Enantioselective synthesis of both enantiomers of vicinal norbornanediamines through the Leuckart reaction of 2-norbornanones

Article abstract of DOI:10.1016/S0957-4166(01)00376-7

The reaction of 2-norbornanones bearing an amine derivative in the bridgehead position with formamide and formic acid yields enantiomerically pure 1,2-norbornane diamides. These compounds are excellent precursors of chiral vicinal diamines, which have been increasingly used due to their numerous applications in medicinal chemistry and asymmetric synthesis. The rigid structure of the norbornane framework allows the study of conformational equilibrium in the formamide groups and its dependence on other substituents in the molecule.

Full text of DOI:10.1016/S0957-4166(01)00376-7

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 12 (2001) 2153–2158
Enantioselective synthesis of both enantiomers of vicinal
norbornanediamines through the Leuckart reaction of
2-norbornanones
Antonio Garc´ıa Mart´ınez,a,* Enrique Teso Vilar,b,* Amelia Garc´ıa Fraileb and
Paloma Mart´ınez-Ruiza
aDepartamento de Qu´ımica Orga´nica I, Facultad de Ciencias Qu´ımicas, Universidad Complutense de Madrid,
Ciudad Universitaria s/n, E-28040 Madrid, Spain
bDepartamento de Qu´ımica Orga´nica y Biolog´ıa, Facultad de Ciencias, UNED, c/Senda del Rey 9, E-28040 Madrid, Spain
Received 20 July 2001; accepted 17 August 2001
Abstract—The reaction of 2-norbornanones bearing an amine derivative in the bridgehead position with formamide and formic
acid yields enantiomerically pure 1,2-norbornane diamides. These compounds are excellent precursors of chiral vicinal diamines,
which have been increasingly used due to their numerous applications in medicinal chemistry and asymmetric synthesis. The rigid
structure of the norbornane framework allows the study of conformational equilibrium in the formamide groups and its
dependence on other substituents in the molecule. © 2001 Published by Elsevier Science Ltd.
1. Introduction
chiral vicinal norbornanediamines have been described
to date.
Chiral, enantiomerically pure vicinal diamines and their
derivatives1 have been increasingly used in asymmetric
catalysis for reactions including epoxidations,2 aldol
condensations,3 nucleophilic additions to carbonyl
compounds4 and Diels–Alder cyclisations.5 The 1,2-
diamine functionality is also present in many natural
products with biological properties such as biotin,
cephalosporin or penicillins,1 as well as in human phar-
maceuticals such as antitumor agents, mainly in the
form of diamine–metal complexes.6,7 Over the past
decades, a large number of chiral cisplatin analogues
based on diaminocyclohexane derivatives, like oxali-
platin or ormaplatin, have been developed and high
activities have been achieved with less toxicity.6 In these
molecules, control of the configuration of the stereo-
genic centres is extremely important, as it has been
reported that the drugs prepared from the separated
diastereoisomers have different antitumor activities.8
The synthesis of chiral diamines with a norbornane
framework is a topic of interest because they constitute
rigid analogues of the diaminocyclohexane moiety.
However, owing to their difficult preparation, only the
synthesis and structural elucidation of 2,3-camphor-
diamine has been recently reported,9 and no other
One of the classical procedures for the synthesis of
amine derivatives is the Leuckart–Wallach reaction,
based on the reductive amination of carbonyl com-
pounds using mixtures of formamide, formic acid or
ammonium formate as reagents.10 The final product is
in all cases a formamide derivative, which, after reduc-
tion or hydrolysis in basic or acid medium, gives the
desired free amine.
* Corresponding authors. E-mail: palmarti@eucmax.sim.ucm.es
Scheme 1.
0957-4166/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0957-4166(01)00376-7

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In the course of our research, we have studied the
mechanism of the Leuckart reaction of 2-norbor-
nanones, and its dependence on the substitution pattern
of the substrate. In previous papers, we have described
the synthesis of both enantiomers, (1R,2R)- and
(1S,2S)-, of N-(3,3-dimethyl-2-formylamino-1-nor-
bornyl)acetamide 3 and ent-3, respectively (Scheme 1)11
through an ionic reaction pathway that includes skele-
ton rearrangements and intramolecular transamida-
tions. This compound is the only product of the
Leuckart reaction of bridgehead 2-oxo-1-norbornyl
acetates11,12 and acetamides 1, 2, 4 and 5 (see Scheme
1). Besides the mechanistic point of view, the relevance
of this process is determined by the interest in these
compounds as intermediates in the synthesis of chiral
diamine derivatives.
dard procedure (HCO2H/HCONH2, 150°C),11 and
monitored by GC/MS analysis. As shown in Scheme 2,
the first step is the acylation of the amine group to give
(1R)-N-(3,3-dimethyl-2-oxo-1-norbornyl)formamide 7.
