Stability of medium-bridged twisted amides in aqueous solutions

Article abstract of DOI:10.1021/jo802192v

"Twisted" amides containing nonstandard dihedral angles are typically hypersensitive to hydrolysis, a feature that has stringently limited their utility in water. We have synthesized a series of bridged lactams that contain a twisted amide linkage but exh

Full text of DOI:10.1021/jo802192v

Stability of Medium-Bridged Twisted Amides in Aqueous Solutions
Michal Szostak, Lei Yao, and Jeffrey Aube´*
Department of Medicinal Chemistry, 1251 Wescoe Hall DriVe, Malott Hall, Room 4070, UniVersity of
Kansas, Lawrence, Kansas 66045-7582
jaube@ku.edu
ReceiVed October 4, 2008
“Twisted” amides containing nonstandard dihedral angles are typically hypersensitive to hydrolysis, a
feature that has stringently limited their utility in water. We have synthesized a series of bridged lactams
that contain a twisted amide linkage but exhibit enhanced stability in aqueous environments. Many of
these compounds were extracted unchanged from aqueous mixtures ranging from the strongly basic to
the strongly acidic. NMR experiments showed that tricyclic lactams undergo reversible hydrolysis at
extreme pH ranges but that a number of compounds in this structure class are indefinitely stable under
physiologically relevant pH conditions; one bicyclic example was additionally water-soluble. We examined
the effect of structure on the reversibility of amide bond hydrolysis, which we attributed to the transannular
nature of the amino acid analogs. These data suggest that medium-bridged lactams of these types should
provide useful platforms for studying the behavior of twisted amides in aqueous systems.
Introduction
cis-trans isomerization processes essential to protein folding,9-11
have been proposed as intermediates in amide bond cleavage,12-14
and are required to experimentally examine the effect of bond
rotation on any type of amide reactivity such as protonation15-17
or other reactions.8,18-20
The most effective way of capturing an amide bond in a
twisted conformation is to constrain it into a bicyclic ring system
in which the amide nitrogen is present at a bridgehead
The amide bond is one of the most important linkages in
nature because of its presence in peptides and protein structures;
its suitability for this central role derives from its resistance to
hydrolysis.1 The stability of planar amide bonds has been
classically explained by resonance delocalization of the nitrogen
lone pair into the carbonyl group (e.g., in the most common
trans isomer, where ω ) 180°, Figure 1a). Although the
resonance model has been the subject of vigorous reexamina-
tion,2-6 the importance of C-N bond rotation on the properties
of the amide bond has been solidly established. Thus, the
nonplanar or “twisted” amides that approach ω angles of 90°
are hypersensitive to hydrolysis and more basic than standard
amides, contain pyramidal rather than planar nitrogen atoms,
and react with protons or electrophiles at nitrogen rather than
oxygen (Figure 1b).7,8 Though much rarer than standard amides,
such species are important because they are encountered during
(7) Greenberg, A. In Structure and ReactiVity; Liebman, J. F., Greenberg,
A., Eds.; VCH: New York, 1988; pp 139-178.
(8) Greenberg, A. In The Amide Linkage: Structural Significance in
Chemistry, Biochemistry, and Materials Science; Greenberg, A., Breneman,
C. M., Liebman, J. F., Eds.; Wiley-Interscience: New York, 2000; pp 47-83.
(9) Shao, H.; Jiang, X.; Gantzel, P.; Goodman, M. Chem. Biol. 1994, 1, 231–
4.
(10) Fischer, G.; Schmid, F. X. In Molecular Chaperones and Folding
Catalysts; Harwood Academic Publishers: Amsterdam, 1999; pp 461-489.
(11) Schmid, F. X. AdV. Protein Chem. 2002, 59, 243–282.
(12) Somayaji, V.; Brown, R. S. J. Org. Chem. 1986, 51, 2676–2686.
(13) Brown, R. S. In The Amide Linkage: Structural Significance in
Chemistry, Biochemistry, and Materials Science; Greenberg, A., Breneman,
C. M., Liebman, J. F., Eds.; Wiley-Interscience: New York, 2000; pp 85-
114.
(14) Mujika, J. I.; Mercero, J. M.; Lopez, X. J. Am. Chem. Soc. 2003, 127,
4445–4453.
(15) Werstiuk, N. H.; Brown, R. S.; Wang, Q. Can. J. Chem. 1996, 74, 524–
532.
(16) Mujika, J. I.; Mercero, J. M.; Lopez, X. J. Phys. Chem. A 2003, 107,
6099–6107.
(17) Ly, T.; Krout, M.; Pham, D. K.; Tani, K.; Stoltz, B. M.; Julian, R. R.
