Dipeptide Surrogates Containing Asparagine-Derived Tetrahydropyrimidinones: Structure and Amide Rotomer Stereochemistry of a Proline Mimic

Article abstract of DOI:10.1021/jo9901790

Novel dipeptide surrogates Fmoc-Xaa-(cyclo)Asn-OBu^t are prepared from asparagine, aromatic aldehydes, and Fmoc-protected amino acid chlorides. Previous studies have focused on basic approaches to key synthetic intermediates containing the (cyclo)Asn fragment for polypeptide preparation. The synthesis of these systems has now been extended to electron-rich aromatic aldehydes, exemplified by p-anisaldehyde. One-dimensional nuclear Overhauser effect experiments confirm the trans configuration around the central secondary amide bond and the boat conformation of the pyrimidinone ring. Variable-temperature NMR at 500 MHz was employed both to obtain thermodynamic data on the cis-trans amide bond interconversion barrier and to probe for the presence of hydrogen bonding. Single-crystal X-ray analysis of Ac-D-Ala-(cyclo)Asn-NHMe shows a high degree of structural similarity to a known proline-containing dipeptide.

Full text of DOI:10.1021/jo9901790

5148
J . Org. Chem. 1999, 64, 5148-5151
Dip ep tid e Su r r oga tes Con ta in in g Asp a r a gin e-Der ived
Tetr a h yd r op yr im id in on es: Str u ctu r e a n d Am id e Rotom er
Ster eoch em istr y of a P r olin e Mim ic
J oseph P. Konopelski,*,1a,b Yunyang Wei,1a,2 and Marilyn M. Olmstead1c,3
Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, and
Department of Chemistry, University of California, Davis, California 95616
Received February 1, 1999
Novel dipeptide surrogates Fmoc-Xaa-(cyclo)Asn-OBut are prepared from asparagine, aromatic
aldehydes, and Fmoc-protected amino acid chlorides. Previous studies have focused on basic
approaches to key synthetic intermediates containing the (cyclo)Asn fragment for polypeptide
preparation. The synthesis of these systems has now been extended to electron-rich aromatic
aldehydes, exemplified by p-anisaldehyde. One-dimensional nuclear Overhauser effect experiments
confirm the trans configuration around the central secondary amide bond and the boat conformation
of the pyrimidinone ring. Variable-temperature NMR at 500 MHz was employed both to obtain
thermodynamic data on the cis-trans amide bond interconversion barrier and to probe for the
presence of hydrogen bonding. Single-crystal X-ray analysis of Ac-D-Ala-(cyclo)Asn-NHMe shows a
high degree of structural similarity to a known proline-containing dipeptide.
In tr od u ction
One of the attractions of the chemical approach to
biological molecules is the ability to prepare derivatives
with novel properties. In recent years there has been
considerable interest in the synthesis of unnatural and/
or conformationally constrained amino acids, peptidomi-
metics,4 and small peptide fragments encompassing these
residues. This interest has been fueled by the desire for
synthetic polypeptides with increased hydrolytic stability,
enzyme inhibition properties, and/or unique conforma-
tional restrictions in comparison to the all-natural coun-
terpart. In addition, these unusual amino acids can be
used to elucidate the relevant conformation(s) of their
biologically active natural counterparts, thereby provid-
ing information concerning the three-dimensional re-
quirements for various recognition events.5 The prepa-
ration of such compounds requires sophisticated, versatile,
and generally applicable synthetic approaches to the
desired structural features.
Of all the common R-amino acids, proline plays a
particular role in peptide secondary structure formation.
Because the pyrrolidine ring of the Pro residue forces the
Φ angle to be centered on -60° ((15°), proline presents
a ready opportunity to change the direction of the
polypeptide chain. Indeed, it has long been recognized
that type I and type II â-turns most often have proline
in the i + 1 position. The lack of a hydrogen on the amide
bond in Xaa-Pro segments significantly lowers the energy
difference between the cis and trans rotomers, while the
activation barrier for isomerization remains significant.
These two unique characteristics (ring and secondary
amide structure) combine to give proline an important
place in the recognition, reactivity, and stability of
polypeptides and proteins. The presence of specific prolyl-
peptidyl cis-trans isomerases6 in living systems also
attests to the importance of the geometry around this
secondary amino acid. It is not surprising, then, that
numerous mimetics and substituted proline analogues
have been prepared in an effort to both understand and
manipulate the topology of reverse turns and cis-trans
isomer ratios in peptides.
