Absolute configurational assignments of secondary amines by CD-sensitive dimeric zinc porphyrin host

Article abstract of DOI:10.1021/ja020520p

A general chiroptical protocol for determination of absolute configuration of secondary amines including acyclic and cyclic aliphatic amines, aromatic amines, amino acids, and amino alcohols is described. The chiral substrate is linked to the achiral carrier moiety (3-N-Boc-amino-propyl-N-Boc-amino) acetic acid 1 (BocHNCH₂CH₂CH₂BocNCH₂COOH), which after deprotection, yields a bidentate conjugate, capable of forming a 1:1 host/guest complex with dimeric zinc porphyrin host 2. As in the cases of primary amines and secondary alcohols reported earlier, the complexation of secondary amine conjugates to porphyrin tweezer host 2 represents a stereodifferentiating process, where the large (L) group at the stereogenic center (assigned on the basis of conformational energies A value) protrudes from the porphyrin binding pocket. This leads to formation of host/guest complexes with a preferred porphyrin helicity that exhibit intense exciton split CD spectra. It was found that the chiral sense of porphyrin twist is clearly controlled by the stereogenic center despite the Z/E conformational complexity around the tertiary amide bond of secondary amine conjugates that has greatly hampered previous configurational assignments. Thus, in cases where there is no ambiguity regarding the relative steric size of substituents, the observed CD couplet can be applied for straightforward assignment of absolute configurations. In addition, to extend the application to more difficult cases a molecular mechanics calculation approach using the Merck Molecular Force Field (MMFFs) was developed; this provides conformational information of host/guest complexes and leads to prediction of preferred porphyrin helicity independent of conformational A values. This chiroptical protocol in combination with molecular modeling represents a general method for configurational assignments of secondary amines.

Full text of DOI:10.1021/ja020520p

Published on Web 08/07/2002
Absolute Configurational Assignments of Secondary Amines
by CD-Sensitive Dimeric Zinc Porphyrin Host
Xuefei Huang,† Naoko Fujioka,† Gennaro Pescitelli,† Frank E. Koehn,‡
R. Thomas Williamson,‡ Koji Nakanishi,*,† and Nina Berova*,†
Contribution from the Department of Chemistry, Columbia UniVersity,
New York, New York 10027, and Department of DiscoVery Analytical Chemistry,
Wyeth Research, 401 North Middletown Road, Pearl RiVer, New York 10965
Received April 10, 2002
Abstract: A general chiroptical protocol for determination of absolute configuration of secondary amines
including acyclic and cyclic aliphatic amines, aromatic amines, amino acids, and amino alcohols is described.
The chiral substrate is linked to the achiral carrier moiety (3-N-Boc-amino-propyl-N-Boc-amino)acetic acid
1 (BocHNCH2CH2CH2BocNCH2COOH), which after deprotection, yields a bidentate conjugate, capable of
forming a 1:1 host/guest complex with dimeric zinc porphyrin host 2. As in the cases of primary amines
and secondary alcohols reported earlier, the complexation of secondary amine conjugates to porphyrin
tweezer host 2 represents a stereodifferentiating process, where the large (L) group at the stereogenic
center (assigned on the basis of conformational energies A value) protrudes from the porphyrin binding
pocket. This leads to formation of host/guest complexes with a preferred porphyrin helicity that exhibit
intense exciton split CD spectra. It was found that the chiral sense of porphyrin twist is clearly controlled
by the stereogenic center despite the Z/E conformational complexity around the tertiary amide bond of
secondary amine conjugates that has greatly hampered previous configurational assignments. Thus, in
cases where there is no ambiguity regarding the relative steric size of substituents, the observed CD couplet
can be applied for straightforward assignment of absolute configurations. In addition, to extend the application
to more difficult cases a molecular mechanics calculation approach using the Merck Molecular Force Field
(MMFFs) was developed; this provides conformational information of host/guest complexes and leads to
prediction of preferred porphyrin helicity independent of conformational A values. This chiroptical protocol
in combination with molecular modeling represents a general method for configurational assignments of
secondary amines.
Introduction
(Figure 1); the conjugate, upon treatment with a dimeric zinc
porphyrin tweezer host 2, yields the macrocyclic complex 3
Secondary amines are of great interest due to their roles as
plant growth regulators, plant growth promoters, antibiotics, and
antitumor agents.1 They have also been extensively utilized as
chiral auxiliaries for asymmetric catalysis, as resolving agents,
and as intermediates for the synthesis of biologically active
substances.2 The studies of their absolute configurations by
chiroptical methods have been hampered due to the lack of an
intense absorption band for direct assignment as well as the
conformational complexity of their most common chromophoric
derivatives, tertiary amides. In the following, we describe a
general method for absolute configurational assignments of
secondary amines, in which a chiral secondary amine is
derivatized with carrier 1 to generate a bidentate conjugate
through zinc amine coordination. Due to stereodifferentiation
leading to preferred porphyrin helicity, the complex exhibits
intense exciton-coupled CD that reflects the absolute configu-
ration of the secondary amine. Furthermore, molecular modeling
of the complex 3 provides a tool for correlating the observed
exciton CD couplet with the absolute configuration.
The absolute configurational assignment of secondary amines
has been studied using circular dichroic (CD) methods. Earlier
approaches were based on derivatization of secondary amines
with fluorescamine or 2-pyridine/pyridine N-oxide.3 The abso-
lute configurations of the amines were empirically correlated
with the sign of the CD bands of the chromophoric derivatives
at longer wavelength (around 320 nm). Recently, Okamoto and
co-workers have synthesized an achiral polyacetylene polymer
bearing carboxylic acid groups.4 Upon formation of acid/base
pairs with chiral amines, the polymer gave an induced CD with
* To whom correspondence should be addressed. E-mail:
kn5@columbia.edu and ndb1@columbia.edu.
† Columbia University.
‡ Wyeth Research.
(1) Saidi, M. R.; Azizi, N.; Zali-Boinee, H. Tetrahedron 2001, 57, 6829-
6832.
(3) Toome, V.; Wegrzynski, B.; Dell, J. Heterocycles 1977, 7, 787-792;
Bettoni, G.; Catsiotis, S.; Perrone, R.; Tortorella, V. Gazz. Chim. Ital. 1977,
107, 111-116; Tortorella, V.; Bettoni, G.; Perrone, R. Gazz. Chim. Ital.
1973, 103, 1083-1098.
(4) Yashima, E.; Matsushima, T.; Okamoto, Y. J. Am. Chem. Soc. 1997, 119,
6345-6359.
(2) Kozma, D. In Optical Resolutions Via Diastereomeric Salt Formation;
Kozma, D., Ed.; CRC Press: Boca Raton, 2002; p 690; Whitesell, J. K.
Chem. ReV. 1989, 89, 1581-1590; Jacques, J.; Collet, A.; Wilen, S. H.
Enantiomers, Racemates, and Resolution; John Wiley & Sons: New York,
1981.
9
10320
J. AM. CHEM. SOC. 2002, 124, 10320-10335
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Configurational Assignments of Secondary Amines
A R T I C L E S
Figure 1. Bidentate conjugate prepared from secondary amine and carrier 1 is capable of forming 1:1 host/guest complex 3 with Zn-porphyrin tweezer 2.
The helicity between the two porphyrins in complex 3, defined as the projection angle between 5 and 15 and 5′-15′ directions, is governed by the absolute
configuration of the secondary amine.
the sign indicating the absolute stereochemistry of the amine.
For secondary amines bearing aromatic chromophores, Smith
and co-workers derived a semiempirical sector rule for absolute
configurational assignments.5 In all of the aforementioned
studies, the CD amplitudes observed are generally very weak
(∆ꢀ < 10). Moreover, the scope of these methods was often
restricted since the sign of CD in these cases may be affected
by other factors such as the type and bulkiness of substituents.
For secondary amines bearing other functional groups,
multiple chromophores can be introduced, and the exciton
chirality method can be applied.6,7 However, this approach has
been encumbered by the conformational heterogeneity of the
tertiary amides generated after chromophoric derivatization.
