Oxazolidin-2-one-containing pseudopeptides that fold into β-bend ribbon spirals

Article abstract of DOI:10.1021/ja056762h

Three sets of oligomers containing the 4-carboxy-5-methyloxazolidin-2-one (Oxd) moiety have been synthesized with the aim of checking whether these molecules are able to fold in ordered structures: A set [Boc-(L-Ala-L-Oxd) _n-OR], B set [Boc-(L-

Full text of DOI:10.1021/ja056762h

Published on Web 01/27/2006
Oxazolidin-2-one-Containing Pseudopeptides That Fold into
â-Bend Ribbon Spirals
Claudia Tomasini,* Gianluigi Luppi, and Magda Monari
Contribution from the Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum
UniVersita` di Bologna, Via Selmi 2, 40126 Bologna, Italy
Received October 3, 2005; E-mail: claudia.tomasini@unibo.it
Abstract: Three sets of oligomers containing the 4-carboxy-5-methyloxazolidin-2-one (Oxd) moiety have
been synthesized with the aim of checking whether these molecules are able to fold in ordered structures:
A set [Boc-(L-Ala-L-Oxd)n-OR], B set [Boc-(L-Ala-D-Oxd)n-OR], and C set [Boc-(Aib-L-Oxd)n-OR] preferential
conformations have been analyzed with IR absorption, NMR, and CD. We have noticed that in these
oligomers three stabilizing effects are active: (i) the rigid Oxd -CO-N(CH<)-CO- moiety, which always
tend to assume a trans conformation; (ii) the formation of Oxd CdO‚‚‚HsRC intramolecolar H-bonds; (iii)
the alternate formation of 1 r 4 intramolecular CdO‚‚‚HsN H-bonds. Through the analysis of the
experimental data, we could demonstrate that only the oligomers of the B set are able to meet all three
requirements listed above. By a deeper insight into the CD spectra, we gathered that the secondary structure
adopted by the B set oligomers is a â-bend ribbon spiral, which is a subtype of the 310-helix.
Introduction
Oligomers capable of folding into defined secondary struc-
tures (“foldamers”) have recently received considerable atten-
tion.1 Indeed, simplified artificial systems based on artificial
backbones designed to fold in secondary structures may be used
for the de novo design of molecules with interesting biological
activities.2
Most unnatural foldamers reported so far have homogeneous
backbones. Heterogeneous backbones, composed of two or more
residue types, can also display well-defined folding behavior,
although this strategy for foldamer design has received relatively
little attention to date.3
Figure 1. Chemical structure of Boc-(Gly-L-Oxd)4-OBn (Boc ) tert-
butoxycarbonyl; OBn ) benzyl).
The exploration of heterogeneous backbones is important
because mixing monomer classes leads to an exponential
increase in the range of potential foldamers. We have recently
introduced some new oligomers, the basic unit of which was
constructed by coupling a 4-carboxy-5-methyloxazolidin-2-one
(Oxd) with Gly (Figure 1).4
Some Gly-L-Oxd oligomers up to the tetramer level were
synthesized and their preferred conformations studied by means
of IR absorption, H NMR, and CD analyses. The combined
(1) Reviews: (a) Seebach, D.; Matthews, J. L. J. Chem. Soc., Chem. Commun.
1997, 2015. (b) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173. (c) Cheng,
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Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem.
ReV. 2001, 101, 3893. (e) Sanford, A. R.; Gong. B. Curr. Org. Chem. 2003,
7, 1649. (f) Huc, I. Eur. J. Org. Chem. 2004, 17. (g) Balbo Block, M. A.;
Kaiser, C.; Khan, A.; Hecht, S. Top. Curr. Chem. 2005, 245, 89. (h) Cheng,
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Patch, J. A.; Barron, A. E. J. Am. Chem. Soc. 2003, 125, 12092. (c)
Arvidsson, P. I.; Ryder, N. S.; Weiss, H. M.; Gross, G.; Kretz, O.; Woessner,
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E. A.; Weisblum, B.; Gellman, S. H. J. Am. Chem. Soc. 2002, 124, 7324.
(f) Umezawa, N.; Gelman, M. A.; Haigis, M. C.; Raines, R. T.; Gellman,
S. H. J. Am. Chem. Soc. 2002, 124, 368. (g) Rueping, M.; Mahajan, Y.;
Sauer, M.; Seebach, D. ChemBioChem 2002, 3, 257.
(3) Selected examples: (a) Hagihara, M.; Anthony, N. J.; Stout, T. J.; Clardy,
J.; Schreiber, S. L. J. Am. Chem. Soc. 1992, 114, 6568. (b) Lokey, R. S.;
Iverson, B. L. Nature 1995, 375, 303. (c) Hamuro, Y.; Geib, S. J.; Hamilton,
A. D. J. Am. Chem. Soc. 1996, 118, 7529. (d) Huck, B. R.; Fisk, J. D.;
Gellman, S. H. Org. Lett. 2000, 2, 2607. (e) Gong, B.; Zeng, H.; Zhu, J.;
Yuan, L.; Han, Y.; Cheng, S.; Furukawa, M.; Parra, R. D.; Kovalevsky A.
Y.; Mills, J. L.; Skrzypczak-Jankun, E.; Martinovic, S.; Smith, R. D.; Zheng,
C.; Szyperski, T.; Zeng, X. C. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 11583.
(f) Violette, A,; Averlant-Petit, M. C.; Semetey, V.; Hemmerlin, C.; Casimir,
R.; Graff, R.; Marraud, M.; Briand, J.-P.; Rognan, D.; Guichard, G. J. Am.
Chem. Soc. 2005, 127, 2156. (g) Hayen, A.; Schmitt, M. A.; Ngassa, F.
N.; Thomasson, K. A.; Gellman, S. H. Angew. Chem., Int. Ed. 2004, 43,
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Chem., Int. Ed. 2004, 43, 511.
1
results demonstrated a tendency to a preferential conformation,
but the strong conformational freedom of the unsubstituted Gly
residue somehow reduced the folding bias of the oligomers.
Therefore, we decided to introduce a methyl group as a
substituent for the Gly residue, thus utilizing L-Ala. Very likely,
the methyl group should be introduced only on one face of the
Gly methylene moiety, because only one side is expected to
afford a stable secondary structure, while the other might
contrast helix formation. Thus, oligomers of both the Boc-(L-
Ala-L-Oxd)n-OBn (L-Oxd ) trans-(4S,5R)-4-carboxy-5-methyl-
oxazolidin-2-one) and the Boc-(L-Ala-D-Oxd)n-OBn (D-Oxd )
trans-(4R,5S)-4-carboxy-5-methyloxazolidin-2-one) series were
synthesized, to check whether the Gly pro-S hydrogen or the
pro-R hydrogen should be replaced (Figure 2). Furthermore,
some even more constrained oligomers of the Aib-L-Oxd series
as well were synthesized and analyzed. Indeed, R-aminoisobu-
(4) Luppi, G.; Soffre`, C.; Tomasini, C. Tetrahedron: Asymmetry 2004, 15,
1645.
9
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J. AM. CHEM. SOC. 2006, 128, 2410-2420
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Oxazolidin-2-one Pseudopeptides That Fold into Spirals
A R T I C L E S
Figure 3. Chemical structure of Boc-(L-Ala-L-Oxd)4-OBn (9a).
Figure 2. Chemical structure of the segment R-Gly-L-Oxd-R′. The pro-R
and the pro-S R-hydrogens of the Gly residue are highlighted.
to the coupling reagents HBTU and DBU, which are very
reactive and water sensitive. HBTU is an activated form of the
classical 1-hydroxybenzotriazole (HOBt) and has been devel-
oped for the solid-phase synthesis of peptides. Recently, Carpino
and co-workers7 demonstrated that it can exist in two forms
which are in equilibrium: the uronium form (more active) and
the guanidinium form (less active). As the presence of DBU
strongly favors the guanidinium form, it is essential to add the
tertiary base always after the two reagents and after HBTU for
the formation of the imide bond.
Scheme 1 a
The L-Ala-L-Oxd oligomers up to the tetramer level were
obtained by removal of the Boc NR-protecting group by reaction
with trifluoroacetic acid (TFA) in dry methylene chloride under
a nitrogen atmosphere, which again is crucial to avoid decom-
position of the resulting trifluoroacetate salt 4a. The corre-
sponding free amine, obtained by treatment of 4a with a weak
base (NaHCO3), proved to be very unstable. Therefore, 4a was
coupled as such, utilizing one more equivalent of TEA during
the coupling reaction.
Simultaneously, the OBn group was removed by reaction with
H2 in the presence of Pd/C (10%) in methanol to give the free
acid 3a. Both deprotections were complete in 2-3 h and
afforded the desired compounds in quantitative yields.
The coupling reaction between 3a and 4a was performed by
activation of 3a with O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-
tetramethyluronium hexafluorophosphate (HATU)8 and TEA in
dry acetonitrile at room temperature under a nitrogen atmosphere
as the reaction mixture is very sensitive to moisture. In this
case, the coupling agent HBTU afforded unsatisfactory yields.
The reaction was complete in 40 min, and Boc-(L-Ala-L-Oxd)2-
OBn (5a) was easily obtained in 95% yield, after flash
chromatography using cyclohexane and ethyl acetate as eluants.
The longer oligomers of the series, 7a and 9a, were obtained
following the same procedure: hydrogenolysis of the ester
moieties of 5a and 7a and coupling with 4a.
Unfortunately, we could not prepare higher oligomers of the
Boc-(L-Ala-L-Oxd)n-OBn series, owing to the low solubility of
both Boc-(L-Ala-L-Oxd)4-OBn (9a) (Figure 3) and Boc-(L-Ala-
L-Oxd)4-OH (10a). This effect usually indicates that the
compound has a high tendency to self-aggregate.
Then we prepared the oligomers of the Boc-(L-Ala-D-Oxd)n-
OBn series, starting from H-D-Oxd-OBn and Boc-L-Ala-OH
(Scheme 2). We followed the same synthetic strategy used for
the preparation of the Boc-(L-Ala-L-Oxd)n-OBn series: each step
was optimized and occurred with high to quantitative yield. In
any case, the use of dry solvents and an inert atmosphere is
crucial for high yield achievement.
a Reagents and conditions: (i) HBTU (1 equiv), DBU (1 equiv), dry
CH3CN, rt, 40 min; (ii) H2, Pd/C (10%), MeOH, rt, 16 h; (iii) TFA (18
equiv), dry CH2Cl2, rt, 2 h; (iv) HATU (1.1 equiv), TEA (3 equiv), dry
CH3CN, 40 min, rt.
tyric acid (Aib) is a CR-tetrasubstituted R-amino acid with two
methyl groups that replace both the pro-R and the pro-S
hydrogens.5
Results and Discussion
1. Synthesis. The pseudopeptide chains have been prepared
in the liquid phase, starting from H-L-Oxd-OBn, H-D-Oxd-OBn,
Boc-L-Ala-OH, and Boc-Aib-OH. While the latter two deriva-
tives are commercially available, L-Oxd-OBn and its enantiomer
were obtained in good yield by reaction of L- or D-threonine
with triphosgene in the presence of NaOH: this is a well-known
procedure which furnished the desired heterocycle in good yield.
