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Controlling the Course of Nucleophilic Additions to ortho-Substituted (6-Anisole)tricarbonyl-chromium Complexes Dienol Ether Formation versus tele-Substitution.

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Controlling the Course of Nucleophilic Additions
to ovtho-Substituted (q6-Anisole)tricarbonylchromium Complexes: Dienol Ether Formation
versus tefe-Substitution **
Hans-Giinther Schmalz,* and K u r t Schellhaas
(tf-Arene)tricarbonylchromium complexes, which were first
described in 1958 by Fischer und Ofele.['] enjoy a multifarious
potential for organic synthesis,"' and the chemistry of this class
of compounds has been the subject of intense investigation for
many years.[3]With our research we want to contribute to the
acceptance of chiral arenetricarbonylchromium complexes as
building blocks for the enantioselective synthesis of bioactive
compounds.['] For this purpose we pursue novel synthetic concepts based on the specific reactivity pattern of arenetricarbonylchromium complexes, and we take synthetically relevant
cases as occasion to explore and tame the often subtle reactivity
of these complexes and to adapt it to other synthetic applications.['I
Recently, we aimed to synthesize the marine natural product
( +)-ptilocaulin (I)[(', 'I following the strategy sketched in
Scheme 1 .I8] As the key step of the projected reaction sequence,
we intended to convert complex 3. which is accessible from
anisoletricarbonylchromium (5). into the enone 2 by reaction
with the C ,-nucleophile 4- -according to a transformation discovered by Semnielhack.'Y.l o ]
OMe
OMe
6
Scheme 2. Prep;iration 01' tlic optically iiclive compleu 3. ;I) LIN[(.S)-CHMePh]l.
TMSCI (Oequiv). THF. - 100 C. 10 111111.b) IIBLILI.THF. -hO+ - 20 C. then
C'tiCI ( 1 . 1 c q u i v ) . -5(l-O c'. then Hi-CH,CH=CHCH, ( 2 c q u i b ) . - 3 0 - 0
C.
3
I1
In order to prepare the enone 2. complex 3 was treated with
2-Iithio-1,l-dithiane (Li-4) in T H F in the presence of hexamethylphosphoric triamide (HMPA) as cosolvent to give the
anionic intermediate 7, which on addition of HCI-free chlorotrimethylsilane (TMSCI). extractive workup, and oxidative decomplexation afforded a mixture containing the dienol ether 8
as the major product.['"' After acidic hydrolysis of the dienol
ether moiety. the diastereomerically pure. crystalline enone 2
was isolated in 53 YOyield (based on 3 ) (Scheme 3) .I' The enantiomeric purity of the product was greater than 95%) c~e,['"]
indicating that the chirality transfer from the planar-chiral
metal complex structure to the chirality center in /l-position to
the carbonyl function in 2 has occurred without racemization.
0
OMe
0
5
5
3
In contrast to all expectations. our initial attempts to achieve
the conversion of 3 into 2 failed completely and led to the insight
that in contrast to 5 ortlro-substituted anisoletricarboiiylchromium derivatives behave unusually, giving rise to t&-substituted
products (e.g. 9; see below) under the routine reaction conditions
(nucleophile addition followed by protonation)
However. we
have now succeeded in finding conditions that effectively suppress
the tek-substitution. We report here (among other things) on the
successful synthesis of the (optically active) enone 2 and demonstrate that the transformation of 3 into 2 occurs under chirality
transfer without loss of stereochemical information.
