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Enantioselective Synthesis of Vicinal Amino Alcohols by Oxa-Michael Addition of ()-N-Formylnorephedrine to Nitroalkenes.

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in direct NMR measurements. Steric interactions between the
receptor phosphorus atom and the NH of amino acid derivatives lead to further reduction in complex stability. Urea derivatives of (S)-alanine and (S)-phenylglycine, (7 and 8, respectively), have association constants of K,,, = 6.4 x lo4 M - ' and 5.8 x
lo4 M - ' , respectively. Due to its better association, the (S)-alanine derivative (S)-7 was selected for competitive titration
against ( S ) - 6 (Kass((S)-6)/Kass((S)-7)
= 3.9). The association
be readily measured by standard
constant of [ ( R ) - l - ( R ) - 6could
NMR titration (K,,, = 4.7 x lo3 M - ' ) . Competitive titration be= 13, a
tween (R)-6 and ( a - 7 afforded Kass((S)-7)/Kass((R)-6)
ratio that, within the experimental error, confirms the previously measured value for the association constant of compound 7.
From the above values the association constant for [ ( R ) - l .
(S)-5] was calculated to be K,,, = 5.7 x l o 5 M - ' (Table 1). This
indicates that the chiral recognition displayed by these complexes is the best in this series: Kays((R)-l)/Kass((S)-l)
= 90. With
(S)-6 this ratio is reduced to 53.
Received: May 2. 1996 [Z9092IE]
German version: Angew. Chem. 1996. 108. 2512-2514
Keywords: enantiomeric resolution
recognition receptors
- hydroxyacids
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(Eds.: K . Nakanishi, N. Berova. R. W. Woody) VCH. New York, 1994. p. 361.
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[lo] R. Huisgen. L. Xingya, Terrahedron Lert. 1983, 24. 4185.
[ t l ] The ratio of the association constants for the complexes formed in the competitive NMR titration was CdlCUkdted according to Equation (a). where 6, is the
chemical shift of the free host. 6, that of the complexed host. and 6 that
Enantioselective Synthesis of Vicinal
Amino Alcohols by Oxa-Michael Addition of
( - )-N-Formylnorephedrine to Nitroakenes**
Dieter Enders,* Andreas Haertwig, Gerhard Raabe, and
Jan Runsink
Enantiomerically pure 1,2-amino alcohols A are characteristic structural features of many natural products and drugs, and
play an important role as chiral building blocks, auxiliaries, and
ligands in transition metal catalyzed reactions.['] They can be
prepared from amino acids (for example, from the chiral pool),
by resolution, or by asymmetric synthesis. Numerous diastereoand enantioselective syntheses have been described that lead to
vicinal amino alcohols for example by C-C['I or C-N bond
or indirectly by hydroboration of enamines.[']
An alternative retrosynthetic analysis leads to a hydroxide
synthon B and a /I-amino cation synthon C by disconnection of
the C - 0 bond. The oxa-Michael additionf6.'] of a chiral synthetic hydroxide equivalent with a removable auxiliary, like
(-)-( 1R,2S)-N-formylnorephedrine ((R,S)-2),to (E)-nitroalkenes ( 1 , as equivalents for C ) should open a new enantioselective
pathway to the title compounds A.
Interestingly, the oxa-analogous Michael addition was published in 1878 by F. Loydl in his work about the synthesis of
malic acid from fumaric acid,[*] five years earlier than the discovery of the actual Michael reaction by Komnenos, Claisen,
and Crismer. After further early work,['] some groups tried later
to find a diastereo- and enantioselective version of this reaction.["] Some intermolecular oxa-Michael additions in which
the chirality information is contained in the acceptor," 'I as well
as several enzymatic variants"
are already available.
Diastereoselective intramolecular oxa-Michael addition is widely used in natural product synthesis.['31 To the best of our
knowledge, the enantioselective intermolecular oxa-Michael addition with removable chirality information in the oxygen-containing nucleophile has not been reported. We have now succeeded in developing such an asymmetric oxa-Michael addition,
by which vicinal amino alcohols are prepared with high asymmetric induction and in good yields by using an enantiomerically pure alcoholate.
[*] Prof. Dr. D. Enders. DipLChem. A. Haertwig. Dr. G. Raabe. Dr. J. Runsink
ic) VCH Verlagsgesellschaft mbH, 0-69451 Weinheim. 1996
Institut fur Organische Chemie der Technischen Hochschule
Professor-Pirlet-Strasse 1 , D-52074 Aachen (Germany)
Fax: Int. code +(241)8888127
e-mail' enders(o
This work was supported by the Deutsche Forschungsgemeinschaft (Leibniz
Prize and Sonderforschungsbereich 380) and by the Fonds der Chemischen
Industrie. We thank the companies Degussa AG, BASF AG, Bayer AG, and
Hoechst AG for the donation of chemicals.
