Asymmetric synthesis of a chiral arsinophosphine via a metal template promoted asymmetric Diels-Alder reaction between diphenylvinylphosphine and 2-furyldiphenylarsine

Article abstract of DOI:10.1016/j.jorganchem.2006.07.025

The asymmetric Diels-Alder reaction between 2-furyldiphenylarsine and diphenylvinylphosphine was achieved stereospecifically by utilizing an organoplatinum reaction promoter containing the ortho-metalated (R)-(1-(dimethylamino) ethyl)-naphthalene as the c

Full text of DOI:10.1016/j.jorganchem.2006.07.025

Journal of Organometallic Chemistry 691 (2006) 4753–4758
www.elsevier.com/locate/jorganchem
Asymmetric synthesis of a chiral arsinophosphine via a metal
template promoted asymmetric Diels–Alder reaction between
diphenylvinylphosphine and 2-furyldiphenylarsine
a
a
a
b
a,
*
Kien-Wee Tan , Fengli Liu , Yongxin Li , Geok-Kheng Tan , Pak-Hing Leung
a
Division of Chemistry and Biological Chemistry, Nanyang Technological University, Nanyang Walk, Singapore 637616, Singapore
b
Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 119260, Singapore
Received 7 June 2006; received in revised form 18 July 2006; accepted 20 July 2006
Available online 1 August 2006
Abstract
The asymmetric Diels–Alder reaction between 2-furyldiphenylarsine and diphenylvinylphosphine was achieved stereospecifically by
utilizing an organoplatinum reaction promoter containing the ortho-metalated (R)-(1-(dimethylamino) ethyl)-naphthalene as the chiral
auxiliary. The optically pure (+)-As–P heterobidentate cycloadduct could be liberated from the template product by successive treatment
with concentrated hydrochloric acid and aqueous potassium cyanide.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Asymmetric Diels–Alder; Chiral metal template; Optically active arsinophosphine; Vinylphosphine; Arsine substituted furan
1. Introduction
moted asymmetric Diels–Alder reaction [24]. We found that
this approach could be applied to the asymmetric synthesis
between diphenylvinylphosphine and various cyclic dienes
e.g. 2-furyldiphenylphosphine promoted by an ortho-meta-
lated platinum(II) complex containing the N,N-dimethyl-
1-(1-naphthyl)ethylamine as the chiral auxiliary [25]. By
using the same reaction promoter, we hereby report a highly
regio- and stereoselective cycloaddition reaction between
the diphenylvinylphosphine and 2-furyldiphenylarsine
which led to the efficient synthesis of the corresponding opti-
cally pure arsinophosphine chelate bearing the oxanorborn-
ene backbone, under mild conditions.
Over the past decade, the design, synthesis and applica-
tions of chiral phosphines in the field of asymmetric catalysis
[1–9] have seen to grow perpetually. In contrast to their
phosphine analogues, chiral arsines are less extensively stud-
ied due to the technical challenges encountered in their syn-
thesis. Hence, their applications have been scarcely reported
even to date [10–13]. Among the various types of chiral
phosphines reported in the literatures, much interests are
being focused on chiral diphosphines [2,14–21]. On the other
hand, the interesting class of chiral bidentate arsinophos-
phines have been shown to be efficient catalyst supporters
[22,23]. Accordingly, the relatively unexplored chemistry
of chiral arsinophosphines is an interesting field that should
be developed further. Our group has previously prepared
various chiral diphosphines containing different functional-
ities with high efficiency by means of the metal-template pro-
2. Experimental
2.1. Materials and procedures
All reactions involving air-sensitive compounds were per-
formed under a positive pressure of purified nitrogen using
standard Schlenk techniques. The solvents were dried over
appropriate drying agents and degassed by standard method.
Bis(l-chloro)-bis{(R)-1-(dimethylamino)ethyl]-napthyl-C2,N}
*
Corresponding author. Fax: +65 63166984.
E-mail address: pakhing@ntu.edu.sg (P.-H. Leung).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.07.025

4754
K.-W. Tan et al. / Journal of Organometallic Chemistry 691 (2006) 4753–4758
4
diplatinum (II) (Rc)-1 [25] and diphenylvinylphosphine
[34] were prepared according to literature methods.