The benzyl group was introduced on the carboxy moiety, by
reaction with benzyl bromide in the presence of triethylamine
(TEA) or diisopropylethylamine (DIEA). Using this procedure,
H-L-Oxd-OBn (1a) and H-D-Oxd-OBn (1b) were easily obtained
in multigram scale.
First, we performed the synthesis of the oligomers of the Boc-
(L-Ala-L-Oxd)n-OR series (R ) OBn, H) (Scheme 1). The
dipeptide Boc-L-Ala-L-Oxd-OBn (2a) was obtained by coupling
Boc-L-Ala-OH with 1a in the presence of N-[(1H-benzotri-
azolyl)(dimethylamino)methylene]-N-methylmethaniminium
hexafluorophosphate N-oxide (HBTU)6 and 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU) in dry acetonitrile. It is crucial to
perform the reaction under nitrogen, as the yield drastically falls
in the presence of air. This effect should probably be ascribed
Luckily, the compounds of this series proved to be more
soluble in organic solvents, so that we could easily prepare Boc-
(L-Ala-D-Oxd)6-OBn (13b) (Figure 4) and the corresponding
(5) (a) Marshall, G. R. In Intra-Science Chemistry Report; Kharasch, N., Ed.;
Gordon & Breach: New York, 1971; pp 305-316. (b) Karle, I. L.; Balaram,
P. Biochemistry 1990, 29, 6747. (c) Toniolo, C.; Benedetti, E. Macromol-
ecules 1991, 24, 4004. (d) Toniolo, C.; Crisma, M.; Formaggio, F.; Peggion,
C. Biopolym. Pept. Sci. 2001, 60, 396.
(6) (a) Fields, C. G.; Lloyd, D. H.; Macdonald, R. L.; Otteson, K. M.; Noble,
R. L. J. Pept. Res. 1991, 4, 95. (b) Dourtoglou, V.; Ziegler, J.-C.; Gross,
B. Tetrahedron Lett. 1978, 19, 1269. (c) Dourtoglou, V.; Gross, B.;
Lambropoulou, V.; Zioudrou, C. Synthesis 1984, 572.
(7) Carpino, L. A.; Imazumi, H.; El-Faham, A.; Ferrer, F. J.; Zhang, C.; Lee,
Y.; Foxman, B. M.; Henklein, P.; Hanay, C.; Mu¨gge, C.; Wenschuh, H.;
Klose, J.; Beyermann, M.; Bienert, M. Angew. Chem., Int. Ed. 2002, 41,
441.
(8) (a) Carpino, L. A. J. Am. Chem. Soc. 1993, 115, 4397. (b) Carpino, L. A.
PCT Int. Appl., 1994, 149 pp.
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J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006 2411

A R T I C L E S
Tomasini et al.
Figure 4. Chemical structure of Boc-(L-Ala-D-Oxd)6-OBn (13b).
Scheme 2 a
We could not obtain oligomers longer than Boc-(Aib-L-Oxd)2-
OBn (5c), owing to the low reactivity of the hindered Aib
R-amino group. Boc-Aib-L-Oxd-Aib-D-Oxd-OBn (7c) was
prepared to compare its properties with those of 5c.
2. Conformational Analysis. Information on the preferred
conformation of the described oligomers in solution was
obtained in structure-supporting solvents (methylene chloride,
1
deuteriochloroform, and methanol), by FT-IR absorption, H
NMR, and CD techniques. An additional interesting piece of
information on the preferred crystal-state conformation was
obtained by X-ray diffraction analysis of Boc-Aib-L-Oxd-OBn.
We analyzed three sets of oligomers: (i) A, Boc-(L-Ala-L-
Oxd)n-OR with n ) 1-4 (R ) Bn, H); (ii) B, Boc-(L-Ala-D-
Oxd)n-OR with n ) 1-6 (R ) Bn, H); (iii) C, Boc-(Aib-L-
Oxd)n-OR with n ) 1-2 and Boc-Aib-L-Oxd-Aib-D-Oxd-OR
(R ) Bn, H).
The FT-IR absorption spectra of the three sets were obtained
as 3 mM solutions in methylene chloride: at this concentration
self-aggregation is usually unimportant. Only 9a has been
analyzed as a 0.5 mM solution, being not very soluble in
methylene chloride.
a Reagents and conditions: (i) HBTU (1 equiv), DBU (1 equiv), dry
CH3CN, rt, 40 min; (ii) H2, Pd/C (10%), MeOH, rt, 16 h; (iii) TFA (18
equiv), dry CH2Cl2, rt, 2 h; (iv) HATU (1.1 equiv), TEA (3 equiv), dry
CH3CN, 40 min, rt.
Figure 5 shows the FT-IR absorption spectra (N-H stretching
region) for all the synthesized compounds, and helps us to detect
non-hydrogen-bonded amide N-H bands (above 3400 cm-1
)
Scheme 3 a
and hydrogen-bonded amide proton bands (below 3400 cm-1).
These results show that the three series behave in very different
ways. Indeed, compounds of the A and the C sets (Figure 5a,c)
show no propensity to form intramolecular CdO‚‚‚HsN
hydrogen bonds, while those of the B set (Figure 5b) form strong
hydrogen bonds in oligomers longer than the trimer 7b.
To further validate these results, the three sets of oligomers
1
were analyzed by H NMR spectroscopy. We have already
demonstrated9 that, in similar pseudopeptides, the formation of
a Ci+1dO‚‚‚HsCi intramolecular H-bond can be spotted simply
by checking the R-proton chemical shift, as the carbonyl
proximity involves a marked deshielding of the proton chemical
shift. In these series we observed this effect only in the L-Ala-
containing sets A and B, because the Aib residues of set C have
no R-hydrogens.
Table 1 lists the R-proton chemical shifts of all the Boc/OBn-
protected pseudopeptides of the A and B sets, compared with
that of Boc-L-Ala-OBn. A strictly comparable trend was
observed for the C deprotected series (not shown). The chemical
shift of the Boc-L-Ala-OBn R-hydrogen is reported in entry 1
and is at 4.4 ppm. In contrast, entries 2-11 show that the
chemical shifts of the L-Ala moiety R-hydrogens of the benzyl
esters of both the A and B sets are always deshielded by about
1 ppm, all resonating between 5.17 and 6.08 ppm.
a Reagents and conditions: (i) HBTU (1 equiv), DBU (1 equiv), dry
CH3CN, rt, 40 min; (ii) H2, Pd/C (10%), MeOH, rt, 16 h; (iii) TFA (18
equiv), dry CH2Cl2, rt, 2 h; (iv) HATU (1.1 equiv), TEA (2 equiv), dry
CH3CN, 40 min, rt.
free acid Boc-(L-Ala-D-Oxd)6-OH (14b). Surprisingly, the
solubility of 13b proved to be higher than that of the shorter
9b.
The last series of oligomers described in this paper was
obtained by coupling Boc-Aib-OH, 1a, and 1b. First, we
prepared Boc-Aib-L-Oxd-OBn (2c), following the same reaction
procedure described in Scheme 1. Then we selectively depro-
tected the carboxyl group by hydrogenolysis to form 3c or the
amino group by acid hydrolysis to form 4c. The latter compound
could be easily isolated as a free amine, being very stable even
at room temperature (Scheme 3).
(9) (a) Bernardi, F.; Garavelli, M.; Scatizzi, M.; Tomasini, C.; Trigari, V.;
Crisma, M.; Formaggio, F.; Peggion, C.; Toniolo, C. Chem.sEur. J. 2002,
8, 2516. (b) Lucarini, S.; Tomasini C. J. Org. Chem. 2001, 66, 727. (c)
Tomasini, C.; Trigari, V.; Lucarini, S.; Bernardi, F.; Garavelli, M.; Peggion,
C.; Formaggio, F.; Toniolo, C. Eur. J. Org. Chem. 2003, 259. (d) Luppi,
G.; Galeazzi, R.; Garavelli, M.; Formaggio, F.; Tomasini C. Org. Biomol.
Chem. 2004, 2, 2181. (e) Luppi, G.; Lanci, D.; Trigari, V.; Garavelli, M.;
Garelli, A.; Tomasini C. J. Org. Chem. 2003, 68, 1982. (f) Tomasini, C.;
Villa M. Tetrahedron Lett. 2001, 42, 5211.
9
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Oxazolidin-2-one Pseudopeptides That Fold into Spirals
A R T I C L E S
Table 1. R-Proton Chemical Shifts (ppm) for Boc-L-Ala-OBn and
Pseudopeptides 2a, 5a, 7a, 9a, 2b, 5b, 7b, 9b, 11b, and 13b in
10-2 M Solutions in CDCl3 at 25 °C
entry
compound
δ(R-CH)
1
2
3
4
5
6
7
8
9
Boc-L-Ala-OBn
4.40a
5.35
5.38, 5.60
Boc-L-Ala-L-Oxd-OBn (2a)
Boc-(L-Ala-L-Oxd)2-OBn (5a)
Boc-(L-Ala-L-Oxd)3-OBn (7a)
Boc-(L-Ala-L-Oxd)4-OBn (9a)
Boc-L-Ala-D-Oxd-OBn (2b)
Boc-(L-Ala-D-Oxd)2-OBn (5b) 5.28, 5.58
Boc-(L-Ala-D-Oxd)3-OBn (7b) 5.17-5.46 (2 H), 5.48-5.58
Boc-(L-Ala-D-Oxd)4-OBn (9b) 5.35-5.58 (4 H)
5.30, 5.42, 5.45
5.25, 5.44 (2 H), 5.47b
5.43
10 Boc-(L-Ala-D-Oxd)5-OBn (11b) 5.18-5.50 (2 H), 5.83, 5.83, 6.07
11 Boc-(L-Ala-D-Oxd)6-OBn (13b) 5.24-5.28, 5.60-5.67 (2 H),
5.75-5.98 (2 H), 5.98-6.08
a Reference 10. b This spectrum was recorded in CD3OD, owing to the
low solubility of compound 9a in CDCl3.
Figure 6. Preferred conformation for the pseudopeptide Boc-L-Ala-(L/D)-
Oxd-OBn (2a/b). The preferential trans conformation of the imide moiety,
which accounts for the anomalous chemical shift of the Ala CR-H proton,
is shown.
a stabilization of the trans-L-pGlu-L-pGlu conformation of about
1.4 kcal mol-1 9a
.