J. Am. Chem. Soc. 2007, 129, 1864–1865.
(1) Amide Linkage: Selected Structural Aspects in Chemistry, Biochemistry,
and Materials Science; Greenberg, A., Breneman, C. M., Liebman, J. F., Eds.;
Wiley-Interscience: New York, 2000.
(2) Lauvergnat, D.; Hiberty, P. C. J. Am. Chem. Soc. 1997, 119, 9478–9482.
(3) Basch, H.; Hoz, S. Chem. Phys. Lett. 1998, 294, 117–125.
(4) Wiberg, K. In The Amide Linkage: Structural Significance in Chemistry,
Biochemistry, and Materials Science; Greenberg, A., Breneman, C. M., Liebman,
J. F., Eds.; Wiley-Interscience: New York, 2000; pp 33-45.
(5) Quinonero, D.; Frontera, A.; Capo, M.; Ballester, P.; Suner, G. A.; Garau,
C.; Deya, P. M. New J. Chem. 2001, 25, 259–261.
(6) Ilieva, S.; Hadjieva, B.; Galabov, B. J. Org. Chem. 2002, 67, 6210–
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10.1021/jo802192v CCC: $40.75
Published on Web 01/29/2009
2009 American Chemical Society
J. Org. Chem. 2009, 74, 1869–1875 1869

Szostak et al.
FIGURE 1. Generalized depictions of (a) a trans amide linkage and (b) a twisted, N-pyramidalized amide. (c) Literature examples of twisted
amides reported by the groups of Stoltz (1),21 Kirby (2),22-24 and Brown (3).13 Approximate half-lives for hydrolysis of these amides in water
solution are indicated. (d) Bridged bicyclic amides examined in the present study. (e) Ball and stick depiction of the X-ray structure of compound
4 and (f) the isolated amide region of this structure along with the two ω values.
position;7,19,25-31 iconic examples include the parent quinucli-
done 121 and the adamantane analog 2 (Figure 1c).22-24 Such
compounds are challenging to prepare, in part as a result of the
extreme lability of their amide bond relative to ordinary amides.
In particular, the predicted sensitivity of these compounds to
hydrolysis has been amply demonstrated; half-lives of hydrolysis
in water of less than 1 min are commonly observed for known
bridgehead lactams. This lability has limited the study of twisted
amide reactivity in water.23,24 In addition, although orthogonal
amides in principle have promise as inhibitors of protease or
isomerase enzymes, this instability has ruled out their study for
such purposes altogether.
There is no reason why the study of amides ought be limited
to fully planar or fully twisted examples. To wit, we have
presented a new class of bridged amides in which the amide
linkage spans a medium size, 9-membered ring.20,32 Prepared
by the intramolecular ring-expansion reaction of a precursor
azidoalkyl ketone, these compounds were shown to exhibit
spectroscopic properties typically associated with other types
of twisted amides and also displayed novel chemistry previously
unobserved in any other standard or twisted amides. We now
report that such medium bridged lactams are also remarkably
stable in aqueous solutions, both kinetically and thermodynami-
cally, and present one example that is both soluble and
indefinitely persistent in pure water.
(19) Bashore, C. G.; Samardjiev, I. J.; Bordner, J.; Coe, J. W. J. Am. Chem.
Soc. 2003, 125, 3268–3272.
(20) Lei, Y.; Wrobleski, A. D.; Golden, J. E.; Powell, D. R.; Aube´, J. J. Am.
Chem. Soc. 2005, 127, 4552–4553.
Results and Discussion
(21) Tani, K.; Stoltz, B. M. Nature 2006, 441, 731–734.
(22) Kirby, A. J.; Komarov, I. V.; Wothers, P. D.; Feeder, N. Angew. Chem.,
Int. Ed. 1998, 37, 785–786.
(23) Kirby, A. J.; Komarov, I. V.; Feeder, N. J. Chem. Soc., Perkin Trans.
2 2001, 522–529.
(24) Kirby, A. J.; Komarov, I. V.; Feeder, N. J. Am. Chem. Soc. 1998, 120,
7101–7102.
(25) Greenberg, A.; Moore, D. T.; DuBois, T. D. J. Am. Chem. Soc. 1996,
118, 8658–8668.
(26) Goodman, M.; Shao, H.; Jiang, X.; Gantzel, P. Peptides: Chemistry,
Structure and Biology-Proceedings of the 14th American Peptide Symposium;
Intercept Ltd.: U.K., 1996; pp 477-478.