Tertiary amide bonds are known to exist in two rotamer forms,
that is, Z and E, usually of close energy, with the ratio depending
upon the steric sizes of substituents. This often leads to weak
CD signals and unreliable or even erroneous assignments. For
example, Van Vranken and co-workers have demonstrated that
bromobenzoylation of an amino glycoside containing a second-
ary amine moiety yielded a tertiary amide, which existed as a
mixture of Z and E conformers in approximately 1:1 ratio. The
original assignment, which did not take this factor into account,
turned out to be incorrect since the observed CD was the sum
of CD contributions from these conformers with opposite
predicted signs.6
this method has undergone remarkable progress with the
development of new reagents and novel procedures. Hoye and
workers have extended the Mosher method to cyclic secondary
amines by derivatization with the (R)- and (S)-enantiomers of
R-methoxy-R-(trifluoromethyl)phenylacetic acid (MTPA).11 How-
ever, the resulting tertiary amides contain significant populations
of two conformers around the tertiary amide bonds. Therefore,
the absolute configurations of secondary amines should be
derived only from chemical shift differences between the two
diastereomers possessing the same amide conformation,11 which
in turn requires a detailed conformational analysis of the two
MTPA derivatives and NMR assignments of all conformers.
Furthermore, this method has not been demonstrated to be
widely applicable to acyclic secondary amines. It is thus highly
desirable that a more general approach for absolute configura-
tional assignment of secondary amines be developed.
In recent years, porphyrins and zinc porphyrins have become
useful as versatile CD reporter groups for structural studies.12
We have developed a protocol for absolute configurational
assignment of diamines utilizing a bis(zinc porphyrin) linked
by a pentanediol linker.13 The resulting achiral zinc porphyrin
tweezer 2 is capable of binding various chiral acyclic R,ω-
diamines through zinc-amine coordination leading to formation
of 1:1 macrocyclic host/guest complexes. Due to stereodiffer-
entiation of substituents at the stereogenic center, the two
porphyrins in the complex adopt a preferred chiral twist, as
evident from the observed bisignate exciton couplet in the
porphyrin Soret band region, the sign of which is governed by
the absolute configuration of the bound diamine.
The modified Mosher NMR method has been extensively
applied to absolute configurational assignments of secondary
alcohols,8 primary amines,9 and carboxylic acids.10 Recently,
In the case of chiral compounds such as monoamines and
monoalcohols with only a single site for attachment of the
chromophoric reporter groups, trifunctional carrier moieties
exemplified by 1 and 4 were developed (Chart 1). They form
(5) Smith, H. E. In Circular Dichroism. Principles and Applications, 2nd ed.;
Berova, N., Nakanishi, K., Woody, R. W., Eds.; Wiley-VCH: New York,
2000; pp 397-430.
(6) Chisholm, J. D.; Golik, J.; Krishnan, B.; Matson, J. A.; Van Vranken, D.
L. J. Am. Chem. Soc. 1999, 121, 3801-3802.
(7) Lo, L. C.; Berova, N.; Nakanishi, K.; Morales, E. Q.; Vazquez, J. T.
Tetrahedron: Asymmetry 1993, 4, 321-330.
(8) Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc.
1991, 113, 4092-4096; Dale, J. A.; Mosher, H. S. J. Am. Chem. Soc. 1973,
95, 512-519; Latypov, S. K.; Seco, J. M.; Quinoa, E.; Riguera, R. J. Org.
Chem. 1996, 61, 8569-8577; Trost, B. M.; Belletire, J. L.; Godleski, S.;
McDougal, P. G.; Balkovec, J. M.; Baldwin, J. J.; Christy, M. E.; Ponticello,
G. S.; Varga, S. L.; Springer, J. P. J. Org. Chem. 1986, 51, 2370-2374.
(9) Seco, J. M.; Quinoa, E.; Riguera, R. J. Org. Chem. 1999, 64, 4669-4675;
Seco, J. M.; Latypov, S. K.; Quinoa, E.; Riguera, R. J. Org. Chem. 1997,
62, 7569-7574; Trost, B. M.; Bunt, R. C.; Pulley, S. R. J. Org. Chem.
1994, 59, 4202-4205; Latypov, S. K.; Seco, J. M.; Quinoa, E.; Riguera,
R. J. Org. Chem. 1995, 60, 1538-1545.
(10) Seco, J. M.; Quinoa, E.; Riguera, R. Tetrahedron: Asymmetry 2001, 12,
2915-2925; Seco, J. M.; Quinoa, E.; Riguera, R. Tetrahedron: Asymmetry
2000, 11, 2781-2791; Ferreiro, M. J.; Latypov, S. K.; Quinoa, E.; Riguera,
R. J. Org. Chem. 2000, 65, 2658-2666; Hoye, T. R.; Hamad, A.-S.; Koltun,
D. O.; Tennakoon, M. A. Tetrahedron Lett. 2000, 41, 2289-2293; Hoye,
T. R.; Koltun, D. O. J. Am. Chem. Soc. 1998, 120, 4638-4643.
(11) Hoye, T. R.; Renner, M. K. J. Org. Chem. 1996, 61, 8489-8495; Hoye,
T. R.; Renner, M. K. J. Org. Chem. 1996, 61, 2056-2064.
(12) Huang, X.; Nakanishi, K.; Berova, N. Chirality 2000, 12, 237-255.
(13) Huang, X.; Rickman, B. H.; Borhan, B.; Berova, N.; Nakanishi, K. J. Am.
Chem. Soc. 1998, 120, 6185-6186.
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A R T I C L E S
Huang et al.
Chart 1
Chart 2
bidentate conjugates with chiral substrates, which are capable
of forming 1:1 complexes with the porphyrin tweezer host
2.14-16 Steric differentiation between the two substituents at the
stereogenic center, assigned large (L) and medium (M), ac-
cording to their conformational energies expressed in A values
as first defined by Winstein and Holness,17 and later by Eliel,18
Bushweller,19 and Lightner,20 leads to a preferred chiral twist
between the porphyrin transition dipole moments;21 this gives
rise to exciton coupled CD with the sign reflecting the absolute
configuration of the substrate.
In the present study on secondary amines, we first tested
carrier 4 utilized in earlier studies of primary amines.14 Although
certain conjugates formed between carrier 4 and model second-
ary amines exhibited exciton CD couplet with large amplitudes,
the signs of these couplets did not necessarily agree with those
expected for the known absolute configurations. We suspected
the conformational flexibility of these conjugates as being the
most likely reason for this inconsistency. To reduce the
conformational freedom, carrier 5 with an N-pyridine oxide
moiety was prepared (Chart 2). It was hoped that the secondary
amine/carrier 5 conjugate might be more rigid through increased
electronic repulsion between the negatively charged oxygen of
the pyridine oxide and the carbonyl oxygen. While all tested
cyclic secondary amines conjugates with carrier 5 yielded
exciton CD couplets with signs in agreement with prediction,
disappointingly the CD of acyclic secondary amine conjugates
were not only much weaker but also exhibited signs inconsistent
with the absolute configuration,22 presumably due to different
Z/E ratios of the conjugates. The unsuccessful attempts with
pyridine derivatives 4 and 5 as carriers prompted us to search
for a carrier that allows more flexibility between the stereogenic
center and the bidentate binding sites, thus leading to a better
chiral recognition. Therefore, carrier 1, developed for successful
absolute configurational assignment of secondary alcohols and
primary amines,15,16 became a prospective carrier moiety, which
would possibly allow the conjugate to bind tweezer 2 and
Scheme 1. Preparation of the Secondary Amine/Carrier 1
Conjugates
provide exciton CD couplets reflecting the absolute configura-
tion regardless of the conformation of the tertiary amide bond.
(14) Huang, X.; Borhan, B.; Rickman, B. H.; Nakanishi, K.; Berova, N. Chem.s
Eur. J. 2000, 6, 216-224.
Results and Discussion:
(15) Kurtan, T.; Nesnas, N.; Li, Y.-Q.; Huang, X.; Nakanishi, K.; Berova, N. J.
Am. Chem. Soc. 2001, 123, 5962-5973.
(16) Kurtan, T.; Nesnas, N.; Koehn, F. E.; Nakanishi, K.; Berova, N. J. Am.
Chem. Soc. 2001, 123, 5974-5982.
Preparation of the Conjugates. The di-Boc-protected carrier
1 was prepared as described previously.15 Accordingly, the
formation of bidentate 2° amine/carrier 1 conjugate 6 was
performed by two different procedures (Scheme 1). Secondary
amines were coupled with carrier 1 to generate the di-Boc-
protected conjugate 7 in greater than 80% yield. Treatment of
7 with trifluoroacetic acid (TFA) and neutralization with Na2-
CO3 afforded the free base form of the conjugate 6 (route A),
which readily formed a 1:1 host/guest complex with zinc
porphyrin tweezer 2. In the case of acid-sensitive compounds
(17) Winstein, S.; Holness, N. J. J. Am. Chem. Soc. 1955, 77, 5562-5578.