[*] Prof. Dr. H.-G Schiiial;.. Dip1 -Cheni. K Schellhaas
rllstlttlt 1i;r 01-ganisckeChrrnie dei- Technischcn Uiiii.cr\itiil
Strazsc des 17 J u n i 135. U-10623 Berlin (C;ei-m;my)
F a x : I n t code - t i 3 0 1 314-71 105
e-inail. schiniil~c i wapO109 chein ~u-berlin.de
[*'I
This work wds \uppoi-tcd by the Volksw.apcnstiftung (ProJcct I 6Y 907) and thc
Fond\ der Cheml\chen Iridurlr~e( a m o n g orher things through ii graduate
scholarship iiwerd t o K S ) We thank the Cheiiietall C h h H for gifts ofchcmicitls and PI-of LI. t. Scinnielliack. Princeton. foi- ii vxluahle discussion
L
Li-4, THF, HMPT
4
Scheme 1. Retrosynthelic m a l y s i s of i + )-pt~locaulin
(1).
OMe
(91 % ee)
2
1
OMe
Following the route described previously.[*] the regio- and
enantioselective preparation of complex 3 was accomplished in
appealing yield after optimization of all individual steps
(Scheme 2)." I ] Starting from the prochiral complex 5. enantioselective deprotonationisilylation[' '. 3 1 provides the monosilylated derivative 6 (91 %I e ~ " ~ ]which
),
in turn is converted into
complex 3 through orrho-deprotonation and Cu-mediated alkylation." 51
-60
r
OMe
+ -30 "C
1
1
HCI, THF
1
With this new procedure, the enantioselective synthesis of the
highly functionalized ptilocaulin precursor 2 is now possible in
only four steps from anisole. To demonstrate that the method is
not limited to ;t single case. we hilve prepared the enone r u c
It should be emphazised that the tdc-substituted product
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9 is formed only in trace amounts under the reaction conditions
described, in contrast to all other conditions tested so far (see
below), where 9 always appears as the major product.
The results obtained so far provide the following picture
(Scheme 4): the anionic $-complex 12, which results from the
stereospecific anti-addition of the nucelophile to the less hindered meta-position of the ortho-substituted anisoletricarbonylchromium substrate 11, may react further by two different
13
15
++
++
?
OMe
16
14
Scheme 4. anti-Addition of a nucleophile (NucO) to I1 and protonation or silylation
of the intermediate 12.
modes depending on the reaction conditions. On addition of
trifluoroacetic acid (TFA), protonation takes place primarily at
the chromium atom (-+ 13),L9]which leads under elimination of
methanol to the teh-substituted product (14),r8] probably according to the mechanism formulated by Rose et al.L19.'01 If 12
is treated with TMSCl in the presence of HMPA, however, the
reaction takes a completely different course. We assume that the
silylation initially takes place at the chromium
The
conversion of the resulting species 15 into the dienol ether 16
then occurs by a still unknown mechanism, however, without
passing through an intermediate of type 13, from which the
tele-substituted product would arise. When the anionic intermediate rue-7 was treated either with Bu,SnCI followed by TFA or
with BF,.Et,O, rac-9 was isolated after hydrolytic workup as
the sole product.r221
Also remarkable is the fact that tele-substitution proceeds with particular efficiency if the primary nucleophilic addition product ruc-7 is treated with EtAlCl,. In this
case the tele-substitution occurs even before hydrolysis of the
reaction mixture (under aprotic conditions!) (Schema 5)
The protocols reported herein allow effective control of the
outcome of nucleophilic additions to ortho-substituted anisoletricarbonylchromium derivatives of type 11. Either dienol ether
of type 16 (or the cyclohexenone derivatives) or tele-substi-
rac-3
4, THF, -40 "C
*
[
rac-7
]
EtAICIz, -40 "C
rac-9
75 %
Scheme 5. EtAICl,-mediated tele-substitution.
Angew. Chem. Int. Ed. Engl. 1996, 35, No. 18
tuted products of type 14 can now be selectively prepared as
required. Current efforts in our laboratory are directed towards
the completion of the synthesis of (+)-ptilocaulin and the exploration of the scope and limitations of our general method.
Experimental Procedure
All reactions were performed in anhydrous solvents in an atmosphcrc of argon.
