Full text of DOI:10.1021/op200038y

TECHNICAL NOTE
pubs.acs.org/OPRD
A Practical Asymmetric Synthesis of Isopropyl
(1R,2S)-Dehydrocoronamate
Wenjun Tang,* Xudong Wei, Nathan K. Yee, Nitinchandra Patel, Heewon Lee, Jolaine Savoie, and
Chris H. Senanayake
Chemical Development, Boehringer Ingelheim Pharmaceuticals Inc., Ridgefield, Connecticut 06877, United States
S Supporting Information
b
ABSTRACT: A novel asymmetric synthesis of isopropyl (1R,2S)-dehydrocoronamate is described from (S)-1,2,4-butanetriol as the
starting material in 28% overall yield. Highlights of this synthetic route include selective cyclopropanation between chiral cyclic
sulfate 5 and diisopropyl malonate (8c), formation of vinylcyclopropane 3c via elimination of halide 4c, selective monohydrolysis of
diisopropyl ester 3c, and Curtius rearrangement of acid 10 to form isopropyl (1R,2S)-dehydrocoronamate TsOH salt 13 in >99% ee.
With the involvement of only three isolations, this chromatography-free process provides a rapid and practical access to (1R,2S)-1-
amino-2-vinylcyclopropane-1-carboxylic acid derivatives.
he chiral vinylcyclopropane amino acid ((1R,2S)-1-amino-2-
vinylcyclopropane-1-carboxylic acid, 1) is an important
T
building block for synthesizing a class of heptatitis C virus NS3
protease inhibitors, most notably represented by BILN 2061
(Figure 1).1 Its synthesis has drawn much attention, and several
synthetic approaches have been reported.2 Beaulieu and co-workers2a
described an efficient synthesis of racemic 1-amino-2-vinylcyclopro-
pane-1-carboxylic acid methyl ester via dialkylation of a glycine Schiff
base using trans-1,4-dibromo-2-butene as the electrophile. By com-
bining an enzymatic resolution protocol, a practical synthesis of
methyl (1R,2S)-dehydrocoronamate was developed which was
successfully demonstrated at multikilogram scales. On the other
hand, little success has been reported on asymmetric synthesis of
chiral amino acid 1, which is in sharp contrast to many asymmetric
synthetic methods of saturated chiral cyclopropane amino acids.3
One method was reported by Fox and colleagues4 utilizing a
palladium-catalyzed asymmetric allylic alkylation reaction. However,
the route was less promising for a large scale synthesis due to the
formation of a major side product during the closure of the
cyclopropane ring. Sala€un and co-workers5 also reported the synth-
eses of vinylcyclopropane aminonitriles via a palladium-catalyzed
tandem alkylation, and only poor enantioselectivities were observed
when chiral ligands were employed. With a chiral phase transfer
catalyst, a stereoselective cyclopropanation between (E)-N-phenyl-
methyleneglycine ethyl ester and trans-1,4-dibromo-2-butene was
also reported.6 However, the isolation of the cyclopropanation
product and the upgrade of its chiral purity required a preparative
supercritical fluid chromatography. Despite these efforts, an efficient
asymmetric synthesis of (1R,2S)-1-amino-2-vinylcyclopropane-1-
carboxylic acid derivatives remains a challenge. Herein we wish to
report a practical and asymmetric synthesis of an isopropyl (1R,2S)-
dehydrocoronamate TsOH salt (13) from a readily available chiral
starting material (S)-1,2,4-butanetriol (6).
Figure 1. BILN 2061 and (1R,2S)-1-amino-2-vinylcyclopropane-1-
carboxylic acid (1).
dicarboxylic ester 3 through a regioselective hydrolysis followed by
a Curtius rearrangement (Scheme 1). Thus, the key challenge is to
develop a practical and economical synthesis of chiral vinylcy-
clopropane dicarboxylic ester 3. Although several asymmetric
methods8,9 toward the synthesis of 3 were reported including the
use of an asymmetric tandem allylic alkylation9a or chiral auxiliaries,7a
most of those methods showed limitations for scale-up due to severe
racemization, a lengthy synthetic route, or employment of expensive
reagents. Herein we disclose a novel asymmetric synthetic approach
toward 3 from an inexpensive and readily accessible chiral material
(S)-1,2,4-butanetriol (6) by a sequence of cyclic sulfate formation,
cyclopropanation, and elimination.
