchalin, S.; Samuel, O.; Kagan, H. B. Tetrahedron Lett. 1988, 29, 6101-6104. (c)
Pyne, S. G.; Boche, G. J. Org. Chem. 1989, 54, 2663-2667. (d) Pyne, S. G.; Dikic, B. J. Chem.
Soc., Chem. Commun. 1989, 826-827. (e) Pyne, S. G.; Dikic, B. J. Org. Chem. 1990, 55, 1932-
1936.
4. Selected molecular dimensions of (2S,RS)-3f are reported in Table 2. Bond lengths and angles fall
all in the expected range (Allen, F.M.; Kennard, O; Watson, D.G.; Brammer, L.; Orpen, A.G.;
Taylor, R.J.; J. Chem. Soc., Perkin Trans. 2, 1987, S1-S19). The two molecules in the unit cell are
configurationally identical but not related by any symmetry operation. The angles found on atoms
N1 and N2 suggest a planar arrangement, which is common for aromatic amines. The high thermal
parameters and the corresponding elongated thermal ellipsoids of atom C(3) and C(6) suggest
disorder involving these atoms. In the crystal, the molecules of (2S,RS)-3f pack arranging the
aromatic rings of adjacent molecules with a favourable herringbone interaction geometry.
Table 2
Bond lengths (Å)
S(1)-O(1)
Bond angles (°)
C(31)-N(1)-C(2)
C(61)-N(2)-C(5)
O(1)-S(1)-C(11)
O(1)-S(1)-C(1)
C(11)-S(1)-C(1)
O(3)-S(2)-C(41)
O(3)-S(2)-C(4)
C(41)-S(2)-C(4)
Torsion angles (°)
C31-N1-C2-C1
1.501(3)
122.9(3)
122.5(3)
107.3(2)
104.8(2)
96.69(17)
108.1(2)
105.2(2)
97.08(18)
S(2)-O(3)
N(1)-C(31)
N(1)-C(2)
N(2)-C(61)
N(2)-C(5)
C(52)-F(4)
C(56)-F(3)
C(26)-F(2)
C(22)-F(1)
1.479(4)
1.376(5)
1.453(5)
1.390(5)
1.442(5)
1.345(6)
1.358(5)
1.356(5)
1.351(6)
-169.39(0.34)
-164.02(0.34)
78.16(0.35)
68.33(0.38)
C61-N2-C5-C4
S1-C1-C2-N1
S2-C4-C5-N2

Molecules 2001, 6
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5. (a) Ojima, I.; Kwon, H. B. J. Am. Chem. Soc. 1988, 110, 5617-5621. (b) Soloshonok, V. A.;
Kacharov, A. D.; Hayashi, T. Tetrahedron 1996, 52, 245-254.
6. Altomare, A.; Cascarano G., Guagliardi A. J. Appl. Cryst. 1993, 26, 343-350.
7. Sheldrick G., SHELXL97, Program for crystal structure refinement; University of Göttingen,
Germany, 1997.
8. Flack H. D. Acta Cryst 1983, A 39, 876-880.
Sample Availability: Available from the authors.
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