This result is not surprising on energetic grounds as this imide
conformation11,12 is the one which allows the two nonbonded
carbonyl oxygens of our semicyclic system to be located the
farthest apart (Figure 6).
Interestingly, in the present series the oligomers of both the
A and B sets follow this trend, regardless of the absolute
configurations of carbons 4 and 5 of the Oxd moiety, which on
the contrary are crucial for the formation of intramolecular Cd
O‚‚‚HsN hydrogen bonds, as deduced by IR absorption spectral
analysis (Figure 5).
Although the oligomers of the C set do not contain R-protons
able to form CdO‚‚‚HsCR H-bonds, the two nonbonded
carbonyl oxygens of each semicyclic system are still located
the farthest apart. This outcome was fully demonstrated by an
X-ray diffraction analysis carried out on a single crystal of 2c.
The solid-state molecular structure of 2c (Figure 7) revealed
that the oxazolidinone and amidic (C10O4 and C9O3) carbonyls
(Figure 7) point in opposite directions. Interestingly, in 2c the
Boc-NHC(CH3)2- chain adopts a conformation different from
that found in tert-butyl N-(2-oxo-2-(4-isopropyl-2-oxo-5,5-
diphenyloxazolidin-3-yl)ethyl) carbamate13 (torsional angle
O3C9C6N1 ) 138.0(4)° in 2c vs 5.8°) because of the two bulky
methyl groups [C7, C8] replacing two hydrogens. No intramo-
lecular CdO‚‚‚HsN bond is formed between the endocyclic
carbonyl and the NH of the Aib residue [dihedral angle between
the O5C10N2O4 and N1C5O2O1 planes, 68.1(2)°] presumably
because this would imply the formation of an unfavored seven-
Figure 5. FT-IR absorption spectra in the N-H stretching region for 3
mM concentration samples of oligomers (a) 2a, 5a, 7a, and 9a in pure
CH2Cl2 at room temperature (the concentration of 9a is 0.5 mM, owing to
its low solubility), (b) oligomers 2b, 5b, 7b, 9b, 11b, and 13b, and (c)
oligomers 2c, 5c, and 7c.
We ascribe this unusual chemical shift to the presence of an
oxazolidin-2-one carbonyl in the vicinity of the R-CH proton
of the L-Ala unit, in such a way that the imide -CO-N< bond
external to the ring system is forced to accommodate the trans
conformation. This conformation is stabilized by CdO‚‚‚HsC
intramolecular H-bonds. We have recently demonstrated by
high-level DFT computational modeling on Boc-(L-pGlu)2-OH
(p-Glu, pyroglutamic acid) that this H-bond is responsible for
(10) Hayashida, O.; Sebo, L.; Rebek, J., Jr. J. Org. Chem. 2002, 67, 8291.
(11) Valle G.; Toniolo C.; Jung, G. Liebigs Ann. Chem. 1986, 1809.
(12) (a) Bellamy L. J. The Infrared Spectra of Complex Molecules; Methuen:
London, 1966. (b) Driessen W. L.; Everstijn, P. L. A. J. Coord. Chem.
1980, 10, 155.
(13) Seebach, D.; Schaeffer, L.; Gessier, F.: Bindscha¨dler, P.; Ja¨ger, C.; Josien,
D.; Kopp, S.; Lelais, G.; Mahajan, Y. R.; Micuch, P.; Sebesta, R.;
Schweizer, B. W. HelV. Chim. Acta 2003, 86, 1852.
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A R T I C L E S
Tomasini et al.
Figure 7. ORTEP drawing of Boc-Aib-L-Oxd-OBn (2c).
Figure 8. Crystal packing of 2c showing the interpenetrating chains
generated by intermolecular CdO‚‚‚HsN hydrogen bonds running along
the c axis.
membered ring. On the other hand, a closer examination of the
crystal packing of the molecule (Figure 8) shows that intermo-
lecular CdO‚‚‚HsN bonds [N1-H10 ) 0.91(6) Å, H10‚‚‚O2′
) 2.19(6) Å, N1-H10‚‚‚O2′ ) 160(5)°] are formed. Moreover,
the NH and CO groups being involved in hydrogen bonds on
opposite sides, the molecules of 2c generate interpenetrating
zigzag chains running along the c axis. The conformational
conclusions extracted from this X-ray diffraction study fit nicely
with those obtained from our FT-IR absorption analysis
discussed above.
The occurrence of intramolecular CdO‚‚‚HsN H-bonds in
oligomers of the A and B sets has been further detected by an
investigation of the DMSO-d6 dependence of NH proton
chemical shifts.14 This solvent is a strong hydrogen-bonding
acceptor, and if it is bound to a free NH proton, it will be
expected to dramatically move its chemical shift downfield. The
results for the DMSO-d6/CDCl3 titrations of the NH protons of
the pseudopeptides 7a, 9b, and 11b are reported in Figure 9.
Figure 9. Variation of NH proton chemical shifts (ppm) of 7a (a), 9b (b),
and 11b (c) as a function of increasing percentages of DMSO-d6 added to
the CDCl3 solution (v/v) (concentration 1 mM).
The outcome of these titrations is in perfect agreement with
the results obtained using FT-IR absorption, as the NH protons
of Boc-(L-Ala-L-Oxd)3-OBn (7a) are very sensitive to DMSO
while all of the NH protons of Boc-(L-Ala-D-Oxd)4-OBn (9b)
and Boc-(L-Ala-D-Oxd)5-OBn (11b) are nearly insensitive to
DMSO, thus confirming that 7a is unable to form an H-bond-
driven secondary structure while 9b and 11b are able to do so.
Unfortunately, we were not able to titrate the longest oligomer
(14) (a) Kopple, K. D.; Ohnishi, M.; Go, A. Biochemistry 1969, 8, 4087. (b)
Martin, D.; Hauthal, H. G. Dimethyl Sulphoxide; Van Nostrand Reinhold:
Wokingham, U.K., 1975.
9
2414 J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006

Oxazolidin-2-one Pseudopeptides That Fold into Spirals
A R T I C L E S
Figure 10. Significative NOE enhancements of 11b obtained by performing
the ROESY experiments on a 10 mM solution in CDCl3 and utilizing a
mixing time of 0.400 s.
of the A set, 9a, under the same conditions owing to its poor
solubility.
Another interesting piece of information has been obtained
by ROESY experiments of a 10 mM solution of 11b in CDCl3
solution (Figure 10). From an inspection of the area between 7
and 9 ppm, many NiH f Ni+1H cross-peaks, which usually
account for the formation of a helix, are visible.15
Further confirmation of helix formation in the longer oligo-
mers of the B set was obtained by recording the CD spectra of
all free acids of the A-C sets in MeOH solution. Although
this technique is intrinsically a low-resolution method,16 it can
furnish useful information on the presence of ordered secondary
structures.17 The per-residue CD spectra are reported in Figure
11.
The per-residue CD bonds of the A and C sets do no exhibit
any significant change from the monomer to the higher
oligomers, thus showing that no configurational effects (besides
the absorption of the chromophore itself) take place. This result
is at variance with that observed with the B set. The spectra of
3b, 6b, 8b, and 10b are nearly superimposable. On the contrary,
an ellipticity increase, associated with a reversal of the Cotton
effect, was observed for 12b and more dramatically for 14b.
These CD spectra are indicative of an ordered secondary
structure and agree well with the results obtained from the IR
1
absorption and H NMR analyses.
To define the nature of the ordered secondary structure formed
by the pseudopeptides Boc-(L-Ala-D-Oxd)5-OH (12b) and Boc-
(L-Ala-D-Oxd)6-OH (14b), we carefully analyzed their CD
spectra in the absence of normalization (Figure 12).
Figure 11. Normalized per-residue CD spectra of (a) Boc-(L-Ala-L-Oxd)n-
OH (n ) 1-4) (3a, 6a, 8a, and 10a), (b) Boc-(L-Ala-D-Oxd)n-OH (n )
1-6) (3b, 6b, 8b, 10b, 12b, and 14b), and (c) 3c, 6c, and 8c (1 mM
concentration in MeOH solution).
The CD spectra of 12b and 14b (Figure 12a) display a
negative CD band centered at about 207 nm followed by a
negative shoulder at about 222 nm, while the ellipticity at 195
nm is positive. The shape of the CD spectrum of 14b does not
change as the concentration decreases, as shown in Figure 12b,
where the ellipticity of 14b in MeOH solution at various
concentrations is reported. This finding excludes formation of
self-associated secondary structures.
Among the ordered peptide secondary structures which can
be formed, we considered the CD spectra of the R- and 310-
helices and the â-pleated sheet conformations. From a general
inspection of the spectra shown in Figure 12, we deduce that
12b and 14b fold in the 310-helix. Indeed, Toniolo and
co-workers18 demonstrated that a 310-helix can be assigned by
its CD spectrum, in agreement with the theoretical calculations.19
(15) Wu¨thrich, K. NMR of Proteins and Nucleic Acids; Wiley: New York, 1986;
p 192.
(16) Gla¨ttli, A.; Daura, X.; Seebach, D.; van Gunsteren, W. F. J. Am. Chem.
Soc. 2002, 124, 12972.
(17) See, for example: (a) Huck, B. R.; Fisk, J. D.; Guzei, I. A.; Carlson, H.
A.; Gellman, S. H. J. Am. Chem. Soc. 2003, 125, 9035. (b) Park, J.-S.;
Lee, H.-S.; Lai, J. R.; Kim, B. M.; Gellman, S. H. J. Am. Chem. Soc. 2003,
125, 8539. (c) Raguse, T. L.; Lai, J. R.; Gellman, S. H. J. Am. Chem. Soc.
2003, 125, 5592. (d) Hart, S. A.; Bahadoor, A. B. F.; Matthews, E. E.;
Qiu, X. J.; Schepartz, A. J. Am. Chem. Soc. 2003, 125, 4022.
9
J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006 2415

A R T I C L E S
Tomasini et al.
Figure 13. Conformation of Boc-(L-Ala-D-Oxd)6-OBn (13b), where the
three stabilizing effects are shown: (a) the rigid -CO-N(CH<)-CO-
moiety, (b) the CdO‚‚‚H-RC H-bonds, and (c) the CdO‚‚‚HsN H-bonds.
12b and 14b a â-bend ribbon spiral structure,20 which may be
considered a subtype of the polypeptide 310 helix, being
stabilized by alternate 1 r 4 intramolecular CdO‚‚‚HsN
H-bonds and having approximately the same fold of the peptide
chain. Indeed, Toniolo and co-workers20b reported the CD
spectrum of (L-Pro-Aib)n, which folds in a â-bend ribbon spiral
structure. The spectrum is very similar to the CD spectrum
reported in Figure 12.21
This outcome may be ascribed to the cooperative effect of
many factors: (i) the rigid -CO-N(CH<)-CO- moiety,
which always tends to assume a trans conformation; (ii) the
formation of CdO‚‚‚HsRC H-bonds; (iii) the alternate forma-
tion of CdO‚‚‚HsN H-bonds. We believe that the combination
of these three effects can hold the oligomers in a well-defined,
rigid conformation (Figure 13).