This compound reacts with formamide in acid medium
to give carbocation 8; the reduction of this cation by
formate ion yields (1R,2R)-N-(3,3-dimethyl-2-formyl-
amino-1-norbornyl)formamide
9 as the sole final
product. Together with 3,3-dimethyl-formamide 7, a
small amount (less than 10%) of (1S)-N-(7,7-dimethyl-
2-oxo-1-norbornyl)formamide ent-12† is detected dur-
ing the course of the reaction, which also gives the
diformamide 9 as the final product. The appearance of
ent-12 can be explained in terms of an equilibrium
between carbocations 8 and 10 (Scheme 2), that com-
petes with the reduction of 8 by formate ion.
The reactivity of ent-12 with formamide/formic acid
has been studied by carrying out the Leuckart reaction
of (1R)-7,7-dimethyl-2-oxo-1-norbornylamine 13. GC/
MS monitoring of this process indicates that the first
intermediate is the 7,7-dimethylformamide 12 (Scheme
3). The characterisation of 12 was completed by isola-
tion from the reaction medium at low conversion ratios,
because it rapidly isomerises to the (1S)-N-(3,3-
dimethyl-2-oxo-1-norbornyl)formamide ent-7 through a
Wagner–Meerwein (W–M) rearrangement and an
intramolecular transamidation. Subsequently, we detect
an almost constant ratio between the two amides (ent-
7/12=3:1), followed by the appearance of diformamide
ent-9 as the sole final product (Scheme 3). According to
this, we can postulate two competitive reaction path-
ways starting from carbocation ent-10: one is the direct
reduction by formate ion, that directly yields ent-9, and
the other is the intramolecular transamidation to give
cation 14. The hydrolysis of one formyl group in this
2. Synthesis of vicinal diamines
Starting from diamide 3, it should be possible to obtain
in one step vicinal primary or secondary diamines
through hydrolysis or reduction of this compound,
respectively.13 However, all attempts to hydrolyse the
bridgehead acetamide group in acid or basic media
were unsuccessful. This prompted us to attempt the
synthesis of the title compounds through the Leuckart
reaction of 3,3- and 7,7-dimethyl-2-oxo-1-norbornyl-
amines 6 and 13, respectively, that bear a free primary
amine group at the bridgehead position. These sub-
strates were prepared in four steps starting from natu-
rally occurring (1R)-camphor and (1R)-fenchone,
respectively.14
The Leuckart reaction of (1R)-(3,3)-dimethyl-2-oxo-1-
norbornylamine 6 was carried out following the stan-
Scheme 2.
† In those cases where the two enantiomers of the same structure are formulated, the prefix ent- is assigned to the (1S)-isomer.

A. G. Mart´ınez et al. / Tetrahedron: Asymmetry 12 (2001) 2153–2158
2155
Scheme 4.
(1R,2R)-3,3-dimethyl-1,2-norbornanediamine 16 in 76%
yield. The opposite enantiomer ent-16 has also been
synthesised starting from diformamide ent-9; the mea-
surement of the [h]2D0 values of both compounds con-
firms the enantioselectivity of the global process.
3. Conformational equilibrium in norbornylformamides
One of the most studied properties of the formamide
group is the conformational equilibrium between trans
and cis isomers, because it constitutes the simplest
model for the understanding of the structural properties
of the peptide bond in proteins.15,16 We have previously
described the conformational equilibrium of 2-nor-
bornylformamides in CDCl3 and found different trans/
cis ratios depending on the substituents linked to the
norbornane framework.11,12 In this paper, we have
studied the behaviour of the synthesised 1-norbornyl-
formamides 7 and 12 in solution, and we can confirm
that the conformational equilibrium is strongly influ-
enced by the shape of the norbornane moiety. Thus, in
the case of 3,3-dimethylformamide 7, we have only
detected traces of the cis isomer in CDCl3 solution,
whereas the 7,7-dimethylformamide 12 shows a trans/
cis ratio=1.8:1. The introduction of a second form-
Scheme 3.
intermediate originates the formamide ent-7, whose
reactivity under these conditions has been studied in
Scheme 2. Using these two possible routes, (1S,2S)-
diformamide ent-9 is obtained as the only final product,
with an overall yield of 49% (98 h) and an endo/exo
ratio=19:1 (1H NMR 300 MHz).