(m, 1H, H6), 2.66 (d, 3H, JPH = 2.1 Hz, NMe), 3.20 (d,
3H, JPH = 3.6 Hz, NMe), 4.80 (qn, 1H, JHH = 4JPH
=
¼
4
3
3
3
0
A Bruker ACF 300 spectrometer was used to record the
1H (300 MHz) and 31P{1H} (121 MHz) NMR spectra.
Optical rotations were measured on the specified solution
in a 1-dm cell at 25 ꢁC with a Perkin–Elmer Model 341
polarimeter. Melting points were determined on a Buchi
B-545 apparatus. Elemental analyses were performed by
the Elemental Analysis laboratory of the Department of
Chemistry at the National University of Singapore.
6.0 Hz, CHMe), 5.10 (dd, 1H, JHH = 4.4 Hz, JHH
3
1:2 Hz, H1), 6.12 (d, 1H, JHH = 5.6 Hz, H3), 6.60 (dd,
3
3
0
1H, JHH = 5.8 Hz, JHH ¼ 1:2 Hz, H2), 6.77 (dd, 1H,
3JHH = 3.2 Hz, JPH = 10.1 Hz, H5), 7.39–7.93 (m, 26H,
2
aromatics); 31P{1H} NMR (CD2Cl2) d 48.0 (s, JPtP
=
3602 Hz).
2.4. Preparation of dichloro {(1a,4a,5a(S))-[4-(diphenyl-
arsino)-5-(diphenylphosphino)-7-oxabicyclo[2.2.1] hept-2-
ene-As,P} platinum(II), (+)-exo-5
2.2. Preparation of 2-furyldiphenylarsine ligand
Sodium diphenylarsenide was prepared by addition of
flatten Na metal (1.00 g, 43.50 mmol) to a stirring solution
of diphenylarsine (4.60 g, 20.00 mmol) in dried THF
(100 mL) for 16 h. The sodium diphenylarsenide thus pre-
pared was added dropwise (over 1 h) to a solution of 2-bro-
mofuran (3.00 g, 20.41 mmol) in THF (100 mL) at ꢀ79 ꢁC
(acetone/dry ice bath). The reaction mixture was warmed
to room temperature and then further refluxed for 3 h.
The solvent was removed under reduced pressure and a sat-
urated ammonium chloride solution (10.00 g in 50 mL of
water) was added slowly into the reaction mixture. The
product was then extracted with diethyl ether. The organic
layer was separated and dried over magnesium sulfate.
The solvent was removed and the residue (brownish oil)
was distilled (bp 120–130 ꢁC; 0.8 mmHg) to give the
The naphthylamine auxillary in the crude reaction prod-
uct (Rc)-exo-4a could be chemoselectively removed by the
addition of concentrated hydrochloric acid (25 mL) to a
solution of the complex (Rc)-exo-4a (0.30 g, 0.30 mmol)
in dichloromethane (60 mL). The mixture was stirred vig-
orously at room temperature for 12 d, washed with water
(3 · 50 mL), and dried (MgSO4). Crystallization of the
crude product from dichloromethane–diethyl ether gave
pure (+)-exo-5 as white crystals: 0.24 g (89% yield); mp
264–266 ꢁC; [a]D = +77.4 (c 0.3, CH2Cl2); Anal. Calc. for
C30H26Cl2OAsPPt: C, 46.5; H, 3.4; Found C, 46.3; H,
1
3.3; H NMR (CD2Cl2) d 1.65–1.75 (m, 1H, H5), 2.04–
0
2.16 (m, 1H, H6 ), 2.36–2.41 (m, 1H, H6), 5.24 (dd,
3
3
3
0
JHH = 4.4 Hz, JHH ¼ 1:2 Hz, H1), 6.19 (d, 1H, JHH
=
3
3
0
5.6 Hz, H3), 6.57 (dd, 1H, JHH = 4.4 Hz, JHH ¼ 1:2 Hz,
H2), 7.39–8.15 (m, 20H, aromatics); 31P{1H} NMR
(CD2Cl2) d 48.9 (s, JPtP = 3476 Hz).