(18) Eliel, E. L.; Wilen, S. H. In Stereochemistry of Organic Compounds;
Wiley: New York, 1994; pp 695-697.
(19) Bushweller, C. H. In Conformational BehaVior of Six-Membered Rings:
Analysis, Dynamics, and Stereoelectronic Effects; Juaristi, E., Ed.; VCH
publishers: New York, 1995; pp 25-58.
(20) Boiadjiev, S. E.; Lightner, D. A. J. Am. Chem. Soc. 2000, 122, 11328-
11339.
(21) Matile, S.; Berova, N.; Nakanishi, K.; Fleischhauer, J.; Woody, R. W. J.
Am. Chem. Soc. 1996, 118, 5198-5206.
(22) Huang, X. In I. Porphyrins in structural studies by circular dichronic
spectroscopy. II. Spectroscopic studies of neurologically actiVe philantho-
toxins. Ph.D. Thesis, Columbia University, New York City, 1999.
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Configurational Assignments of Secondary Amines
A R T I C L E S
Figure 2. (a) CD spectrum of complex 1-9/2 formed between conjugate 1-9 (20 equiv) and Zn-porphyrin tweezer 2 (1 µM) in methylcyclohexane (MCH).
ACD denotes the amplitude of the CD exciton couplet, which represents the difference in ∆ꢀ between the peak and the trough. (b) Job plot of the complex
1-9/2 with the maximum CD amplitude observed when the molar fraction of host 2 is 0.5 indicates that the stoichiometry of binding is 1:1 (total host and
guest concentration is 1µM).
such as 1-O-methyl-N-methyl-L-acosamine, an alternative route
was designed by derivatizing the secondary amine with bro-
moacetic acid (route B). Treatment of the bromoacetamide 8
with 1,3-diaminopropane gave the free amine conjugate 6
(Scheme 1). Although typical reactions were carried out with
milligram quantities of the secondary amine, it was feasible to
form the conjugate with as little as 10 µg of chiral substrate
followed by the use of a Pasteur pipet column for purification
prior to UV-vis and CD measurements (see Supporting
Information).
free tweezer 2 to the 1:1 host/guest macrocyclic complex (Figure
3a,d). It is only after the guest concentration exceeds 200 equiv
that UV-vis maxima of the complex are shifted to longer
wavelengths beyond 422 nm (Figure 3b, 200-1000 equiv
curves). The red-shift beyond 422 nm points to an increase in
the population of the flexible 1:2 host/guest complexes. The
binding constant (Ka) between host 2 and guest 1-9 is determined
to be 5 × 106 M-1 through nonlinear curve-fitting of the
absorbance changes at 422 nm with increasing amount of guest
1-9 based on a 1:1 stoichiometry. The value of Ka is similar to
that measured for the primary amine conjugate of carrier 1.15
Formation and Stoichiometry of Host/Guest Complexes.
Mixing of porphyrin tweezer host 2 with the bidentate guest
conjugate 1-9 (1-9 denotes the conjugate formed between carrier
1 and secondary amine 9), formed between (S)-N-benzyl-R-
phenyethylamine 9 and carrier 1, gave rise to an intense positive
exciton CD couplet with an amplitude ACD ) +1078 in
methylcyclohexane (MCH) (Figure 2a). A Job plot was obtained
from CD spectra measured with different molar fractions of the
tweezer host 2 with a fixed total amount of host and guest. It is
advantageous to monitor CD rather than UV-vis changes for
the construction of the Job plot, since in this manner only the
host/guest complex is selectively detected while the uncom-
plexed host remains CD silent. From the Job plot, it is evident
that the stoichiometry for the binding of porphyrin tweezer 2
with conjugate 1-9 is 1:1 (Figure 2b) with the maximum CD
amplitude observed when the molar fraction of the host 2 was
0.5.
The formation of host/guest complexes accompanying the
addition of conjugate 1-9 to porphyrin tweezer 2 is also
corroborated by changes in the corresponding CD spectra. The
CD amplitudes of the host/guest complex increase as the guest
concentration increases from 0.2 to 2 equiv and reaches a
maximum at 2 equiv, indicating formation of the 1:1 complex
(Figure 3c). The CD amplitudes, while varying little between 2
and 500 equiv of guest concentration, decrease above 500 equiv
(Figure 3c), suggesting an increase in the population of the 1:2
complex devoid of exciton coupling. Twenty equivalents of the
guest were used as the optimal amount for the following CD
investigations. In all cases, the CD spectra were measured in
different solvents, such as MCH, hexane, benzene, toluene,
methylene chloride, and acetonitrile with the data for complex
1-9/2 shown in Table 1. The complexes consistently exhibited
similar amplitudes in apolar solvents, that is, MCH, hexane,
benzene, toluene, with subsequent CD data given only for MCH
and hexane. It is noteworthy that, without exception, the sign
of CD exciton couplets for a complex remained the same in all
solvents tested.
Formation of the 1:1 macrocyclic complex between porphyrin
tweezer 2 and the bidentate conjugate guest is also supported
by changes in UV-vis spectra observed with different amounts
of guest (Figure 3). With increasing concentration of the guest
such as conjugate 1-9, the UV-vis maxima are shifted from
416 nm (Figure 3a, 0 equiv curve) to 422 nm with a shoulder
at 435 nm (Figure 3a, 2-40 equiv curve). The absorption
maximum of the complex formed between a monoamine, such
as isopropylamine, and porphyrin tweezer 2 is at 428 nm.23 The
blue-shifted band of 1-9/2 complex with an absorption maximum
at 422 nm and the red-shifted shoulder at 435 nm arise from
high-energy (in-phase) and low-energy (out-of-phase) transi-
tions, respectively, due to the strong exciton coupling between
the two nearby porphyrins held in a close-to-parallel orienta-
tion.23 This difference clearly indicates the transition from the
CD of the Host/Guest Complexes. The reason that the
conjugate/porphyrin tweezer 2 complex yields an exciton-
coupled CD spectrum with a specific sign is due to steric
differentiation of the three groups at the stereogenic center of
the conjugate by porphyrin tweezer 2. The two substitutents at
the stereogenic center are assigned large (L) and medium (M)
according to their steric sizes based on their respective confor-
mational energy A values,17-20 while the third group is hydrogen.
Upon addition of conjugate 1-9 to porphyrin tweezer 2, a 1:1
host/guest complex is formed via coordination of the amino
nitrogens of 1-9 to the zinc atoms of the porphyrins. In this
complex, porphyrin P-1 binds with the primary amino group
of the conjugate, while porphyrin P-2 preferably approaches
(23) Huang, X.; Borhan, B.; Berova, N.; Nakanishi, K. J. Indian Chem. Soc.
1998, 75, 725-728.
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A R T I C L E S
Huang et al.
Figure 3. UV-vis and CD of 1-9/2 complex with different equivalents of 1-9 in MCH. (The concentration of tweezer 2 is kept constant at 1 µM in all
cases). (a) UV-vis of 2 with 0-40 equiv of 1-9. (b) UV-vis of 2 with 80-1000 equiv of 1-9. (c) Change in CD amplitudes with different equivalents of
1-9. (d) Schematic presentation of the equilibria between porphyrin tweezer 2, 1:1 macrocyclic host/guest complex, and flexible 1:2 host/guest complexes.
the secondary amine from the face of M and H groups, to avoid
unfavorable steric interaction with the bulky L group (Figure
4). This stereoselection would render the L group pointing away
from the binding pocket formed by the two porphyrins, leading
in the case of conjugate 1-9 to a clockwise twist between the
transition dipole moments of the two porphyrins, which are
along the 5-15 direction of the zinc tetraphenylporphyrins.21
(For detailed discussion about conformation of the host/guest
complex, see the Conformational Analysis section). The over-
riding clockwise twist between the two porphyrins leads to a
positive CD couplet, which is in full agreement with the
observed positive CD couplet with an amplitude of +1078 in
MCH. Thus, the absolute configuration of the amine can be
correlated with the sign of the exciton CD couplet by following
the simple rule: if a positive exciton-coupled CD spectrum is
observed, the L, M, and H groups are arranged in a clockwise
fashion with the amino group in the rear in the Newman
projection, and vice versa. This is fully consistent with the
primary amine and secondary alcohol conjugates as established
previously (conjugates 1-9 vs 1-10, 1-15 vs 1-16 in Table 2).15
The method described here is applicable to chiral secondary
amines containing various substituents on the nitrogen (Table
2). While varying the substituents on nitrogen from hydrogen
(conjugate 1-10) to methyl (conjugate 1-11) or ethyl groups
(conjugate 1-12) does not significantly affect the CD amplitudes,
introduction of a benzyl moiety (conjugate 1-9) leads to a great
enhancement in the CD amplitude. It appears that the presence
of two aromatic groups, one at the stereogenic center and the
other on the nitrogen, is necessary for this enhancement, since
the enhancement is much smaller where only one aromatic group
is involved (conjugate 1-16 vs 1-11, 1-16 vs 1-17). The possible
reasons for the observed enhancement of CD amplitude will be
discussed in the Molecular Modeling section. Tertiary amides
can be formed selectively in the presence of free hydroxyl
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Configurational Assignments of Secondary Amines
A R T I C L E S
Table 1. CD Amplitudes of Complex 1-9/2 in Various Solvents (Concentrations of Tweezer 2 and Conjugate 1-9 Are 1 and 20 µM,
Respectively)
Figure 4. Complex formation between conjugate 1-9 and tweezer 2 leads to two conceivable conformations with opposite sense of twists. The predominant
conformation I, the one in which the L (large) group is protruding from the porphyrin binding pocket, gives rise to an CD exciton couplet representing the
sense of twist between the two porphyrins and hence the absolute configuration at the stereogenic center.