As we have mentioned before, our strategy for the
synthesis of enantiopure vicinal diamines is based on
the transformation of 1,2-norbornanediamides 3 and 9.
Reduction of 3 was first attempted with LiAlH4;14
under various different conditions, a complex mixture
was obtained in all cases, and no free diamine could be
isolated. The reduction of the amide groups was then
achieved by treatment of the substrate with BH3·THF
complex in THF, which afforded (1R,2R)-3,3-dimethyl-
1-ethylamino-2-methylaminonorbornane 15 after 24 h
(63%) (Scheme 4).
1
amide moiety complicates the H NMR spectrum of
diformamide 9, and four conformers have been
detected in CDCl3 solution in a ratio of (trans,trans):
(trans,cis):(cis,trans):(cis,cis)=7.2:1.4:1.5:1, respectively.
To identify these rotamers, we have considered the
proton shift of the C(2) formyl groups, which must be
very similar to that of 1-acetamide-2-formamide 3, and
All attempts to carry out the hydrolysis of 3 in basic or
acid media were unsuccessful. Using diformamide 9 as
starting compound, the treatment with concentrated
KOH yields a complex mixture because of oxidation of
the free amine in the reaction medium. Thus, we carried
out the hydrolysis of 9 by reaction with boiling cc. HCl,
where the deprotection of both formamide groups
implies the stabilisation of the resulting diamine
through the corresponding dihydrochloride salt.
Finally, this procedure has allowed us to obtain
3
the different JH-N-CO-H of the cis and trans isomers
1
(10–13 and 0–2 Hz, respectively).11 The H NMR spec-
trum (CDCl3, 500 MHz) of diformamide ent-9, with the
assignment made through the formyl signals, is shown
in Fig. 1.
In summary, we have developed an expeditious method
for the enantioselective synthesis of new chiral 1,2-nor-
bornanediamines. Starting from two different naturally
occurring 2-norbornanones, we can obtain the two

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A. G. Mart´ınez et al. / Tetrahedron: Asymmetry 12 (2001) 2153–2158
Figure 1.
enantiomers of the target molecules while controlling
the configuration of the stereogenic centres.
pounds. Lower yields were observed after long reaction
times due to formation of dark-coloured polymeric
products. On completion of the reaction, a saturated
aqueous NaHCO3 solution (20 mL) was added and the
mixture was extracted with CH2Cl2 (4×30 mL). The
organic layer was washed with water (30 mL) and brine
(20 mL), and dried over anhydrous MgSO4. After
filtration, the solvent was evaporated in vacuo. The
residue was analysed by GC/MS and NMR (endo/
exo=19:1, 1H NMR 300 MHz, CDCl3). The crude
reaction mixture was decolourised with charcoal and
recrystallised in MeOH/Et2O; this procedure gave in all
cases the pure endo-epimer. To isolate the N-(2-oxo-1-
norbornyl)formamides 7 and 12, the reaction was
stopped at low conversion ratio (15–30 min). The mix-
ture was hydrolysed with water (20 mL) and extracted
with CH2Cl2 (3×20 mL), and the organic layer succes-
sively washed with 10% HCl solution (30 mL) and
saturated NaHCO3 (30 mL). After drying with MgSO4
and removal of the solvent, the crude product was
purified by column chromatography (silica gel, hex-
ane:ethyl acetate, 1:1), followed by recrystallisation of
the formamide in hexane.
