Highly enantio- and diastereoselective construction of 1,2-disubstituted cyclopentane compounds by dirhodium(II) tetrakis[N-phthaloyl-(S)-tert-leucinate] -catalyzed C-H insertion reactions of α-diazo esters

Article abstract of DOI:10.1002/adsc.200505201

A highly enantio- and diastereoselective intramolecular C-H insertion reaction of α-diazo esters has been achieved with the use of dirhodium(II) tetrakis[N-phthaloyl-(S)-tert-leucinate] as a catalyst, providing exclusively cis-2-arylcyclopentane-1-carboxy

Full text of DOI:10.1002/adsc.200505201

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DOI: 10.1002/adsc.200505201
Highly Enantio- and Diastereoselective Construction of
1,2-Disubstituted Cyclopentane Compounds by Dirhodium(II)
ꢀ
Tetrakis[N-phthaloyl-(S)-tert-leucinate]-Catalyzed C H Insertion
Reactions of a-Diazo Esters
Kazushi Minami, Hiroaki Saito, Hideyuki Tsutsui, Hisanori Nambu,
Masahiro Anada, Shunichi Hashimoto*
Graduate School of Pharmaceutical Sciences, Hokkaido University, Sapporo 060-0812, Japan
Fax: (þ81)-11-706-4981, e-mail: hsmt@pharm.hokudai.ac.jp
Received: May 16, 2005; Accepted: June 13, 2005
Dedicated to the memory of the late Professor Kenji Koga.
Abstract: A highly enantio- and diastereoselective
intramolecular C H insertion reaction of a-diazo es-
Rh2(5S-MEPY)4 (1a), Rh2(4S-MEOX)4 (1b), and
Rh2(4S-MPPIM)4 (1c) is characteristic of intramolecu-
ꢀ
ꢀ
ters has been achieved with the use of dirhodium(II)
tetrakis[N-phthaloyl-(S)-tert-leucinate] as a catalyst,
providing exclusively cis-2-arylcyclopentane-1-car-
boxylates in up to 95% ee with no evidence of alkene
formation.
lar C H insertion reactions of diazoacetates and diazo-
acetamides,[4] while Daviesꢀ dirhodium(II) carboxylate
catalysts such as Rh2(S-DOSP)4 (2) are exceptionally ef-
ꢀ
fective for intermolecular C H insertion reactions with
aryl and arylvinyl diazoacetates when hydrocarbons are
used as a solvent.[5] Dirhodium(II) carboxylate catalysts
developed in this laboratory, such as Rh2(S-PTTL)4 (3a),
Rh2(S-PTPA)4 (3b), Rh2(S-PTA)4 (3c) are especially
ꢀ
Keywords: asymmetric catalysis; C C bond forma-
tion; C H insertion; chiral dirhodium(II) carboxy-
lates; cyclization; a-diazo esters
ꢀ
ꢀ
well suited for intramolecular C H insertion reactions
of a-diazo-b-keto esters,[6] a-methoxycarbonyl-a-diazo-
ꢀ
The development of catalytic enantioselective C C
bond forming reactions has been a subject of intensive
investigation in the field of synthetic organic chemis-
try.[1] Among the wide variety of transition metal com-
plexes used to catalyze a broad spectrum of transforma-
tions of a-diazocarbonyl compounds, dirhodium(II)
complexes have distinguished themselves as superior
ꢀ
ꢀ
catalysts in C H insertion reactions that form C C
bonds in which a new stereogenic center is created at
an unactivated carbon atom.[2] Over the past fifteen
years, substantial progress has been made in the
development of chiral dirhodium(II) carboxylate and
carboxamidate complexes as catalysts for enantioselec-
ꢀ
tive intramolecular and even intermolecular C H inser-
tion reactions via a rhodium(II)-carbene intermediate
(Scheme 1).[3] A number of systems have been reported
to provide enantioselectivities in greater than 90% ee;
however, there still remains a need for additional devel-
opment in terms of the scope with respect to the type of
a-diazocarbonyl compounds. The salient ability of
Doyleꢀs dirhodium(II) carboxamidate catalysts such as Scheme 1.
Adv. Synth. Catal. 2005, 347, 1483 – 1487
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1483

Kazushi Minami et al.