$ 15.00-t 2510
AnQew. Chem. Inl. Ed. E n d . 1996.35. No. 20
c I2
The alcoholate is obtained from (-)-(I R,2S)-N-formylnorephedrine ((R,S)-2)[14Jby reaction with sodium hydride. It
reacts highly diastereoselectively (de: 93 to 2 98 YO)at - 78 "C
with the (E)-nitroalkenes la-e to give the nitro ethers 3a-e in
good yields (60--87 Yo)(Table 1, Scheme 1 ) . Virtually diastereomerically pure products (de: 96 to 2 98 %) can be obtained for
(R,S, R)-3a-c by using flash chromatography.
Table 1. Yields, diastereomeric excesses, and rotational values of the nitro ethers
(R.S.R)-3 obtained by asymmetric conjugated hydroxylation.
Yield [a]
cle [a, b]
94 ( 2 9 6 )
93 (97)
96 (298)
2 9 8 [c]
94 ICI
60 (56)
75 (72)
87 (84)
67 [CI
85 [CI
[a] Values in parentheses obtained after separation of the diastereomers by column
Chromatography (SiO,, ether o r etherihexane 2!1 up to l / l ) . [b] The de values are
obtained by ' H N M R spectroscopy (3a,e 300 MHz; 3b-d 500 MHz). [c] After purification by column chromatography (SiO,. ether/hexane 3: 1). separation of the
diastereomers 3e was not possible.
Fig. 1. Structure of the nitro ether (R.S,R)-3b
the crystal (Schakal plot) 1171
side reactions. The auxiliary is reduced to (S)-N-formylaniphetamine during the ether cleavage and can be recovered. The overall yield of this procedure is 34-53% over four steps with high
enantiomeric excesses (ee:94 to 2 98 % ; Table 2).
Table 2. Yields, diastereomeric excesses, and rotational values of the N-Bocprotected amino alcohols 5, and the yields of the N-Boc-protected amino ethers
34 - 53% (4 steps)
Yield (4)
Yield ( 5 )
2 9 6 [a]
2 9 6 [a]
2 9 8 [bl
2 9 8 [b]
94 ICI
(c. CHCI,)
lee = 94 - 2 98%1
1 ) ( W - 2 ,NaH,
69 - 76%
1. NaBH,. P d G MeOHI
THF, 0
-23.2 (0.6)
-16.2 (1.1)
- 19.4 (0.9)
-28.8 (1.2)
- 19.4 (1.3)
H Y ' A O H
[a] Determined by ' H N M R spectroscopy with (-)-(R)-1-(9-anthryl)-2,2.2-trlfluoroethanol as cosolvent. I31 Determined by G C analysis on a chiral stationary
phase (heptakis(2.3.6-tri-O-methyl)-~-cyclodextrin~polysiloxane,
25 m x 0.25 mm)
and by 'H N M R spectroscopy with (-)-(R)-l-(9-anthryl)-2.2.2-trifluoroethanolas
cosolvent. [c] Determined after conversion to the corresponding esters with MTPA
(Mosher's reagent) by 'H N M R spectroscopy [20].
Scheme 1 . Enantioselective synthesis of 1.2-amino alcohols 5.
The preferred conformation in solution and hence the relative
and absolute configuration of the newly formed stereogenic center can be determined by comparison of the 'H N M R spectra of
both diastereomers-NOE measurements, coupling constants,
and ring current effects-to be ( R ) in the case of the major
diastereomer and to be ( S ) for the minor diastereomer. These
results were confirmed by the X-ray structure determination of
(R,S,R)-3b (Fig. 1).["]
The reduction of the nitro group to the amino function is
carried out with sodium tetrahydridoborate and a PdjC catalyst
in methanol/THF (1 : I ) ; the hydroxylamine is the intermediate
product.['81 The introduction of the Boc protecting group is
carried out in the methanol-enriched solvent with di-tertbutyldicarbonate (Boc,O) and triethylamine; the isolation of
the free amine is unnecessary. The N-Boc-protected amino
ethers 4a-e are obtained after flash chromatography in good
yields (80-84%).
The cleavage of the ethers with sodium in liquid ammonia at
-78'C leads to the protected amino alcohols 5a-e without
racemization and in good yields (69-75
The reaction
must be stopped after 15 minutes to avoid loss of products by
In summary, the described asymmetric oxa-Michael addition
under conjugate nucleophilic hydroxylation of (E)-nitroalkenes
opens a new efficient entry to virtually enantiomerically pure
vicinal amino alcohols. Since the chiral ephedra alkaloid
norephedrine is readily available, it is possible to synthesize both
enantiomers of vicinal amino alcohols and thus broadly applicable chiral building blocks.[211
Experimental Procedure
(-)-(R,S,R)-3a-e: A solution of (-)-N-formylnorephedrine (215 mg, 1.2 mrnol) in
T H F (15 mL) was added at room temperature to a suspension of sodium hydride
(24 mg. 1 mmol) in T H F (10 mL). This suspension was cooled to -78 "C and the
(Ef-nitroalkene l a - e (1 mmol) in T H F (1 mL) was added (the reaction was monitored by TLC). The reaction was stopped after 24-96 h by quenching with glacial
acetic acid and then allowed to warm up to room temperature After aqueous
work-up, extraction with ether (4 x 20 mL), washing with saturated NaHCO, (aq),
and drying with Na,SO,. yellow. oiiy crude products were obiained that could he
purified by flash chromatography (SiO,, ether or etherihexane 3/1 up to l i l ) to give
colorless oils (R,S,R)-E,d o r solids (R,S,R)-3a,b,e.