4. Experimental
4.1. General
1H and 13C NMR spectra were obtained using Varian
VXR 300S, Brucker AMX500 and Brucker AC250
spectrometers, with tetramethylsilane as internal stan-
dard. Capillary GC/MS analyses were completed on a
Shimadzu QP-17A (column type: TRB-1, 30 m) cou-
pled to a Shimadzu QP-5000 mass spectrometer (EI, 60
eV). Melting points were recorded using a Gallenkamp
apparatus; values are uncorrected. Molecular rotations
were obtained using a Perkin–Elmer 241 spectropolar-
imeter. Elemental analysis was completed on a Perkin–
Elmer 2400 CHN analyser. For the preparation of the
amines 6 and 13 from (1R)-camphor and (1R)-fen-
chone, respectively, see Ref. 14. For the synthesis of
compound 3, see Refs. 11 and 12.
4.2. Leuckart reaction of 1-amino-2-norbornanones
4.3. (1R,2R)-N-(3,3-Dimethyl-2-formylamino-1-nor-
A mixture of the corresponding 1-amino-2-norbor-
nanone (2.0 mmol), formamide (32.2 mmol) and formic
acid (17.9 mmol) was heated to 150°C. The reaction
was monitored by GC/MS analysis until total disap-
pearance of the starting ketone and intermediate com-
bornyl)formamide 9
Following the procedure described above, the Leuckart
reaction of (1R)-1-amino-3,3-dimethyl-2-norbornanone
6 gave 0.24 g of 9 (60% yield, white prisms) as the final

A. G. Mart´ınez et al. / Tetrahedron: Asymmetry 12 (2001) 2153–2158
2157
1
product. H NMR analysis in CDCl3 solution showed
4.6. (1R)-N-(7,7-Dimethyl-2-oxo-1-norbornyl)formamide
12
four conformational isomers whose relative ratios
(trans,trans):(trans,cis):(cis,trans):(cis,cis) were 7.2:1.4:
1.5:1, respectively. Owing to the complexity of the
spectrum, not all the signals were assigned. Mp 142.9–
145.3°C. [h]2D0 −58.8 (c 0.50, MeOH). IR (KBr) w 3260,
3050, 2990, 2950, 1670, 1650, 1550, 1390 cm−1. 1H
NMR (500 MHz, CDCl3) l 8.27 (s, 1H trans,trans, 1H
cis,trans), 8.15 (d, J=12.3 Hz, 1H cis,trans), 8.09 (d,
J=1.8 Hz, 1H trans,cis), 8.08 (d, J=12.0 Hz, 1H
cis,cis), 8.04 (d, J=1.5 Hz, 1H trans,trans), 7.90 (d,
J=11.4 Hz, 1H cis,cis), 7.89 (d, J=11.4 Hz, 1H
trans,cis), 7.50–6.80 (m, 1H), 6.50 (br. s, 1H), 3.98 (dm,
J=9.0 Hz, 1H trans,cis, 1H cis,cis), 3.90 (dd, J=7.3
Hz, J=1.5 Hz, 1H trans,trans, 1H cis,trans), 3.13 (dm,
J=11.1 Hz, 1H cis,cis), 2.61 (dm, J=10.2 Hz, 1H
trans,cis), 2.17 (dd, J=9.9 Hz, J=1.5 Hz, 1H
trans,trans, 1H cis,trans), 2.10–1.40 (m, 6H), 1.16 (s, 3H
trans,trans, 3H cis,trans), 1.13 (s, 3H trans,cis), 1.11 (s,
3H cis,cis), 0.93 (s, 3H trans,trans), 0.90 (s, 3H cis,cis),
0.89 (s, 3H trans,cis, 3H cis,trans) ppm. 13C NMR (75
MHz, CDCl3). l trans,trans-Conformer: 163.3, 161.5,
66.7, 62.8, 45.9, 38.8, 38.0, 30.8, 25.3, 22.8, 21.0 ppm.
trans,cis-, cis,trans- and cis,cis-Conformers: 165.8,
165.7, 163.7, 163.4, 162.6, 161.2, 65.5, 65.3, 65.0, 67.8,
61.7, 61.6, 45.6, 45.5, 45.2, 40.4, 39.8, 38.3, 38.1, 38.0,
Following the procedure described above, formamide
12 was isolated from the reaction medium as intermedi-
ate of the Leuckart reaction of (1R)-1-amino-7,7-
dimethyl-2-norbornanone 13. trans/cis (CDCl3)=1.8:1.