We have recently prepared a series of dipeptide sur-
rogates of the type Fmoc-Xaa-(cyclo)Asn-OBut, where
“(cyclo)Asn” is a novel cyclic derivative of the common
amino acid asparagine.7 In our initial studies, we out-
lined a substantial amount of basic chemistry of these
systems and have demonstrated methods for their incor-
poration into polypeptide units. In addition, the depro-
tection of the (cyclo)Asn residue to the native amino acid
could be readily achieved by mild acid treatment at
aqueous reflux temperature. During the course of this
work, we discovered that the Φ angle of the pyrimidinone
ring in these systems was within the range expected for
a proline dipeptide. Herein, we outline additional experi-
ments designed to explore the structure of this proline
mimic.
(1) (a) University of California, Santa Cruz. (b) Phone 831-459-4676;
FAX 831-459-2935; email joek@chemistry.ucsc.edu. (c) University of
California, Davis.
(2) On leave from the Institute of Chemical Engineering, Nanjing
University of Science and Technology, PRC.
(3) To whom correspondence concerning X-ray crystallography
should be addressed.
(4) (a) Gante, J . Angew. Chem., Int. Ed. Engl. 1994, 33, 1699-1720.
(b) Giannis, A.; Kolter, T. Angew. Chem., Int. Ed. Engl. 1993, 32, 1244-
67.
(6) Chen, J . K.; Schreiber, S. L. Angew. Chem., Int. Ed. Engl. 1995,
34, 953-69.
(7) Konopelski, J . P.; Filonova, L. K.; Olmstead, M. M. J . Am. Chem.
Soc. 1997, 119, 4305-6.
(5) For a recent example, see: Smith, A. B., III; Keenan, T. P.;
Holcomb, R. C.; Sprengeler, P. A.; Guzman, M. C.; Wood, J . L.; Carroll,
P. J .; Hirschmann, R. J . Am. Chem. Soc. 1992, 114, 10672-4.
10.1021/jo9901790 CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/17/1999

Stereochemistry of a Proline Mimic
J . Org. Chem., Vol. 64, No. 14, 1999 5149
Resu lts a n d Discu ssion
work in our laboratory11 and in others12 had firmly
established the utility of aliphatic aldehydes as important
components in this cyclization reaction, and our own
work with electron-poor aromatic aldehydes had been
quite successful.7 However, earlier work on our one-pot
cyclization procedure13 with benzaldehyde or electron-
rich aromatic aldehydes gave poor yields of initial mate-
rial and/or poor stability of the isolate products. Coupling
of the asparagine imine derived from benzaldehyde with
Fmoc-Ala-Cl afforded a 61% yield of desired product,
essentially identical with the yield of 3a formation. A
more modest 51% yield of 3b was obtained when imine
2b was employed in the two-step protocol. Use of the
imine formed from 3,4-dimethoxybenzaldehyde led to low
yields of desired product. Thus, this two-step approach
to the heterocycle is somewhat more tolerant of electron-
rich aromatic rings than is the one-step approach and
extends the limits of the substitution pattern at C2.
Or igin of Isom er s. As mentioned previously, 3a exists
as a single isomer at room temperature in CDCl3.
Conversely, 3b and the majority of the dipeptide sur-
rogates prepared in our initial investigation display peaks
The synthesis of the key compounds in this study
follows the route outlined below and in our initial
publication. Asparagine tert-butyl ester (1) is employed
as the starting material. This ester is commercially
available or easily prepared by a three-step literature
procedure from asparagine.8 By the latter method both
enantiomers are readily available. Imine 2, prepared by
the condensation of 1 with the requisite aldehyde in the
presence of trimethylorthoformate,9 is extremely clean
and forms in quantitative yield. Formation of the desired
dipeptide surrogate evolves from the coupling of 2 with
the acid chloride derivative of the desired N-terminal
amino acid. These amino acid chlorides, which are
protected as Fmoc derivatives, are crystalline compounds
as described by Carpino.10
1
in the H NMR indicative of multiple isomers that do not
interconvert on the time scale of the instrument at room
temperature. We briefly investigated Fmoc-Tyr-(cyclo)-
Asn-OBut by variable-temperature NMR in our initial
studies and observed a coalescence temperature of ap-
proximately 75 °C.7 These results were interpreted as
evidence of cis-trans isomerization about the secondary
amide bond. A similar compound was investigated by
Snieckus and displayed overall coalescence at 100 °C,14
while the corresponding bond in a Xaa-Pro dipeptide has
been characterized with a barrier to rotation of ap-
proximately 20 kcal/mol,15 displaying an even higher
coalescence temperature.