(R.S,R)-4a-e: To a suspension of nitro ether 3 (1 mmol) and PdiC (50 mg. 10%) in
methanol/THF (111, 20 mL), NaBH, (151 mg. 4 mmol) was added at 0 C . The
reaction mixture was kept at this temperature for 2 h and then allowed to warm up
to room temperature and stirred for 3 d. The PdjC catalyst was removed by filtration over Celite and washed with methanol (30 mL). Boc,O (218 mg, 1 mmol) and
triethylamine (5 mL) were added t o this solution (monitored by TLC). After 1-2 h,
the solvent was removed and the residue was dissolved in CH,CI,, washed with
water and brine, and dried with Na,SO,. After removal of the solvent. the obtained
crude oils were purified by flash chromatography (SO,. ether/hexane l / l up to 1/3).
(R)-5a-e: Sodium (58 mg. 2.5 mmol) was added to liquid ammonia (25 mL) at
-78°C. Then the N-Boc-protected amino ether 4a-e (1 mmol) i n THF (10mL)
was slowly added. After 15 min the reaction was stopped by addition of saturated
NH,CI (aq) at - 78 "C. After removal of the ammonia, the residue was dissolved in
CH,CI, and dried with Na,SO,. The solvent was removed and the crude product
purified by flash chromatography (SiO>.etherihexane l / l up to 211) affording the
compounds 5 as colorless liquids (5a-c) or solids (5d.e). The (S)-N-formylamphetamine can also be isolated in yields of 70-75%.
Received: May 9. 1996
Revised version: July 8, 1996 [Z91081E]
German version: Angew. Chem. 1996, 108, 2540- 2542
Keywords: amino alcohols - asymmetric syntheses
additions * nitroalkenes . synthetic methods
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[l5] Suitable crystals were obtained from etherihexane mixtures at room temperature. monoclinic. space group P2, (no. 4). a =7.392(1), h = 8.984(6), c =
12.202(1)& /j =106.728(6),; V =776.04A3, Z = 2, M,,,, = 280.33, p =
1.200 gcm-'. Total number of electrons per unit cell is 300. The data were
collected on an Enraf-Nonius CAD4-diffractometer,Mo,. radiation (graphitemonochromator. i = 0.71069 .&). The structure was solved by direct methods
(GENSIN. GENTAN from XTAL3.2 [16]). The positions of the hydrogen
atoms were calculated in ideal positions. 1576 observed reflections [1>2u(I)],
181 parameters refined. R = 0.053 (Rw = 0.034). Maximum residual electron
density f 0 . 3 e k ' . The configuration of C2 was obtained by using the known
configuration of C3 and C4. Crystallographic data (excluding structure factors) for the structure(s) reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication no.
CCDC-179-91. Copies of the data can be obtained free of charge on application to The Director, CCDC. 12 Union Road, Cambridge CB2 lEZ, UK (fax:
Int. code +(1223) 336-033; e-mail: teched(
[16] S. R. Hall, H. D. Flack. J. M. Stewart XTAL3.2 Reference Manual, Universities of Western Australia, Geneva, and Maryland, Lamb. Perth 1992.
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1211 All new compounds gave appropriate spectroscopic data (IR, NMR, MS) and
correct elemental analyses.
Thiolate Bridged Nickel- Iron Complexes
Containing both Iron(o) and Iron(@ Carbonyls""
Chia-Huei Lai, Joseph H. Reibenspies, and
Marcetta Y Darensbourg*
The impressive synergism developed in the last decade
between synthetic inorganic and biophysical chemists with
regard to understanding the function and spectroscopic signature of nickel in [NiFe] hydrogenase has gained even more
momentum by the report of a protein X-ray crystal structure
in 1995.['' Crystals isolated from Desulfovibrio gigas in a
mixture of inactive (oxidized) and ready enzymatic states
were investigated. The electron density around nickel was
interpreted as a two-cysteine-bridged Ni-Fe bimetallic active
site, which has a Ni-Fe distance of 2.7 8, and is removed by
more than 6 8, from the closest Fe,S, cluster in the electron
Prof. Dr. M. Y. Darensbourg. C.-H. Lai, Dr. J. H. Reibenspies
Department of Chemistry. Texas A& M University
College Station. TX 77843-3255 (USA)
Fax: Int. code f(409) 845-0158
e-mail: marcetta(n'
Financial support from the National Science Foundation for this work (CHE
94-15901)and the X-ray diffractometer and crystallographic computing system
CHE-8513273, as well as contributions from the R. A. Welch Foundation are
gratefully acknowledged. We thank Professor Paul Lindahl for discussion and
Jason Smee (TAMU) for his help with CACHe molecular modeling programs.
0570-083319613520-2390$15.00+ .2510
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nitroalkenes, synthesis, formylnorephedrine, michael, amin, additional, enantioselectivity, oxa, vicinal, alcohol
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