6: To a solution of (S,S)-di(1-phenylethy1)amine(272 mg, 1.1 mmol) in THF
,
(30mL)at -70°C wasaddednBuLi(0,74mL, l.l6mmol, 1 . 5 6 ~ i n h e x an e)and
stirring was continued for 1 h while warming to -40°C. Then into the vigorously
stirred solution at - 100 "C was injected first TMSCl (0.83 mL, 6.6 mmol) immediately followed by the rapid injection of a solution of 5 (269 mg, 1.1 mmol) in 3 mL
of TH E After 10 min the cooling bath was removed, and the reaction mixture was
diluted with tBuOMe and washed twice with 2N HCI, and then with saturated
aqueous solutions of NaHCO, and NaCI. The dried (KzC03)organic solution was
evaporated in vacuo, and the crude product was purified on a chromatotron (eluent:
hexane/ethyl acetate 5/1) to give 318 mg (1.01 mmol, 91 %) ofcompound 6 (91 % ee
[14]) as a greenish-yellow powder. M.p. 99-100°C; [a]:' = - 209 (c = 1.0 in
CHCI,); FT-IR (KBr): G = 1942,1885,1865,1842cm-I; 'HNMR (400 MHz, CDCIJ: 6 = 0.31 (s, 9H), 3.73 (s, 3H), 4.79 (1, J = 6 Hz, 1 H), 4.97 (d, J =7 Hz, I H),
5.57 (dd, J = 6, 1.5 Hz, 1 H), 5.68 (pseudo dt, J, = 6 Hz, Jd = 1.5 Hz, 1 H);
NMR (67.5 MHz, CDCI,): 6 = - 0.7(q), 55.3(q), 73.4(d), 85.0(d), 88.8(s),
95.9(d), 101.8(d), 147.5(s), 233.2(s).
3: To a solution of 6 (275 mg, 0.869 mmol) in THF (10 mL) at -60°C was added
nBuLi (0.59 mL, 0.913 mmol, 3.56 M in hexane), and stirring was continued for 1 h
while warming to - 20 "C. After cooling to - 50 "C, CuCl(98 mg, 0.986 mmol) was
added. While the mixture was stirred for 1.5 hand allowed to warm to 0 "C,it slowly
turned greenish-black. The solution was then cooled to -50 "C, treated with crotyl
bromide (0.18 ml, 1.74 mmol), and stirred for 2 h a t 0 "C. The reaction mixture was
then diluted with fBuOMe and washed several times with small portions of 2~ HCl
(until the green color of the aqueous layer no longer intensified). The organic
solution was washed with saturated aqueous solutions of NaHCO, and NaCI, dried
(K,CO,), and the solvent was removed in vacuo to give an orange oil. Purification
on the chromatotron (hexane/ethyl acetate 1011) afforded 252 mg (0.68 mmol,
78 YO)of complex 3 as a dark yellow oil, which solidified upon standing. An analytical sample was crystallized from hexane at - 18 "C: M.p. 55-56°C; [a];' = - 190
(c = 1.0 in CHCI,); FT-IR (KBr): iJ=1960,1883 cm-'; 'H NMR (400 MHz, CD-
CI3):S=0.36(~,9H),1.75(dd,J=6,1Hz,3H),3.19(d,J=6Hz,2H),3.74(~,
3H), 4.88 (I, J = 6.5 Hz, 1 H), 5.40 (dd, J = 6.5, 1.5 Hz, 1H), 5.50-5.67 (m, 3H);
',C NMR (67.5 MHz, CDCI,): S = O.l(q), 17.9(q), 32.l(t), 62.8(q), 87.8(d),
92.6(s), 97.2(d), 99.6(d), 103.9(s), 127.8(d), 129.l(d), 146.6(s), 233.6(s).