Bearing these requirements in mind, we can understand why
only oligomers of the B set form a â-bend ribbon spiral while
the oligomers of the A set do not.
Figure 14 shows that only the oligomers of the B set are able
to meet all three requirements listed above. Indeed, the rigid
trans conformation of the -CO-N(CH<)-CO- moiety and
the CdO‚‚‚HsRC H-bond are present in both sets of pseudopep-
tides, so that the -L-Ala-L-Oxd- and -L-Ala-D-Oxd- sequences
can be considered as rigid bicyclic moieties. As a result of this
outcome, the side chains of the B set are cis, so that the
carbonyls and the NH groups can form 1 r 4 intramolecular
CdO‚‚‚HsN H-bonds. On the other hand, in the -L-Ala-L-Oxd-
sequence, the two side chains are trans, so that a H-bond cannot
be formed (Figure 14).
Figure 12. (a) CD spectra of Boc-(L-Ala-D-Oxd)5-OH (12b) and Boc-(L-
Ala-D-Oxd)6-OH (14b) in MeOH solution (3 mM concentration). (b) CD
spectra of 14b at different concentrations in MeOH solution.
Table 2. Selected Molar Ellipticity Values for
Boc-(L-Ala-D-Oxd)5-OH (12b) and Boc-(L-Ala-D-Oxd)6-OH (14b) in
MeOH Solution at Different Concentrations at 25 °C
concn
3
3
3
entry compd (mM, in MeOH)
[
θ]195
×
10-
[
θ]207
×
10-
[
θ]222
×
10-
R ) [θ]222/[θ]207
1
2
4
5
6
7
12b
14b
14b
14b
14b
14b
1
1
0.5
0.2
0.1
0.05
21.16
35.59
42.46
59.79
69.08
15.5
-40.59
-161.19
-151.82
-140.69
-121.14
-100.74
-21.38
-55.09
-51.88
-42.02
-28.88
-29.33
0.53
0.34
0.34
0.30
0.24
0.29
Conclusions
In this paper we reported the synthesis and conformational
analysis of pseudopeptides containing alternate L-Ala or Aib
residues and L-Oxd or D-Oxd residues. These pseudopeptides
have been synthesized in the liquid phase, utilizing a uronium
salt (HATU or HBTU) as the coupling agent under an inert
atmosphere, which turned out to be crucial for good to excellent
yields. The conformational analysis, performed in solution (IR
Both theoretical and experimental studies point to the following
main characteristics of the 310-helix: (a) a negative CD band
centered near 207 nm; (b) a negative shoulder in the vicinity of
222 nm; (c) an R ) [θ]222/[θ]207 ratio much weaker than the
value reported for the R-helical peptides, which usually exhibit
an R value of about 1; (d) a positive maximum at 198 nm.
1
absorption, H NMR, 13C NMR, CD) and in the crystal state
(20) (a) Venkataram Prasad, B. V.; Balaram, P. Int. J. Biol. Macromol. 1982,
4, 99. (b) Di Blasio, B.; Pavone, V.; Saviano, M.; Lombardi, A.; Nastri,
F.; Pedone, C.; Benedetti, E.; Crisma, M.; Anzolin, M.; Toniolo, C. J. Am.
Chem. Soc. 1992, 114, 6273. (c) Crisma, M.; Anzolin, M.; Bonora, G. M.;
Toniolo, C.; Benedetti, E.; Di Blasio, B.; Pavone, V.; Saviano, M.;
Lombardi, A.; Nastri, F.; Pedone, C. Gazz. Chim. Ital. 1992, 122, 239. (d)
Rebuffat, S.; Goulard, C.; Hlimi, S.; Bodo, B. J. Pept. Sci. 2000, 6, 519.
(e) Segalas, I.; Prigent, Y.; Davoust, D.; Bodo, B.; Rebuffat, S. Biopolymers
1999, 50, 71. (f) Crisma, M.; Valle, G.; Bonora, G. M.; Toniolo, C.;
Cavicchioni, G. Int. J. Pept. Protein Res. 1993, 41, 553. (g) Moretto, V.;
Valle, G.; Crisma, M.; Bonora, G. M.; Toniolo, C. Int. J. Biol. Macromol.
1992, 14, 178. (h) Benedetti, E.; Di Blasio, B.; Pavone, V.; Pedone, C.;
Toniolo, C.; Crisma, M. Biopolymers 1992, 32, 453. (i) Karle, I. L.; Flippen-
Anderson, J.; Sukumar, M.; Balaram, P. Proc. Natl. Acad. Sci. U.S.A. 1987,
84, 5087.
Table 2 displays the most relevant ellipticity values for 12b
and 14b.
Our experimental CD data meet all of the requirements for
the 310-helix. More precisely, we can attribute to our oligomers
(18) (a) Toniolo, C.; Polese, A.; Formaggio, F.; Crisma, M.; Kamphuis, J. J.
Am. Chem. Soc. 1996, 118, 2744. (b) Toniolo, C.; Formaggio, F.; Tognon,
S.; Broxterman, Q. B.; Kaptein, B.; Huang, R.; Setnicka, V.; Keiderling,
T. A.; McColl, I. H.; Hecht, L.; Barron, L. D. Biopolymers 2004, 75, 32.
(c) Formaggio, F.; Baldini, C.; Moretto, V.; Crisma, M.; Kaptein, B.;
Broxterman, Q. B.; Toniolo, C. Chem.sEur. J. 2005, 11, 2395.
(21) Yoder, G.; Keiderling, T. A.; Formaggio, F.; Crisma, M.; Toniolo, C.
Biopolymers 1995, 35, 103.
(19) Manning, M. C.; Woody, R. W. Biopolymers 1991, 31, 569.
9
2416 J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006

Oxazolidin-2-one Pseudopeptides That Fold into Spirals
A R T I C L E S
pure in 88% yield (0.89 g) as a waxy solid after silica gel chromatog-
raphy (cyclohexane/ethyl acetate, 9:1, as eluant): [R]D -44.1 (c 1.0,
1
CH2Cl2); IR (CH2Cl2, 3 mM) ν ) 3439, 1794, 1755, 1718 cm-1; H
NMR (CDCl3, 400 MHz) δ 1.31 (d, 3H, J ) 7.6 Hz), 1.42 (s, 9H),
1.51 (d, 3H, J ) 6.4 Hz), 4.50-4.58 (m, 2H), 5.14 (br s, 1H), 5.17
(AB, 2H, J ) 12.0 Hz), 5.35 (dq, 1H, J ) 7.6 Hz), 7.25-7.40 (m,
5H); 13C NMR (CDCl3, 100 MHz) δ 17.8, 20.9, 28.1, 48.8, 61.4, 67.9,
73.5, 79.7, 128.2, 128.4, 128.7, 134.3, 151.1, 154.9, 167.4, 173.9. Anal.
Calcd for C20H26N2O7 (406.43): C, 59.10; H, 6.45; N, 6.89. Found:
C, 59.12; H, 6.50; N, 6.85.
Boc-L-Ala-L-Oxd-OH (3a). To a solution of Boc-L-Ala-L-Oxd-OBn
(2 mmol, 0.81 g) in methanol (40 mL) was added 10% palladium on
charcoal, and the mixture was stirred under a hydrogen atmosphere
for 2 h. Then the catalyst was filtered on a Celite pad, and the mixture
was concentrated. The corresponding acid Boc-L-Ala-L-Oxd-OH was
obtained pure in quantitative yield without any further purification: mp
142 °C dec; [R]D -25.5 (c 1.0, CH2Cl2); IR (CH2Cl2, 3 mM) ν ) 3437,
1792, 1713 cm-1; 1H NMR (CDCl3, 400 MHz) δ 1.38 (d, 3H, J ) 7.6
Hz), 1.43 (s, 9H), 1.56 (d, 3H, J ) 6.2 Hz), 4.57 (d, 1H, J ) 5.6 Hz),
4.68-4.75 (m, 1H), 5.27 (dq, 1H, J ) 7.6 Hz), 6.75 (d, 1H, J ) 7.6
Hz), 10.00 (br s, 1H); 13C NMR (CDCl3, 100 MHz) δ 18.0, 28.3, 50.2,
61.7, 73.9, 82.3, 151.6, 157.7, 170.6, 173.2. Anal. Calcd for C14H24N2O7
(332.35): C, 50.59; H, 7.28; N, 8.43. Found: C, 50.55; H, 7.20; N,
8.35.
Figure 14. (a) A â-bend ribbon spiral formed by the Boc-(L-Ala-D-Oxd)n-
OR oligomers (B set) and (b) the impossibility to form an ordered secondary
structure for the Boc-(L-Ala-L-Oxd)n-OR oligomers (A set).
(X-ray diffraction), unambiguously proved that the oligomers
having the general formula Boc-(L-Ala-D-Oxd)n-OR (n ) 1-6,
R ) Bn or H) fold in ordered structures with n g 5. With a
deeper insight into the CD spectra, we could demonstrate that
the secondary structure adopted is a â-bend ribbon spiral, which
is a subtype of the 310-helix. Its formation should be ascribed
to the cooperative effect of three factors: (i) the rigid Oxd
-CO-N(CH<)-CO- moiety, which always tends to assume
a trans conformation; (ii) the formation of Oxd CdO‚‚‚Hs RC
intramolecolar H-bonds; (iii) the alternate formation of 1 r 4
intramolecular CdO‚‚‚HsN H-bonds, favored by the two side
chains NHR and C(O)-NHR, both pointing in the same
direction.
H-L-Ala-L-Oxd-OBn‚CF3CO2H (4a). A solution of 2a (2 mmol,
0.83 g) and TFA (36 mmol, 2.78 mL) in dry methylene chloride (20
mL) was stirred for 4 h at room temperature, then the volatiles were
removed under reduced pressure, and the product was obtained pure
in quantitative yield without any further purification: 1H NMR (CDCl3,
300 MHz) δ 1.56 (d, 3H, J ) 6.3 Hz), 1.65 (d, 3H, J ) 5.6 Hz), 4.58-
4.78 (m, 2H), 5.13-5.38 (m, 4H), 7.22-7.52 (m, 5H), 7.75 (br s, 3H).