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acetamides,[7] and aryl diazoacetates.[8] While the high- {[a]2D3: ꢀ44.38 (c 1.20, MeOH); lit.,[17] [a]D23: ꢀ48.98 (c
ꢀ
est enantioselectivity in intramolecular C H insertion 1.2, MeOH) for the (1R,2R)-enantiomer]. Not unex-
reactions of a-diazo ketones remains 73% ee with the pectedly, an increase in the reaction temperature was ac-
use of chiral ortho-metalated arylphosphine-dirhodiu- companied by a significant decrease in enantioselectivi-
´
m(II) catalysts developed by Lahuerta and Perez-Prie- ty as well as the formation of a large amount of (Z) al-
to,[9,10] enantioselective reactions of a-diazo esters (a-al- kene 7a, but somewhat surprisingly, not even a trace of
kyl-a-diazoacetates) have remained unexplored. Here- trans isomer 6a could be detected (entries 2 and 3). A
in we report the first successful example of an enantiose- survey of solvents at ꢀ788C revealed that toluene was
ꢀ
lective intramolecular C H insertion reaction of a-di- the optimal solvent for this transformation. Although
azo esters, in which Rh2(S-PTTL)4 (3a) provides cis-2- dichloromethane and ether exhibited essentially the
arylcyclopentane-1-carboxylates as the sole product in same rate, enantioselectivity and cis selectivity as those
up to 95% ee.
found with toluene (95% and 94% ee, entries 4 and 5),
ꢀ
Rhodium(II)-catalyzed intramolecular C H inser- the reactions in these solvents produced small amounts
tion reactions of a-diazo esters, developed and exten- of (Z) alkene 7a. Using toluene as the solvent, we next
sively advanced by the Taber group, offer a powerful evaluated the abilities of other chiral dirhodium(II) car-
route to the production of substituted cyclopentanes[11] boxylates, Rh2(S-PTPA)4(3b), Rh2(S-PTA)4(3c), and
and tetrahydrofurans[12] in natural product synthesis. Rh2(S-PTV)4 (3d) (entries 6–8). Compared with cataly-
Since rhodium(II)-carbene intermediates, generated sis by Rh2(S-PTTL)4, reactions with these catalysts at
ꢀ
from a-diazo esters bearing a C H bond adjacent to
ꢀ788C required significantly longer times to reach
the diazo carbon, tend to form a,b-unsaturated esters completion. These reactions were also found to provide
via a 1,2-hydride shift,[13] the efficiency of these reactions a mixture of cis and trans cyclopentane products 5a and
relies on the judicious choice of conditions in which b- 6a with 5a as the major constituent, together with small
hydride elimination can be suppressed and C H inser- amounts of (Z) alkene 7a. While similar high levels of
tion can be favored. Taber and Joshi recently reported asymmetric induction as was found for Rh2(S-PTTL)4
ꢀ
ꢀ
that the ratio of C H insertion to b-hydride elimination were maintained with the cis isomer 5a, a sharp drop
products is significantly influenced by the electronic na- in enantioselectivity was observed with the trans isomer
ture and the steric bulk of the bridging ligands of dirho- 6a.[18] Somewhat disappointingly, switching the catalyst
dium(II) catalysts.[14] Compared with Rh2(O2CCH3)4, from Rh2(S-PTTL)4 to Rh2(S-BPTTL)4 (3e)[19] charac-
ꢀ
which generally favors C H insertion, the use of terized by an extension of the phthalimido wall with
Rh2(O2CCF3)4 with a more electron-withdrawing ligand one additional benzene ring had no beneficial effect in
would generate a more electrophilic and thus more reac- this system, and the cis cyclopentane product 5a of
tive rhodium(II)-carbene intermediate that would 67% ee was obtained as the sole product (entry 9).
greatly favor b-hydride elimination via an early transi- Although no explanation for the advantage of
tion state, while that of Rh2(O2CCPh3)4 with an excep- Rh2(S-PTTL)4 (3a) can presently be offered, 3a proved
tionally bulky ligand also increases the proportion of to be the catalyst of choice for this transformation in
b-hydride elimination via an entropically less demand- terms of rate and selectivity, as well as product yield.[20,21]
ing pathway for steric reasons. In their studies, the find- As expected from the robust nature and high reactivity
ꢀ
ing that with Rh2(S-PTPA)4 (3b), C H insertion can of 3a, 0.05 mol % of 3a was found to catalyze the reac-
compete effectively with b-hydride elimination is of par- tion in 9 h without compromising either the yield or
ticular interest, although asymmetric induction has not enantioselectivity (entry 10).
