Synthesis of enantiopure pyrrolidine-derived peptidomimetics and oligo-β-peptides via nucleophilic ring-opening of β-lactams

Article abstract of DOI:10.1021/jo061189l

(Chemical Equation Presented) The synthesis of the two enantiomers of pyrrolidine-derived spiro β-lactams by resolution with D- and L-Boc phenylalanine is described. The potential of these optically active spiro β-lactams on the synthesis of peptidomimetics as analogues of melanostatin is evaluated. Theoretical studies of several models, at the Becke3LYP/6-31+G* level of theory, together with previous experimental evidences from our group, gathered by NMR, allow us to design structures that can efficiently mimic some biologically active peptide-type molecules. On the other hand, the spiro β-lactams have shown their utility in the preparation of β-peptides. As an example, a homo-tetra-β-peptide was synthesized. This research will continue in the future in order to obtain higher peptides with potential biological activity.

Full text of DOI:10.1021/jo061189l

Synthesis of Enantiopure Pyrrolidine-Derived Peptidomimetics and
Oligo-â-peptides via Nucleophilic Ring-Opening of â-Lactams†
Alberto Mac´ıas,‡ Antonio Mora´n Ramallal,‡ Eduardo Alonso,‡ Carlos del Pozo,*,§ and
Javier Gonza´lez*,‡
Departamento de Qu´ımica Orga´nica e Inorga´nica, UniVersidad de OViedo, Julia´n ClaVer´ıa 8,
33006 OViedo, Spain, and Departamento de Qu´ımica Orga´nica, UniVersidad de Valencia,
46100-Burjassot, Valencia, Spain
fjgf@unioVi.es; carlos.pozo@uV.es
ReceiVed June 9, 2006
The synthesis of the two enantiomers of pyrrolidine-derived spiro â-lactams by resolution with D- and
L-Boc phenylalanine is described. The potential of these optically active spiro â-lactams on the synthesis
of peptidomimetics as analogues of melanostatin is evaluated. Theoretical studies of several models, at
the Becke3LYP/6-31+G* level of theory, together with previous experimental evidences from our group,
gathered by NMR, allow us to design structures that can efficiently mimic some biologically active
peptide-type molecules. On the other hand, the spiro â-lactams have shown their utility in the preparation
of â-peptides. As an example, a homo-tetra-â-peptide was synthesized. This research will continue in
the future in order to obtain higher peptides with potential biological activity.
Introduction
The field of peptidomimetics is experiencing considerable
development because of their potential as precursors for small,
but efficient biologically important compounds that could lead
to new therapeutically useful drugs. These compounds act by
mimicking the activity of some biologically relevant peptides
being, however, resistant to the hydrolytically activity of
proteases. In addition, an important feature of peptidomimetics
is the introduction of conformational restrictions that still allow
them to be recognized by the potential acceptors interacting with
them. There are a number of possible approaches for achieving
this goal, but the introduction of cycles that bridge different
parts of the molecule is one of the most popular alternatives.
This strategy was pioneered by Freidinger1 and followed by a
number of practitioners in this field.2 We have shown previ-
FIGURE 1. Spiro â-lactam forming a type II â-turn 1, and melano-
statin 2.
ously3 that the introduction of a combination of a four-membered
ring (such as a â-lactam) with a five-membered ring (such as
pyrrolidine-derived ring) could produce a quite stable structure,
able to form a type II â-turn (1, Figure 1).2c,4
The detailed analysis by NMR and the density-functional
calculations indicate that 1 adopts in solution a type II â-turn
conformation. In view of the potential interest of these types of
compounds, we decided to further explore other possible spiro
â-lactam-containing structures.
† Dedicated to Prof. Dr. V´ıctor M. Riera Gonza´lez on the occasion of his
retirement from the Universidad de Oviedo.
‡ Universidad de Oviedo.
§ Universidad de Valencia.