2: To a solution of 1,3-dithidne (71 mg 0.594mmoI) in THF (1 mL) and HMPA
(0.94 mL) at - 60 "C was added a 1.56 M solution (0.38 mL) of nBuLi in hexane, and
stirring was continued for 1.5 h while warming to -30 "C. At -70°C a solution of
complex 3 (200 mg, 0.54 mmol) in TH F (2 mL) was added dropwise over a period
of 5 min, and the mixtore was allowed to warm slowly to - 30 "C within 2 h. At this
point freshly distilled TMSCl(0.68 mL, 5.4 mmol) was added, and the mixture was
stirred for 12h at 25 "C before it was diluted with tBuOMe and washed twice with
2 N HCl. The organic layer was then exposed to the sunlight and air until it decolorized completely, and washed with saturated aqueous solutions of NaHCO, and
NaCl. The dried (K2C03) solution was filtered through a pad of Celite, and the
solvent was removed in vacuo to give a clear, slightly yellow oil (dienol ether), which
was dissolved in a mixture of THF (20 mL) and 2 N HCI (10 mL) and heated for 1 h
to 80 "C. The cooled mixture was diluted with tBuOMe, washed as described above,
and dried with MgSO,. After removal of the solvent, the crude product was purified
on the chromatotron (hexanelethyl acetate IOjl) to yield 98 mg (0.287 mmol; 53 %)
of the enone 2 as a colorless solid. An analytical sample was crystallized from hexane
at -18°C. M.p. 157°C; [a]:'= 46 (c = 0.4 in CHCI,); FT-IR (KBr): B =1660,
1593,1245,839;'HNMR(400MHz,CDC1,):6
= 0.10(~,9H),1.60(d,J=4.5Hz,
3H), 1.87 (m, I H ) , 2.03-2.19 (m, 2H), 2.22-2.34 (m, 2H), 2.51 (m, l H ) , 2.69-
2.89(m,6H),4.05(d,J=9Hz,lH),5.27-5.40(m,2H),6.99(dd,J=5,3Hz,
1H); I3CNMR (100 MHz, CDCI,): 6 = - 1.5(q), 17.8(q), 25.8(t), 28.3(t), 29.3(t),
29.7(t), 29.8(t), 40.6(d), 48.6(d), 49.2(d), 127.0(d), 127.8(d), 140.5(s), 354.5(d),
203.9(s); HR-MS: m/z = 340.13508 (as calculated for C,,H,,OS,Si).
rac-9: To a solution of 1,3-dithiane (71 mg, 0.594 mmol) in THF (2 mL) at -60°C
was added a 1.56 M solution of nBuLi (0.38 mL, 0.594 mmol) in hexane. Stirring was
continued for 2.5 h (-+ -2O"C), the mixture was recooled to -60 "C, and a solution of rac-3 (200 mg, 0.54 mmol) in TH F (2 mL) was added dropwise over a period
of 5 min. After 2.5 h (- 60 + 30 "C) the mixture was cooled to - 78 "C, and a 1 M
solution of EtAICI, (1.62mL, 1.62mmol) in hexane was added dropwise. The
mixture was stirred for 2 h at - 78 "C, quenched by careful addition of 2~ HCl, and
diluted with tBuOMe. The organic layer was washed with 2~ HCI, saturated
aqueous NaHCO, and brine, and dried with K,CO,, and the solvent was removed
in vacuo. Chromatography (chromatotron; hexane/ethyl acetate l0jl) gave 186 mg
of rac-9 0.406 mmol; 75%) besides some starting material (rac-3). An analytical
sample of rac-9 was crystallized from hexane at - 18 "C. M.p. 120°C; 'H NMR
(400MHz,C,D,):6=0.23(s,9H),1.42(m,1H),1.60(m,1H),1.64(d,J=3Hz,
3H), 2.26 (m. 1H), 2.45-2.58 (m, 3H),3.14,3.44 (each dd, J = 8,3.5 Hz, 1H), 4.76
0 VCH Verlagsgesehchaft mbH. 0-69451 Weinhezm, 1996
-
0570-0833/96/3518-2147$15.00+ ,2510
2147
COMMUNICATIONS
(d, J = 3 Hz, I H ) , 5.07 (s, I H), 5.15 (s, I H ) , 5.52, 5.65 (each m, I H ) , 5.86 (d,
J = 3 Hz, 1 H). For further characteristic data of rac-9 see ref. 181.