Mp 150.5–153.0°C (white crystals). [h]2D0 −40.2 (c 0.49,
MeOH). IR (CHCl3) w 3450, 2980, 1750, 1510, 1390
1
cm−1. H NMR (300 MHz, CDCl3) l 8.28 (d, J=1.8
Hz, 1H trans), 8.25 (d, J=13.2 Hz, 1H cis), 5.90 (br. s,
1H), 3.19 (td, J=12.3 Hz, J=3.6 Hz, 1H), 2.55–2.40
(m, 1H), 2.22–2.00 (m, 3H), 1.56–1.32 (m, 2H), 1.27 (s,
3H trans), 1.12 (s, 3H cis), 0.91 (s, 3H cis), 0.86 (s, 3H
trans) ppm. 13C NMR (62.5 MHz, CDCl3) l trans-
Conformer: 213.3, 161.2, 72.9, 48.4, 41.2, 40.4, 26.6,
21.6, 19.1, 18.9 ppm. cis-Conformer: 211.9, 163.7, 71.9,
47.3, 41.7, 40.6, 27.7, 22.4, 19.1, 18.9 ppm. MS m/e
+
’
(%B) 181 (M , 12), 153 (59), 137 (21), 112 (54), 108
(27), 84 (38), 69 (25), 55 (24), 41 (100). Anal. calcd for
C10H15NO2: C, 66.27; H, 8.34; N, 7.73. Found: C,
66.13; H, 7.94; N, 7.59%.
4.7. (1R,2R)-3,3-Dimethyl-1-ethylamino-2-methyl-
aminonorbornane 15
37.9, 30.7, 26.2, 25.8, 25.4, 25.1, 25.0, 24.2, 21.5, 21.4,
21.2 ppm. MS (%B): 152 (M −NHCOH, 2), 150 (3),
110 (100), 82 (29), 68 (8), 55 (10), 41 (20). Exact mass
calcd 210.1368. Found 210.1360. Anal. calcd for
C11H18N2O2.: C, 62.83; H, 8.63; N, 13.32. Found C,
62.27; H, 8.28; N, 13.11%.
A solution of (1R,2R)-N-3,3-dimethyl-2-formylamino-
1-norbornyl)acetamide 311 (0.40 g, 2.0 mmol) in THF
(20 mL) was slowly added to a BH3·THF solution (1
M, 12 mL, 12 mmol) at 0°C under Argon. The mixture
was heated under reflux and monitored by GC until
disappearance of the starting material, then cooled to
0°C and carefully hydrolysed with cc. HCl (20 mL).
After washing with diethyl ether (2×20 mL), the
aqueous layer was basified with NaOH solution (30%)
and extracted with diethyl ether (4×30 mL). The
organic layer was washed with brine (30 mL) and dried
over KOH, and the solvent evaporated in vacuo. Yield:
0.21 g, 63% (colourless liquid). The obtained diamine
was very pure, but oxidises readily in air. Thus, the
characterisation was carried out on the corresponding
hydrochloride, prepared by bubbling gaseous HCl
through a solution of diamine in Et2O, filtration and
recrystallisation from MeOH/Et2O. Mp >170°C
(decomp). [h]2D0 −1.6 (c 1.47, MeOH). IR (KBr) w 3020,
2740, 1620, 1515, 1490, 1350 cm−1. 1H NMR (300
MHz, CD3OD) l 3.55 (s, 1H), 3.30–3.18 (m, 2H), 2.88
(s, 3H), 2.25 (dm, J=10.3 Hz, 1H), 2.0–1.8 (m, 6H),
1.47 (t, J=7 Hz, 3H), 1.31 (s, 3H), 1.25 (s, 3H) ppm.