1
product as a colourless viscous oil: yield 4.47 g (73%); H
3
3
NMR (CDCl3): d 6.44 (dd, 1H, JHH = 1.8 Hz, JHH
=
3
1.2 Hz, OC@CH), 6.52 (d, 1H, JHH = 3.2 Hz, Ph2AsC
3
@CH), 7.34–7.46 (m, 10H, aromatics), 7.68 (d, 1H, JHH
1.6 Hz, OCH).
=
2.5. Liberation of the free ligand {(1a,4a,5a(S))-[4-
(diphenylarsino)-5-(diphenylphosphino)-7-
oxabicyclo[2.2.1] hept-2-ene-As,P}, (+)-exo-6
2.3. Synthesis of {R-1-[1-(dimethylamino)ethyl]-2-
naphthalenyl-C,N} {(1a,4a,5a(S))-[4-(diphenylarsino)-5-
(diphenylphosphino)-7-oxabicyclo[2.2.1] hept-2-ene-As,P}
platinum(II) tetrafluoroborate, (Rc)-exo-4a
A mixture of the dichloro complex (+)-exo-5 (0.03 g,
0.04 mmol) in dichloromethane (20 mL) and aqueous
potassium cyanide (2.00 g, 5 mL) was stirred vigorously
for 2 h. The aqueous phase was separated, and the organic
layer was washed with water (3 · 10 mL) and dried over
magnesium sulfate. Removal of the solvent under vacuum
gave ligand (+)-exo-6 as an air-sensitive white solid in 81%
yield (0.017 g); [a]D = +26.2 (c 0.7, CH2Cl2); 31P {1H}
NMR (CDCl3): d ꢀ9.7 (s).
The dimeric complex (Rc)-1 (0.68 g, 0.79 mmol) dissolved
in dichloromethane (500 mL) was treated with diphenylvi-
nylphosphine (0.34 g, 1.58 mmol) to give the monophos-
phine complex (Rc)-2. This solution was directly treated
with silver tetrafluoroborate (0.50 g, 2.57 mmol) in water
(2 mL) to generate the cationic complex (Rc)-3 which
was directly treated with 2-furyldiphenylarsine (0.47 g,
1.58 mmol). The resulting mixture was stirred vigorously
at room temperature for 2 h, filtered (to remove silver chlo-
ride), washed with water, and then dried (MgSO4). The reac-
tion was monitored by 31P NMR spectroscopy and was
found to complete in 22 d. Slow addition of diethyl ether
to the crude reaction mixture gave complex (Rc)-exo-4a as
pale yellow micro-crystals: (1.09 g, 70%); mp 276–278 ꢁC
(decomp.); [a]D = +92.9 (c 0.4, CH2Cl2); Anal. Calc. for
C44H42NOAsPPtBF4: C, 53.4; H, 4.3; N, 1.4; Found C,
3. Results and discussion
The arsine substituted cyclic diene, 2-furyldiphenylar-
sine was obtained as a colourless oil in 73% yield via the
nucleophilic addition of diphenylarsenide to 2-bromofu-
ran. In the absence of a transition metal ion, no reaction
was observed between diphenylvinylphosphine and 2-furyl-
diphenylarsine. In the presence of the chiral reaction pro-
moter (Rc)-1, however, the asymmetric cycloaddition
reaction proceeded smoothly at room temperature. As
shown in Scheme 1, the addition of diphenylvinylphos-
phine to a stoichiometric amount of (Rc)-1 generated the
1
53.0; H, 4.4; N, 1.4; H NMR (CD2Cl2) d 1.94 (d, 3H,
3
0
JHH = 6.4 Hz, CHMe), 1.99–2.10 (m, 1H, H6 ), 2.37–2.40

K.-W. Tan et al. / Journal of Organometallic Chemistry 691 (2006) 4753–4758
4755
monophosphine complex (Rc)-2. Treatment of the crude
complex (Rc)-2 with silver tetrafluoroborate removed the
kinetically stable chloro ligand to generate the relatively
reactive cationic species (Rc)-3. In order to optimize the
efficiency, the tetrafluoroborato species generated (Rc)-3
was not isolated in routine synthesis. The diphenylvinyl-
phosphine complex prepared earlier was dissolved in a
large volume of dichloromethane [1.02 g (1.58 mmol) in
500 mL] and then treated directly with a stoichiometric
quantity of 2-furyldiphenylarsine at room temperature.
The Diels–Alder reaction was monitored by the 31P
NMR spectroscopy and was found to complete in 22 d.