been explored.[15]
We then investigated the scope of this catalytic proc-
ꢀ
At the outset, we explored the intramolecular C H in- ess with respect to the substituents at the insertion site.
sertion of methyl 2-diazo-6-phenylhexanoate (4a)[16] in Aside from essentially complete cis selectivity, a high
toluene using 1 mol % of Rh2(S-PTTL)4 (3a). The reac-
tion proceeded smoothly to completion at ꢀ788 C with-
in 0.5 h, giving methyl cis-2-phenylcyclopentane-1-car-
boxylate (5a) as the sole product in 85% yield (Table 1,
entry 1). Gratifyingly, no signs of trans isomer 6a or al-
kene product 7a could be detected in the crude reaction
mixture by NMR spectroscopy. The enantioselectivity
of this reaction was determined to be 95% by HPLC
with a connected series of Daicel Chiralcel OJ-H and
Chiralpak AS-H columns. The preferred absolute ster-
eochemistry of 5a {[a]1D9: þ98.88 (c 1.12, CHCl3) for
95% ee} was established as (1S,2R) by its transformation
[(1) NaOMe, MeOH, reflux; (2) LiAlH4, THF, 08C]
to the known trans-2-phenylcyclopentane-1-methanol Scheme 2.
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Enantio- and Diastereoselective Construction of 1,2-Disubstituted Cyclopentanes
COMMUNICATIONS
[a]
ꢀ
Table 1. Enantioselective C H insertion reactions of a-diazo esters 4 catalyzed by chiral dirhodium(II) carboxylates.
Entry Substrate
R
Rh(II) catalyst
Solvent T [ 8C] Time [h] Yield[b] [%] 5:6:7[c]
ee [%][d]
5
6
1
2
3
4
5
6
7
8
9
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4c
H
H
H
H
H
H
H
H
H
H
MeO
Cl
Rh2(S-PTTL)4 (3a)
Rh2(S-PTTL)4 (3a)
Rh2(S-PTTL)4 (3a)
Rh2(S-PTTL)4 (3a)
Rh2(S-PTTL)4 (3a)
Rh2(S-PTPA)4 (3b)
Rh2(S-PTA)4 (3c)
Rh2(S-PTV)4 (3d)
toluene
toluene
toluene
CH2Cl2
ether
toluene
toluene
toluene
ꢀ78
ꢀ45
0
0.5
0.2
0.1
0.5
0.5
7
30
6
0.5
9
0.5
0.5
85
85
80
76
73
73
69
80
76
82
85
81
>99:– :–
91:– :9
82:– :18
93:– :7
97:– :3
95
–
–
–
–
–
90[e]
81[e]
95[e]
94[e]
ꢀ78
ꢀ78
ꢀ78
ꢀ78
ꢀ78
ꢀ78
ꢀ78
ꢀ78
ꢀ78
56: 37: 07 89[e] 27[e, f]
73: 19: 08 90[e] 56[e, f]
77: 20: 03 95[e] 22[e, f]
Rh2(S-BPTTL)4 (3e) toluene
>99:– :–
>99:– :–
>99:– :–
>99:– :–
67
95
92
93
–
–
–
–
10[g]
11
12
Rh2(S-PTTL)4 (3a)
Rh2(S-PTTL)4 (3a)
Rh2(S-PTTL)4 (3a)
toluene
toluene
toluene
[a]
ꢀ
Typical procedure for C H insertion reaction (entry 1): 3a·2 EtOAc (2.8 mg, 0.002 mmol) was added to a solution of 4a
(46.5 mg, 0.20 mmol) in toluene (1.0 mL) at ꢀ788C. After 0.5 h, the mixture was concentrated and the residue was purified
by column chromatography (silica gel, hexane/EtOAc¼15: 1).
[b]
[c]
[d]
Combined yield of 5, 6 and 7.
1
Determined by H NMR analysis of the crude reaction mixture.
Determined by HPLC [column: Chiralcel OJ-H followed by Chiralpak AS-H, eluent: 100: 1 hexane/i-PrOH, flow rate:
1.0 mL/min, detection: UV (254 nm)].
[e]
[f]
Determined after removal of (Z) alkene 7 by dihydroxylation [cat. OsO4, NMO, t-BuOH-acetone-H2O (1: 4: 2)].
Preferred absolute stereochemistry of 6a was secured by HPLC comparison with (1R,2R)-6a prepared from (1S,2R)-5a
(NaOMe, MeOH, reflux)[18]
0.05 mol % of 3a was used.