A one-pot synthesis of chiral cyclic sulfate 5 was developed from
(S)-1,2,4-butanetriol (6) as the starting material on modification of
Special Issue: Asymmetric Synthesis on Large Scale 2011
Our synthetic strategy toward the chiral vinylcyclopropane amino
ester 2 is illustrated in Scheme 1. We believe that 2 could be
synthesized by a reported method3,7 from a chiral vinylcyclopropane
Received: February 12, 2011
Published: March 14, 2011
r
2011 American Chemical Society
1207
dx.doi.org/10.1021/op200038y Org. Process Res. Dev. 2011, 15, 1207–1211
|

Organic Process Research & Development
TECHNICAL NOTE
a reported procedure10 (Scheme 2). Thus, chiral triol 6 was
reacted with SOCl2 in dichloroethane at reflux for 4 h to form
quantitatively a chloro-substituted cyclic sulfite 7, which was
then exposed to NaIO4 in the presence of a catalytic amount
product 4b over the cyclopentane side-product 9b (entries 5, 7,
and 9). A significant solvent effect was observed. Toluene was found
to be beneficial for the formation of 4b. Thus, with toluene as the
solvent and LiOtBu as the base, the cyclopropane di-tert-butyl
product 4b was obtained in 80% isolated yield (entry 9). Under
similar conditions, the cyclopropane diisopropyl ester 4c was
obtained in 75% isolated yield (entry 10).
Next, the key elimination of the chlorocyclopropane diester 4 to
form a vinylcyclopropane diester 3 was studied with various bases.12
Since the selective monohydrolysis of a diisopropyl ester is more
of RuCl3 xH2O to form 5 in 83% isolated yield and >97% ee.
3
The distilled chiral cyclic sulfate 5 is stable and can be stored
in a refrigerator for months without noticeable decomposi-
tion or racemization.
With chiral cyclic sulfate 5 in hand, we then studied the
cyclopropanation between 5 and dimethyl malonate (8a). Interest-
ingly, in addition to the desired product 4a, a cyclopentane side-
product 9a11 was also formed under the conditions of NaH/DME
(Scheme 3). Apparently, there was a competitive intramolecular
cyclization through pathway b during the course of the reaction. We
envisioned that the solvent and base could have a significant impact
on the selectivity of ring closures. Since both dimethyl malonate
(8a) and the cyclopropanation product 4a are very susceptible to
trans-esterification or hydrolysis under various basic conditions, we
chose bulkier esters such as di-tert-butyl malonate (8b) and
diisopropyl malonate (8c) as the starting material to study the base
and solvent effect. As shown in Table 1, the selectivity of the
cyclization products was closely related to the solvent and the base
employed. Although weak bases such as K2CO3 and Cs2CO3 did
not promote the desired reaction (entries 1ꢀ2), stronger bases such
as NaH, KOtBu, and LiOtBu led to the formation of cyclization
products at rt (entries 3ꢀ9). Among the various bases screened,
LiOtBu provided the best selectivity of the cyclopropanation
Table 1. Base and solvent effects in cyclopropanation
ratio of 4b:9b
or 4c:9c
yield of 4b
or 4c (%)
entrya
solvent
base
1b
2b
3
DMF
DMF
DME
DME
DME
THF
K2CO3
Cs2CO3
NaH
--
0
0
--
1:3
1:2
1:1
1:1
1:1
4:1
10:1
10:1
20
20
30
36
50
50
80
75
4
KOtBu
LiOtBu
KOtBu
LiOtBu
KOtBu
LiOtBu
LiOtBu
5
Scheme 1. Synthetic strategy of (1R,2S)-1-amino-2-vinylcy-
clopropane-1-carboxylic acid ester 2
6
7
THF
8
toluene
toluene
toluene
9
10c
a The reactions were carried out at rt in the presence of 2.2 equiv of base
with di-tert-butyl malonate (8b) as the starting material for 12 h unless
otherwise specified; ratios of 4b vs 9b or 4c vs 9c were determined by
HPLC on a C-18 reversed-phase column; isolated yields. b The reactions
were carried out at 100 °C for 12 h. c The reaction was carried out with
diisopropyl malonate (8c) as the starting material.