The presence of the methyl singlet for the anisole
functionality in 3b afforded the opportunity to obtain
thermodynamic data on this system. At room tempera-
ture, a pair of singlets corresponding to the -OMe
functionality in the two isomers of 3b are visible at δ
3.730 and 3.649 in an approximate ratio of 1:1 in DMSO-
d6. Overall coalescence was observed at 65 °C. These data
correspond to an activation energy (∆Gq) for cis-trans
isomerization of 16.9 kcal/mol, significantly lower than
that of an Xaa-Pro dipeptide.
Compound 3a had been synthesized for our initial
report and shown to exist as a single isomer in solution.
By contrast, the compound with the L-Ala residue at the
N-terminus (3a with (S)-Me) was a mixture of amide
isomers at room temperature in CDCl3. To identify which
amide isomer corresponded to 3a and to explore in detail
the conformation of the pyrimidinone ring in comparison
with previous structural studies on proline dipeptides,
we prepared the corresponding acetamide/methylamide
derivative 5. To this end, 3a was treated with HNMe2,
Con for m a tion of Tr ia m id e. Triamide 5 is unique in
the synthetic series from 3a in that it is appreciably
water soluble and exhibits NMR signals indicative of two
isomers at room temperature in D2O. Although water
solubility is expected for similar Xaa-Pro triamides, the
presence of a minor amide isomer in this series of proline
mimics was not.
Nuclear Overhauser experiments were carried out on
4 to determine key structural features of the major
isomer in solution. As shown in the diagram, the trans
amide isomer should bring the methyl and/or asymmetric
center hydrogen of the D-alanine residue in close proxim-
followed by direct acetylation with Ac2O/pyridine to afford
4 in 80% yield. Reaction of 4 with TFA followed by
coupling with MeNH2 using a mixed anhydride protocol
gave 5 in 74% yield.
It was of interest to extend the substitution pattern
at C2 to include electron-rich aromatic rings. Previous
(11) Chu, K. S.; Negrete, G. R.; Konopelski, J . P.; Lakner, F. J .; Woo,
N.-T.; Olmstead, M. M. J . Am. Chem. Soc. 1992, 114, 1800-12.
(12) J uaristi, E.; Lo´pez-Ruiz, H.; Madrigal, D.; Ram´ırez-Quiro´s, Y.;
Escalante, J . J . Org. Chem. 1998, 63, 4706-10.
(13) Lakner, F. J .; Chu, K. S.; Negrete, G. N.; Konopelski, J . P. Org.
Synth. 1995, 73, 201-14.
(14) Beaulieu, F.; Arora, J .; Veith, U.; Taylor, N. J .; Chapell, B. J .;
Snieckus, V. J . Am. Chem. Soc. 1996, 118, 8727-8.
(15) Creighton, T. E. Proteins: Structures and Molecular Properties,
2nd ed.; Freeman: New York, 1993; p 181.
(8) Ko¨nig, W.; Kernebeck, K. Liebigs Ann. Chem. 1979, 227-47.
(9) Look, G. C.; Murphy, M. M.; Campbell, D. A.; Gallop, M. A.
Tetrahedron Lett. 1995, 36, 2937-40.
(10) For a review, see: Carpino, L. A. Acc. Chem. Res. 1996, 29,
268-74.

5150 J . Org. Chem., Vol. 64, No. 14, 1999
Konopelski et al.
ity to the C2 hydrogen of the pyrimidinone. A 1D NOE
experiment on 4 indicated such a close approach of the
alanine asymmetric center to the C2 proton, as depicted
by the arrows in the diagram, thereby setting the major
isomer as trans in this compound.
Nuclear Overhauser experiments were also used to
determine the structure of the pyrimidinone ring. Previ-
ously, we7,11 and others14,16 have obtained X-ray crystal-
lographic data to support the boat conformation of the
heterocyclic ring, but to our knowledge, there has never
been conclusive data on the solution conformation of
these systems. Irradiation of a sample of 5 in the area of
the C5 protons (δ 2.3-2.6) afforded the expected en-
hancement of the C6 (asymmetric) proton, as well as an
enhancement of the ortho protons on the C2 aromatic
substituent, as indicated in the diagram. This result
could only arise from the boat conformation, which places
the C2 axial substituent (in this case the 4-chloro-3-
nitrophenyl ring) in close proximity to the axial hydrogen
on C5, the other “flagpole” substituent. We had proposed
a close approach of the cis substituents on these two
centers to explain our results in the formation of dihy-
dropyrimidinones in a palladium-catalyzed reaction we
studied some time ago.7 However, this is the first defini-
tive evidence for the preference in solution of the boat
conformation in these compounds.