groups in the chiral substrates (conjugates 1-18-1-21). The
existence of multiple chiral centers in these conjugates does
not interfere with the results since the amino containing
stereogenic center is the sole determinant of the sign of exciton
CD couplet.
N-Me amino acids have also been included in this study since
they are important constituents of many biologically active
peptides and are often critical for their activities.24 The carboxyl
moiety is assigned M since its conformational energy (∼5 kJ/
mol) is smaller than that of the alkyl or aryl substituents (>7.3
kJ/mol).18
conjugate 1-22a, the predicted negative CD couplet (0 h curve)
completely inverted after 4 h with a much larger amplitude
(Figure 5).25 Moreover, the rate of CD sign inversion was highly
dependent upon the size of the substituent groups, for example,
in (S)-N-Me-valine/carrier 1 conjugate the sign inversion took
3 days, while with (S)-N-Me-alanine/carrier 1 conjugate a
positive CD was observed immediately after preparation. It was
found that upon deprotonation of the TFA salt of the amino
ester conjugate with carrier 1, the ester conjugate underwent
cyclization to yield diketopiperazine 23 in quantitative yield
(Figure 5). After isolation, the diketopiperazines functioned as
bidentate guests forming 1:1 host/guest complexes with tweezer
2, which exhibited exciton coupled CD spectra with signs
consistent with their absolute stereochemistry (Table 3). This
unexpected transformation has prompted our ongoing studies
of utilizing tweezer 2 for absolute configurational assignment
However, in case of N-Me amino esters, unexpected chemical
transformation of the conjugates took place, which greatly
affected the CD. For example, with (S)-N-Me-phenylalanine
(24) Dreyfus, M.; Harri, E.; Hofmann, H.; Kobel, H.; Pache, W.; Tscherter, H.
Eur. J. Appl. Microb. 1976, 3, 125; Takesako, K.; Ikai, K.; Haruna, F.;
Endo, M.; Shikamanka, K.; Sono, E.; Nakamura, T.; Kata, I.; Tamaguchi,
H. J. Antibiot. 1991, 44, 919; Luo, Y.; Evindar, G.; Fishlock, D.; Lajoie,
G. A. Tetrahedron Lett. 2001, 42, 3807-3809.
(25) With other conjugates, CD amplitudes of the complexes formed with
tweezer 2 did not change significantly with time.
9
J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002 10325

A R T I C L E S
Huang et al.
Table 2. Structures and Schematic Representations of Secondary
Amines and CD Data of the Conjugate/Tweezer 2 Complexes
(Concentrations of Tweezer 2 and the Conjugates Are 1 and 20
µM, Respectively)
Figure 5. Upon deprotonation of TFA salt of conjugate 1-22a (CD t ) 0
h curve), it cyclized to yield diketopiperazine 23 (CD t ) 4 h curve), which
gave opposite sign of exciton-coupled CD upon complexing with Zn-
porphyrin tweezer 2.
Table 3. CD Amplitudes of the Diketopiperazines/Tweezer 2
Complexes in MCH (Concentrations of Tweezer 2 and
Diketopiperazines Are 1 and 20 µM, Respectively)
Previously we have reported that the conjugates of primary
amino esters (e.g., 1-27) and hydroxy esters (e.g., 1-28) with
carrier 1 also yielded exciton coupled CD when complexed with
porphyrin tweezer 2 (Figure 6).15,16 Interestingly, it was found
that they did not cyclize to yield diketopiperazine or diketo-
morpholine under the experimental conditions. The lack of
cyclization products in these cases is attributed to the propensity
of secondary amide in conjugate 1-27 and ester in conjugate
1-28 to adopt the Z conformation exclusively.15,16 The reactive
*From ref 15.
(26) Wang, H.; Ganesan, A. Tetrahedron Lett. 1997, 38, 4327-4328; Cui, C.-
B.; Kakeya, H.; Osada, H. Tetrahedron 1996, 52, 12651-12666.
(27) Li, W. R.; Peng, S.-Z. Tetrahedron Lett. 1998, 39, 7373-7376.
(28) Kim, H.-O.; Nakanishi, H.; Lee, M. S.; Kahn, M. Org. Lett. 2000, 2, 301-
302; Golebiowski, A.; Klopfenstein, S. R.; Shao, X.; Chen, J. J.; Colson,
A.-O.; Grieb, A. L.; Russell, A. F. Org. Lett. 2000, 2615-2617.
of diketopiperazines that have been found to be useful as cell
cycle inhibitors,26 tyrosine kinase inhibitors,27 and â-turn
mimetics;28 the results will be reported in due course.
9
10326 J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002

Configurational Assignments of Secondary Amines
A R T I C L E S
Figure 6. Conjugates 1-27 and 1-28 do not cyclize under experimental conditions due to the predominant Z conformations of the amide or ester bonds,
respectively.
Table 4. Structures and Schematic Representations of N-Me
conformations leading to diketopiperazine 29 or diketomorpho-
Amino Acid Amides and CD Data of the Conjugate/Tweezer 2
line 30 are presumably E (Figure 6).
Complexes (Concentrations of Tweezer 2 and the Conjugates Are
1 and 20 µM, Respectively)
On the basis of these results, and to prevent the formation of
diketopiperazines, a modified protocol was developed in which,
before conversion to conjugate, the N-Me amino acid amides
were prepared either through direct amide formation from the
free carboxylic acids or aminolysis of the corresponding amino
esters.29 The absolute configuration of N-Me amino acid amides
can be easily determined by considering the amide moiety as
the M group and the alkyl or aromatic moieties as L on the
basis of their respective conformational energies (Table 4).
The chiroptical method described here was found to be
applicable to cyclic secondary amines as well (Table 5). If a
carboxyl substituent is present in the chiral substrate (compounds
36, 37, 40, 41, 42), the prior conversion into an amide is
necessary to avoid formation of diketopiperazine derivatives.
The method has been applied to compounds containing various
ring sizes from four- to six-membered rings including pipe-
colinic acid (41)30 and a tetrahydroisoquinoline derivative (42),31
which are constituents of important biologically active com-
pounds.
This method, however, led to unexpected results with amines
containing additional secondary alkyl groups such as cyclohexyl
and isopropyl on nitrogen (compounds 43 and 44). The sign of
observed CD of the complexes formed between conjugates 1-43
and 1-44 and porphyrin tweezer 2 turned out to be opposite to
that predicted for the corresponding conjugate carrying a primary
alkyl group (namely conjugates 1-9 and 1-11) (Table 6). We
find that application of the present chiroptical method to
secondary amines containing cyclohexyl and isopropyl on
nitrogen is possible in conjunction with molecular modeling due
to unusual conformational features of this class of compounds
(see the Molecular Modeling section).
amines. The starting amines are converted into bidentate
conjugates with carrier 1 in high yields. Upon mixing of the
conjugates with the host tweezer 2, a 1:1 host/guest complex is
formed, which exhibits an exciton CD couplet in the porphyrin
Soret band region, with the sign diagnostic of the absolute
configuration.
In summary, a chiroptical microscale protocol has been
developed for absolute configurational assignment of secondary
Structural Studies and Conformational Analysis of the
Host/Guest Complexes
(29) Ghosh, A. K.; Lee, H. Y.; Thompson, W. J.; Culberson, C.; Holloway, M.
K.; Mckee, S. P.; Munson, P. M.; Duong, T. T.; Smith, A. M.; Darke, P.