Scheme 2. Synthesis of chiral cyclic sulfate 5
Scheme 3. Competitive ring closures during cyclopropanation
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dx.doi.org/10.1021/op200038y |Org. Process Res. Dev. 2011, 15, 1207–1211

Organic Process Research & Development
TECHNICAL NOTE
Scheme 4. Formation of vinylcyclopropane 3c via
elimination
Scheme 5. A one-pot synthesis of monoacid salt 12
Scheme 6. A one-pot synthesis of isopropyl (1R,2S)-dehy-
drocoronamate TsOH salt (13)
Table 2. Selective hydrolysis of diester 3c with various bases
diester 3c and diacid 11 without chromatographic separation, we
sought to isolate a salt of 10 by reacting with an organic base. The
dibenzylamine salt 12 was found to be highly crystalline and easy to
isolate. More gratifyingly, the optical purity of 12 could be increased
from 94% ee to >99% ee by a simple recrystallization from
iPrOHꢀwater. To our delight, no isolation is necessary at the stage
of cyclopropanation, elimination, and selective hydrolysis. We
successfully isolated salt 12 in >99% ee from cyclic sulfate 5 in
one pot in 50% overall yield (Scheme 5).
The final transformation of monoacid 12 to amino ester 2 via
Curtius rearrangement was straightforward. A salt-break of 12 was
successfully accomplished with 15% (w/w) H3PO4 solution as the
reagent. Use of HCl or H2SO4 as the acid failed to provide good phase
separation. The resulting acid 10 underwent Curtius rearrangement
by reacting sequentially with ethyl chloroformate, NaN3, and tert-
butanol to form smoothly the Boc-protected amino ester, which was
further treated with TsOH in isopropanol to deprotect its Boc group
and form the TsOH salt 13. As a white crystalline solid, compound 13
is easy to handle for its further synthetic applications. A one-pot
sequence of salt-break, Curtius rearrangement, Boc deprotection, and
salt formation was developed, and the desired product 13 was isolated
in 65% overall yield and in >99% ee (Scheme 6). The stereochemistry
of 13 was further confirmed by converting to its corresponding
methyl ester with treatment of MeONa/MeOH, which was consis-
tent on every aspect with the reported data.2
In summary, a novel and efficient synthesis of isopropyl (1R,2S)-
dehydrocoronamate has been developed from (S)-1,2,4-butane-
triol as the starting materials in a 28% overall yield. As no column
chromatographic separation is needed, this process is amenable for
scale-up activities. It is noteworthy that this is the first asymmetric
synthetic process of chiral vinylcyclopropane amino acid derivatives,
which features a sequence of cyclopropanation between chiral cyclic
sulfate 5 and diisopropyl malonate (8c), olefination via elimination
of halide 4c, selective monohydrolysis of diisopropyl ester 3c,
and final Curtius rearrangement to form amino ester salt 13. A
number of challenging development issues were addressed,
including the careful selection of diisopropyl malonate, the base
and solvent choice for cyclopropanation, choice of base for
elimination, base identification for selective monohydrolysis,
and salt formation of the monoacid 15. With only three isola-
tions involved, the new synthetic method provides a rapid and
economical access to isopropyl (1R,2S)-dehydrocoronamate,
which would facilitate its further applications in both synthetic
organic chemistry and medicinal chemistry.
entry
base (equiv)
T (°C)
10:11:3ca
10:11b
1
2
3
4
5
LiOH (1.05)
LiOH (1.05)
NaOH (1.2)
KOH (1.2)
40
50
40
40
40
62:10:28
55:8:36
77:19:4
83:8:9
10:1
6:1
17:1
27:1
35:1
Me4NOH (1.2)
90:5:5
a The ratios of 10:11:3c were determined after 6ꢀ12 h by HPLC on a
C-18 reversed-phase column. b The ratios of 10:11 were determined
after 1 h.
controllable than that of a di-tert-butyl ester13,7b for the following
step, we focused on the elimination of the diisopropyl 4c to form the
vinylcyclopropane diisopropyl ester 3c (Scheme 4). It was found
that strong sterically hindered potassium bases such as KOtBu or
KHMDS were necessary for this transformation, while organic bases
such as DBU or 2-tert-butyl-1,1,3,3-tetramethylguanidine (Barton’s
base) were ineffective, providing no trace amount of elimination
product. Unfortunately, partial racemization of the vinylcyclopro-
pane product was observed when KHMDS was employed as the
base, providing 3c in ∼70% ee. Less racemization (92% ee) was
achieved when a weaker base KOtBu was used. In order to avoid the
partial trans-esterification observed with KOtBu as the base, we
sought to employ KOiPr as the base for elimination. Gratifyingly, we
found that the in situ formed KOiPr from KHMDS/iPrOH in THF
led to the formation of 3c in almost quantitative yield with only
limited racemization (94% ee). We were optimistic to enhance its
optical purity in the following steps by crystallizations.