Received: March 15, 1996 [Z 8928IEl
German version: Angew. Chem. 1996, 108, 2277-2280
-
-
Keywords: arene complexes asymmetric syntheses chromium
compounds nucleophilic additions
-
[l] E. 0. Fischer, K. Ofele, Z . Naturforsch. B 1958, 13, 458.
[2] Review: L. S. Hegedus, Trunsition Metals in the Synthesis of Complex Organic
Molerules, University Science Books, Mill Valley 1994. Chapter 10.
[3] Selected recent work: a) E. P. Kiindig, A. Ripa, R. Liu, G. Bernardinelli, J.
Org. Chem. 1994, 59, 4773; b) K. Kamikawa, T. Watanabe, M. Uemura, ibid.
1996,61,1375; c) M. Brands, H. G. Wey, J. Bruckmann, C. Kriiger, H. Butenschon, Chem. Eur. J. 1996,2, 182.
[4] a) H:G. Schmalz, J. Hollander, M. Arnold, G. Diirner, Tetrahedron Lett. 1993,
34,6259; b) H:G. Schmalz, M. Arnold, J. Hollander, J. W. Bats, Angew. Chem.
1994,106,77; Angew. Chem. Int. Ed. Engl. 1994,33,109; c) H:G. Schmalz, A.
Schwarz, G. Diirner, Tetrahedron Lett. 1994, 35, 6861 ; d) H.-G. Schmalz, A.
Majdalani, T. Geller, J. Hollander, J. W. Bats, ibid. 1995, 36, 4777.
[5] An example is the radical cyclization of arenetricarbonylchromium complexes
discovered recently in our laboratory: a) H:G. Schmalz, S. Siegel, J. W Bats,
Angew. Chem. 1995, 107, 2597; Angew. Chem. Int. Ed. Engl. 1995, 34, 2383;
b) H.-G. Schmalz, S. Siegel, A. Schwarz, Tetrahedron Lett. 1996, 37, 2947.
[6] Isolation and structure elucidation: G. C. Harbour, A. A. Thymiak, K. L.
Rinehart, Jr., P. D. Shaw, R. G. Hughes, Jr., S. A. Mizsak, J. H. Coats, G. E.
Zurenko, L. H. Li, S. L. Kuentzel, J. Am. Chem. Soc. 1981, 103, 5604.
[7] Synthetic work: a) B. B Snider, W. C. Faith, Tetrahedron Lett. 1983, 24, 861 ;
h) J. Am. Chem. Sor. 1984, 106, 1443; c) W. R. Roush, A. E. Walts, ibid. 1984,
106, 721; d) A. E. Walts, W. R. Roush, Telrahedron 1985, 41, 3463; e) T. Uyehara, T. Furuta, Y. Kabasawa, J. Yamada, T. Kato, J. Chem. Soc. Chem. Commun. 1986, 539; f) T. Uyehara, T. Furuta, Y Kahasawa, J. Yamada, T. Kato,
Y Yamamoto. J. Org. Chem. 1988, 53, 3669; g) A. Hassner, K. S. K. Murthy,
Tetrahedron Lett. 1986, 27, 1407; h) M. Asaoka, M. Sakurai, H. Takei, ihid.
1990,31,4759.
[8] H:G. Schmalz, K. Schellhaas, Tetrahedron Lett. 1995, 36, 5511.
[9] a) M. F. Semmelhack, J. J. Harrison, Y. Thebtaranonth, J. Org. Chem. 1979,44,
3275; h) M. F. Semmelhack, H. T. Hall, Jr., R. Farina, M. Yoshifuji, G. Clark,
T. Bargar, K. Hirotsu, J. Clardy, J. Am. Chem. Sor. 1979,101, 3535; c) M. F.
Semmelhack, G. R. Clark, J. L. Garcia, J. J. Harrison, Y. Thebtaranonth, W.