(27) Yamada, S. In The Amide Linkage: Structural Significance in Chemistry,
Biochemistry, and Materials Science; Greenberg, A., Breneman, C. M., Liebman,
J. F., Eds.; Wiley-Interscience: New York, 2000; pp 215-246.
(28) Sparks, S. M.; Vargas, J. D.; Shea, K. J. Org. Lett. 2000, 2, 1473–
1475.
(29) Chow, C. P.; Shea, K. J.; Sparks, S. M. Org. Lett. 2002, 4, 2637–2640.
(30) Sparks, S., M.; Chow, C., P.; Zhu, L.; Shea, K. J. J. Org. Chem. 2004,
69, 3025–3035.
(31) Ribelin, T. P.; Judd, A. S.; Akritopoulou-Zanze, I.; Henry, R. F.; Cross,
J. L.; Whittern, D. N.; Djuric, S. W. Org. Lett. 2007, 9, 5119–5122.
Two kinds of bridged lactams were used in this study: the
tricyclic derivative 4 and several bicyclic examples 5 (Figure
1d). Lactam 4 contains a heavily pyramidalized nitrogen atom
33
with C-C(O)-N-C dihedral angles of -153.4° and 62.0°
and a Dunitz-Winkler twist angle34 of τ ) 50.7° (the τ value
for a planar amide is 0° and a fully orthogonal one is 90°). A
previously reported analog of 5 (where R1 ) C(O)-pyrrolidinyl
(32) Golden, J. E.; Aube´, J. Angew. Chem., Int. Ed. 2002, 41, 4316–4318.
(33) Compound 4 was previously published,20 but a new crystal structure
was determined for this work. The crystal of lactam 4 (C18H20NOBr) used for
the X-ray structure determination was a pseudo-merohedral twin having two
domains with nearly equal (56%/44%) volumes. These domains were related
by a 180° rotation about the [-1 0 1] direction of the monoclinic unit cell chosen
for the final refinement. This monoclinic unit cell had a ) c and metrically
mimicked a C-centered orthorhombic unit cell for which the intensity data gave
Rsym ) 0.079 when averaged according to orthorhombic Laue symmetry. See
cif file (Supporting Information) for details.
(34) Winkler, F. K.; Dunitz, J. D. J. Mol. Biol. 1971, 59, 169–82.
1870 J. Org. Chem. Vol. 74, No. 5, 2009

Stability of Medium-Bridged Twisted Amides
FIGURE 2. Proposed species for compound 4 in organic (ethyl acetate) and aqueous layers of different pH.
and the seven-membered ring containing a double bond20 has τ
) 50.3. Lactams were prepared via the intramolecular ring
insertion of an alkyl azide into a carbonyl group as previously
reported,20,32 generally as the minor component of a mixture
also containing the regioisomeric fused lactam; the additional
six-membered ring (labeled A in Figure 1d) or tert-butyl group
were incorporated to enhance the proportion of bridged lactam
obtained in this synthesis.
in Table 1, similar results were obtained when the lactams were
challenged by extremely acidic or basic conditions, longer
dissolution times in acid, and higher temperatures in base. In
fact, the only irreversible chemical reaction that was observed
in these experiments took place when 4 was placed in aqueous
HCl/acetonitrile at 80 °C for 23 h, which led to the regioselective
cleavage of one of the C-N bonds adjacent to the amide bond,
affording 7 (Figure 2). This unconventional mode of amide
hydrolysis is emblematic of the unusual reactivity imparted by
the twisted amide linkage.20
TABLE 1. Extraction Studies of Lactam 4
entry
1
conditionsa
time, temp
20 h, rt
result
The recovery of high quantities of amide from this range of
aqueous solutions is consistent with two possible scenarios. The
first would be that the amide linkage is thermodynamically
stabilized in these molecules. Thus, dissolution of 4 in an
aqueous environment could result in reversible amide hydroly-
sis,24 with reclosure of the amide occurring in the presence of
a large excess of water, permitting re-extraction of the original
compound. Some of the species that would be expected in water
under acid or basic conditions are depicted in Figure 2. The
second possibility is that the amide bond is kinetically stabilized
toward hydrolysis.
1:4 H2O/CH3CN
>85% recovery
of 4
2
3
4
5
6
1:8 aq NaOH/CH3CN
(pH ca. 14)
1:24 aq NaOH/CH3CN
(pH ca. 14)
1:8 aq HCl/CH3CN
(pH ca. 1)
1:8 aq HCl/CH3CN
(pH ca. 1)
20 h, rt
>85% recovery
of 4
22 h, 80 °C
20 h, rt
>80% recovery
of 4
>80% recovery
of 4
8 days, rt
23 h, 80 °C
>85% recovery
of 4
conversion to 7
(95% yield)
1:24 aq HCl/CH3CN
(pH ca. 1)
a The pH values noted refer to the aqueous layers.