(1) Freidinger, R. M.; Veber, D. F.; Perlow, D. S.; Brooks, J. R.;
Saperstein, R. Science 1980, 210, 656.
(2) For a review, see: (a) Gante, J. Angew. Chem., Int. Ed. Engl. 1994,
33, 1699. (b) Giannis, A.; Kolter. T. Angew. Chem., Int. Ed. Engl. 1993,
32, 1244. (c) Robinson, J. A. Synlett. 1999, 4, 429.
(3) Alonso, A.; Lo´pez-Ortiz, F.; Pozo, del C.; Peralta, E.; Mac´ıas, A.
Gonza´lez, J. J. Org. Chem. 2001, 66, 6333.
(4) Muller, G.; Hessler, G.; Decornez, H. Y. Chem. Int. Ed. Engl. 2000,
39, 894.
10.1021/jo061189l CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/07/2006
J. Org. Chem. 2006, 71, 7721-7730
7721

Mac´ıas et al.
FIGURE 2. Optically active â-lactams 3 and â-peptides 4.
FIGURE 3. Spiro â-lactams 5-8 studied theoretically at the
Becke3LYP/6-31+G* level of theory.
More specifically, we decided to undertake the preparation
of an optically active analogue of the neuropeptide melanostatin
(PLG) 2 (see Figure 1), which has a related structure with the
peptidomimetic 1 following the methodology we described
previously, but making the necessary modifications that will
allow us to obtain the C4-unsubstituted system.5 The importance
of the preparation of these analogues lies in the fact that they
could serve as starting structures for the development of new
types of compounds with the ability to modulate the activity of
the dopamine receptor.6 The essential step relies on the synthesis
of optically active â-lactams (+)-3 and (-)-3 (Figure 2).
On the other hand, we wish to demonstrate the utility of the
optically active â-lactams 3 on the preparation of â-peptides 4
(Figure 2) bearing a functionalized five-membered ring in its
structure. Analogues of these compounds have shown to form
secondary structures with a helix-type conformation and could
have interesting biological properties.7
TABLE 1. Hydrogen Bond Lengths (Å), Energetic Preferences
(kcal mol-1), and Torsional Angles (deg) in Spiro â-Lactams and
Related Compounds
d(CdO‚‚‚HN) ∆E(anti-syn)
φi+1
ψi+1
φi+2
ψi+2
0.0
-8.5
ideala
2-syn
5-syn
6-syn
7-syn
8-syn
-
2.030
-
1.2
-60
-69
-3.6 127.9
-38.3 123.0 101.5 -10.4
-50.6 123.6 104.5 -13.6
-57.8 122.9 108.6 -13.6
120
113
80
109
90.4 -22.7
2.346
2.067
2.014
2.019
2.3
4.6
4.7
2.1
a See ref 9.
Results and Discussion
Molecular Modeling Studies. Before starting the synthesis
of analogues of 2, we carried out a theoretical study, at the
Becke3LYP/6-31+G* level of theory, of the different structures
in which we were interested (5-8, Figure 3), to test the
possibility that these structures could be better candidates than
the system containing the spiro 5-4 structure, previously studied
in our group.3
Information (SI) for the energies and Cartesian coordinates).
The stationary points located were characterized as minima, by
performing the corresponding frequency calculations. All the
calculations were carried out with the Gaussian 98 suite of
programs.8 The optimized structures (showing some selected
geometrical parameters) are shown in Figure 4.
As can be seen, the syn conformations form a intramolecular
hydrogen bond, which determine the formation of a â-turn.9
Attending to the length of this hydrogen bond, the most
favorable situation is in the case of melanostatin (2-syn), 7-syn,
and 8-syn analogues. A summary of the results obtained are
collected in Table 1.