.
[g]
enantioselectivity was consistently observed with either the reasons for the reversal of diastereo- and enantiose-
electron-donating or electron-withdrawing groups pres- lection are not clear at this time, it is noteworthy that
ent at the para position on the benzene ring (92% and high levels of asymmetric induction can be achieved re-
93% ee, entries 11 and 12). Again, no evidence of alkene gardless of the type of substituents at the insertion site.
formation was observed. Catalyst 3a was also found to
ꢀ
catalyze the C H insertion of a-diazo ester 4d contain-
ing a benzene ring on the tether to give methyl cis-2,3-di-
hydro-1-phenyl-1H-indene-2-carboxylate (5d) {[a]2D4:
ꢀ69.38 (c 1.24, CHCl3)] as the sole product in 85% yield
with 92% ee (Scheme 2).[22] On the other hand, the op-
posite diastereoselectivity was observed with 4e bearing
a butyl group at the insertion site, where the reaction af-
forded a 10:90 mixture of cis and trans cyclopentane
products 5e and 6e, with no trace of alkene (Scheme 3).
The sense and extent of asymmetric induction for trans
isomer 6e were determined to be (1S,2S) and 94% ee
by HPLC, using a connected series of Daicel Chiralpak
IA, Chiralcel OD-H (Â2) and Chiralpak AD-H col-
umns after conversion to the corresponding p-bromo-
benzoate [(1) LiAlH4, THF, 08C; (2) p-bromobenzoyl
chloride, cat. DMAP, pyridine, CH2Cl2, 08C].[23] While Scheme 3.
Adv. Synth. Catal. 2005, 347, 1483 – 1487
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asc.wiley-vch.de
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Kazushi Minami et al.
COMMUNICATIONS
[2] For a book and reviews, see: a) M. P. Doyle, Chem. Rev.
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In summary, we have developed the first highly enan-
tio- and diastereoselective intramolecular C H inser-
ꢀ
tion reaction of a-diazo esters by using Rh2(S-PTTL)4
as the catalyst, in which no evidence of a,b-unsaturated
esters derived from a 1,2-hydride shift was observed.
The present catalytic protocol provides an attractive
and powerful access to optically active cyclopentane
building blocks. Further studies on the scope of the reac-
tion as well as mechanistic and stereochemical studies
are currently in progress.
Experimental Section
ꢀ
Representative Procedure for the Intramolecular C H
Insertion (Entry 1 in Table 1)
Rh2(S-PTTL)4 ·2 EtOAc (2.8 mg, 0.002 mmol, 1 mol %) was
added to a solution of 4a (46.5 mg, 0.20 mmol) in toluene
(1.0 mL) at ꢀ788C. After 0.5 h, the mixture was concentrated
and the residue purified by column chromatography (silica gel,
hexane/EtOAc¼15:1) to give methyl (1S,2R)-cis-2-phenylcy-
clopentane-1-carboxylate (5a) as a colorless oil; yield: 34.7 mg
(85%); Rf ¼0.39 (5:1 hexane/EtOAc); [a]1D9: þ98.88 (c 1.12,
[4] a) M. P. Doyle, A. van Oeveren, L. J. Westrum, M. N.
Protopopova, T. W. Clayton, Jr. J. Am. Chem. Soc.
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1
CHCl3); IR (neat): n¼1732, 1200, 1171, 700 cmꢀ1; H NMR
˜
Roos, F. Canas, D. A. Pierson, A. van Basten, P. Müller,
(400 MHz, CDCl3): d¼1.70 (m, 1H), 1.95–2.15 (m, 5H), 3.16
(ddd, J¼6.2, 9.0, 9.0 Hz, 1H, C2-H), 3.22 (s, 3H, CO2CH3),
3.41 (ddd, J¼7.1, 9.0, 9.0 Hz, 1H, C1-H), 7.15–7.28 (m, 5H,
ArH); 13C NMR (100 MHz, CDCl3): d¼24.8 (CH2), 28.6
(CH2), 31.2 (CH2), 49.2 (CH)?, 49.8 (CH), 50.9 (CH3), 126.3
(CH), 127.8 (CH), 127.9 (CH), 141.5 (C), 174.9 (C); EI-HR-
MS: m/z¼204.1151 [calcd. for C13H16O2 (Mþ) 204.1150];
anal. calcd. for C13H16O2: C 76.44, H 7.90; found: C 76.33, H
7.98.