We addressed this issue by dissolving the twisted amides in
D2O/THF-d8 solutions at neutral, acidic, or basic pH and directly
observing these solutions via 13C NMR spectroscopy (Figure 3
and Table 2). The chemical shift of the signal arising from the
carbonyl carbon provided evidence for the kinetic stability under
neutral conditions, with other species being observed under
strongly basic or acidic condition. Thus, the 13C NMR signal
for the carbonyl carbon in CDCl3 is at 187 ppm (Figure 3a);
upon dissolution in 1:1 D2O/THF-d8, this signal moves slightly
downfield to 189 (Figure 3b). When 4 is dissolved in other
organic solvents or in 1:1 D2O/THF-d8, the carbonyl signal
always appears between 185 and 190 ppm. At pH ca. 14, this
The first indication that this class of compounds had unusual
properties came from a simple set of extraction experiments.
Thus, samples of tricyclic lactams 4 were dissolved in aqueous
acetonitrile solutions, vigorously stirred for ca. 20 h, and then
extracted with an organic solvent (ethyl acetate). A twisted
amide with a short half-life toward hydrolysis would be expected
to afford amino acid under such conditions. In sharp contrast
to this expectation, it proved possible to recover unchanged
compound 4 in high (generally >85%) yield after this treatment
(it is likely that the recoveries were <100% due to modest
solubility in the acetonitrile-containing aqueous layer). As shown
J. Org. Chem. Vol. 74, No. 5, 2009 1871

Szostak et al.
FIGURE 3. 13C NMR spectra of compound 4 dissolved in (a) CDCl3, (b) 1:1 D2O/THF-d8, (c) 1:6 DCl (1 N in D2O)/THF-d8, and (d) 1:6 NaOD
(1 N in D2O)/THF-d8.
9).36 However, solvent effects could modulate the location of
these peaks, leaving open the possibility that we are simply
observing N-protonation of 4 in acid/THF as opposed to ring
opening (obviously, in acid/DMSO both occurrences provide
the only reasonable explanation for the data). This would still
be remarkable, however, given the extreme tendence of most
twisted lactams to quickly hydrolyze under these conditions.
In addition, we assigned the small signal observed at 106 ppm
to the hydrate 4·H2O, also protonated on nitrogen. Finally, the
unconventional conversion of lactam 4 to alcohol 7 entails
nonreversible attack of water on a carbon adjacent to the amine
in the N-protonated 4; the group that specifically undergoes this
attack is that revealed by X-ray to have the most severe deviation
from planarity (ω ) -62°; Figure 1f).
TABLE 2. 13C NMR Carbonyl Shifts of Lactam 4 and Its
Derivatives
shift
(ppm)
entry
conditions
assignment
1
2
3
4
5
CDCl3
THF-d8
DMSO-d6
1:1 D2O/THF-d8
1:6 DCl (1 N in D2O)/THF-d8
4
4
4
4
187.1
185.0
186.2
189.6
6 (conjugate acid) 178.5
4·H2O (hydrate) 106.1
6 (conjugate acid) 178.5
4 (conjugate acid) 176.9
6
1:6 DCl (1 N in D2O)/DMSO-d6
4·H2O (hydrate)
104.9
7
8
9
1:6 NaOD (1 N in D2O)/THF-d8
6 (conjugate base) 182.3
1:6 NaOD (1 N in D2O)/DMSO-d6 6 (conjugate base) 179.0
CDCl3 4 (conjugate acid) 176.9
The presence of the various species noted in Figure 2 was
also supported by mass spectrometry (Table 3). Thus, MS
measurements were taken from samples prepared as described
above using neutral, strongly basic, or strongly acidic conditions.