As can be seen, the spiro system containing the pyrrolidine
ring 7 is apparently the most favored, both in terms of the
hydrogen bond length and the energetic preference for the syn
conformation, having a â-turn. In addition, the analysis of the
dihedral angles (see the figure in Table 1 for the definition of
the dihedral angles) indicates that compounds 7-syn and 8-syn
are close to the ideal case for a type II â-turn, while 5-syn and
6-syn are far from the ideal values, especially for the parameter
φi+1. It seems that the introduction of three- and four-membered
rings in the bicyclic system significantly alters the molecular
geometry with respect to the ideal cases.
The geometries of the structures 2, 5-8 were fully optimized
at the Becke3LYP/6-31+G* level of theory (see Supporting
(5) The synthesis of the C4 unsubstituted â-lactams was accomplished
using formaldehyde-derived imines, as described previously. See: (a)
Kamiya, T.; Oku, T.; Nakaguchi, O.; Takeno, H.; Hashimoto, M. Tetra-
hedron Lett. 1978, 51, 5119. (b) Nakaguchi, O.; Oku, T.; Takeno, H.;
Hashimoto, M. Kamiya, T. Chem. Pharm. Bull. 1987, 35, 3985. (c) Cainelli,
G.; Galletti, P.; Giacomini, D. Synlett. 1998, 611. (d) Cainelli, G.; Galletti,
P.; Giacomini, D. Tetrahedron Lett. 1998, 39, 7779.
(6) (a) Khalil, E. M.; Ojala, W. H.; Pradhan, A.; Fair, V. D.; Gleason,
W. B.; Mishra, R. K.; Johnson, R. L. J. Med. Chem. 1999, 42, 628. (b)
Khalil, E. M.; Pradhan, A.; Ojala, W. H.; Gleason, W. B.; Mishra, R. K.;
Johnson, R. L. J. Med. Chem. 1999, 42, 2977. (c) Dolbeare, K.; Pontoriero,
G. F.; Gupta, S. K.; Mishra, R. K.; Johnson, R. L. J. Med. Chem. 2003, 46,
727. (d) Palomo, C.; Aizpurua, J. M.; Benito, A.; Miranda, J. I.; Fratila, L.
A.; Matute, C.; Domercq, M.; Gago, F.; Martin-Santamar´ıa, S.; Linden, A.
J. Am. Chem. Soc. 2003, 125, 16243. (e) Saitton, S.; Del Tredici, A. L.;
Mohell, N.; Vollinga, R. C.; Bostrom, D.; Kihlberg, J.; Luthman, K. J.
Med. Chem. 2004, 47, 6595. (f) Khasanov, A. B.; Ramirez-Weinhouse, M.
M.; Webb, T. R.; Thiruvazhi, M. J. Org. Chem. 2004, 69, 5766. (g)
Bitterman, H.; Gmeiner, P. J. Org. Chem. 2006, 71, 97. (h) Grison, C.;
Coutrot, P.; Gene`ve, S.; Didierjean, C.; Marraud, M. J. Org. Chem. 2005,
70, 10753.
The geometry of the melanostatin 2, in the syn conformation,
forming a â-turn was optimized at the same level of theory, to
(7) (a) Hintermann, T.; Seebach, D. Chimia 1997, 51, 244. (b) Gademann,
K.; Ernst, M.; Hoyer, D.; Seebach, D. Angew. Chem., Int. Ed. Engl. 1999,
38, 1223. (c) Werder, M.; Hauser, H.; Abele, S.; Seebach, D. HelV. Chim.
Acta 1999, 82, 1774. (d) Porter, E. A.; Wang, X.; Lee, H.; Weisblum, B.;
Gellman, S. H. Nature 2000, 404, 565.
(8) The calculations were carried out with the Gaussian 98 (Revision
A.9) M. J. Frisch et al. Gaussian, Inc. Pittsburgh, PA, 1998.