F igu r e 1. Stereoview of Ac-D-Ala-(cyclo)Asn-NHMe‚2H2O.
The water molecules have been removed for clarity.
Ta ble 1. Com p a r ison of Str u ctu r a l Da ta betw een 5 a n d
Ac-Tyr -P r o-NHMe18
Ac-D-Ala-(cyclo)Asn-NHMe‚2(H2O) Ac-Tyr-Pro-NHMe‚2(H2O)
D-Ala
Φ
Ψ
Ω
73
-147
-177
Tyr
Φ
Ψ
Ω
-71
149
-178
(cyclo)Asn
% cis in H2O
Φ
Ψ
Ω
-71
150
174
Pro
Φ
Ψ
Ω
-75
144
-178
24
% cis in H2O 35
be expected for these residues from opposite enantiomeric
series. However, the structural data for the two cyclic
amino acids are nearly identical in both sign and
magnitude, indicating a high degree of similarity. Like-
wise, the percent cis content in D2O is quite similar.
In conclusion, the initial suggestion that the (cyclo)-
Asn residue functions as a proline mimic has been
validated. Amide bond isomer distribution and ring
conformation are supported by both solution- and solid-
phase data. In addition, we have documented the ∆Gq of
amide bond isomerization, which is lower than the
corresponding value for proline itself. Extensions of this
work to tripeptides and tetrapeptides is currently un-
derway in our laboratory and will be reported in due
course.
The temperature dependence of the N-H proton
chemical shift of amides in polypeptides has emerged as
an important measure of structure brought on by hydro-
gen bonding.17 The data obtained for 5 in DMSO-d6
indicate no significant hydrogen bonding over the tem-
perature range of 25-50 °C. These results are consistent
with the trans geometry around the D-Ala-(cyclo)Asn
amide bond.
The evidence given above supports a structure for 5
in which the ring is in a boat conformation with trans
geometry around the secondary amide bond and no
secondary structure due to hydrogen bonding. Nonethe-
less, we sought to confirm these data and obtain direct
comparison with other structures in the literature by
single-crystal X-ray analysis. The results are shown in
Figure 1, accompanied by a comparison of structural data
between 5 and Ac-Tyr-Pro-NHMe as obtained by Scher-
aga (Table 1).18 The stereoview in Figure 1 highlights the
central trans amide bond. As can be seen in the Table,
the dihedral angles about the N-terminal amino acids are
equal in magnitude but opposite in rotation, as would
Exp er im en ta l Section 19
Im in e 2b. To a solution of H-Asn-OBut (1, 0.150 g, 0.80
mmol) in 4.0 mL of trimethylorthoformate was added the
p-anisaldehyde (0.80 mmol). The resulting mixture was stirred
at room temperature for 16 h. The solvent was removed in
vacuo. Methylene chloride (3 × 2 mL) was added and removed
in vacuo to remove trimethylorthoformate completely. The
residue (0.240 g, 98%) was used directly in the next reaction.
1H NMR (DMSO-d6) δ 1.37 (s, 9 H), 2.68 (dd, J ) 15.1, 5.9 Hz,
2 H), 3.79 (s, 3 H), 4.20 (t, J ) 6.7 Hz, 1 H), 6.82 (s, 1 H), 6.99
(d, J ) 8.5 Hz, 2 H), 7.36 (s, 1 H), 7.67 (d, J ) 8.4 Hz, 2 H),
8.24 (s, 1 H).
F m oc-D-Ala -(cyclo-MeOC6H 4CH )-Asn -OBu t (3b ). To
imine 2b (0.201 g, from 0.122 g, 0.65 mmol of H-Asn-OBut) in
10 mL of benzene was added pyridine (0.08 mL, 1.0 mmol),
followed by Fmoc-D-Ala-Cl (0.283 g, 0.86 mmol). The mixture
was stirred at 65 °C for 5 h. Solid byproducts were removed
by filtration and washing with benzene. The solvent was then
removed in vacuo, and the residue was purified by column
(16) Seebach, D.; Lamatsch, B.; Amstutz, R.; Beck, A. K.; Dobler,
M.; Egli, M.; Fitzi, R.; Gautschi, M.; Herrado´n, B.; Hidber, P. C.; Irwin,
J . J .; Locher, R.; Maestro, M.; Maetzke, T.; Mourin˜o, A.; Pfammatter,
E.; Plattner, D. A.; Schickli, C.; Schweizer, W. B.; Seiler, P.; Stucky,
G.; Petter, W.; Escalant, J .; J uaristi, E.; Quintana, D.; Miravitlles, C.;
Molins, E. Helv. Chim. Acta 1992, 75, 913-34.