Boc-(L-Ala-L-Oxd)2-OBn (5a). To a stirred solution of 3a (1 mmol,
0.33 g) and HATU (1 mmol, 0.38 g) in dry acetonitrile (10 mL) under
an inert atmosphere was added a mixture of 4a (1 mmol) and Et3N (3
mmol, 0.44 mL) in dry acetonitrile (10 mL) at room temperature. The
solution was stirred for 40 min under and inert atmosphere, and then
acetonitrile was removed under reduced pressure and was replaced with
ethyl acetate. The mixture was washed with brine, 1 N aqueous HCl
(3 × 30 mL), and 5% aqueous NaHCO3 (1 × 30 mL), dried over
sodium sulfate, and concentrated in vacuo. The product was obtained
pure after silica gel chromatography (cyclohexane/ethyl acetate, 8:2,
as eluant) in 95% yield: mp 128-131 °C; [R]D -70.2 (c 1.0, CH2Cl2);
Experimental Section
Routine NMR spectra were recorded with spectrometers at 400, 300,
or 200 MHz (1H NMR) and at 100, 75, or 50 MHz (13C NMR).
Chemical shifts are reported in δ values relative to the solvent peak of
CHCl3, set at 7.27 ppm. Infrared spectra were recorded with an FT-IR
spectrometer. Melting points were determined in open capillaries and
are uncorrected.
IR (CH2Cl2, 3 mM) ν ) 3430, 1792, 1756, 1704 cm-1 1H NMR
;
(CDCl3, 400 MHz) δ 1.42-1.46 (m, 15 H), 1.55 (d, 3 H, J ) 5.6 Hz),
1.57 (d, 3H, J ) 5.6 Hz), 4.37 (d, 1H, J ) 4.4 Hz), 4.56 (d, 1H, J )
4.4 Hz), 4.60 (dq, 1H, J ) 4.4, 5.6 Hz), 4.76 (dq, 1H, J ) 5.6 Hz),
5.04 (d, 1H, J ) 7.2 Hz), 5.21 (AB, 2H, J ) 12.0 Hz), 5.38 (dq, 1H,
J ) 7.2 Hz), 5.60 (dq, 1H, J ) 7.2 Hz), 6.49 (d, 1H, J ) 7.2 Hz),
7.30-7.41 (m, 5H); 13C NMR (CDCl3, 75 MHz) δ 17.4, 18.1, 20.7,
21.1, 28.3, 48.5, 49.1, 61.4, 62.7, 68.2, 73.8, 74.1, 79.9, 128.4, 128.7,
128.9, 134.3, 151.1, 151.6, 155.1, 166.8, 167.4, 172.6, 174.7. Anal.
Calcd for C28H36N4O11 (604.24): C, 55.62; H, 6.00; N, 9.27. Found:
C, 55.67; H, 6.04; N, 9.23.
Boc-(L-Ala-L-Oxd)2-OH (6a). For the synthetic procedure from 5a
see the above preparation of 3a: quantitative yield; mp 116-119 °C
dec; [R]D -56.8 (c 1.0, CH2Cl2); IR (Nujol) ν ) 3434, 3379, 1793,
1708 cm-1; 1H NMR (CD3OD, 200 MHz) δ 1.34 (d, 3H, J ) 7.2 Hz),
1.42 (s, 9H), 1.46 (d, 3H, J ) 7.2 Hz), 1.54 (d, 6H, J ) 6.8 Hz), 4.53
(d, 1H, J ) 4.4 Hz), 4.55 (d, 1H, J ) 4.4 Hz), 4.72 (dq, 2H, J ) 4.4,
6.8 Hz), 5.21 (q, 1H, J ) 7.2 Hz), 5.43 (q, 1H, J ) 7.2 Hz); 13C NMR
(CDCl3, 100 MHz) δ 16.7, 17.3, 20.7, 21.2, 28.7, 49.7, 50.1, 62.7,
63.3, 75.9, 76.1, 80.6, 153.4, 153.8, 157.9, 170.2, 171.0, 174.0, 175.6.
Anal. Calcd for C21H30N4O11 (514.19): C, 49.02; H, 5.88; N, 10.89.
Found: C, 49.11; H, 5.83; N, 10.78.
High-quality infrared spectra (64 scans) were obtained at 2 cm-1
resolution using a 1 mm NaCl solution cell and a Nicolet 210 FT-
infrared spectrometer. All spectra were obtained in 3 mM solutions in
dry CH2Cl2 at 297 K. All compounds were dried in vacuo, and all the
sample preparations were performed in a nitrogen atmosphere.
1
High-quality H NMR spectra were recorded with a Varian Inova
600. Measurements were carried out in CDCl3 and in DMSO-d6 using
tetramethylsilane as the internal standard. Proton signals were assigned
by COSY spectra. Data for conformational analysis are obtained with
2D NOESY spectra with typical mixing times of 1.0 s.
The CD spectra were obtained on a Jasco J-810 spectropolarimeter.
Cylindrical fused quartz cells of 0.02 cm path length were used. The
values are expressed in terms of [θ]T, the total molar ellipticity (deg
cm2 dmol-1).
Boc-L-Ala-L-Oxd-OBn (2a). To a stirred solution of Boc-L-Ala-
OH (0.47 g, 2.5 mmol) in acetonitrile (20 mL) were added HBTU (0.98
g, 2.6 mmol), then L-Oxd-OBn (1a) (0.59 g, 2.5 mmol), and last DBU
(0.75 mL, 5 mol). The mixture was stirred for 30 min, and then
acetonitrile was removed under reduced pressure and was replaced with
ethyl acetate. The mixture was washed with brine, 1 N aqueous HCl
(3 × 30 mL), and 5% aqueous NaHCO3 (1 × 30 mL), dried over
sodium sulfate, and concentrated in vacuo. Product 2a was obtained
Boc-(L-Ala-L-Oxd)3-OBn (7a). For the synthetic procedure from 6a
and 4a see the above preparation of 5a: 80% yield; mp 187-189 °C
9
J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006 2417

A R T I C L E S
Tomasini et al.
dec; [R]D -143 (c 1.0, CH2Cl2); IR (CH2Cl2, 3 mM) ν ) 3418, 1791,
(d, 3H, J ) 6.2 Hz), 4.42 (d, 1H, J ) 3.4 Hz), 4.62-4.80 (m, 1H),
5.35-5.49 (m, 1H), 6.38 (d, 1H, J ) 6.2 Hz), 9.25 (br s, 1H); 13C
NMR (CDCl3, 75 MHz) δ 18.7, 21.2, 28.2, 49.1, 61.8, 74.1, 80.7, 151.6,
155.5, 170.1, 173.7. Anal. Calcd for C14H24N2O7 (332.35): C, 50.59;
H, 7.28; N, 8.43. Found: C, 50.62; H, 7.52; N, 8.40.
H-L-Ala-D-Oxd-OBn‚CF3CO2H (4b). For the synthetic procedure
from 2b see the above preparation of 4a: quantitative yield; 1H NMR
(CDCl3, 300 MHz) δ 1.54 (d, H, J ) 6.3 Hz), 1.65 (d, 3H, J ) 6.9
Hz), 4.55 (d, 1H, J ) 3.9 Hz), 4.62 (dq, 1H, J ) 3.9, 6.3 Hz), 5.13-
5.32 (m, 4H), 7.32-7.45 (m, 5H), 7.82 (br s, 3H).
Boc-(L-Ala-D-Oxd)2-OBn (5b). For the synthetic procedure from
3b and 4b see the above preparation of 5a: 85% yield; mp 199-203
°C; [R]D +3.6 (c 1.0, CH2Cl2); IR (CH2Cl2, 3 mM) ν ) 3445, 1790,
1720, 1699 cm-1; 1H NMR (CDCl3, 600 MHz) δ 1.40 (d, 3H, J ) 6.0
Hz), 1.42 (s, 9H), 1.49 (d, 3H, J ) 7.0 Hz), 1.52 (d, 3H, J ) 6.0 Hz),
1.53 (d, 3H, J ) 6.2 Hz), 4.42 (d, 1H, J ) 4.8 Hz), 4.45 (d, 1H, J )
4.2 Hz), 4.57 (dq, 1H, J ) 4.2, 6.3 Hz), 4.72 (dq, 1H, J ) 6.0 Hz),
5.10 (br s, 1H), 5.23 (AB, 2H, J ) 12.2 Hz), 5.28 (dq, 1H, J ) 6.0,
6.9 Hz), 5.57 (dq, 1H, J ) 7.0 Hz), 7.14 (d, 1H, J ) 7.0 Hz), 7.30-
7.43 (m, 5H); 13C NMR (CDCl3, 75 MHz) δ 17.0, 20.9, 21.0, 28.2,
48.7, 49.3, 62.0, 62.9, 67.9, 73.8, 75.0, 80.2, 128.2, 128.6, 134.7, 151.2,
151.7, 155.4, 166.8, 167.3, 172.3, 174.3. Anal. Calcd for C28H36N4O11
(604.24): C, 55.62; H, 6.00; N, 9.27. Found: C, 55.67; H, 6.04; N,
9.23.
1
1754, 1701 cm-1; H NMR (CD3OD, 400 MHz) δ 1.39 (d, 3H, J )
7.2 Hz), 1.41 (d, 3H, J ) 7.2 Hz), 1.47 (s, 9H), 1.48 (d, 3H, J ) 7.2
Hz), 1.56 (d, 3H, J ) 6.0 Hz), 1.59 (d, 3H, J ) 6.0 Hz), 1.60 (d, 3H,
J ) 6.0 Hz), 4.56-4.63 (m, 4H), 4.69 (d, 1H, J ) 4.4 Hz), 4.77 (dq,
1H, J ) 4.4, 6.0 Hz), 5.21-5.30 (m, 3H), 5.42 (q, 1H, J ) 7.2 Hz),
5.45 (q, 1H, J ) 7.2 Hz), 7.30-7.41 (m, 5H); 13C NMR (CD3OD, 100
MHz) δ 16.6, 16.7, 17.3, 20.6, 21.0, 49.6, 49.8, 50.1, 62.8, 63.2, 68.9,
80.6, 129.5, 129.6, 129.7, 136.4, 153.2, 153.7, 153.8, 157.9, 169.3,
170.0, 170.2, 173.9, 174.1, 175.5. Anal. Calcd for C36H46N6O15
(802.30): C, 53.86; H, 5.78; N, 10.47. Found: C, 53.92; H, 5.82; N,
10.52.