(9) The different types of turns have analyzed by Rose, G. D.; Gierasch,
L. M.; Smith, J. A. AdV. Protein. Chem. 1985, 37, 1. See, also: Wilmot,
C. M.; Thornton, J. M. J. Mol. Biol. 1988, 203, 221.
7722 J. Org. Chem., Vol. 71, No. 20, 2006

Peptidomimetics and Oligo-â-peptides
FIGURE 4. Becke3LYP/6-31+G* optimized structures, 2 and 5-8 (lengths are in Å).
imines was carried out in the following way: ketene 12 (see
Scheme 1) was generated by dehydrochlorination of the acid
chloride with triethylamine at -40 °C for 45 min, and then a
dichloromethane solution of triazine 11 and boron trifluoride
etherate (which depolymerize the triazine) was added. After 12
h, the corresponding spiro â-lactams 9 were obtained, with yields
around 25-59%.
The â-lactams 9 can also be used in the preparation of
analogues of N-sulfonyl-â-lactams, which in some cases have
shown antibiotic activity (Scheme 2).10 The oxidative removal
of the PMP group from â-lactam 9b gives the N-unsubstituted
derivative 9e, which upon treatment with the complex pyridine-
sulfur trioxide gave the N-sulfonyl â-lactam 9f.
FIGURE 5. â-Lactam 9 and â-aminoesters and â-amino acids
intermediates 10.
SCHEME 1. Synthesis of Spiro â-Lactams 9 via Staudinger
Reaction of Methyleneimines Derived from 11
Optical Resolution of the Racemic Mixture (()-3. To
obtain the enantiomerically pure spiro-â-lactam 3 (see Figure
2) as the pivotal intermediate on the synthesis of PLG analogues
and â-peptides, we decided to resolve the corresponding racemic
mixture. The â-lactam 9b (R1 ) PMP; Scheme 2) was subjected
to hydrogenolysis, to give the unprotected pyrrolidine system
(()-3 (Scheme 3). The racemic mixture (()-3 was then treated
with L-Boc-Phe-OH or D-Boc-Phe-OH, to give the correspond-
ing salts 14 and 13, respectively, which precipitates in the
reaction medium. An additional crystallization gave the enan-
tiopure salt, which upon treatment with NaHCO3, gave the
optically pure â-lactams (-)-3 and (+)-3, respectively.
This synthetic procedure allows us to obtain each of the two
enantiomers of the desired â-lactam 3, in a procedure that can
be carried out on a multigram scale.
Assignment of the Absolute Configuration to the â-Lac-
tams 3. After we carried out the resolution of the racemic
mixture of the â-lactams 3, we had undertaken the task of
determining their absolute configuration. Previously, the optical
purity of the enantiomers obtained was assessed by HPLC
analysis of the â-lactams (-)-3 and (+)-3 (see SI).
be able to make comparisons with the other possible analogues;
also, the anti conformer was fully optimized.
On the basis of these calculations and the experimental
information available from our previous experiments,3 we
decided to accomplish the synthesis of the C4 unsubstituted and
optically active â-lactam derivatives 9 (Figure 5), as a valuable
intermediates in the preparation of different types of â-turn-
inducing peptidomimetics, analogues to melanostatin 2, and also
as intermediates for the synthesis of the â-aminoesters and
â-amino acids 10 (Figure 5), which could be used in the
preparation of â-peptides 4.
Synthesis of â-Lactams 3. The synthesis of C4 unsubstituted
â-lactams involves the use of the extremely unstable imines of
formaldehyde. These compounds, which are formed by con-
densation of a primary amine with an 30% aqueous solution of
formaldehyde, rapidly trimerize to give the corresponding
hexahydro-1,3,5-triazines, 11 (see Scheme 1).
The method chosen for solving the problem of the absolute
configuration, involved the analysis of the NOESY spectra of
(10) (a) Du¨ckheimer, W.; Blumbach, J.; Lattrell, R.; Scheunemann, K.