13C NMR (75 MHz, CD3OD) l 68.7, 67.2, 44.7, 39.1,
+
’
4.4. (1R)-N-(3,3-Dimethyl-2-oxo-1-norbornyl)formamide
7
Following the procedure described above, formamide 7
was isolated from the reaction medium as an intermedi-
ate of the Leuckart reaction of (1R)-1-amino-3,3-
dimethyl-2-norbornanone 6. Mp 72.4–75.4°C (white
crystals). [h]2D0 −8.6 (c 0.97, MeOH). IR (CHCl3) w 3400,
1
2990, 2740, 1750, 1700, 1520, 1390 cm−1. H NMR (300
MHz, CDCl3) l 8.21 (d, J=2.2 Hz, 1H), 6.20 (br. s,
1H), 2.60 (dd, J=10.5 Hz, J=2.0 Hz, 1H), 2.57–2.43
(m, 1H), 2.30–2.05 (m, 1H), 2.00–1.80 (m, 3H), 1.44–
1.24 (m, 1H), 1.12 (s, 3H), 1.11 (s, 3H) ppm. The
formyl group signals showed traces of the cis-isomer in
CDCl3, but no other signals were located. 13C NMR
(62.5 MHz, CDCl3) l 217.3, 160.8, 68.0, 46.1, 43.4,
38.6, 26.6, 24.4, 23.6, 21.7 ppm. MS m/e (%B): 153
+
’
(M −C2H4, 9). 138 (4), 124 (4), 110 (100), 108 (20), 93
38.5, 35.0, 32.4, 29.4, 23.0, 21.0, 18.4, 9.8 ppm. MS m/e
+
’
(11), 85 (32), 69 (51), 57 (20), 41 (85). Anal. calcd for
C10H15NO2: C, 66.27; H, 8.34; N, 7.73. Found: C,
66.55; H, 8.36; N, 7.69%.
(%B): 165 (M −C2H5, 6), 110 (100), 94 (3), 82 (6), 42
(17). Anal. calcd for C12H26Cl2N2: C, 53.53; H, 9.73; N,
10.40. Found: C, 52.99; H, 9.75; N, 10.39.
4.5. (1S,2S)-N-(3,3-Dimethyl-2-formylamino-1-nor-
bornyl)formamide ent-9
4.8. (1R,2R)- and (1S,2S)-3,3-Dimethyl-1,2-norbor-
nanediamine 16
Following the procedure described above, the Leuckart
reaction of (1R)-1-amino-7,7-dimethyl-2-norbornanone
13 gave 0.20 g of ent-9 (49% yield, white prisms) as
final product. [h]2D0 +57.9 (c 0.64, MeOH).
(1R,2R)-N-(3,3-Dimethyl-2-formylamino-1-norbornyl)-
formamide 9 (0.30 g, 1.4 mmol) was suspended in cc.
HCl (15 mL) and heated at 100°C for 12 h. After
cooling and washing with diethyl ether (2×20 mL), the

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A. G. Mart´ınez et al. / Tetrahedron: Asymmetry 12 (2001) 2153–2158
aqueous layer was basified and continuously extracted
with diethyl ether (24 h). The organic layer was dried
over KOH, filtered and the solvent was evaporated in
vacuo. Yield: 0.16 g, 76% (colourless liquid). The
obtained diamine was very pure, but oxidises readily in
air. Thus, characterisation was carried out on the corre-
sponding hydrochloride, prepared by bubbling gaseous
HCl through a solution of diamine in Et2O, filtration
and recrystallisation from MeOH/Et2O. Mp >250°C
(decomp). [h]2D0 +1.5 (c 0.67, MeOH). IR (KBr) w 3500–
2500, 1600, 1565, 1515, 1480, 1340, 1060 cm−1. 1H
NMR (300 MHz, CD3OD) l 3.35 (d, J=1.8 Hz, 1H),
2.19 (dm, J=10.3 Hz, 1H), 2.05–1.65 (m, 6H), 1.22 (s,
3H), 1.09 (s, 3H) ppm. 13C NMR (75 MHz, CD3OD) l
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64.9, 63.0, 46.6, 40.8, 40.0, 30.6, 25.4, 24.3, 20.3 ppm.
+
’
MS m/e (%B): 137 (M −NH3, 5), 122 (7), 82 (100), 56
(7), 41 (8). Anal. calcd for C9H20Cl2N2: C, 47.58; H,
8.87; N, 12.33. Found: C, 47.88; H, 9.05; N, 12.30%.
The same procedure was applied in the hydrolysis of
ent-9 to obtain ent-16, [h]2D0 −1.2 (c 1.39, MeOH).
Acknowledgements
We thank the DGCYT (Spain) for financial supports of
this work (Grant PB 97-0264).
14. Mart´ınez, A. G.; Teso Vilar, E.; Garc´ıa Fraile, A.; de la
Moya Cerero, S.; Mart´ınez Ruiz, P.; Subramanian, L. R.
Tetrahedron: Asymmetry 1996, 7, 1257–1260.
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