(17) For a discussion, see: Kessler, H. Angew. Chem., Int. Ed. Engl.
1982, 21, 512-23.
(18) Montelione, G. T.; Arnold, E.; Meinwald, Y. C.; Stimson, E. R.;
Denton, J . B.; Huang, S.-G.; Clardy, J .; Scheraga, H. A. J . Am. Chem.
Soc. 1984, 106, 7946-58.
(19) For generic experimental details, consult ref 11. Details of the
syntheses of 3a and Fmoc-Tyr-(cyclo)Asn-OBut are given in the
Supporting Information to ref 7.

Stereochemistry of a Proline Mimic
J . Org. Chem., Vol. 64, No. 14, 1999 5151
chromatography on silica gel using methylene chloride/ether
(7:3) as eluent to afford 0.197 g (51%, based on H-Asn-OBut)
Ac-D-Ala -(cyclo-Cl,NO2-C6H3CH)-Asn -NHMe (5). Com-
pound 4 (0.375 g, 0.80 mmol) was dissolved in trifluoroacetic
acid (TFA, 8 mL). The resultant solution was stirred at room
temperature for 1 h. TFA was removed in vacuo. Methanol (3
× 3 mL) was added to the residue and evaporated in vacuo
prior to drying.
as a white solid: mp 103-106 °C; [R]25 ) +4.1 [c ) 1.18,
D
1
CHCl3]; H NMR (CDCl3) δ 1.34 and 1.40 (s, 9 H), 1.42-1.47
(m, 3 H), 2.46∼2.86 (m, 2 H), 3.72 and 3.77 (s, 3 H), 4.13-
4.40 (m, 3 H), 4.57-4.66 (m, 1 H), 4.76 and 5.27 (t, J ) 6.5
Hz, 1 H), 5.77 and 5.82 (d, J ) 8.5 Hz, 1 H), 6.29 and 7.09 (br
s, 1 H), 6.85 (d, J ) 8.0 Hz, 2 H), 7.31-7.42 (m, 6 H), 7.58-
7.61 (m, 2 H), 7.76 (d, J ) 7.5 Hz, 2 H), 8.21 (br s, 1 H); 13C
NMR (CDCl3) δ 18.5, 19.2, 27.8, 28.0, 33.0, 47.1, 47.3, 54.0,
54.9, 55.3, 63.8, 65.9, 67.3, 82.4, 83.4, 114.0, 114.2, 120.1, 125.3,
127.2, 127.8, 127.9, 128.0, 130.1, 130.4, 136.2, 141.4, 143.8,
143.9, 144.1, 149.7, 155.6, 156.4, 159.7, 159.9, 168.4, 169.1,
170.0, 171.1, 172.3, 174.6; IR (thin film) 2977, 1650, 1512,
1450, 1205, 1148, 740 cm-1. According to the 1H NMR
integration of the OCH3 peaks, the conformational isomer ratio
is 2:1 in CDCl3 and 1:1 in DMSO-d6. Dynamic 1H NMR in
DMSO-d6 shows the coalescence of OCH3 peaks at 65 °C, and
the activation free energy (∆Gq) of rotation around the amide
bond is 70.54 kJ /mol (16.86 kcal/mol); HRMS calcd for [M +
1]+ C34H38N3O7 600.2710, found 600.2712.
The above product (0.336 g) was dissolved in THF (8 mL)
at 0 °C, followed by addition of N-methylmorpholine (0.10 mL,
0.910 mmol) and ethyl chloroformate (0.09 mL, 0.94 mmol).