P. Polleux, J. Am. Chem. Soc. 1994, 116, 4507; c) M. P.
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A. G. H. Wee, Z. Wang, S. C. Duncan, Org. Lett. 2003,
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The enantiomeric excess of 5a was determined to be 95% ee
by HPLC with a Daicel Chiralcel OJ-H column followed by
Daicel Chiralpak AS-H column (100:1 hexane/i-PrOH,
1.0 mL/min): tR (minor)¼12.9 min for (1R,2S) enantiomer; tR
(major)¼14.7 min for (1S,2R) enantiomer.
[5] a) H. M. L. Davies, Aldrichmica Acta 1997, 30, 107;
b) H. M. L. Davies, Eur. J. Org. Chem. 1999, 2459;
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Antoulinakis, J. Organomet. Chem. 2001, 617, 47;
e) H. M. L. Davies, Ø. Loe, Synthesis 2004, 2595;
f) H. M. L. Davies, Q. Jin, J. Am. Chem. Soc. 2004, 126,
10862.
[6] a) S. Hashimoto, N. Watanabe, T. Sato, M. Shiro, S. Ike-
gami, Tetrahedron Lett. 1993, 34, 5109; b) S. Hashimoto,
N. Watanabe, S. Ikegami, Synlett 1994, 353; c) T. Takaha-
shi, H. Tsutsui, M. Tamura, S. Kitagaki, M. Nakajima, S.
Hashimoto, Chem. Commun. 2001, 1604.
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Synlett 1994, 1031; b) M. Anada, N. Watanabe, S. Hashi-
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1999, 1775.
Acknowledgements
This research was partially supported by the Uehara Memorial
Foundation and a Grant-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan. We thank Ms. H. Matsumoto, A. Maeda, S. Oka, and M.
Kiuchi of the Center for Instrumental Analysis, Hokkaido Uni-
versity, for technical assistance in the MS and elemental analyses.
References and Notes
[1] For recent monographs, see: a) Comprehensive Asym-
metric Catalysis, (Eds.: E. N. Jacobsen, A. Pfaltz, H. Ya-
mamoto), Springer, Berlin, 1999; b) Catalytic Asymmet-
ric Synthesis, 2nd edn., (Ed.: I. Ojima), Wiley-VCH, Chi-
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[8] H. Saito, H. Oishi, S. Kitagaki, S. Nakamura, M. Anada,
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Adv. Synth. Catal. 2005, 347, 1483 – 1487

Enantio- and Diastereoselective Construction of 1,2-Disubstituted Cyclopentanes
COMMUNICATIONS
´
[9] a) F. Estevan, K. Herbst, P. Lahuerta, M. Barberis, J. Per-
[18] The preferred absolute stereochemistry of 6a in each
case was established as (1R,2R) by HPLC comparison
with the trans isomer derived from (1S,2R)-5a via C1-ep-
imerization (NaOMe, MeOH, reflux). (1R,2R)-6a: a col-
orless oil; Rf ¼0.39 (5:1 hexane/EtOAc); [a]2D5: –1038 (c
ez-Prieto, Organometallics 2001, 20, 950; see also: b) P.
Müller, P. Polleux, Helv. Chim. Acta 1994, 77, 645.
ꢀ
[10] The enantiotopically selective aromatic C H insertion of
a-diazoketones (up to 98% ee) has been achieved with
the use of Rh2(S-PTTL)4 (3a), although an accurate char-
acterization of the reaction pathway is electrophilic aro-
matic substitution: a) N. Watanabe, Y. Ohtake, S. Hashi-
moto, M. Shiro, S. Ikegami, Tetrahedron Lett. 1995, 36,
1491; b) N. Watanabe, T. Ogawa, Y. Ohtake, S. Ikegami,
S. Hashimoto, Synlett 1996, 85; c) H. Tsutsui, Y. Yamagu-
chi, S. Kitagaki, S. Nakamura, M. Anada, S. Hashimoto,
Tetrahedron: Asymmetry 2003, 14, 817.