The cycloadduct complex (Rc)-exo-4a was obtained as pale
yellow micro-crystals in 70% isolated yield: [a]D = +92.9 (c
0.4, CH2Cl2). Prior to purification, the 31P NMR spectrum
of the crude cycloadducts in CD2Cl2 exhibited a sole singlet
signal at d 48.0 (JPtP = 3602 Hz). No other 31P NMR sig-
nals could be detected from the 121 MHz NMR spectrum,
thus indicating that only a single diastereomer was gener-
ated in the Diels–Alder reaction. It is noteworthy that the
amount of solvents used in this reaction plays a vital role
towards the reaction selectivity. At higher concentration,
the reaction became less stereoselective. For example when
the reaction was conducted using 0.14 g of (Rc)-3 and
0.06 g of 2-furyldiphenylarsine in 30 mL of CH2Cl2, the
31P NMR spectrum showed that two isomeric products
were generated in the ratio of 1:3.
Fig. 1. Molecular structure and absolute stereochemistry of the cationic
complex (Rc)-exo-4a.
regio-selectivity is consistent with a similar P–As complex
containing the chiral naphthylamine auxiliary [27]. Selected
bond lengths and angles are given in Table 1.The Pt–As(1)
˚
and As–C(15) bond distances of 2.436(1) and 1.959(9) A
observed are longer than those recorded for the analogous
diphosphine complex in which the Pt–P bond distance is
2.327(2) with the corresponding P–C bond length of
A single crystal X-ray analysis of (Rc)-exo-4a confirmed
the coordination chemistry of the new As–P bidentate che-
late. The absolute stereochemistries at C(15), C(18) and
C(20) are S, R and S, respectively (Fig. 1). This assignment
is based upon both anomalous scattering and by internal
reference to the known stereocentre at C(11) of the naph-
thylamine ligand. The phosphorus atom is positioned trans
to the r-donating NMe2 group while the arsenic analogue is
trans to the p-accepting naphthalene carbon atom. Such
˚
1.850(7) A [25]. Such phenomenon is clearly attributed to
the different electronic properties between the phosphorus
and arsenic donors [26]. Additionally, the coordination
geometry at the platinum centre is slightly distorted square-
planar with angles ranging from 79.6(3)–98.2(2) and
174.0(3)–175.4(2) [28–33]. The bond angle at the bridgehead
oxygen [95.8(8)ꢁ] is typical of oxanorbornene skeletons
[25,28,33].
Me
Me
Me
Me
Me
Me
Me
R
Me
R
Me
R
N
N
N
Cl
P
Solvent
Cl
Ph2P
AgBF4
+
Pt
Pt
Pt
-
P
BF4
2
Ph
Ph
Ph
Ph
(Rc)-2
(Rc)-1
(Rc)-3
Me
Me
O
AsPh2
Ph2
As
3
O
6
Ph2
As
Ph2
3
3
O
6
Me
R
N
+
As
S
S
2
1
conc. HCL
KCN
2
1
Cl
Cl
2
1
S
S
Pt
O
6
Pt
S
S
R
R
R
P
Ph2
P
P
H
-
H
Ph2
H
BF4
Ph2
(Rc)-exo-4a
(+)-exo-5
(+)-exo-6
Scheme 1.