After the mixture stirred at 0 °C for 10 min, MeNH2 (0.50 mL,
2 M solution in THF) was added. The resulting solution was
stirred at room temperature for 20 h, and THF was removed
in vacuo. The residue was purified by column chromatography
on silica gel using methylene chloride/methanol as eluent to
afford 0.255 g (74%) of desired material: mp 130-133 °C; [R]25
D
) -14.2 [c ) 0.59, MeOH]; 1H NMR (DMSO-d6) δ 1.18 and
1.21 (d, J ) 6.5 Hz, 3 H), 1.45 and 1.83 (s, 3 H), 2.10-2.28 (m,
2 H), 2.51 and 2.56 (d, J ) 4.0 Hz, 3 H), 4.31 and 4.88 (m, 1
H), 4.66 and 5.07 (m, 1 H), 6.26 and 6.70 (d, J ) 6.0 Hz, 1 H),
7.77-7.81 (m, 1 H), 7.88 and 8.24 (d, J ) 4.5 Hz, 1 H), 8.00 (d,
J ) 4.0 Hz, 1 H), 8.12 and 8.55 (s, 1 H), 8.45∼8.47 (m, 1 H),
8.97 and 9.29 (d, J ) 5.0 Hz, 1 H); 13C NMR (MeOH-d4) δ 17.2,
21.8, 26.4, 34.0, 35.1, 55.5, 56.0, 64.0, 65.9, 67.1, 125.1, 127.1,
132.7, 141.3, 141.7, 149.3, 171.9, 172.2, 172.5, 172.9, 174.6,
Ac-D-Ala -(cyclo-Cl,NO2-C6H 3CH )-Asn -OBu t (4). Com-
pound 3a (0.65 g, 1.000 mmol) was dissolved in Me2NH (25
mL, 2 M solution in THF). The resultant solution was stirred
at room temperature for 20 min. Solvent was removed in
vacuo. The residue was dissolved in THF (1 mL), and petro-
leum ether (5 mL) was added to precipitate the desired
product, which was separated from the liquid, washed with
additional petroleum ether, and dried.
175.9; IR 3325, 3075, 2950, 1688, 1650, 1538, 1425, 1062 cm-1
;
HRMS calcd for [M + 1]+ C17H21N5O6Cl 426.1180, found
426.1178.
Ack n ow led gm en t. Partial support of this work by
the American Cancer Society (DHP-79) is gratefully
acknowledged. Grants for the purchase of NMR equip-
ment from NSF (BIR-94-19409) and the Elsa U. Pardee
Foundation are gratefully acknowledged. We wish to
express our sincere thanks to the China Scholarship
Council for support for Y.W. In addition, we thank our
collaborators in the group of Professor G. Cardillo
(Bologna) for many stimulating discussions and NATO
(Collaborative Research Grant CRG 950049) for funding
of that collaboration.
The above product (0.407 g) was dissolved in methylene
chloride (20 mL) and treated with pyridine (0.16 mL, 1.98
mmol) and acetic anhydride (0.20 mL, 2.12 mmol). The
resultant solution was stirred at room temperature for 22 h.
Solvent was removed in vacuo. The resultant residue was
purified by column chromatography on silica gel using meth-
ylene chloride/methanol as eluent to give the desired acetate
(0.375 g, 80%) as a white solid: mp 112-113 °C; [R]25D ) +22.5
1
[c ) 1.24, MeOH]; H NMR (CDCl3) δ 1.40 (s, 9 H), 1.43 (d, J
) 6.0 Hz, 3 H), 1.98 (s, 3 H), 2.63 (dd, J ) 8.0, 16.5 Hz, 1 H),
2.83 (dd, J ) 7.0, 16.0 Hz, 1 H), 4.50-4.65 (m, 1 H), 5.50 (t, J
) 7.5 Hz, 1 H), 6.81 (d, J ) 4.5 Hz, 1 H), 7.03 (d, J ) 7.5 Hz,
1 H), 7.51 (d, J ) 8.5 Hz, 1 H), 7.64 (dd, J ) 2.0, 8.0 Hz, 1 H),
7.95 (s, 1 H), 8.01 (d, J ) 4.0 Hz, 1 H); 13C NMR (CDCl3) δ
17.6, 22.5, 27.6, 27.7, 29.6, 33.3, 45.7, 54.4, 63.1, 83.9, 123.8,
127.0, 131.3, 132.1, 139.3, 147.7, 169.5, 170.8, 175.3; IR 1651,
1539, 1261, 1154, 1050 cm-1; HRMS calcd for [M + 1]+
C20H26N4O7Cl 469.1490, found 469.1490.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures for the synthesis of 1 and other compounds not
1
specifically numbered in the text, as well as H NMR spectra
for key compounds, and X-ray data for 5. This material is
available free of charge via the Internet at http://pubs.acs.org.
J O9901790