Boc-(L-Ala-L-Oxd)3-OH (8a). For the synthetic procedure from 7a
see the above preparation of 3a: quantitative yield; mp 173-174 °C
dec; [R]D -77.0 (c 1.0, CH2Cl2); IR (Nujol) ν ) 3409, 1775, 1718,
1683 cm-1; 1H NMR (CD3OD, 400 MHz) δ 1.39 (d, 3H, J ) 6.8 Hz),
1.46 (s, 9H), 1.49 (d, 3H, J ) 7.2 Hz), 1.51 (d, 3H, J ) 7.2 Hz),
1.56-1.61 (m, 9H), 4.53-4.64 (m, 5H), 4.77 (dq, 1H, J ) 4.4, 6.4
Hz), 5.25 (q, 1H, J ) 6.8 Hz), 5.44 (q, 1H, J ) 7.2 Hz), 5.48 (q, 1H,
J ) 7.2 Hz); 13C NMR (CD3OD 100 MHz) δ 16.6, 17.2, 17.6, 20.6,
21.1, 21.2, 28.6, 49.5, 49.6, 50.1, 62.8, 63.2, 63.3, 76.0, 76.1, 76.3,
80.6, 153.5, 153.7, 153.9, 157.9, 170.0, 170.2, 171.2, 173.9, 174.1,
175.5. Anal. Calcd for C29H40N6O15 (712.26): C, 48.87; H, 5.66; N,
11.79. Found: C, 48.91; H, 5.71; N, 11.79.
Boc-(L-Ala-L-Oxd)4-OBn (9a). For the synthetic procedure from 8a
and 4a see the above preparation of 5a: 78% yield; mp 210 °C dec;
[R]D -127.5 (c 1.0, MeOH); IR (CH2Cl2, 2 mM) ν ) 3414, 1798,
Boc-(L-Ala-D-Oxd)2-OH (6b). For the synthetic procedure from 5b
see the above preparation of 3a: quantitative yield; mp 138 °C dec;
[R]D -16.0 (c 1.0, MeOH); IR (Nujol) ν ) 3548, 3361, 1788, 1674
cm-1; 1H NMR (CD3OD, 200 MHz) δ 1.35 (d, 3H, J ) 6.8 Hz), 1.42-
1.46 (m, 12H), 1.52 (d, 3H, J ) 5.8 Hz), 1.55 (d, 3H, J ) 6.0 Hz),
4.48-4.78 (m, 4H), 5.30 (q, 1H, J ) 7.4 Hz), 5.55 (q, 1H, J ) 6.8
Hz); 13C NMR (CD3OD, 50 MHz) δ 17.7, 18.0, 20.8, 21.2, 28.7, 49.9,
50.3, 63.1, 64.6, 76.1, 76.5, 80.7, 153.4, 153.7, 157.5, 169.7, 171.2,
173.6, 175.2. Anal. Calcd for C21H30N4O11 (514.19): C, 49.02; H, 5.88;
N, 10.89. Found: C, 49.11; H, 5.83; N, 10.78.
Boc-(L-Ala-D-Oxd)3-OBn (7b). For the synthetic procedure from
6b and 4b see the above preparation of 5a: 82% yield; mp 128-131
°C; [R]D -25.9 (c 0.7, CH2Cl2); IR (CH2Cl2, 3 mM) ν ) 3440, 3401,
3390, 1793, 1721, 1692 cm-1; 1H NMR (CDCl3, 400 MHz) δ 1.35 (d,
3H, J ) 6.8 Hz), 1.40 (s, 9H), 1.42-1.55 (m, 15H), 4.45 (d, 1H, J )
4.4 Hz), 4.48 (d, 1H, J ) 3.6 Hz), 4.61-4.74 (m, 4H), 5.17-5.46 (m,
5H), 5.48-5.58 (m, 1H), 7.28-7.39 (m, 5H), 7.47 (br s, 1H), 7.51 (br
s, 1H); 13C NMR (CDCl3, 75 MHz) δ 15.8, 16.4, 17.1, 20.5, 20.6,
20.8, 28.2, 48.6, 48.8, 49.4, 61.6, 62.4, 62.6, 68.5, 74.2, 75.1, 75.3,
80.5, 128.4, 128.5, 128.7, 134.6, 151.6, 151.9, 155.8, 168.2, 168.4,
172.5, 173.0, 174.6. Anal. Calcd for C36H46N6O15 (802.30): C, 53.86;
H, 5.78; N, 10.47. Found: C, 53.88; H, 5.76; N, 10.45.
1
1771, 1718, 1686, 1607 cm-1; H NMR (CD3OD, 400 MHz) δ 1.38
(d, 3H, J ) 7.2 Hz), 1.41 (d, 3H, J ) 6.8 Hz), 1.45-1.50 (m, 15H),
1.56 (d, 3H, J ) 6.8 Hz), 1.59 (d, 9H, J ) 6.0 Hz), 4.55-4.62 (m,
6H), 4.69 (d, 1H, J ) 4.8 Hz), 4.77 (dq, 1H, J ) 4.8, 6.4 Hz), 5.25
(AB, 2H, J ) 12.4 Hz), 5.22-5.28 (m, 1H), 5.44 (q, 2H, J ) 7.2 Hz),
5.47 (q, 1H, J ) 6.8 Hz), 7.35-7.41 (m, 5H); 13C NMR (CD3OD, 100
MHz) δ 16.6, 16.7, 17.3 (two signals), 20.6 (three signals), 21.0, 28.6,
49.5, 49.6, 49.9, 50.1, 62.8, 63.1, 63.2, 63.5, 69.0, 74.8, 75.5, 76.1,
76.3, 80.7, 129.4, 129.5, 129.6, 136.4, 153.2, 153.7, 157.8, 157.9, 169.3,
169.9, 170.0, 170.2, 173.9, 174.0, 174.1, 175.5. Anal. Calcd for
C44H56N8O19 (1000.37): C, 52.80; H, 5.64; N, 11.19. Found: C, 52.89;
H, 5.57; N, 11.21.
Boc-(L-Ala-L-Oxd)4-OH (10a). For the synthetic procedure from
9a see the above preparation of 3a: quantitative yield; mp 188-192
°C dec; [R]D -118.0 (c 1.0, MeOH); IR (Nujol) ν ) 3412, 1792, 1769,
1
1721, 1685 cm-1; H NMR (CD3OD, 400 MHz) δ 1.38 (d, 3H, J )
6.8 Hz), 1.47 (s, 9H), 1.49 (d, 9H, J ) 7.2 Hz), 1.57-1.61 (m, 12H),
4.55 (d, 1H, J ) 4.0 Hz), 4.56-4.63 (m, 6H), 4.76 (dq, 1H, J ) 4.4,
6.4 Hz), 5.25 (q, 1H, J ) 7.2 Hz), 5.41-5.52 (m, 3H); 13C NMR (CD3-
OD, 100 MHz) δ 16.5, 16.6, 16.7, 17.3, 20.6 (three signals), 21.2, 28.6,
49.4, 49.6, 49.7, 50.1, 63.1 (two signals), 63.2, 63.3, 76.1 (two signals),
76.3 (two signals), 80.7, 153.6, 153.8, 153.9, 157.9, 170.0, 170.1, 170.2,
171.5, 173.9, 174.0, 174.1, 175.5. Anal. Calcd for C37H50N8O19
(910.32): C, 48.79; H, 5.53; N, 12.30. Found: C, 48.75; H, 5.57; N,
12.33.
Boc-L-Ala-D-Oxd-OBn (2b). For the synthetic procedure from 1a
see the above preparation of 2a: 90% yield; mp 105-107 °C; [R]D
-1.5 (c 1.0, CH2Cl2); IR (CH2Cl2, 3 mM) ν ) 3429, 1795, 1752, 1704
cm-1; 1H NMR (CDCl3, 200 MHz) δ 1.45 (m, 12H), 1.54 (d, 3H, J )
6.6 Hz), 4.46 (d, H, J ) 4.4 Hz), 4.58 (dq, H, J ) 4.4, 6.6 Hz), 5.23
(s, 2H), 5.30 (br s, 1H), 5.43 (dq, 1H, J ) 6.2, 7.8 Hz), 7.27-7.40 (m,
H); 13C NMR (CDCl3, 75 MHz) δ 18.1, 21.1, 28.2, 49.2, 61.9, 67.9,
73.7, 79.6, 128.3, 128.7, 134.6, 151.1, 154.6, 167.4, 173.9. Anal. Calcd
for C20H26N2O7 (406.43): C, 59.10; H, 6.45; N, 6.89. Found: C, 59.07;
H, 6.48; N, 6.88.
Boc-(L-Ala-D-Oxd)3-OH (8b). For the synthetic procedure from 7b
see the above preparation of 3a: quantitative yield; mp 177 °C dec;
[R]D -9.3 (c 1.0, MeOH); IR (Nujol) ν ) 3579, 3411, 3358, 1780,
1
1721, 1676 cm-1; H NMR (CD3OD, 600 MHz) δ 1.22 (d, 3H, J )
6.0 Hz), 1.26-1.32 (m, 15H), 1.38 (d, 3H, J ) 7.2 Hz), 1.40 (d, 3H,
J ) 7.2 Hz), 1.42 (d, 3H, J ) 6.0 Hz), 4.32-4.35 (m, 1H), 4.38-4.40
(m, 2H), 4.50-4.63 (m, 3H), 5.14-5.17 (m, 1H), 5.24-5.27 (m, 1H),
5.42-5.45 (m, 1H); 13C NMR (CD3OD 50 MHz) δ 17.0, 17.8, 18.2,
20.9, 21.0, 21.2, 28.8, 49.5, 50.2, 50.4, 63.3, 64.4, 64.5, 76.2, 76.7,
76.8, 80.9, 153.5, 153.7, 157.5, 169.8, 170.2, 171.5, 173.9, 174.0, 175.4,
187.7. Anal. Calcd for C29H40N6O15 (712.26): C, 48.87; H, 5.66; N,
11.79. Found: C, 48.90; H, 5.64; N, 11.74.
Boc-(L-Ala-D-Oxd)4-OBn (9b). For the synthetic procedure from
8b and 4b see the above preparation of 5a: 83% yield; mp 120 °C;
[R]D -51.7 (c 1.0, CH2Cl2); IR (CH2Cl2, 3 mM) ν ) 3441, 3350, 3287,
1
1788, 1742, 1708, 1672 cm-1; H NMR (CDCl3, 400 MHz) δ 1.37-
Boc-L-Ala-D-Oxd-OH (3b). For the synthetic procedure from 2b
see the above preparation of 3a: quantitative yield; mp 56-58 °C;
[R]D -3.1 (c 1.0, CH2Cl2); IR (CH2Cl2, 3 mM) ν ) 3436, 1799, 1747,
1713 cm-1; 1H NMR (CDCl3, 200 MHz) δ 1.32-1.45 (m, 12H), 1.56
1.59 (m, 33H), 4.42-4.75 (m, 5H), 4.91-5.02 (m, 3H), 5.18-5.30
(m, 3H), 5.35-5.58 (m, 4H), 5.62-5.90 (m, 2H), 5.96-6.08 (m, 1H),
7.30-7.42 (m, 6H); 13C NMR (CDCl3, 75 MHz) δ 19.3, 21.1, 21.2,
21.3, 28.5, 49.2 (four signals), 62.1, 62.8, 74.5, 75.2, 75.5, 82.3, 128.8,
9
2418 J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006

Oxazolidin-2-one Pseudopeptides That Fold into Spirals
A R T I C L E S
129.1, 129.3, 135.1, 151.2, 152.2, 167.2, 168.3, 169.2, 173.3, 174.0.