4756
K.-W. Tan et al. / Journal of Organometallic Chemistry 691 (2006) 4753–4758
Table 1
Table 2
Selected bond lengths (A) and angles (ꢁ) for (+)-exo-5
˚
˚
Selected bond lengths (A) and angles (ꢁ) for (Rc)-exo-4a
Pt–C(1)
Pt–P(1)
As(1)–C(15)
As(1)–C(33)
P(1)–C(21)
O(1)–C(15)
C(15)–C(16)
C(16)–C(17)
2.059(7)
2.254(2)
1.959(9)
1.922(8)
1.82(1)
1.42(1)
1.50(1)
1.32(2)
1.55(2)
Pt–N(1)
Pt–As(1)
P(1)–C(20)
As(1)–C(39)
P(1)–C(27)
O(1)–C(18)
C(15)–C(20)
C(17)–C(18)
2.113(7)
2.437(1)
1.877(9)
1.94(1)
1.81(1)
1.48(1)
1.55(3)
1.47(2)
1.53(1)
96.4(2)
Pt–As(1)
Pt–Cl(1)
As(1)–C(4)
As(1)–C(13)
P(1)–C(19)
C(1)–C(2)
C(2)–C(3)
C(4)–C(5)
C(1)–O(1)
As(1)–Pt–Cl(1)
P(1)–Pt–Cl(1)
P(1)–Pt–As(1)
C(4)–As(1)–Pt
C(5)–C(4)–As(1)
C(4)–O(1)–C(1)
C(3)–C(4)–C(5)
C(2)–C(1)–C(6)
C(4)–C(5)–C(6)
2.307(6)
2.357(2)
1.970(6)
1.928(6)
1.833(6)
1.50(1)
1.309 (9)
1.564(8)
1.574(8)
89.78(5)
176.40(6)
88.89(4)
106.5(2)
111.7(4)
95.1(4)
Pt–P(1)
Pt–Cl(2)
As(1)–C(7)
P(1)–C(5)
P(1)–C(25)
C(1)–C(6)
C(3)–C(4)
C(5)–C(6)
2.218(1)
2.344(2)
1.920(5)
1.871(6)
1.808 (6)
1.528(9)
1.496(8)
1.567(8)
1.332(1)
176.59(4)
90.11(5)
91.40(6)
111.9(2)
112.6(4)
105.9(6)
105.6(6)
101.0(5)
C(18)–C(19)
C(19)–C(20)
C(4)–O(1)
C(1)–Pt–As(1)
C(1)–Pt–N(1)
N(1)–Pt–P(1)
C(15)–As(1)–Pt
C(20)–C(15)–As(1)
C(16)–C(15)–C(20)
C(16)–C(17)–C(18)
C(16)–C(17)–C(18)
C(19)–C(20)–C(15)
C(15)–O(1)–C(18)
174.0(3)
C(1)–Pt–P(1)
N(1)–Pt–As(1)
P(1)–Pt–As(1)
C(20)–P(1)–Pt
C(15)–C(20)–P(1)
C(17)–C(16)–C(15)
C(17)–C(18)–C(19)
C(20)–C(19)–C(18)
C(19)–C(24)–C(15)
As(1)–Pt–Cl(2)
P(1)–Pt–Cl(2)
Cl(2)–Pt–Cl(1)
C(5)–P(1)–Pt
C(4)–C(5)–P(1)
C(2)–C(3)–C(4)
C(3)–C(2)–C(1)
C(1)–C(6)–C(5)
79.6(3)
175.4(2)
105.6(3)
111.2(6)
109.3(8)
107.3(11)
106.2(5)
100.6(7)
95.8(8)
98.2(2)
86.03(6)
112.9(3)
113.5(6)
105.0(12)
104.7(10)
102.3(8)
102.7(4)
106.6(5)
107.2(5)
99.7(4)
The C(4)–O(1)–C(1) oxa-bridgehead angle is recorded as
95.1(4)ꢁ. It is noteworthy that the bridgehead C–O bonds
in the As–P chelating exo-cycloadduct (Rc)-exo-4a, are
not affected by the strong acidic conditions. As desired,
the absolute configurations of the three chiral carbon cen-
tre at C(1), C(4) and C(5) [R, S and S, respectively] are
identical to their counterpart in the original template com-
plex (Rc)-exo-4a (Table 3).
The optically pure arsinophosphine bidentate ligand
could be liberated stereospecifically from (+)-exo-5 by
treatment of the dichloro complex with aqueous potassium
cyanide. The heterobidentate ligand (+)-exo-6 was thus
obtained as an air-sensitive white solid in 81% yield:
[a]D = +26.2 (c 0.7, CH2Cl2). The 31P NMR spectrum of
(+)-exo-6 in CDCl3 exhibited a sharp signal at d ꢀ9.7. In
order to establish the optical purity of the liberated ligand,
the heterobidentate ligand was re-coordinated to (Rc)-1,
followed by the treatment with stoichiometric amount of
The chiral naphthylamine auxiliary in (Rc)-exo-4a could
be chemoselectively removed by treatment of the As–P
coordination complex with concentrated hydrochloric acid.
The neutral dichloro complex (+)-exo-5 thus obtained as
white crystals in 89% yield; [a]D = +77.4 (c 0.3, CH2Cl2).
The 31P NMR spectrum of (+)-exo-5 in CD2Cl2 exhibited
a singlet resonance at d 48.9 (JPtP = 3476 Hz). The molec-
ular structure of the dichloro complex is shown in Fig. 2.