Wulff, A. Yamashita, Tetrahedron 1981, 37, 3957; d) M. F. Semmelhack in
Comprehensive Organometal/ic Chemistry 11,Vol. 12 (Eds.: E. W Ahel, F, G. A.
Stone, G. Wilkinson), Pergamon, NY, 1995. p. 979; e) for an application in a
synthesis of (+)-acorenone B (preparation of a spirocyclic, 5,5-disuhstituted
cyclohexenone derivative) see: e) M. F. Semmelhack, A. Yamashita, J. Am.
Chem. Soc. 1980, 102, 5924.
[lo] For a review on nucleouhilic additions to arene comulexes. see M. F. Semmelhack in Comprehensive Organic Synthesis, Vol. 4 (Eds.: B. M. Trost, I. Fleming), Pergamon, Oxford, 1991, p. 517.
All new compounds were characterized by the usual spectroscopic methods
and gave correct elemental analyses.
a) D. A. Price, N. S. Simpkins, A. M. MacLeod, A. P. Watt, J. Org. Chem.
1994, 59, 1961; b) Tetrahedron Lett. 1994, 35, 6159; c) H . G . Schmalz, K.
Schellhaas, ibid. 1995, 36, 5515.
The enantioselectivity of the reaction was enhanced over that of earlier procedures [I 21 by reducing the reaction temperature. The absolute configuration
given for 6 is based on the assignment of Simpkins [12a].
The enantiomeric excesses were determined by HPLC on a Chiralcel OJcolumn (Diacel). The separation conditions were optimized with racemic samples.
a) M. F. Semmelhack, A. Zask, J. Am. Chem. Soc. 1983, 105, 2034; for a
different procedure for the preparation of rac-3, see b) M. F. Semmelhack,
J. Bisaha, M. Caarny, ibid. 1979, 101, 768.
After extractive workup, the crude product (dienol ether) still contains significant amounts of (undefined) chromium complexes. A complete decomplexation is achieved by exposing the yellow solution to the sunlight for some time.
Thereafter, GLC-MS and 'H NMR analysis of the product mixture indicates
that 8 is the major component (beside small amounts of 2, some decomplexed
9, and several unidentified by-products).
The trans-configuration of2 follows from the 'H NMR coupling constants; the
signals were assigned by means of a COSY spectrum.
This product was obtained under analogous conditions in a yield comparable
to that of 2; for rue-10: m.p. 98°C (hexane); FT-IR (KBr): i =1652, 1597,
1343, 1246, 841(s)cm-'; 'HNMR(400MHz, CDC1,): 6 = 0.12(s, 9H), 1.05
(d, J = 7 Hz, 3H), 1.89 (m. 1 H), 2.09 (m, 1 H), 2.26 (ddd, J = 20, 10, 2 Hz,
IH), 2.45 (pseudo sept., J = 5 Hz, 1 H), 2.69-2.91 (m, 6H), 4.00 (d, J =
10 Hz. 1 H), 7.06 (pseudo dd, J = 5 , 2 Hz, 1 H); 13C NMR (67.5 MHz,
CDCl,): 6 = - 0.7(q), 10.3(q), 25.9(t), 28.3(t), 29.7(t), 29.8(t), 41.1(d),
2148
0 VCH Veria,rs,reseilsrhafi mhH. 0-6945f Weinheim. 1996
42.9(d), 49.3(d), 140.2(s), 155.5(d), 205.1 (s); HR-MS: m/r=300.10378 as
calcd. for C,,H,,OS,Si.
1191 a) J.-C. Boutonnet, F. Rose-Munch, E. Rose, Tetrahedron Lett. 1985.26.3989;
b) J. P. Djukic, F. Rose-Munch, E. Rose, F. Simon, Y Dromzee, Organomelullics 1995, 14, 2027, and references therein.