With vinylcyclopropane diisopropyl ester 3c in hand, we turned
our attention to its selective monohydrolysis with various bases
(Table 2). It is important to note that the less sterically hindered
ester which is trans to the vinyl group in compound 3c was
preferentially hydrolyzed to provide the monoacid 10.14 Indeed,
no formation of epi-10 was observed during the reaction course.
A dramatic base effect on the selectivity of the monohydrolysis
product 10 over the double-hydrolysis product 11 was observed,
and bulkier bases provided higher selectivities. While only moderate
selectivity was observed with LiOH as base (entries 1 and 2),
significant improvement was observed when KOH was employed
(entry 4). The use of Me4NOH as the base was very effective, and
the desired monoacid 10 was formed in 90% yield after the reaction
mixture was stirred at 40 °C for 6 h with H2O/isopropanol as the
solvent (entry 5). In order to isolate the pure monoacid 10 from
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Organic Process Research & Development
TECHNICAL NOTE
’ EXPERIMENTAL SECTION
waterꢀisopropanol to provide (1S,2S)-2-vinyl-cyclopropane-1,1-di-
carboxylic acid isopropyl ester dibenzylamine salt (12) as a white
crystalline solid (21.0 g, 50%, >99% ee). 12: [R]20D = ꢀ19.8 (c 1.0,
MeOH); 1H NMR (400 MHz, CD3OD): δ = 7.44 (m, 10H), 5.39
(m, 1H),5.19(m,1H),4.99(m, 2H),4.16(s, 4H),2.39(dd,J= 15.6,
8.6 Hz, 1H), 1.37 (m, 2H), 1.24 (d, J = 6.3 Hz, 3H), 1.20 (d, J = 6.2
Hz, 3H); 13C NMR (100 MHz, CD3OD): δ = 176.1, 171.3, 136.7,
133.6, 131.0, 130.3, 130.2, 69.6, 52.0, 39.7, 31.0, 22.2, 21.0, 19.6; ESI-
MS: m/z 199 [M ꢀ Bn2NH þ H]þ, 198 [Bn2NH þ H]þ; HRMS
calcd for C24H30NO4 396.2169, found 396.2212.
Isopropyl (1R,2S)-1-Amino-2-vinylcyclopropanecarboxylate
p-Toluenesulfonic Acid Salt (13).15. To a suspension of (1S,2S)-
2-vinyl-cyclopropane-1,1-dicarboxylic acid isopropyl ester dibenzyla-
mine salt (12, 18 g, 45.5 mmol, 1 equiv) in MTBE (200 mL) was
added a 15% H3PO4 solution (w/w, 200 mL), and the resulting
mixture was stirred at rt for 10 min. The organic phase was separated,
washed with 5% NaCl (100 mL), and concentrated to a minimum
amount of volume (∼20 mL). To the residue was added acetone
(100 mL) and triethylamine (5.1 g, 50.1 mmol, 1.1 equiv), followed
by ethyl chloroformate (5.4 g, 50 0.1 mol, 1.1 equiv) at ꢀ5 °C over
5 min. The resulting mixture was stirred at ꢀ5 to 0 °C for 10 min.
Upon the complete formation of the mixed anhydride, sodium azide
(5.9 g, 91.0 mmol, 2 equiv) in water (60 mL) was added at ꢀ5 to
0 °C over 5 min. The mixture was further stirred at ꢀ5 to 0 °C for
10 min. Toluene (200 mL) and water (200 mL) were added. The
toluene layer was separated, washed with water (100 mL) and brine
(100 mL), and concentrated to an ∼50 mL total volume. To a
refluxed mixture of toluene (80 mL) and tert-butanol (80 mL) was
added the aforementioned toluene solution over 20 min. Gas was
generated instantaneously during the addition. The mixture was
further stirred at reflux (∼85 °C) for 3 h, then concentrated to a
minimum amount of volume, and retreated with 2-propanol
(S)-4-(2-Chloro-ethyl)-[1,3,2]dioxathiolane 2,2-Dioxide (5).