Selected bond lengths and angles are given in Table 2.
Table 3
Crystallographic data for complexes (Rc)-exo-4a and (+)-exo-5
(Rc)-exo-4a
(+)-exo-5
Formula
C
988.58
44H42AsBF4NOPPt
C30H26AsCl2OPPt
774.39
P212121
Orthorhombic
10.1554(4)
15.8084(7)
17.1282 (8)
2749.8(2)
4
Formula weight
Space group
Crystal system
P212121
Orthorhombic
11.7784(6)
15.6568(8)
21.159(1)
3901.9(3)
4
˚
a (A)
˚
b (A)
˚
c (A)
3
˚
V (A )
Z
T (K)
223(2)
1.683
0.71073 (Mo)
4.534
223(2)
1.871
0.71073 (Mo)
6.572
qcalc. (g cmꢀ3
)
˚
k (A)
l (mmꢀ1
)
Flack parameters
R1 (obsd data)a
wR2 (obsd data)b
0.001(10)
0.0756
0.1054
0.011(6)
0.0368
0.0659
P
P
a
Fig. 2. Molecular structure and absolute stereochemistry of the neutral
dichloro complex (+)-exo-5.
R1 ¼ kF oj ꢀ jF ck= jF oj.
P
p
2
2
b
wR2 ¼
f
½wðF 2o ꢀ F 2c Þ ꢁg; wꢀ1 ¼ r2ðF 2oÞ þ ðaPÞ þ bP.

K.-W. Tan et al. / Journal of Organometallic Chemistry 691 (2006) 4753–4758
4757
Ph2
As
3
O
6
2
1
S
S
P
Ph2
R
H
(+)-exo-6
AgBF4
(Rc )-1
Me
Me
Me
Me
Ph2
P
Ph2
As
3
O
6
H
6
O
3
Me
Me
R
R
N
N
R
+
+
2
1
1
2
S
S
S
S
Pt
Pt
+
As
Ph2
R
P
Ph2
H
-
-
BF4
BF4
(Rc)-exo-4a
(Rc)-exo-4b
Scheme 2.
silver tetrafluoroborate (Scheme 2). This re-coordination
process generated two distinct regioisomers [(Rc)-exo-4a
and (Rc)-exo-4b (Scheme 2)]. The 31P NMR spectrum of
the re-coordination crude products in CDCl3 indeed
showed two singlet resonances in equal ratio at d 48.0
(JPtP = 3586 Hz) and 55.2 (JPtP = 1797 Hz) respectively.
The former being identical to that recorded by (Rc)-exo-
4a and the large Pt–P coupling constant observed is attrib-
uted to the trans influence exerted by the NMe2 group. On
the other hand, the latter shows a smaller Pt–P coupling
constant which is the characteristic indication for this class
of platinum complex with the ascribed to the PPh2 group
coordinated trans to the strong p-accepting naphthalene
carbon atom. Hence the signal at d 55.2 is unambiguously
assigned to the regioisomer (Rc)-exo-4b, which was not
generated directly from the Diels–Alder reaction. Since
there are no other 31P signals arising from the other possi-
ble diastereomers, this re-coordination process established
that the liberated arsinophosphine ligand is optically pure.
In comparison, the cycloaddition reaction between the
arsenic cyclic diene with diphenylvinylphosphine is rela-
tively slower than its phosphorus counterpart which
requires 6 d to complete the analogous reaction [25]. It is
well-established that most classic cycloaddition reactions
require activation of the cyclic dienes or dienophiles. The
current studies thus provide a direct comparison of the
reactivities between a phosphorus substituted furan diene
and its arsenic counterpart. In subsequent work, it will be
shown that the information obtained in this preliminary
study is crucial to the rational utilization of the stereoelec-
tronic properties of the arsenic ligand in some stereochem-
ically highly demanding organic transformations.
4. Supplementary data
Crystallographic data for the structural analysis has
been deposited with the Cambridge Crystallographic
Data Centre, CCDC 608020 for compound (Rc)-exo-4a
and CCDC 608021 for compound (+)-exo-5. Copies of
this information may be obtained free of charge from:
The Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK. Fax. (int code) +44 (1223) 336 033 or Email:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.
ac.uk.
Acknowledgement
We are grateful to the National University of Singapore
for the support of a scholarship to K.W.T.
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