Aliquots from each experiment were diluted by the solvents
noted in the table to prepare them for ionization. These results
supported the qualitative data as discerned from the NMR work
described above. Thus, only starting lactam 4 was observed in
MS measurements from samples of 4 dissolved in D2O/THF,
regardless of whether samples were diluted by THF or water
prior to injection. Under basic conditions (entries 3-6) the
solvent used for ionization played an important role in determin-
ing whether parent lactam 4 (MW 346) or the corresponding
amino acid (MW 364) was observed. Thus, when the samples
prepared from strongly basic NaOD and THF were dissolved
in nonpolar solvents (THF or CH3CN), only 4 could be detected
by MS (entries 3 and 4). In contrast, however, when the same
signal is replaced by an upfield signal at 182 ppm. In strong
base (Figure 3d), the most reasonable assignment for this signal
is for the carboxylate anion arising from clean conversion to
the conjugate base of the amino acid 6 (Figure 2). This
assignment is consistent with normal values for simple car-
boxylic acids. For example, the carbonyl of acetic acid in CDCl3
appears at 178.1 ppm and the corresponding signal for the
conjugate base is at 181.5 in aqueous solution.35 In mixtures of
DCl (pH ca. 1) and either THF-d8 (Figure 3c) or DMSO-d6,
several species can be observed. In the former, the predominant
peak occurs at 178 ppm (entry 5), whereas two peaks in the
same range appear when DMSO is used as the cosolvent (176.9
and 178.5, entry 6). We assign the more downfield of these
peaks to the N-protonated form of 6 shown in Figure 2 and the
176.9 peak in DMSO to the protonated form of lactam 4. This
tentative assignment is based on the fact that the corresponding
signal in fully characterized salts of 4 and closely related
compounds appear between 176-177.5 ppm in CDCl3 (entry
(36) N-Protonation is a common and distinctive feature of twisted amides
(regular amides protonate predominantly on nitrogen). The conjugate acid of
compound 3 can be made by simply adding toluenesulfonic acid to the amide in
ether; the resulting salt has been purified by recrystallization and characterized
by X-ray crystallography. Lei, Y. Ph.D. Dissertation, University of Kansas, 2006.
(35) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectroscopic
Identification of Organic Compounds; John Wiley and Sons: New York, 1991.
1872 J. Org. Chem. Vol. 74, No. 5, 2009

Stability of Medium-Bridged Twisted Amides
TABLE 3. ESI MS Experiments with Lactam 4a
entry
conditions
solvent used for ionization
exact mass (es) observed
assignments
1
2
1:1 D2O/THF
THF
346.0818
4
1:1 D2O/THF
H2O
346.0797
4
3
1:6 NaOD (1 N in D2O)/THF
1:6 NaOD (1 N in D2O)/THF
1:6 NaOD (1 N in D2O)/THF
1:6 NaOD (1 N in D2O)/THF
1:6 DCl (1 N in D2O)/THF
1:6 DCl (1 N in D2O)/THF
1:6 DCl (1 N in D2O)/THF
1:6 DCl (1 N in D2O)/THF
THF
346.0856
4
4
CH3CN
346.0792
4
5
H2O
364.0918
6
6
DMSO
364.0920
6
7
THF
346.0752; 364.0903 (ratio ca. 1:1)
346.0722; 364.0913
346.0761; 378.1022 (ratio ca. 3:1)
346.0776; 364.0916 (ratio ca. 1:1)
4 and 6
4 and 6
8
H2O
9
MeOH/H2O/HCOOH
CH3CN
4 and the methyl ester of 6
4 and 6
10
a Relevant HRMS calculations: HRMS calcd for C18H21BrNO (M+ + H) 346.0806 (compound 4); HRMS calcd for C18H23BrNO2 (M+ + H) 364.0912
(compound 6); and HRMS calcd for C19H25BrNO2 (M+ + H) 378.1069 (methyl ester of compound 6).
FIGURE 4. (a) Results of treating 5a with strong acid or base. (b) X-ray structure of amino acid 8.
TABLE 4. Extraction Studies of Bicyclic Lactams
entry
lactam
solvent
time, conditions
result
1
2
3
4
5
6
7
8
5a
5a
5a
5b
5b
5c
5c
5c
1: 1 D2O/THF-d8
aq HCl (1.0N)
aq NaOH (1.0N)
buffer (pH 4)/CH3CN
buffer (pH 10)/CH3CN
D2O
13 d, rt
0.25 h, rt
3 h, rt
2 h, rt
2 h, rt
6 d, rt
2 h, rt
2 h, rt
ca. 50% recovery of 5a
conversion to 8
conversion to 8
>90% recovery of 5b
>90% recovery of 5b
>90% recovery of 5c
>90% recovery of 5c
>90% recovery of 5c
buffer (pH 4)
buffer (pH 10)
samples were diluted by either water or DMSO, no parent ion
corresponding to 4 was observed. Instead, weak signals corre-
sponding to 6 were present in each spectra (we ascribe the
weakness of these signals to the relative difficulty of forming a
positive ion in the MS from zwitterions 6). Samples in which
4 was dissolved in 1 N aqueous DCl/THF mixtures generally
FIGURE 5. 13C NMR spectra of lactam 5c in (a) CDCl3 and (b) D2O.