H. Angew. Chem., Int. Ed. Engl. 1985, 24, 180. (b) Stewart, W. J. Nature
1971, 229, 174. (c) Hashimoto, M.; Komori, T.; Kamiya, T. J. Am. Chem.
Soc. 1976, 98, 3023. (d) Slucharsky, A.; Dejneka, T.; Gordon, E. M.;
Weaver, E. R.; Koster, W. H. Heterocycles 1984, 21, 191. (e) Cimarusti,
C. M.; Sykes, R. B. Chem. Ber. 1983, 116, 2.
The [2 + 2]-cycloaddition reaction between the ketene
derived from Cbz-L-proline acid chloride and the methylene-
J. Org. Chem, Vol. 71, No. 20, 2006 7723

Mac´ıas et al.
FIGURE 6. PM3-optimized structures of the Mosher amides I and II.
SCHEME 2. Synthesis of Spiro Analogs of N-sulfonyl-â-lactams, 9f
SCHEME 3. Optical Resolution of â-Lactam (()-9b
of the configuration of the stereocenter of â-lactam (-)-3. The
assignment of the 1H signals in the spectra of the amides I and
II was based on the basis of the analysis of the two-dimensional
NMR experiments: COSY, HMQC, HMBC, and NOESY. The
absolute stereochemistry of the quaternary center in â-lactam
(-)-3 was established from the data of the 2D NOESY spectra
of amides I and II.
The 2D NOESY spectrum of amide II is shown in Figure 7.
The most conclusive signal in the amide II (see Figure 7) is
the correlation observed between the ortho hydrogens of the
phenyl group coming from the Mosher moiety [H12, δ ) 7.56
ppm, (d)] with one of the methylenic hydrogens (CH2) of the
â-lactam fragment [H5â, δ ) 4.48 ppm, (d)], which implies that
the phenyl ring and the â-lactam-CH2, are in the same side.
Here, we are using the R/â notation in order to differentiate the
hydrogens placed bottom side (R-hydrogens) from the ones
placed upper side (â-hydrogens).
SCHEME 4. Mosher’s Amides, I and II of (-)-3
In addition, H5â shows two cross-peaks with the hydrogens
H1â[δ ) 2.77 ppm, (ddd)] and H2â [δ ) 1.63 ppm, (ddd)]. From
this result, and taking into account that the absolute stereo-
chemistry of the Mosher fragment is known, we could assign
the (R) absolute configuration to the stereogenic center of the
â-lactam (-)-3.
The analysis of the 2D NOESY spectrum of the amide I,
allowed us to confirm the assignment previously made. In this
case, as it could be expected, it is not observed NOE between
the phenyl ring and the CH2 of the â-lactam moiety, because
two fragments are oriented in opposite directions (see Figure
8).
It is interesting to note that the inversion on the absolute
configuration of the stereogenic center of the Mosher reagent
produces an inversion on the chemical shifts of the methylenic
hydrogens of the carbon atom in the R position with respect to
the nitrogen of the pyrrolidine ring (H1), owing to the shielding
effect exerted for the phenyl ring. Thus, the signals that in II
appear at δ ) 2.7 (H1â) and δ ) 3.84 ppm (H1R), now, in I,
appear at δ ) 3.47 (H1â) and δ ) 2.80 ppm (H1R). As can be
seen in Figure 9, the phenyl ring of the Mosher reagent shields
the hydrogen R in the pyrrolidine ring in I and the hydrogen â
in the pyrrolidine ring in II.
the corresponding Mosher’s amides of (+)-3 and (-)-3, as
shown in Scheme 4.11
As the starting point, we optimized the geometry of the two
diastereomeric amides I and II, using the PM3 method (see
Figure 6),12 assuming an (R) configuration for the stereocenter
of the â-lactam (-)-3.
This model allowed us to tentatively establish what NOEs
could be expected for each of the amides I and II, as a function
(11) a) Dale, J. A.; Mosher, H. S. J. Am. Chem. Soc. 1972, 95, 512. (b)
Dale, J. A.; Dull, D. L.; Mosher, H. S. J. Am. Chem. Soc. 1969, 34, 2543.