To a 3-L three-necked flask equipped with a condenser and an
additional funnel were charged (S)-1,2,4-butanetriol (100 g, 0.94
mol) and dichloroethane (600 mL). To the mixture at rt was added
thionyl chloride (336.3 g, 2.827 mol, 3 equiv) over 30 min. The
resulting mixture was stirred at reflux for 4 h and then concentrated
under a reduced pressure. The residue was further treated with
MeCN (300 mL), dichloromethane (300 mL), and water (400 mL)
to form a biphasic solution. To the mixture at 10 °C was added
RuCl3 xH2O (2.4 g) followed by the addition of NaIO4 (282.1 g,
3
1.32 mol, 1.4 equiv) in portions over 30 min while controlling the
reaction temperature below 25 °C. After the addition, the mixture
was further stirred at rt for 1 h and then quenched by the addition of
MTBE (500 mL) and water (500 mL). The organic phase was
separated, filtered though a pad of Celite; washed sequentially with
water (500 mL), 10% Na2SO3 solution (500 mL), and brine
(300 mL); and then concentrated and distilled under vacuum
(130ꢀ135 °C/6 mmHg) to provide the desired pure product 6
as a colorless oil (145 g, 83%, >97% ee). 5: [R]20D = ꢀ60.4 (c 1.0,
CHCl3); 1H NMR (500 MHz, CDCl3): δ = 5.21 (m, 1H), 4.81 (m,
1H), 4.41 (m, 1H), 3.69 (m, 2H), 2.43 (m, 1H), 2.16 (m, 1H); 13C
NMR (125 MHz, CDCl3): δ = 79.9, 72.4, 39.2, 34.9. EI-MS: m/z
186 [M þ H]þ; HRMS calcd for C4H8ClO4S þ CH3OH
219.0088, found 219.0111.
(1S,2S)-2-Vinyl-cyclopropane-1,1-dicarboxylic Acid Isopropyl
Ester Dibenzylamine Salt (12). To a solution of (S)-4-(2-chloro-
ethyl)-[1,3,2]dioxathiolane 2,2-dioxide (5, 21.8 g, 117 mmol, 1.1
equiv) and diisopropyl malonate (4c, 20.0 g, 106 mmol, 1.0 equiv) in
toluene (350 mL) at ꢀ10 °C was added lithium tert-butoxide (18.7 g,
234 mmol, 2.2 equiv). The mixture was stirred at ꢀ10 °C for 20 min,
then allowed to warm to rt over 30 min, and further stirred at rt for
12 h. To the mixture was added a 2 N NaOH solution (200 mL), and
the resulting mixture was stirred at rt for 10 min. The organic phase
was separated, washed sequentially with water (200 mL) and brine
(200 mL), and concentrated to give the crude cyclopropane product
4c as a colorless oil, which was further treated with THF (40 mL) to
form a solution. To a solution of KHMDS in THF (0.9 M, 236 mL,
212.5 mmol, 2.0 equiv) at rt was charged isopropanol (12.8 g, 212.5
mmol, 2.0 equiv) over 5 min. The resulting slurry was stirred at rt for
10 min. To the mixture at rt was then charged the aforementioned
THF solution of 4c over 10 min. The resulting solution was then
warmed to 40 °C and stirred at this temperature for 4 h. Upon
complete elimination monitored by HPLC, the mixture was cooled to
0 °C and quenched by the addition of a 1 N HCl solution (200 mL).
To the mixture was added MTBE (300 mL), and the organic phase
was separated, washed with water (200 mL) and brine (200 mL), and
concentrated to a minimum volume to provide the crude vinylcyclo-
propane product 3c as a light yellow oil. To the residue were added
isopropanol (120 mL) and water (24 mL) to form a clean solution.