J. Org. Chem. Vol. 74, No. 5, 2009 1873

Szostak et al.
led to well-behaved MS spectra in which both 4 or 6 could be
observed regardless of how the samples were prepared for the
MS experiments. Interestingly, the ratio of 4 over 6 was higher
when the samples were prepared using water as a diluent versus
either THF or CH3CN. Additionally, the methyl ester of 6 was
observed when the MS experiments were prepared by dissolu-
tion with a ca. 95:4:1 MeOH/water/aqueous formic acid mixture
(entry 9). Overall, these experiments are consistent with the
assignments made from the NMR experiments.
(pH ca. 10) conditions. However, the phenyl-substituted analog
5b could be completely recovered following exposure to mixed
solvents.
A lactam containing an R-methylthio group and lacking the
tert-butyl substituent (5c) was fully water-soluble, which allowed
its study in pure aqueous solutions at pH ) 4, 7, and 10. Under
all of these conditions, lactam 5c was stable throughout the
course of study (Table 4, entries 6-8). The kinetic stability of
this compound was further demonstrated by comparing its 13C
NMR spectra taken in CDCl3 and D2O, which were similar
(Figure 5). These studies establish that the enhanced stability
of these “mid-range” twisted amides is not solely due to
scaffolding/reversibility effects but may also depend on their
degree of twist or steric effects arising from embedding the
amide carbonyl group into the middle of the ring system.
These data are consistent with both kinetic and thermody-
namic stability at neutral pH. When the amides are subjected
to strong acid or base, hydrolysis occurs, but the amide bond is
able to reform even in a medium containing 50% water. It is
possible that small amounts of unobserved amide persist in water
under extreme pH conditions, but the high yields of recovered
bridged amide jibe with the NMR results only if reclosure of
the amino acid form is possible. Numerous attempts were made
to retrieve validated samples of zwitterions 6 or derivated thereof
by concentrating the aqueous samples (acid or basic solutions)
and examining the residues by NMR. However, only starting
lactam 4 was observed under these conditions, strongly sug-
gesting that the removal of water from these samples by any
means is sufficient to shift the equilibrium of the compound
back to lactam 4.
Summary
This new class of medium bridged lactams will further the
study of the effect of dihedral angle and twist value on chemical
reactivity. These compounds are clearly twisted amides due to
their substantial twist values of ca. 60°, characteristic spectral
and chemical properties (i.e., N-protonation), and the fact that
they undergo unprecedented chemical reactivity such as adjacent
N-C cleavage (e.g., 4 f 7).20 However, they offer superior
kinetic and thermodynamic water stability relative to known
classes of twisted amides and are readily amenable to synthesis
and structural variation. It has been suggested that twisted
amides would provide an attractive platform for the study of
enzymatic folding and proteolysis properties.1 However, existing
systems were sufficiently unstable in water and/or insufficiently
diversifiable that nearly all aqueous studies of twisted amides
to date have been limited to their hydrolytic behavior. We
anticipate that medium bridged amides will enhance the range
of chemical and biological studies possible with compounds
containing unconventional amide linkages.
We hypothesize that the bridged amides are stabilized by a
scaffolding effect of the medium ring. In addition, once the
lactam bond is hydrolyzed, the resulting amino acid has the
carboxylic acid and amine moieties on the opposite sides of a
medium-size ring where they should be subject to strong
proximity effects. While transannular reactions across medium-
or large-size rings have been frequently observed, the formation
of a twisted amide in water is highly unusual. The most relevant
prior observations are the spontaneous formation of the quinu-
clidone system under mass spectrometry conditions17 and a
single example of amide bond formation in the highly con-
strained adamantane system in acid.24
Experimental Section
The role of conformation in constraining the open nine-
membered amino acid form was of interest as species 6 in Figure
2 exhibits in/out isomerism.37 Thus, the six-membered ring holds
the carboxylic acid in place following amide hydrolysis to the
medium-size ring. The importance of this was underlined when
exposure of lactam 5a, lacking this ring, to strongly acidic or
basic conditions did not permit reisolation of the unchanged
amide, as was the case for compound 4. Removal of all solvent
did afford a quantitative yield of amino acid 8. Here, X-ray
crystallography established that this compound was able to
undergo a conformational change that afforded an amino acid
in which the carboxylic acid has now flipped to the outside
perimeter of the nine-membered ring, where it is unable to reach
the amide group (Figure 4).38 These observations are consistent
with the proposed role of transannular interactions in contribut-
ing to the reversibility of the amide bond formation.