L.; Zugay, J. A.; Emini, E. A.; Schleif, W. A.; Huff, J. R.; Anderson, P. S.
J. Med. Chem. 1994, 37, 1177-1188.
(30) Scott, J. D.; Williams, R. M. Tetrahedron Lett. 2000, 41, 8413-8416; Scott,
J. D.; Williams, R. M. J. Am. Chem. Soc. 2002, 41, 8413-8416.
(31) Sa´nchez, R.; Luna, H.; Pe´rez, H. I.; Manjarrez, N.; Sol´ıs, A. Tetrahedron:
Asymmetry 2001, 12, 1399-1401.
To better understand the correlation between the absolute
configuration of the secondary amine, the conformational
complexity of corresponding amides, and the genesis of por-
phyrin helicity in host/guest complexes, NMR and molecular
modeling studies of these complexes have been performed.
9
J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002 10327

A R T I C L E S
Huang et al.
Table 5. Structures and Schematic Representations of Cyclic
Secondary Amines and CD Data of the Conjugate/Tweezer 2
Complexes (Concentrations of Tweezer 2 and the Conjugates Are
1 and 20 µM, Respectively)
6.16 ppm, adopts Z-amide conformation, while the minor
conformer with an H-1 methine quartet at 5.13 ppm, adopts
E-amide conformation (1-9E) (Figure 7a).
Upon complexation with tweezer 2, the proton resonances
of 1-9 undergo a series of important diagnostic shifts (Table
7).32 The signals from the propanediamine linker, H-5′′, H-6′′,
and H-7′′, are shifted upfield by at least 6 ppm due to the effect
of the porphyrin ring current,33 with the 6′′-H showing the largest
shift of -7.35 ppm (Figure 7b). The fact that H-6′′, located in
the middle of the chain, experiences the largest shift demon-
strates that both amine nitrogens are coordinated to zinc
porphyrins and that the conjugate is sandwiched between the
two porphyrins of the zinc porphyrin tweezer. Complexation
to a single porphyrin would result in substantially smaller upfield
shifts, especially for the protons farther removed from the
coordination site.33
The host/guest complex 1-9/2 (1-9/2 denotes the complex
formed by conjugate 1-9 and porphyrin tweezer 2) showed two
sets of signals in a 2:1 ratio for conjugate 1-9 (Figure 7c and
Table 7). These are assigned by 2D ROESY measurements as
arising from the Z and E conformers of the bound conjugate.
To slow the dynamic equilibrium between the multiple confor-
mations of the complexes, ROESY experiments were carried
out at 270 K. Lowering the temperature of the sample resulted
in minimal changes in chemical shifts for the guest signals,
whose assignments were confirmed by 2D TOCSY and HSQC.34
In the major conformer of the bound guest, the amide
conformation is assigned as Z, on the basis of NOE correlations
observed between one of the H-1′ benzyl protons at 2.28 ppm
with the propanediamine protons at -2.93 ppm (H-3′′) and
-3.99 ppm (H-5′′). The H-1 methine at 4.56 ppm, deshielded
by a syn relationship to the amide carbonyl, showed only intra-
guest NOEs to the 1-Me at 0.55 ppm and to adjacent H-3
aromatic protons at 6.32 ppm (see Figure S in Supporting
Information). Strong NOE interactions between the 1-Me group
at 0.55 ppm and pyrrole protons of the porphyrin moiety at 8.60
and 8.91 ppm were observed, indicating a close proximity
between these two groups in the host/guest complex (Figure
7d). Conversely, in the bound minor E conformer the H-1
methine quartet of the guest at 2.75 ppm showed NOE
interactions with H-3′′ at -2.79 ppm and with H-5′′ at -3.88
ppm. The minor conformer failed to exhibit any NOE interac-
tions between host 2 and the guest.
Table 6. Structures and Schematic Representations of Secondary
Amines Bearing Additional N-secondary Alkyl Groups and CD
Data of the Conjugate/Tweezer 2 Complexes (Concentrations of
Tweezer 2 and the Conjugates Are 1 and 20 µM, Respectively)
Because the three groups attached to the stereogenic center
are differentially accommodated within the intra-tweezer pocket
depending on their steric sizes, it is conceivable that they would
experience different ring current-induced upfield shifts from zinc
porphyrin tweezer 2.15 On the basis of our model (Figure 4,
see also the Molecular Modeling section), the hydrogen, being
the smallest group, will lie between the porphyrins, and be
subjected to the strongest ring current effect; the large group L
will protrude outside of the pocket in a region of relatively
(32) The NMR measurements were carried out on a sample consisting of 2.8
mM tweezer in the presence of 0.7 equiv of conjugate 1-9 in 0.60 mL of
CDCl3, at 300 and 270 K. The NMR signals of the complex of 1-9 with
tweezer 2 were assigned by TOCSY, DEPT-HSQC, and ROESY experi-
ments.
(33) Abraham, R. J.; Bedford, G. R.; McNeillie, D.; Wright, B. Org. Magn.
Reson. 1980, 14, 418-425; Abraham, R. J.; Medforth, C. J. Magn. Reson.
Chem. 1990, 28, 343-347; Abraham, R. J.; Marsden, I. Tetrahedron 1992,
48, 7489-7504; Cross, K. J.; Crossley, M. J. Aust. J. Chem. 1992, 45,
991-1004.
(34) Wilker, D.; Liebfritz, D.; Kerssebaum, R.; Bermel, W. Magn. Res. Chem.
1993, 31, 287-292.
NMR Studies of Host/Guest Complexes
NMR Assignments of Conjugate 1-9 Upon Complexation
with Tweezer 2. The 1H NMR spectrum of the free-base form
of conjugate 1-9 in CDCl3 shows the presence of a 2:1 mixture
of two conformers arising from rotation around the amide bond.
The major conformer (1-9Z), with an H-1 methine quartet at
9
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Configurational Assignments of Secondary Amines
A R T I C L E S
Figure 7. Structures of 1-9Z and 1-9E conjugates. Double-headed arrows indicate the intraguest NOE correlations observed in 1-9/2 complex that are
diagnostic of the amide bond conformation. (a) 1H NMR spectrum of free conjugate 1-9 in CDCl3 (500 MHz) at 300 K. (b) 1H NMR spectrum of complex
1
1-9/2 from -6 ppm to -2.5 ppm in CDCl3 (500 MHz) at 300K. (c) H NMR spectrum of complex 1-9/2 from 0 to 5.5 ppm in CDCl3 (500 MHz) at 300
K. (d) 2D ROESY spectrum of complex 1-9/2 in CDCl3 (500 MHz) at 270 K, indicating NOE correlation between 1-Me of 1-9Z and porphyrin pyrrole
protons.
Table 7. 1H NMR Chemical Shifts δ (ppm) in CDCl3 (600 MHz) at 300 K for Major and Minor Conformers of Conjugates 1-9 and 1-9/2
Complex, and Ring Current Shift ∆δ ) δ1-9 - δ1-9/2 (ppm) upon Complexation
major conformer
minor conformer
proton
free conjugate 1-9Z
bound conjugate 1-9Z/2
∆δ
free conjugate 1-9E
bound conjugate 1-9E/2
∆δ
1
6.16
1.46
7.07
7.27
7.25
3.66
3.60
≈7.3
≈7.3
≈7.3
4.36
4.19
2.75
1.60
2.58
4.56
0.55
6.32
7.19
7.19
2.95
2.28
5.95
6.83
1.60
0.89
0.75
0.08
0.06
0.71
1.32
1.35
0.47
0.19
7.29
7.96
6.74
7.35
6.78
S
M
5.13
1.47
7.12
7.20
7.18
3.66
3.60
≈7.3
≈7.3
≈7.3
4.95
4.01
2.79
1.64
2.68
2.75
0.60
6.25
6.95
7.17
3.51
2.82
6.02
6.95
2.38
0.87
0.87
0.25
0.01
0.15
0.78
1.28
0.35
0.23
7.74
7.78
6.67
7.50
6.70
S
M
1-Me
3
4
0.30
L
0.38
L
}
}
5
1′a
1′b
3′
4′
5′
3′′a
3′′b
5′′
6′′
7′′
7.11
7.07
-2.93
-3.77
-3.99
-5.75
-4.20
-2.79
-3.77
-3.88
-5.86
-4.02
weaker effect, while for the medium group M an intermediate
shielding is predicted. This would offer an experimental method
to assign their relative steric sizes independent of the confor-
mational energy values. This is corroborated by the NMR shift
differences of conjugate 1-9 before and after complexation with
tweezer 2 (Table 7). For the major conformer, the observed ring
current shift is 1.60 ppm for the hydrogen, 0.89 ppm for the M
group methyl, and on the average 0.30 for the L group phenyl
protons. The same trend is observed for the minor conformer,
as the observed ring current shift is 2.38 ppm for the hydrogen,
0.87 ppm for the M group methyl, and on the average 0.38
ppm for the L group phenyl protons. Thus, in faVorable cases
9
J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002 10329

A R T I C L E S
Huang et al.