Tetramethylammonium hydroxide pentahydrate (20.5 g, 112 mmol,
1.1 equiv) was added in one portion, and the mixture was heated to
40 °C and stirred at this temperature for 6 h. At ∼95% conversion by
HPLC, the mixture was quenched by the addition of a 2 N HCl
solution (200 mL) and heptane (200 mL), then concentrated under
a reduced pressure to remove most of the organic phase, and further
treated with heptane (200 mL). After passing through a pad of Celite,
the organic phase was separated and concentrated to an ∼200 mL
total volume. To the mixture was added dibenzylamine (20.9 g, 106.3
mmol, 1.0 equiv), and the resulting slurry was further stirred at 0 °C
for 30 min and then filtered. The white solid was recrystallized from
(20 mL). TsOH H2O (9.52 g, 50.1 mol, 1.1 equiv) was added to
3
the solution, and the mixture was stirred at 50 °C for 12 h before
cooled to rt. To the solution was added isopropyl acetate (100 mL),
and the mixture was then concentrated to a minimum amount of
volume (∼20 mL). The residue was further treated with isopropyl
acetate (100 mL), stirred at 0 °C for 10 min, and then filtered. The
cake was washed with isopropyl acetate (25 mL ꢁ 2) and dried at rt
under a reduced pressure to give isopropyl (1R,2S)-1-amino-2-
vinylcyclopropanecarboxylate p-toluenesulfonic acid salt (13) as a
white crystalline solid (10.1 g, 29.6 mmol, 65%). 13: [R]20D = þ27.4
(c 1.0, MeOH); 1H NMR (400 MHz, CDCl3): δ = 8.59 (s, 1H),
7.74 (d, J = 7.7 Hz, 2H), 7.15 (d, J = 7.7 Hz, 2H), 5.59 (m, 1H), 5.17
(d, J = 17.1 Hz, 1H), 5.07 (d, J = 10.3 Hz, 1H), 4.96 (m, 1H), 5.54
(dd, J = 17.8, 8.8 Hz, 1H), 2.35 (s, 3H), 1.91 (dd, J = 10.0, 6.4 Hz,
1H), 1.18 (d, J = 6.2 Hz, 3H), 1.14 (d, J = 6.2 Hz, 3H); 13C NMR
(100 MHz, CDCl3): δ = 141.3, 140.5, 131.8, 128.9, 126.0, 119.2,
70.8, 30.4, 21.6, 19.2; ESI-MS: m/z 170 [M ꢀ TsOH þ H]þ;
HRMS calcd for C9H16NO2 170.1175, found 170.1195.
’ ASSOCIATED CONTENT
S
Supporting Information. Analysis of the enantiomeric
b
purities of 12 and 13; NMR spectra of compound 5, 3c, 12, and
13. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
wenjun.tang@boehringer-ingelheim.com
1210
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Organic Process Research & Development
TECHNICAL NOTE
’ REFERENCES
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(9) (a) Hayashi, T.; Yamamoto, A.; Ito, Y. Tetrahedron Lett. 1988,
29, 669. (b) Carson, C. A.; Kerr, M. A. Angew. Chem., Int. Ed. 2006, 45, 6560.
(10) Hercouet, A.; Bessiꢁeres, B.; Le Corre, M. Tetrahedron: Asymmetry
1996, 7, 1267.
(11) The formation of 9a was observed upon neutralization of the
basic media. Its water-soluble sulfate salt A is fairly stable under strong
basic conditions, which can be easily removed by aqueous wash.
(12) For synthesis of vinylcyclopropane via elimination of chloride, see:
Sylvestre, I.; Ollivier, J.; Sala€un, J. Tetrahedron Lett. 2001, 42, 4991.
(13) Gassman, P. G.; Schenk, W. N. J. Org. Chem. 1977, 42, 918.
(14) The relative stereochemistry of monoacid 10 was confirmed by
NOE between the iPr group and the vinyl proton; see Table 2.
(15) Although a number of examples of Curtius rearrangements are
reported, the scale-up activities with extra cautions, sufficient safety
assessments, and procedural as well as engineering controls are highly
recommended. For some recent examples, see: (a) Caron, S.; Dugger,
R. W.; Ruggeri, S. G.; Ragan, J. A.; Ripin, D. H. B. Chem. Rev. 2006,
106, 2943. (b) Yue, T.-Y.; McLeod, D. D.; Albertson, K. B.; Beck, S. R.;
Deerberg, J.; Fortunak, J. M.; Nugent, W. A.; Radesca, L. A.; Tang, L.;
Xiang, C. D. Org. Process Res. Dev. 2006, 10, 262. (c) Ende, D. J.; DeVries,
K. M.; Clifford, P. J.; Brenek, S. J. Org. Process Res. Dev. 1998, 2, 382.
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