Anal. Calcd for C44H56N8O19 (1000.37): C, 52.80; H, 5.64; N, 11.19.
Found: C, 52.77; H, 5.62; N, 11.22.
The mixture was washed with brine, 1 N aqueous HCl (3 × 30 mL),
and 5% aqueous NaHCO3 (1 × 30 mL), dried over sodium sulfate,
and concentrated in vacuo. Product 2c was obtained pure in 60% yield
(0.63 g) as a waxy solid after silica gel chromatography (cyclohexane/
ethyl acetate, 9:1, as eluant): mp 114-115 °C; [R]D -54.4 (c 1.0, CH2-
Cl2); IR (CH2Cl2, 3 mM) ν ) 3448, 1793, 1754, 1712 cm-1; 1H NMR
(CDCl3, 300 MHz) δ 1.42 (s, 9H, t-Bu), 1.47 (d, 3H, J ) 6.0 Hz,
CH3-Oxd), 1.56 (s, 3H, CH3-Aib), 1.60 (s, 3H, CH3-Aib), 4.48-4.60
(m, 2H, CHN + CHO), 5.22 (AB, 2H, J ) 12.1 Hz, OCH2Ph), 5.44
(s, 1H, NH), 7.23-7.40 (m, 5H, Ph); 13C NMR (CDCl3, 100 MHz) δ
21.1, 23.9, 26.6, 28.5, 57.5, 63.4, 68.1, 73.6, 80.1, 128.6, 128.9, 134.9,
151.2, 155.1, 168.7, 174.9. Anal. Calcd for C21H28N2O7 (420.46): C,
59.99; H, 6.71; N, 6.66. Found: C, 59.97; H, 6.74; N, 6.65.
Boc-(L-Ala-D-Oxd)4-OH (10b). For the synthetic procedure from
9b see the above preparation of 3a: quantitative yield; mp 174 °C
dec; [R]D -18.9 (c 0.8, MeOH); IR (Nujol) ν ) 3403, 3269, 1789,
1669 cm-1; 1H NMR (CD3OD, 400 MHz) δ 1.34 (d, 3H, J ) 6.8 Hz),
1.40 (d, 3H, J ) 7.2 Hz), 1.41-1.45 (m, 15H), 1.49 (d, 3H, J ) 6.4
Hz), 1.50 (d, 3H, J ) 6.4 Hz), 1.51 (d, 3H, J ) 7.6 Hz), 1.52 (d, 3H,
J ) 6.8 Hz), 4.61-4.78 (m, 8H), 5.30 (q, 1H, J ) 6.8 Hz), 5.42 (q,
1H, J ) 6.8 Hz), 5.48-5.56 (m, 1H), 5.60-5.72 (m, 1H); 13C NMR
(CD3OD, 100 MHz) δ 16.4, 16.5, 17.2, 17.4, 19.6, 19.7, 19.8, 20.1,
27.6, 48.6, 48.8, 48.9, 49.1, 61.8, 62.6, 62.8, 63.1, 75.0, 75.2, 75.3,
75.4, 79.4, 152.2, 152.3, 152.4, 152.6, 163.7, 168.7, 168.8, 172.1, 172.3.
Anal. Calcd for C37H50N8O19 (910.32): C, 48.79; H, 5.53; N, 12.30.
Found: C, 48.70; H, 5.61; N, 12.25.
Boc-(L-Ala-D-Oxd)5-OBn (11b). For the synthetic procedure from
10b and 4b see the above preparation of 5a: 78% yield; mp 210 °C;
[R]D -73.2 (c 1.0, CH2Cl2); IR (CH2Cl2, 3 mM) ν ) 3354, 3296, 1790,
1741, 1707, 1669 cm-1; 1H NMR (CDCl3, 400 MHz) δ 1.35-1.62 (m,
39H), 4.41-4.88 (m, 7H), 4.91-5.08 (m, 3H), 5.18-5.56 (m, 5H),
5.83 (dq, 1H, J ) 6.2 Hz), 5.93 (dq, 1H, J ) 6.0 Hz), 6.07 (dq, 1H, J
) 6.0 Hz), 7.30-7.42 (m, 5H), 7.85-8.35 (m, 4H); 13C NMR (CDCl3,
75 MHz) δ 18.0, 18.1, 18.3, 19.1, 19.5, 20.1, 20.6, 20.7, 20.9, 21.3,
28.4, 48.9 (four signals), 74.9, 78.7, 128.3, 128.9, 129.2, 134.3, 150.4,
150.9, 151.3, 151.8, 155.0, 167.4, 168.1, 168.5, 168.7, 170.0, 171.3,
171.5, 172.1, 172.4. Anal. Calcd for C52H66N10O23 (1198.43): C, 52.08;
H, 5.55; N, 11.68. Found: C, 52.11; H, 5.57; N, 11.70.
Boc-Aib-L-Oxd-OH (3c). For the synthetic procedure from 2c see
the above preparation of 3a: quantitative yield; mp 204-205 °C; [R]D
-77.1 (c 1.0, acetone); IR (CH2Cl2, 3 mM) ν ) 3445, 1792, 1745,
1
1706, 1673 cm-1; H NMR (CDCl3, 200 MHz) δ 1.42 (s, 9H, t-Bu),
1.51 (d, 3H, J ) 6.2 Hz, CH3-Oxd), 1.56 (s, 3H, CH3-Aib), 1.62 (s,
3H, CH3-Aib), 4.47 (d, 1H, J ) 2.2 Hz, CHN-Oxd), 4.77 (dq, 1H, J )
2.2, 6.2 Hz, CHO), 6.30 (s, 1H, NH); 13C NMR (acetone-d6, 50 MHz)
δ 19.6, 22.4, 25.3, 27.0, 56.4, 62.2, 72.8, 77.9, 150.0, 153.9, 168.8,
173.6. Anal. Calcd for C14H22N2O7 (330.33): C, 50.90; H, 6.71; N,
8.48. Found: C, 50.86; H, 6.73; N, 4.51.
H-Aib-L-Oxd-OBn (4c). A solution of 2a (1 mmol, 0.42 g) and
TFA (18 mmol, 1.39 mL) in dry methylene chloride (10 mL) was stirred
for 4 h at room temperature, and then the volatiles were removed under
reduced pressure and replaced with pure methylene chloride (20 mL).
The mixture was washed with brine and with 5% aqueous NaHCO3 (1
× 30 mL), dried over sodium sulfate, and concentrated in vacuo. The
product was obtained pure in 95% yield (0.30 g) without any further
purification: [R]D +3.0 (c 0.8, CH2Cl2); IR (CH2Cl2, 3 mM) ν ) 3444,
1789, 1754, 1711 cm-1; 1H NMR (CDCl3, 300 MHz) δ 1.21 (d, 3H, J
) 6.0 Hz, CH3-Oxd), 1.47 (s, 6H), 4.62 (dq, 1H, J ) 4.6, 6.0 Hz),
4.82 (d, 1H, J ) 4.6 Hz), 5.23 (s, 2H), 7.23-7.40 (m, 5H); 13C NMR
(CDCl3, 75 MHz) δ 20.1, 24.8, 30.8, 59.2, 59.8, 66.3, 67.8, 72.8, 128.5,
128.8, 129.0, 134.2, 152.8, 167.1, 178.4. Anal. Calcd for C16H20N2O5
(320.34): C, 59.99; H, 6.29; N, 8.74. Found: C, 60.01; H, 6.31; N,
8.76.
Boc-(L-Ala-D-Oxd)5-OH (12b). For the synthetic procedure from
11b see the above preparation of 3a: quantitative yield; mp 180 °C
dec; [R]D -56.4 (c 1.0, MeOH); IR (Nujol) ν ) 3410, 1779, 1686,
1627 cm-1; 1H NMR (CD3OD, 300 MHz) δ 1.43-1.48 (m, 24H), 1.51-
1.58 (m, 15H), 4.47-5.15 (m, 10H), 5.35-5.45 (m, 2H), 5.85-6.15
(m, 3H); 13C NMR (CD3OD, 75 MHz) δ 18.1, 18.9, 20.3, 21.2, 21.3,
21.4, 21.5, 21.6, 29.2, 50.4, 50.6, 50.7, 63.2, 63.8 (four signals), 76.5,
76.7, 80.6, 153.6, 154.1, 170.2, 170.4, 170.5, 170.6, 172.9, 173.0, 174.3.
Anal. Calcd for C45H60N10O23 (1108.38): C, 48.74; H, 5.45; N, 12.63.
Found: C, 48.81; H, 5.49; N, 12.59.
Boc-(L-Ala-D-Oxd)6-OBn (13b). For the synthetic procedure from
12b and 4b see the above preparation of 5a: 70% yield; mp 185 °C
dec; [R]D -101.0 (c 0.1, CHCl3); IR (CH2Cl2, 3 mM) ν ) 3352, 3295,
Boc-(Aib-L-Oxd)2-OBn (5c). To a stirred solution of 3c (1 mmol,
0.33 g) and HATU (1 mmol, 0.38 g) in dry acetonitrile (10 mL) under
an inert atmosphere was added a mixture of 4c (1 mmol, 0.32 g) and
Et3N (2 mmol, 0.29 mL) in dry acetonitrile (10 mL) at room
temperature. The solution was stirred for 40 min under an inert
atmosphere, and then acetonitrile was removed under reduced pressure
and was replaced with ethyl acetate. The mixture was washed with
brine, 1 N aqueous HCl (3 × 30 mL), and 5% aqueous NaHCO3 (1 ×
30 mL), dried over sodium sulfate, and concentrated in vacuo. The
product was obtained pure after silica gel chromatography (cyclohexane/
ethyl acetate, 8:2, as eluant) in 55% yield (0.35 g): mp 136-138 °C;
[R]D -63.2 (c 1.0, CH2Cl2); IR (CH2Cl2, 3 mM) ν ) 3446, 3375, 1789,
1726, 1710 cm-1; 1H NMR (CDCl3, 600 MHz) δ 1.27 (d, 3H, J ) 6.2
Hz), 1.45 (s, 9H), 1.54 (d, 3H, J ) 6.2 Hz), 1.63 (s, 3H), 1.67 (s, 6H),
1.72 (s, 3H), 3.99 (dq, 1H, J ) 1.4, 6.2 Hz), 4.38-4.45 (m, 1H), 4.63
(d, 1H, J ) 5.4 Hz), 4.92 (AB, 2H, J ) 12.2 Hz), 5.48 (s, 1H), 5.51
(d, 1H, J ) 1.4 Hz), 7.01-7.31 (m, 6 H); 13C NMR (CDCl3, 100 MHz)
δ 20.2, 20.3, 21.4, 22.8, 23.7, 26.4, 28.3, 57.2, 60.0, 64.5, 64.8, 65.7,
73.8, 79.8, 128.4, 128.7, 128.8, 134.6, 151.3, 154.0, 154.8, 166.4, 167.1,
175.0, 175.6. Anal. Calcd for C30H40N4O11 (632.66): C, 56.95; H, 6.37;
N, 8.86. Found: C, 56.92; H, 6.40; N, 8.83.