(c) Seco, J. M.; Quin˜oa´, E.; Riguera, R. Chem. ReV. 2004, 104, 17.
(12) The PM3 method was used as implemented in Gaussian 98.
Synthesis of the Peptidomimetic (-)-20. After obtaining
the â-lactam (-)-3 we proceeded to carry out the synthesis of
7724 J. Org. Chem., Vol. 71, No. 20, 2006

Peptidomimetics and Oligo-â-peptides
FIGURE 7. NOESY spectrum (CDCl3) of the Mosher amide II.
FIGURE 8. NOESY spectrum (CDCl3) of the Mosher amide I.
(-)-20, the optically active analogue of melanostatin (see Figure
1), as described in Scheme 5.
The structure of (-)-19 was assigned through the analysis
1
of the H, 13C, DEPT, COSY, HSQC, and HMBC spectra.1H
After the coupling of the L-Boc-proline moiety, (-)-15 was
subjected to the oxidative removal of the PMP group. The
reaction with benzyl bromoacetate and further hydrogenolysis
gave the acid 18, which was coupled with O-benzylglycine to
introduce the side chain, affording (-)-19. The NMR study of
(-)-19 conducted in CDCl3 showed that this compound is able,
in solution, to adopt a type II â-turn conformation (Figure 10).
NMR spectrum of (-)-19 showed an equilibrium mixture of
rotamers about the carbamate bond. The 1H NMR chemical shift
of the NHa amide proton (δ ) 8.73 and δ ) 8.80 ppm) is in
the expected range for a hydrogen-bonded proton which is in
agreement with the previously described theoretical studies and
the correlations observed in the NOESY spectra (Figure 11).4
Thus, the amide proton NHa shows a correlation with proton
J. Org. Chem, Vol. 71, No. 20, 2006 7725

Mac´ıas et al.
It is interesting to note that the ring opening of the spiro
â-lactam has to be carried out under mild conditions, as
described previously,13 using KCN in catalytic amounts to
promote the â-lactam opening. The availability of the Boc-
protected â-aminoester (+)-10a and the â-amino acid (+)-10b,
allows us to proceed with the reiterative coupling, to obtain the
desired tetra-â-peptide, (-)-4c.
In addition, we investigated the possibility of developing new
peptidomimetics containing phosphorus or sulfur; thus, the
geometry of compounds 22 and 23 was fully optimized (see
Figure 12), and the results obtained indicate that these structures
could be good starting points for developing new peptidomi-
metics, along the lines presented in the current work.
FIGURE 9. Opposite shielding effects of the phenyl ring on the H1
hydrogens in I and II.
Conclusions
In summary, this research has established the utility of the
optically pure spiro â-lactams for both the synthesis of optically
active analogues of melanostatin, as (-)-20, and the new type
of functionalized â-peptides 4. The synthetic procedure devel-
oped allows us to obtain any of the two enantiomers of the
critical intermediate 3 in a procedure that can be carried out in
a multigram scale.
Experimental Section
FIGURE 10. â-Turn conformation of (-)-19 and NMR correlations.
SCHEME 5. Synthesis of the Melanostatin Analog(-)-20
Preparation of Hexahydro-1,3-5-triazines 11a-c. These com-
pounds were prepared according to the procedure described by
Hegedus and co-workers.14 To a solution of the corresponding
primary amine (60 mmol) in a mixture (150 mL) of ethyl acetate/
water (1:1), cooled at 0 °C, an aqueous solution (5 mL) of
formaldehyde (37%) is added. The reaction mixture is stirred for 2
h at 0 °C, and then the organic layer is separated, washed with
water (50 mL), and dried with anhd Na2SO4. The solvent is removed
under vacuum, and a white solid is obtained. This solid is washed
once with diethyl ether. Following this procedure, the hexahydro-
1,3-5-triazines 11a-c (Scheme 1) were prepared.