6-(Methylthio)-1-azabicyclo[4.3.1]decan-10-one (5c) and 9a-
(Methylthio)hexahydro-1H-pyrrolo[1,2-a]azepin-5(6H)-one. To
a solution of 2-(3′-azidopropyl)-2-(methylthio)cyclohexanone (0.0910
g, 0.40 mmol, 1.0 equiv, Supporting Information) in CH2Cl2 (8.0
mL, 0.05 M) was added TfOH (0.18 mL, 2.0 mmol, 5.0 equiv) in
one portion at 0 °C, and the resulting solution was stirred at 0 °C
for 2.5 min. The reaction was quenched with saturated NaHCO3
(10 mL) and extracted with CH2Cl2 (3 × 20 mL). The organic layer
was washed with brine (1 × 20 mL), dried (Na2SO4), and
concentrated. Flash chromatography (1/2 EtOAc/hexanes, followed
by EtOAc) afforded compound 5c as a pale yellow oil (Rf ) 0.57,
1/1 EtOAc/hexanes), yield 65% (0.0525 g, 0.26 mmol) and 9a-
(methylthio)hexahydro-1H-pyrrolo[1,2-a]azepin-5(6H)-one as a
colorless oil (Rf ) 0.31, 1/1 EtOAc/hexanes), yield 15% (0.0120
1
g, 0.06 mmol). Compound 5c: H NMR (400 MHz, CDCl3) δ
1.53-1.79 (complex, 4H), 1.80-1.99 (complex, 4H), 2.05-2.14
(complex, 4H), 2.22-2.29 (m, 1H), 2.80-2.86 (m, 1H), 3.18-3.24
(m, 1H), 3.43 (dt, J ) 2.8, 12.0 Hz, 1H), 3.86-3.94 (m, 1H); 13C
NMR (100 MHz, CDCl3) δ 12.8, 22.5, 24.3, 26.5, 36.4, 40.1, 47.9,
50.6, 57.0, 182.4; 1H NMR (500 MHz, D2O) δ 1.37-1.46 (m, 1H),
1.49-1.64 (complex, 2H), 1.65-1.71 (complex, 2H), 1.78-1.86
The determination of the limits of thermodynamic stability
in simple bicycles such as 5a allowed us to examine the kinetic
stability of this series of amides through extraction experiments
using organic solvents (either CH3CN or tetrahydrofuran (THF))
and buffer (Table 4). Although the unsubstituted amide 5a was
largely stable in neutral solutions (with a half-life of ca. 13 days,
as determined by NMR), it underwent irreversible conversion
to amino acid under even moderately acidic (pH ca. 4) or basic
(37) Alder, R. W.; East, S. P. Chem. ReV. 1996, 96, 2097–2111.
(38) The crystal of compound 8 contained 0.25 mol of cocrystallized NaCl
per mole of [C13H26NO2][Cl]. The Na ion was disordered over four general
position sites and was therefore included in the structural model with an
occupancy factor of 0.25. See cif file (Supporting Information) for details.
1874 J. Org. Chem. Vol. 74, No. 5, 2009

Stability of Medium-Bridged Twisted Amides
(m, 1H), 1.88-1.94 (m, 1H), 1.95-1.98 (m, 1H), 2.00 (s, 3H),
2.07-2.14 (m, 1H), 2.20-2.25 (m, 1H), 2.84-2.89 (m, 1H),
3.15-3.19 (m, 1H), 3.40 (dt, J ) 2.9, 12.2 Hz, 1H), 3.61-3.66
(m, 1H); 13C NMR (125 MHz, D2O) δ 11.5, 22.0, 23.1, 25.0, 35.5,
39.8, 49.0, 50.3, 58.0, 186.2; IR (neat) 2927, 2860, 1686, 1445,
1173 cm-1; HRMS calcd for C10H18NOS (M+ + H) 200.1109,
found 200.1107.
9a-(Methylthio)hexahydro-1H-pyrrolo[1,2-a]azepin-5(6H)-
one: 1H NMR (400 MHz, CDCl3) δ 1.45-1.57 (m, 1H), 1.62-1.90
(complex, 4H), 1.97-2.22 (complex, 6H), 2.41-2.47 (m, 1H),
2.50-2.56 (m, 1H), 3.20 (dt, J ) 2.2, 13.8 Hz, 1H), 3.48-3.57
(m, 1H), 3.68-3.75 (m, 1H); 13C NMR (100 MHz, CDCl3) δ 13.2,
21.1, 23.7, 24.9, 37.2, 39.3, 43.1, 49.6, 174.7; IR (neat) 2926, 1632,
1429, 1406 cm-1; HRMS calcd for C10H18NOS (M+ + H) 200.1109,
found 200.1105.
and the combined organic layers were washed with water (1 × 10
mL) and brine (1 × 10 mL), dried (Na2SO4), and concentrated.