1
1.17, CHCl3) for 95% ee; H NMR (400 MHz, CDCl3):
d¼1.73–1.89 (m, 3H), 1.96 (m, 1H), 2.10–2.22 (m,
2H), 2.84 (ddd, J¼7.9, 7.9, 7.9 Hz, 1H, C2-H), 3.35
(ddd, J¼7.9, 9.4, 9.4 Hz, 1H, C1-H), 3.60 (s, 3H,
OCH3), 7.17–7.30 (m, 5H, ArH); 13C NMR (100 MHz,
CDCl3): d¼25.2 (CH2), 30.9 (CH2), 35.1 (CH2), 49.8
(CH), 51.6 (CH3), 52.0 (CH), 126.2 (CH), 127.1 (CH),
128.3 (CH), 143.8 (C), 176.2 (C). The enantiomeric
excess of 6a was determined by HPLC with a Daicel
Chiralcel OJ-H column followed by Daicel Chiralpak
AS-H column (100:1 hexane/i-PrOH, 1.0 mL/min): tR
(major)¼17.8 min for (1R,2R) enantiomer; tR (mi-
nor)¼20.5 min for (1S,2S) enantiomer.
[11] a) D. F. Taber, M. J. Hannessy, J. P. Louey, J. Org. Chem.
1992, 57, 436; b) D. F. Taber, K. K. You, J. Am. Chem.
Soc. 1995, 117, 5757; c) D. F. Taber, K. K. You, A. L.
Rheingold, J. Am. Chem. Soc., 1996, 118, 547; d) D. F.
Taber, S. C. Malcolm, J. Org. Chem. 1998, 63, 3717.
[12] a) D. F. Taber, Y. Song, Tetrahedron Lett. 1995, 36, 2587;
b) D. F. Taber, Y. Song, J. Org. Chem. 1996, 61, 6706;
c) D. F. Taber, Y. Song, J. Org. Chem. 1997, 62, 6603.
[13] a) N. Ikota, N. Takamura, S. D. Young, B. Ganem, Tetra-
hedron Lett. 1981, 22, 4163; b) D. F. Taber, R. J. Herr,
S. K. Pack, J. M. Geremia, J. Org. Chem. 1996, 61, 2908.
[14] D. F. Taber, P. V. Joshi, J. Org. Chem. 2004, 69, 4276.
[19] S. Kitagaki, M. Anada, O. Kataoka, K. Matsuno, C.
Umeda, N. Watanabe, S. Hashimoto, J. Am. Chem. Soc.
1999, 121, 1417.
[20] As expected from Taberꢀs work,[14] the reaction with Rh2
(4S-MPPIM)4 (1c) (toluene, 238C, 3 h) provided exclu-
sively (Z) alkene 7a in 80% yield.
[21] The results for Rh2(S-DOSP)4 (2) are as follows: toluene,
ꢀ
¼
[15] In their studies of intramolecular N H insertion reac-
–788C, 8 h, 73% yield, 5a:6a:7a 61 (71% ee):25 (64%
¼
tions of a-diazo esters, McKervey and co-workers report-
ed that dirhodium(II) tetrakis[(S)-mandelate]-catalyzed
ee):14; hexanes, –608C, 5 h, 73% yield, 5a:6a:7a
(67% ee):31 (92% ee):34.
35
ꢀ
the N H insertion of methyl 6-benzyloxycarbonylami-
[22] The enantiomeric excess of 5d was determined by HPLC
with a Daicel Chiralcel OJ-H column (100:1 hexane/i-
ꢀ
no-2-diazohexanoate competed with C H insertion and
ꢀ
¼
alkene formation, where the C H insertion product cy-
PrOH, 1.0 mL/min): tR 25.3 min for major enantiomer;
¼
clopentane with the cis stereochemistry of the substitu-
ents was produced in up to 28% ee: C. F. Garcia, M. A.
McKervey, T. Ye, Chem. Commun. 1996, 1465.
[16] a-Diazo ester 4a was prepared from 1-iodo-4-phenylbu-
tane according to the procedure of Taber:[11a,13b] (1)
NaH, methyl acetoacetate, DMF, 808C, 3 h; (2) MsN3,
Et3N, MeCN, 238C, 12 h.
tR 28.5 min for minor enantiomer. The absolute stereo-
chemistry was not determined.
[23] The peak assignment [750:1 hexane/i-PrOH, 1.0 mL/
min, tR (major)¼38.1 min for (1S,2S) enantiomer; tR (mi-
nor)¼42.6 min for (1R,2R) enantiomer] was carried out
by comparison with [(1S,2S)-2-butylcyclopentyl]methyl
p-bromobenzoate prepared from the known (1S,2S)-
6e.[17]
[17] C.-L. Fang, H. Suemune, K. Sakai, J. Org. Chem. 1992,
57, 4300.
Adv. Synth. Catal. 2005, 347, 1483 – 1487
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1487