1
1787, 1739, 1706, 1667 cm-1; H NMR (CDCl3, 400 MHz) δ 1.39-
1.49 (m, 18H), 1.49-1.62 (m, 27H), 4.46-4.59 (m, 7H), 4.73-4.78
(m, 1H), 4.95-4.98 (m, 1H), 5.00-5.03 (m, 1H), 5.05-5.09 (m, 1H),
5.24 (br s, 3H), 5.24-5.28 (m, 1H), 5.60-5.67 (m, 2H), 5.75-5.98
(m, 2H), 5.98-6.08 (m, 1H), 7.32-7.41 (m, 5H), 7.94 (br s, 1H), 8.15
(br s, 1H), 8.31 (br s, 1H), 8.47 (br s, 1H), 8.53 (br s, 1H); 13C NMR
(CD3CN, 100 MHz) δ 17.5, 17.8, 20.8, 21.0, 21.1, 28.5, 48.9, 49.4,
49.8, 62.1, 62.6, 63.3, 68.6, 75.2, 76.0, 76.2, 77.6, 129.2, 129.5, 129.6,
136.3, 153.0, 169.1, 169.6, 170.7, 173.6. Anal. Calcd for C60H76N12O27
(1396.50): C, 51.57; H, 5.48; N, 12.03. Found: C, 51.61; H, 5.46; N,
12.08.
Boc-(L-Ala-D-Oxd)6-OH (14b). For the synthetic procedure from
13b see the above preparation of 3a: quantitative yield; mp 230 °C
dec; [R]D -118.7 (c 0.4, CH2Cl2); IR (Nujol) ν ) 3280, 1789, 1709,
1660 cm-1; 1H NMR (CD3CN, 600 MHz) δ 1.38-1.59 (m, 45H), 4.37-
4.62 (m, 6H), 4.64-5.26 (m, 7H), 5.28-6.18 (m, 6H), 7.09 (br s, 1H),
8.05-8.58 (br s, 4H); 13C NMR (CD3OD, 100 MHz) δ 16.7, 17.6,
17.8, 19.9, 20.1, 20.3, 28.0, 49.0, 61.4, 62.1, 75.2, 76.8, 152.1, 168.9,
171.4, 172.5. Anal. Calcd for C53H70N12O27 (1306.45): C, 48.70; H,
5.40; N, 12.86. Found: C, 48.63; H, 5.41; N, 12.89.
Boc-(Aib-L-Oxd)2-OH (6c). For the synthetic procedure from 5c
see the above preparation of 3a: quantitative yield; mp 204-206 °C;
[R]D -115.5 (c 1.0, CH2Cl2); IR (CH2Cl2, 3 × 10-3 M) ν ) 3443,
1792, 1706 cm-1; 1H NMR (CDCl3, 200 MHz) δ 1.27 (d, 3H), 1.43 (s,
9H), 1.55 (d, 3H, J ) 6.2 Hz), 1.60 (s, 3H), 1.65 (s, 3H), 1.70 (s, 3H),
1.73 (s, 3H), 4.56 (dq, 1H, J ) 1.4, 6.2 Hz), 4.60 (dq, 1H, J ) 5.0, 6.2
Hz), 4.80 (d, 1H, J ) 5.0 Hz), 5.41 (d, 1H, J ) 1.4 Hz), 5.59 (s, 1H),
Boc-Aib-L-Oxd-OBn (2c). To a stirred solution of Boc-Aib-OH
(0.51 g, 2.5 mmol) in acetonitrile (20 mL) were added HBTU (0.98 g,
2.6 mmol), then 1a (0.59 g, 2.5 mmol), and finally DBU (0.75 mL, 5
mol). The mixture was stirred for 30 min, and then acetonitrile was
removed under reduced pressure and was replaced with ethyl acetate.
9
J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006 2419

A R T I C L E S
Tomasini et al.
6.58 (br s, 2H); 13C NMR (CDCl3, 50 MHz) δ 20.3, 20.5, 21.7, 23.0,
24.0, 26.5, 28.5, 57.4, 59.8, 64.7, 65.0, 65.8, 74.2, 76.5, 77.2, 77.8,
80.5, 151.4, 154.1, 155.0, 167.1, 168.5, 174.9, 175.7. Anal. Calcd for
C23H34N4O11 (542.54): C, 50.92; H, 6.32; N, 10.33. Found: C, 50.88;
H, 6.30; N, 10.38.
equipped with a CCD detector using Mo KR radiation. Cell dimensions
and the orientation matrix were initially determined from a least-squares
refinement on reflections measured in 3 sets of 20 exposures, collected
in 3 different ω regions, and eventually refined against all data. A full
sphere of reciprocal space was scanned by 0.3° ω steps, with the
detector kept at 5.0 cm from the sample. The software SMART21 was
used for collecting frames of data, indexing reflections, and determi-
nation of lattice parameters. The collected frames were then processed
for integration by the SAINT program,22 and an empirical absorption
correction was applied using SADABS.23 The structure was solved by
direct methods (SIR 9724) and subsequent Fourier synthesis, and refined
by full-matrix least squares on F2 (SHELXTL25), using anisotropic
thermal parameters for all non-hydrogen atoms. All hydrogen atoms,
except the amidic hydrogen which was located in the Fourier map and
refined isotropically, were added in calculated positions, included in
the final stage of refinement with isotropic thermal parameters, U(H)
) 1.2Ueq(C) [U(H) ) 1.5Ueq(C-Me)], and allowed to ride on their carrier
carbons. Crystallographic parameters for 2c are reported in the
Supporting Information.
Boc-Aib-L-Oxd-Aib-D-Oxd-OBn (7c). To a stirred solution of 3c
(1 mmol, 0.33 g) and HATU (1 mmol, 0.38 g) in dry acetonitrile (10
mL) under an inert atmosphere was added a mixture of H-Aib-D-Oxd-
OBn (1 mmol, 0.32 g) and Et3N (2 mmol, 0.29 mL) in dry acetonitrile
(10 mL) at room temperature. The solution was stirred for 40 min under
an inert atmosphere, and then acetonitrile was removed under reduced
pressure and was replaced with ethyl acetate. The mixture was washed
with brine, 1 N aqueous HCl (3 × 30 mL), and 5% aqueous NaHCO3
(1 × 30 mL), dried over sodium sulfate, and concentrated in vacuo.
The product was obtained pure after silica gel chromatography
(cyclohexane/ethyl acetate, 8:2, as eluant) in 53% yield (0.34 g) as a
waxy solid: [R]D -63.2 (c 1.0, CH2Cl2); IR (CH2Cl2, 1 × 10-3 M) ν
) 3437, 1787, 1728 cm-1; 1H NMR (CDCl3, 300 MHz) δ 1.24 (d, 3H,
J ) 6.9 Hz), 1.43 (s, 9H), 1.53 (d, 3H, J ) 6.3 Hz), 1.62 (s, 3H), 1.65
(s, 3H), 1.68 (s, 6H), 4.50 (dq, 1H, J ) 1.5, 6.3 Hz), 4.60-4.65 (m,
1H), 4.77 (d, 1H, J ) 4.5 Hz), 5.20 (d, 1H, J ) 12.0 Hz), 5.26 (d, 1H,
J ) 12.0 Hz), 5.33 (d, 1H, J ) 1.5 Hz), 5.52 (br s, 1H), 7.22-7.40
(m, 6H); 13C NMR (CDCl3, 100 MHz) δ 20.1, 20.2, 21.5, 22.7, 23.7,
26.4, 28.3, 57.2, 60.3, 64.4, 65.9, 68.3, 73.8, 79.8, 128.5, 128.7, 128.8,
134.5, 151.3, 153.5, 154.9, 166.5, 167.2, 175.1, 176.1. Anal. Calcd for
C30H40N4O11 (632.66): C, 56.95; H, 6.37; N, 8.86. Found: C, 56.98;
H, 6.41; N, 8.83.
Boc-Aib-L-Oxd-Aib-D-Oxd-OH (8c). For the synthetic procedure
from 7c see the above preparation of 3a: quantitative yield; mp 147
°C dec; [R]D -67.0 (c 0.2, MeOH); IR (Nujol) ν ) 33373, 3173, 1794,
1732, 1705 cm-1; 1H NMR (CD3OD, 400 MHz) δ 1.40-1.44 (m, 9H),
1.51 (s, 3H), 1.53 (d, 3H, J ) 6.4 Hz), 1.60 (s, 3H), 1.64 (s, 3H), 1.68
(s, 3H), 4.46 (dq, 1H, J ) 1.6, 7.2 Hz), 4.55 (d, 1H, J ) 7.2 Hz), 4.72
(dq, 1H, J ) 1.6, 6.0 Hz), 5.41 (d, 1H, J ) 1.6 Hz); 13C NMR (CDCl3,
100 MHz) δ 20.7, 21.8, 22.2, 22.8, 23.7, 27.0, 27.8, 28.8, 54.6, 58.2,
60.2, 65.0, 65.2, 65.8, 75.1, 152.7, 154.7, 168.6, 169.7, 176.3, 176.4.
Anal. Calcd for C23H34N4O11 (542.54): C, 50.92; H, 6.32; N, 10.33.
Found: C, 50.98; H, 6.38; N, 10.27.
Acknowledgment. We thank MIUR (Grant PRIN 2004),
CNR-ISOF, and the University of Bologna (Funds for Selected
Topics) for financial support.
Supporting Information Available: Crystallographic data and
structure refinement parameters for 2c and a dilution series of
1H NMR spectra of 11b in CDCl3. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA056762H
(22) SMART & SAINT Software Reference Manuals, version 5.051 (Windows
NT Version); Bruker Analytical X-ray Instruments Inc.: Madison, WI,
1998.
(23) Sheldrick, G. M. SADABS, program for empirical absorption correction;
University of Go¨ttingen: Germany, 1996.
(24) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, C.;
Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl.
Crystallogr. 1999, 32, 115.
(25) Sheldrick, G. M. SHELXTLplus (Windows NT Version) Structure Deter-
mination Package, Version 5.1; Bruker Analytical X-ray Instruments Inc.:
Madison, WI, 1998.
X-ray Structure Determination. The X-ray intensity data for 2c
were measured on a Bruker AXS SMART 2000 diffractometer,
9
2420 J. AM. CHEM. SOC. VOL. 128, NO. 7, 2006