Flash chromatography (1/4 EtOAc/hexanes) afforded compound 7
1
as a colorless film, yield 95% (19.1 mg, 0.53 mmol). H NMR
(400 MHz, CDCl3) δ 1.41-1.53 (m, 2H), 1.65-1.77 (m, 2H),
1.81-2.05 (m, 3H), 2.21-2.36 (br, 1H), 2.52 (dd, J ) 4.8, 11.9
Hz, 1H), 2.79-2.84 (m, 1H), 3.06 (dd, J ) 1.4, 10.0 Hz, 1H),
3.16 (dt, J ) 3.6, 12.1 Hz, 1H), 3.24-3.29 (m, 1H), 3.51-3.58
(m, 1H), 3.60-3.67 (m, 1H), 5.56-5.59 (m, 1H), 6.04-6.09 (m,
1H), 6.26-6.34 (br, 1H), 7.01 (d, J ) 8.3 Hz, 2H), 7.46 (d, J )
8.2 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 25.3, 29.1, 29.8, 33.7,
36.5, 40.4, 44.3, 46.1, 48.8, 119.6, 128.7, 128.9, 130.5, 130.6, 141.4,
171.3; IR (neat) 3223, 3135, 2995, 2890, 1630, 1455, 795, 705
cm-1; HRMS calcd for C18H23BrNO2 (M+ + H) 364.0912, found
364.0910.
General Procedure for Extraction Studies of Lactam 4.
Lactam 4 was dissolved in a specified amount of CH3CN at room
temperature. After addition of water, aqueous HCl, or aqueous
NaOH the reaction mixture was vigorously stirred under conditions
specified in Table 1. Reactions were cooled to room temperature
(if necessary) and extracted with EtOAc (4 × 10 mL). Combined
organic layers were washed with water (1 × 10 mL) and brine (1
× 10 mL), dried (Na2SO4), and concentrated to afford the title
compound. For entries 4-6 (Table 1) the reactions were neutralized
with saturated NaHCO3 before extraction with EtOAc.
Recovery of Lactam 4 from H2O/CH3CN Mixture. According
to the general procedure, lactam 4 (40.0 mg, 0.116 mmol) was
dissolved in 6.0 mL of CH3CN, and 1.5 mL of H2O was added.
After stirring at room temperature for 20 h the reaction mixture
was extracted with EtOAc (4 × 10 mL), and combined organic
layers were washed with water (1 × 10 mL) and brine (1 × 10
mL), dried (Na2SO4), and concentrated to afford 34.9 mg (0.101
mmol) of the title compound. Yield 87%.
Synthesis of Amino Acid 8 under Basic Conditions. A 10-
mL round-bottom flask was charged with lactam 5a (20.0 mg, 0.096
mmol, 1.0 equiv) and NaOH (1.0 N in H2O) (0.10 mL, 0.096 mmol,
1.0 equiv). The flask was stirred for 3 h. The solvent was
evaporated, and the flask was left under vacuum overnight to give
the title compound as a white solid. Yield: 100% (24.0 mg, 0.096
1
mmol). Mp > 300 °C; H NMR (400 MHz, D2O) 0.87 (s, 9H),
1.18-1.42 (m, 3H), 1.46-1.57 (m, 1H), 1.64-1.92 (m, 5H),
2.22-2.35 (m, 1H), 2.66-2.79 (m, 2H), 2.81-2.96 (m, 2H); 13C
NMR (100 MHz, D2O) 20.2, 24.9, 26.6, 27.7, 28.7, 33.5, 41.2,
42.0, 46.6, 48.2, 186.3; IR (KBr) 3370, 2880, 1525, 1375 cm-1
;
HRMS calcd for C13H26NO2 (M+ + H) 228.1963, found 228.1958.
Acknowledgment. We thank the National Institute of General
Medical Sciences (GM-49093) for financial support and Victor
Day for X-ray crystallography.
Supporting Information Available: Experimental details,
characterization data for new compounds, and cif files for
compounds 4 and 8. This material is available free of charge
via the Internet at http://pubs.acs.org.
Conversion of Lactam 4 to Compound 7. According to the
general procedure, lactam 4 (19.2 mg, 0.056 mmol) was dissolved
in 6.0 mL of CH3CN, and 0.25 mL of 1.0 N HCl was added. After
stirring at room temperature for 30 min, the reaction was refluxed
for 23 h. The reaction was cooled to room temperature, basified
with saturated NaHCO3, and extracted with EtOAc (4 × 10 mL),
JO802192V
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