NMR spectra, making assignments of L and M groups from
chemical shift differences difficult, if not impossible.
Choice of Computational Method. The conformations of
the host/guest complexes were studied by carrying out molecular
mechanics (MM) calculations with the Merck molecular force
field (MMFF94)35 utilizing MacroModel 7.1.36,37 Various MM
methods have been successfully applied to modeling porphyrin-
derived structures.38 MMFF94 differs from the most frequently
utilized Allinger-type MM force fields (MM2 and MM3) mainly
for its peculiar expressions of van der Waals (VdW) and
electrostatic interactions.35 The choice of MMFF94, whose
accuracy was first checked with known structures of porphyrins
with axial amino ligands,39 is based on the possibility of treating
zinc ions explicitly (as parametrized in the MacroModel
package) within a non-bonded model where the interaction
between zinc and nitrogen is only of VdW and electrostatic
nature. As a result, the coordination of zinc ions to the amine
nitrogens of the conjugate could be reproduced without resorting
to dummy atoms with constrained geometry, which will not bias
the geometrical freedom around the Zn-N bonds. The “static”
version of MMFF94 was employed (referred to as MMFFs) to
ensure the planarity of amide and aromatic nitrogen atoms.35
The conformational space was sampled by means of the
Metropolis Monte Carlo (MC) algorithm implemented in
MacroModel.37
Modeling of Free Conjugates. The molecular conformations
of the free conjugates 1-9, 1-10, 1-11, and 1-43 were investigated
by MC/MMFFs calculations in CHCl3. In the calculated MMFFs
structures of the free secondary amine conjugates, the confor-
mation around the amide bond is found to be strongly dependent
on the substrate, as summarized in the following:
Figure 8. (a) 1H NMR spectrum of free conjugate 1-43 in CDCl3 (500
MHz) at 300 K, indicating the presence of only E conformer. (b) 1H NMR
spectrum of complex 1-43/2 in CDCl3 (500 MHz) at 300 K. (c) H NMR
1
spectrum of complex 1-43/2 in CDCl3 (500 MHz) at 223K.
where the proton resonances of L and M groups in the complex
can be clearly assigned, the difference in the ring current-
induced upfield shifts proVides experimental eVidence for
establishing the relatiVe steric size of the groups attached to
the stereogenic center.
(1) For the primary amine conjugate 1-10, only the Z
conformer is found.
(2) For secondary amine conjugates 1-9 and 1-11 the Z
conformation is favored with an approximate 2:1 Z/E ratio
calculated from the MMFFs energy differences at room tem-
perature for both compounds. Figure 9 reports the lowest-energy
structures for conjugate 1-9 in the two amide conformations.
(3) For the N-cyclohexyl conjugate 1-43, only the E confor-
mation is found within 10 kJ/mol.
These calculation results nicely corroborate experimental
NMR results of the free conjugates presented above.
Modeling of Conjugate/Tweezer Complexes: General
Approach and Results. MC/MMFFs calculations were per-
NMR of Other Conjugates and Host/Guest Complexes.
Free conjugate 1-13 bearing N-Me (Table 2) and the cyclic
secondary amine conjugate 1-37 (Table 5) in CDCl3 showed
the presence of both Z and E conformers in 3:1 and 1:1 ratio,
respectively, at room temperature, while only the E conformer
was detected for the N-cyclohexyl 1-43 conjugate (Figure 8a).
The NMR spectra of the complex 1-43/2 showed very broad
signals for the guests (Figure 8b), as was the case with
complexes 1-11/2 and 1-37/2. This was rationalized by the
existence of multiple bound guest conformations undergoing
rapid exchange inside the host/guest complexes. Lowering the
temperature to -50 °C reduced the line broadening but
unfortunately revealed extremely complicated NMR spectra
(Figure 8c), which rendered further detailed analysis of these
particular substrates impossible.
(35) Halgren, T. A. J. Comput. Chem. 1996, 17, 616-641; Halgren, T. A.;
Nachbar, R. B. J. Comput. Chem. 1996, 17, 587-615; Halgren, T. A. J.
Comput. Chem. 1996, 17, 553-586; Halgren, T. A. J. Comput. Chem. 1996,
17, 520-552; Halgren, T. A. J. Comput. Chem. 1996, 17, 490-519.
(36) Mohamadi, F.; Richards, N. G. J.; Guida, W. C.; Liskamp, R.; Lipton, M.;
Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C. J. Comput. Chem.
1990, 11, 440-467.
(37) Chang, G.; Guida, W. C.; Still, W. C. J. Am. Chem. Soc. 1989, 111, 4379-
4386.
(38) Marques, H. M.; Brown, K. L. Coord. Chem. ReV. 2002, 225, 123-158.
(39) The MMFFs optimized structure for the complex pyridine-Zn(TPP) has a
N-Zn distance of d(N-Zn) ) 2.291 Å and the Zn atom protrudes by d(N4-
Zn) ) 0.300 Å from the average plane defined by the four pyrrole nitrogens.
The average values for the 18 entries found in the Cambridge Crystal-
lographic Database (CCD), relative to Zn-tetraphenylporphyrins axially
bound to pyridine derivatives, are d(N-Zn) ) 2.169 ( 0.04 Å and d(N4-
Zn) ) 0.327 ( 0.04 Å. More interestingly, in the MC/MMFFs structures
for the host/guest complexes, the corresponding average values for the
amino groups of the linker are d(N-Zn) ) 2.245 ( 0.010 Å and d(N4-
Zn) ) 0.265 ( 0.015 Å. Only two entries are found in the CCD for Zn-
tetraphenylporphyrins axially bound to aliphatic amines; they have d(N-
Zn) ) 2.2415 ( 0.020 Å and d(N4-Zn) ) 0.26 ( 0.12 Å, in very good
agreement with our calculations. (TPP ) tetraphenylporphyrin).
Molecular Modeling of Host/Guest Complexes. Molecular
modeling studies of host/guest complexes offer an independent
tool to predict the expected sign of exciton-coupled CD in
addition to conformational energy values (A values) and NMR
methods presented above. This will be especially helpful in cases
where the assignment of L and M groups cannot be ac-
complished unambiguously because either their A values are
similar or simply unavailable. Moreover, as discussed above,
in many cases the host/guest complexes exhibit complicated
9
10330 J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002

Configurational Assignments of Secondary Amines
A R T I C L E S
This observation agrees well with the conformational hetero-
geneity demonstrated by NMR spectra of the host/guest
complexes. It must be stressed that any discrete species detected
by NMR would in principle correspond to an ensemble of fast
exchanging structures.
Because of the incomplete parametrization of MMFFs as well
as the complexity of the system leading to questionable absolute
accuracy of the calculated energies, the lowest-energy minimum
cannot be taken as an entirely faithful representation of the host/
guest complex. It is more appropriate to take into account all
the structures within an energy window arbitrarily set to 10 kJ/
mol, and to focus on the distribution of structures characterized
by the sign and value of the projection angle θ, rather than on
the absolute energies. This quantity refers to the projection angle
between the transition dipole moments of the two porphyrins
running in the 5-15 direction (defined as in Figures 1 and
11),21,40 which is indicative of the sign of the expected exciton
couplet, that is, a positive twist leads to a positive couplet, and
vice versa. Following an approach similar to cluster-type
analysis,41 we plotted distribution graphs of θ values among
all minima within 10 kJ/mol, corresponding approximately to
30-40 optimized structures resulting from the MC calculations
(3 times 1000 steps). In these graphs (Figure 10), the preferred
sign of projection angle θ for a given complex can be inferred
by counting the total number of structures presenting positive
and negative θ. Moreover, the value of θ corresponding to the
higher probability cluster detectable in all cases can be taken
as the “most probable” θ value adopted in the host/guest
complexes. For all compounds inVestigated, the predicted
preferred sign of θ based on calculated distribution agreed with
the sign of the obserVed exciton CD couplet. In most situations,
the preferred sign also agrees with the predicted sign based on
conformational A values.18 In the two cases (1-43/2 and 1-44/2
complexes) where prediction based on A values seems to fail,
MC/MMFFs calculations correctly predicted the observed sign
of the porphyrin twist. Thus, the current calculation method in
conjunction with the experimental CD host/guest complexes can
be used as a reliable tool for assigning the absolute configuration
of the secondary amine. We strongly suggest employing this
MC/MMFFs calculation approach when an unambiguous pre-
diction of relative steric size of substituents is not possible solely
on the basis of conformational energies A values and NMR data.