[20] There are several examples of nucleophilic tele- and cinesubstitutions (S,Ar
reactions) to Cr(CO),-complexed arenes [lo], hut only in very few cases does
a methoxy group act as a leaving group. See, for instance, refs. [4b, 8, 19 b].
1211 It is known that the reaction of anionic q5-complexesof type 12 with Ph,SnCl
may lead to (isolable) stannylated products with a structure similar to 15: E. P.
Kiindig, A. F. Cunningham, Jr., P. Paglia, D. P. Simmons, G. Bernardinelli,
Helv. Chim. Actu. 1990, 73, 386.
[22] In these cases, a characteristic red color of the reaction mixture appeared,
which is also observed when TFA is added to solutions of anionic intermediates
of type 12.
[23] This was secured by 'H NMR spectroscopy of a sample that had been removed
under exclusion of air (prior to the hydrolysis of the reaction mixture), freed
from all volatile components in vacuo, and redissolved in C,D,.
Cyclopeptide Libraries as New Chiral Selectors
in Capillary Electrophoresis
Giinther Jung," Heike Hofstetter, Susanne Feiertag,
Dieter Stoll, Oliver Hofstetter, Karl-Heinz Wiesmiiller,
and Volker Schurig*
For the chromatographic separation of enantiomers a single
chiral selector is added to the mobile phase or used as stationary
phase. Multicomponent chiral selectors have been applied
rarely, for example in inclusion gas chromatography['] and in
complexation gas chromatography.['] Combinatorial chemi ~ t r y [furnishes
~]
defined libraries that may be utilized as novel
multicomponent chiral additives for the mobile phase or as multicomponent chiral stationary phases in pressure-driven or electrically driven separation systems. Thus, the time-consuming
screening of a multitude of individual potential chiral selectors
can be avoided by employing a selector library. Sublibraries
with reduced heterogeneity can be employed to identify the
components with the best selector properties. The search for the
most efficient selectors is indicated because each component is
highly dilute. Yet the use of the whole library may prove beneficial if cooperative effects between the components affect the
enantioselectivity.
Capillary electrophoresis with chiral additives in the mobile
phase is an efficient technique for separating water-soluble
enantiomers in small sample volumes. Enantiomeric resolution
has been achieved with proteins such as bovine serum albumin
(BSA),I4]0rosomucoid,[~1ovomucoid,[6.1' caseine,['] and cellulase'81 as well as with cyclic structures such as the macrocyclic
antibiotics vancomycin and rifamycin B,I9, l o ]crown ethers,["]
and cyclodextrins[l'] serving as single chiral selectors. The
mechanisms of retention and enantiodifferentiation by proteins
and peptides are not yet well-under~tood.['~~
However, the selector systems described here, which are based on cyclopeptides,
may provide first hints. Firstly, as the structures of conformationally constrained cyclopeptides in solution can be determined
[*I Prof. Dr. G. Jung, Prof. Dr. V. Schurig, DipLChem. H. Hofstetter,
Dip1.-Biochem. 0. Hofstetter
Institut fur Organische Chemie der Universitit
Auf der Morgenstelle 18, D-72076 Tiibingen (Germany)
Fax: Int. code +(7071) 29-6925
e-mail: guenther.jung@tuebingen.de
Dr. S. Feiertag, Dip].-Chem. D. Stoll, Dr. K.-H. Wiesmiiller
Naturwissenschaftliches und Medizinisches Institut
an der Universitlt Tubingen, D-72762 Reutlingen (Germany)
0570-083319413518-2148$15.00+ .2SiO
Anpew. Chem. Int. Ed. Enpi. 1996, 35. No. 18
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course, substitution, formation, anisole, complexes, chromium, telef, ethers, dienol, additional, versus, substituted, controlling, nucleophilic, tricarbonyl, ortho
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