Modeling of Conjugate/Tweezer Complexes: Analysis of
Calculated Structures. (A) Complex 1-9/2 (Figures 10c and
11). This complex was especially worthy of investigation for
the purpose of comparison with experimental conformational
information, since its structure is the only one accessible in detail
through NMR studies. A large preference for supramolecular
conformations with positive θ values (86%) is found (Figure
Figure 9. Lowest-energy structures (calculated with MMFFs in CHCl3)
and relative energies for conjugate 1-9 in the two amide conformations
1-9Z and 1-9E with energy for the Z isomer set to 0.
Chart 3
(40) The porphyrin Soret transition is doubly degenerate and is more correctly
described as a circular oscillator polarized in the whole chromophoric plane
(Hsu, M.-C.; Woody, R. W. J. Am. Chem. Soc. 1971, 93, 3515-3525).
However, in many situations the exciton interaction between two porphyrins
may be interpretated in terms of an effective transition dipole moment,
which in the current case is directed along the 5-15 direction.21 The
theoretical basis of this approach and its application to different bisporphyrin
derivatives is currently under investigation: Pescitelli, G., et al., in
preparation.
(41) Watts, C. R.; Mezei, M.; Murphy, R. F.; Lovas, S. J. Biomol. Struct. Dyn.
2001, 18, 733-748; Ripoll, D. R.; Liwo, A.; Scheraga, H. A. Biopolymers
1998, 46, 117-126; Levy, S.; Schuyler, S. C.; Maglothin, R. K.; Staehelin,
L. A. Biopolymers 1996, 38, 251-272; Wang, C. G.; Langer, T.; Kamath,
P. G.; Gu, Z.-Q.; Skolnik, P.; Fryer, R. I. J. Med. Chem. 1995, 38, 950-
957; Bryant, S. D.; Balboni, G.; Guerini, R.; Salvadori, S.; Tomatis, R.;
Lazarus, L. H. Biol. Chem. 1997, 378, 107-114.
formed on the complexes between conjugates 1-9, 1-10, 1-11,
1-16, 1-32, 1-41, 1-43, 1-44 (Chart 3) and tweezer host 2. Due
to the complexity and multiple degrees of freedom of the system,
in all cases investigated a number of local minima were found
within a limited energy window, usually about 10-15 structures
within 10 kJ/mol over a total of 1000 MC steps. Moreover,
repetition of the calculation from the same starting structure
and with the same parameters and convergence criteria oc-
casionally led to a partially different set of minima. This renders
seeking the actual absolute energy minimum an impractical task.
9
J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002 10331

A R T I C L E S
Huang et al.
Figure 10. Distribution of θ values of the MC/MMFFs calculated host/guest complexes within 10 kJ/mol of the lowest-energy conformation for various
conjugates (see Chart 3 for structures). The probability of structures with negative and positive values of θ is reported as percentage. Numbers in bold are
for the prevailing sign, also summarized in (g). Arrows indicate the higher probability cluster, to which the “most probable” θ value is referred.
10c). The calculated structure shown in Figure 11c,d also
characterized by a positive twist between the two interacting
porphyrins (side view, Figure 11d) is the lowest-energy
conformation within the most probable cluster (θ ≈ +17°). The
amide conformation of the guest in the complex is Z, which
nicely corroborates the NMR results. In less than 10% of all
1-9/2 complexes, the guests adopt E-amide conformation, but
interestingly even in this conformation the two porphyrins
mostly retain the positive twist. In the most probable structures
with the guest in the Z conformation, the H-1 is always syn to
the carbonyl (Figures 11c,d); the position of the N-benzyl group
is also in agreement with the observed NOEs between protons
3′/1-Me, 1′/1-Me, and 1′/3. Except for the complexes of 1-43/2
and 1-44/2 (vide infra), the structure depicted in Figure 11c,d
is a good representation of a general host/guest arrangement
valid for all complexes investigated, which is described in detail
9
10332 J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002

Configurational Assignments of Secondary Amines
A R T I C L E S
Figure 11. (a) Complex 1-9/2. (b) Schematic representation of the preferred conformation with a positive θ value. (c) Top view and (d) front view for the
most probable calculated MC/MMFFs structure belonging to the higher-probability cluster in Figure 10c. (Blue) L group; (green) M group; (orange) N-benzyl
group. Proton numberings are the same as in Figure 7 and Table 7. Dotted double-headed arrows depict intraguest (black) and host/guest (red) NOE's.
as follows: (a) the methylene of the N-benzyl group (orange)
is directed toward P-2; (b) the chiral center lies approximately
in the middle between the 10,20/10′,20′ phenyls (Figure 11a)
of the two porphyrins; (c) the M methyl group (green) points
toward P-1, reaching in most cases a distance of less than 4 Å
with respect to the closest pyrrole protons, which agrees well
with the observed host/guest NOE; (d) the L phenyl group (blue)
lies in the middle between the two porphyrins, pointing outward
(i.e., far from the porphyrin core) and upward in Figure 11d
(i.e., toward 15′ phenyl). Thus, the calculation method lends
itself as a means of differentiating the M and L groups on the
basis of their positions and orientations in the host/guest
complex.
stereogenic center and its substituents more outward and under
a stronger influence of 10,20/10′,20′ phenyls, which are
perpendicular to the porphyrin planes. It is likely that the
ultimate effect of this augmented structural strain leads to greater
steric recognition.
(B) Complex 1-10/2 (Figure 10a). The calculated structure
of primary amine conjugate 1-10 complexed with tweezer 2, in
terms of both θ dispersion (Figure 10a) and conformation of
the host/guest complex (not shown), is consistent with the
structures described previously for primary amines.15,16 An
almost complete prevalence of structures with positive θ values
(96%) and a strongly preferred cluster with θ ) +38° are found.
A comparison of the molecular model for 1-10/2 with the ones
for secondary amines 1-9/2 and 1-11/2, suggests that the most
evident structural difference is that in 1-10/2 the stereogenic
center and its substituents lie closer to P-2, with the N-H proton
pointing directly toward one of the pyrroles. The N-H bond is
almost perpendicular to a pyrrole plane, with an H/plane distance
of about 2.70 Å; this suggests a H/π interaction,43 which could
be partially responsible for a more rigid and conformationally
homogeneous complex, as observed by NMR.16 The H/π
interaction is also supported by the strong ring current shift of
The reasons for the surprisingly strong CD (A ) +1078)
observed for tweezer complex of the N-benzyl conjugate 1-9
remain unclear, especially because the sign preference and the
dispersion of θ values are quite similar to those of N-Me
conjugate 1-11 (Figure 10c vs 10b). Moreover, π-π interactions
between the N-Bn and phenyl at the stereogenic center do not
seem to play an obvious role since these two groups are quite
distant in the minimized complex structures (Figure 11c,d), and
no NOE between their protons is observable. Only minor
geometrical differences are apparent between the molecular
models of 1-9/2 and 1-11/2. In 1-9/2, the sterically more
demanding benzyl group enlarges the whole tweezer chiral
pocket more than in the N-Me analogue 1-11/2,42 pushing the
(42) The acute angle between the average porphyrin planes is about 42° in 1-9/2
and 30° in 1-13/2.
(43) Del Bene, J. E.; Cohen, I. J. Am. Chem. Soc. 1978, 100, 5285-5290;
Tarakeshwar, P.; Choi, H. S.; Kim, K. S. J. Am. Chem. Soc. 2001, 123,
3323-3331.
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A R T I C L E S
Huang et al.
Figure 12. (a) Complex 1-43/2. (b) Schematic representation of the preferred conformation with a negative θ. (c) Top view and (d) front view for the most
probable calculated MC/MMFFs structure belonging to the higher-probability cluster in Figure 10d. (Blue) L group; (green) M group; (orange): N-cyclohexyl
group.
the N-H (up to 3.70 ppm), the largest reported for protons of
the chiral substrate.
one predicted on the basis of conformational energy A values
for phenyl and methyl groups (Table 6). A very heterogeneous
conformational situation is found for the complex 1-43/2, as
depicted in Figure 10d; however, structures with negative values
of θ are strongly dominating (90%). The favored amide
conformation in the complex (>90% of minima within 10 kJ/
mol) is found to be E, as in the case of the isolated guest (see
above). The calculated structure of the N-isopropyl bearing
conjugate 1-44/2 complex (Table 6) shows similar tendency
(results not shown). These findings emphasize the usefulness
of our calculations for predicting the preferred porphyrin
helicity: no discrepancies between the absolute configuration
predicted by concurrent use of CD spectroscopy and MC/
MMFFs conformational analysis on conjugate/tweezer com-
plexes haVe been found in this study.
Inspection of a representative structure of the 1-43/2 complex
belonging to the cluster around θ ≈ -37° of molecular models
(Figure 12) shows clear differences as compared with the
common structure represented by the 1-9/2 complex (Figure 11).
In 1-43/2, the N-cyclohexyl group (orange) is directed toward
P-1, enlarging the whole tweezer pocket, the methyl group M
(green) is directed toward P-2, and the phenyl group L (blue)
is pointing downward (i.e., toward the benzoate rings). It should
be emphasized that these effects only partially result from the
favored E conformation. It is clear that a completely different
discrimination mechanism is operating in this case, which is in
(C) Complexes 1-32/2 and 1-41/2 (Figure 10e,f). Confor-
mational energy values are not available for complexes 1-32/2
and 1-41/2. For conjugate 1-32, the difference between con-
formational A values18 of methyl group (L) (A ) 7.28 kJ/mol),
and butyl amide group M (A value of approximate 4.6-5.4 kJ/
mol for esters), is quite small. This leads to a reduced
stereodifferentiation, which is reflected in the weak observed
CD couplet (amplitude ) -26 in MCH, undetectable in hexane).
Calculated θ values are very dispersed (Figure 8e), but still a
large preference (80%) for structures with negative θ values
emerges. For conjugate 1-41, as well as for all cyclic compounds
1-36-1-42, the A values for the endocyclic CH2 group are not
available; the homogeneity of the CD results for all of these
compounds justifies assigning M to the CH2 and L to the
exocyclic substituent (Table 5). This hypothesis is substantiated
by MC/MMFFs calculations for the complex 1-41/2, which
results in a sharp cluster with θ ) +14° (Figure 10f) within a
distribution where positive values are clearly favored (61%).
(D) Complex 1-43/2 (Figures 10d and 12). The molecular
modeling study of 1-43/2 (Figure 12) was carried out to shed
some light on the anomalous behavior of the class of derivatives
with secondary N-alkyl groups. In fact, the two examples
considered in the current study (1-43 and 1-44) exhibited
porphyrin CD exciton couplets with the sign opposite to the
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Configurational Assignments of Secondary Amines
A R T I C L E S
Cl2 (1 mL) was then added to yield the free amine solution of conjugate
1-9 (3.54 mM). An aliquot of 10 µL of the latter solution (20 equiv)
was added to the prepared porphyrin tweezer 2 solution to afford
tweezer 2/conjugate 1-9 host/guest complex. The UV-vis and CD
spectra were recorded at 25 °C and corrected for background. The CD
spectrum was measured in millidegrees and normalized into ∆ꢀ
(L‚mol-1‚cm-1).
Computational Section. Molecular modeling calculations were
executed with MacroModel 7.1 package (Schro¨dinger, Inc., Portland,
OR) including Maestro 3.0 as GUI, on a Dell Precision 330 workstation.
All molecular mechanics calculations were run using the native MMFFs
(MMFF94s) in vacuo with default parameters and convergence criteria,
except for the maximum number of minimization steps, set to 50 000.
Monte Carlo conformational searches were run with default parameters
and convergence criteria, sampling all the structures within 10 kJ/mol
over 1000 fully optimized steps. All possible torsional angles were
varied during each step, except for the porphyrin ring dihedral angles
and the porphyrin-10,15,20 phenyl torsions. Zn2+ ions were placed in
the middle of porphyrin rings, with -1 charge assigned to one pair of
opposite nitrogen atoms; no bonds or other restraints were used. Guest
molecules were placed with amine nitrogens close to Zn2+ ions; no
bonds or other restraints were used, except for a N-Zn distance check
set to 2.2 ( 0.5 Å to prevent host/guest dissociation. A typical 1000-
step calculation requires approximately 8-10 h.
All the structures resulting from the above calculations with energies
within 10 kJ/mol were considered, usually 10-15 structures over 1000
steps; duplicate structures were tallied with their frequency of occur-
rence. The minima were collected in occurrence graphs as shown in
Figure 10 in the main text, and the sign preference of projection angle
θ determined by considering the total probability for θ of assuming
positive and negative values. Figures 11 and 12 in the main text refer
to the lowest-energy structures within the most probable clusters, which
in many cases include 60-80% of all structures with a given sign of
θ.
E/Z ratios (amide conformations) of free guests were calculated
through the Boltzmann formula and absolute MMFFs energies in CHCl3
(GB/SA solvation model) for the two lowest-energy structures corre-
sponding to amide conformers with the linker moiety in the same
conformation.
keeping with the exceptional behavior of compounds with
secondary N-alkyl groups in terms of the relationship between
the observed CD couplet and relative sizes of substituents at
the stereogenic center.
Conclusions
A general microscale protocol utilizing zinc porphyrin tweezer
2 to determine the absolute stereochemistry of secondary amines
is described. This approach is applicable to cyclic and acyclic
aliphatic amines, aromatic amines, amino acids, and amino
alcohols. The chiral substrate is derivatized with an achiral
carrier moiety 1 to generate a bidentate conjugate, capable of
forming 1:1 host/guest complex with zinc porphyrin tweezer
host 2. The absolute configuration of the secondary amine can
be readily determined from the sign of the exciton-coupled CD
spectrum of the host/guest complex: a positive CD couplet
corresponds to a clockwise arrangement of the L, M, and H
groups at the stereogenic center in the Newman projection with
amino group in the rear, and vice versa. The assignment of large
(L) and medium (M) moieties at the stereogenic center is based
on their conformational energies. This follows the general trend
established previously with primary amines and secondary
alcohols,15,16 even though the complex formed by secondary
amine conjugate with tweezer 2 has shown much greater
conformational heterogeneity, which has hampered previous
attempts for absolute configurational assignment of secondary
amines. NMR analysis of some complexes demonstrated that
observed differences in the upfield ring current-induced chemical
shifts can provide useful information for assignment of the
relative steric size of substituents linked to the stereogenic center.
Furthermore, conformational studies of host/guest complexes
by molecular mechanics calculation using MC/MMFFs revealed
that this approach is a reliable tool for predicting the preferred
porphyrin helicity upon complexation. In all cases, the sign of
calculated preferred porphyrin twist was found to be in
agreement with the observed CD exciton chirality. The molec-
ular modeling of the host/guest complex can therefore be utilized
in conjunction with the chiroptical procedure as a useful method
for absolute configurational assignments, particularly when the
relative steric sizes of substituents L and M are ambiguous.
Acknowledgment. This research is supported by NIH Grants
GM 34509 and GM 36564. G. Pescitelli is indebted to the Italian
C.N.R. Grant (203.03.26) for financial support.
Experimental Section
Supporting Information Available: General procedures for
the conjugate preparation at both the milligram and microgram
scales; procedures for CD measurements of the host/guest
complex at the microgram scale; procedures for obtaining the
Job plot and binding constant; general procedure for transami-
dation; procedures for synthesis of 12, 16, 17, and diketopip-
Zinc porphyrin tweezer 2 is commercially available from TCI
(Japan).
General Procedure for Preparation of Host/Guest Complex 3
for CD Measurement. In a typical experiment, tweezer 2 solution (1
µM) was prepared by the addition of a 10 µL aliquot of tweezer 2 (0.1
mM in anhydrous CH2Cl2) solution to the solvent (1 mL) where CD is
going to be measured. The free amine solution of conjugate 1-9 (1.8
mg, 3.54 µmol) was prepared from its TFA salt after the addition of
0.5 mL of MeOH followed by solid Na2CO3 (10 mg). The solvent
(MeOH) was then evaporated under a stream of argon followed by
placement under high vacuum (0.2 Torr) for 20 min. Anhydrous CH2-
1
erazines 23-26, H NMR and MS data of conjugates 1-9 to
1-11, 1-14, 1-16 to 1-21, 1-31 to 1-44; 2D ROESY spectrum
of complex 1-9/2 (Figure S) (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
JA020520P
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J. AM. CHEM. SOC. VOL. 124, NO. 35, 2002 10335