General Procedure for the Preparation of Spiro â-Lactams
9a,c.3,13 To a stirred solution of the N-benzyloxycarbonyl L-proline
acid chloride (2 mmol) in dry dichloromethane (20 mL) cooled to
-40 °C, was added dropwise dry triethylamine (0.41 mL, 3 mmol).
The solution became yellow to confirm that the ketene was formed.
After 45 min at low temperature, a purple solution of the triazine
11a,c (0.67 mmol) and BF3‚OEt2 (0.25 mL, 2 mmol), previously
mixed in CH2Cl2 (10 mL), was added dropwise. The mixture was
allowed to warm slowly to room temperature overnight and then
quenched with saturated aqueous NaHCO3. The aqueous layer was
extracted twice with CH2Cl2 (15 mL); the combined organic layers
were washed with brine (20 mL) and dried over anhydrous Na2-
SO4. The solution was then concentrated and purified by column
chromatography over SiO2 with the appropriated mixture of ethyl
acetate/hexane.
(()-2-Benzyl-5-benzyloxycarbonyl-2,5-diazaspiro[3,4]octan-
1-one (9a). Rf ) 0.25 (hexane/EtOAc 1:1), 350 mg of 9a, 50%
yield, white foam. 1H and 13C NMR show the presence of rotamers
about the carbamate bond in 1:1 ratio. 1H NMR (300 MHz,
CDCl3): δ 1.70-2.15 (m, 3H), 2.34 (m, 1H), 3.01, 3.06 (d, J )
4.8; J ) 4.6 Hz, 1H), 3.35-3.78 (m, 3H), 4.27-4.70 (m, 2H),
4.98-5.21 (m, 2H), 7.00-7.40 (m, 10H). 13C NMR (75 MHz,
CDCl3): δ 22.7, 23.3, 33.5, 34.8, 45.5, 45.8, 47.6, 48.3, 52.9, 54.1,
67.0, 67.7, 72.2, 73.0, 127.5, 127.6, 127.9, 128.0, 128.2, 128.4,
HS of â-lactam moiety at δ ) 3.95 and δ ) 4.03 ppm,
suggesting the presence of a â-turned conformation. Moreover,
the tert-butoxycarbonyl group (δ ) 1.37 and δ ) 1.40 ppm)
correlates with methylene protons of benzyl group (Hc) at δ )
5.13 ppm, which is also consistent with a â-turn conformation
(Figure 10).
Synthesis of the Oligo-â-peptide (-)-4c. As stated before,
we want to demonstrate the utility of the optically active
â-lactams 3 on the preparation of â-peptides 4 (Figure 2),
because of the potential biological activity of these compounds.7
The synthesis of â-peptides 4 (see Figure 2 and Scheme 6) was
carried out with the standard techniques for peptide coupling.
(13) Mac´ıas, A. Alonso, E.; Pozo, del C. Venturini, A.; Gonza´lez, J. J.
Org. Chem. 2004, 69, 7004.
(14) Hegedus, L. S.; D’Andrea, S. J. Org. Chem. 1988, 53, 3113.
(15) Floy, D. M.; Fritz, W.; Pluscec, J.; Weaver, E. R.; Cimarusti, C.
M. J. Org. Chem. 1982, 47, 5160.
7726 J. Org. Chem., Vol. 71, No. 20, 2006

Peptidomimetics and Oligo-â-peptides
FIGURE 11. 2D NMR correlations observed in (-)-19.
SCHEME 6. Synthesis of â-Peptides 4a
a (a) ClCO2Bn; (b) CAN; (c) (Boc)2O; (d) KCN catalyst, MeOH; (e) NaOH 2 N, THF; (f) CF3CO2H, CH2Cl2, 0 °C; (g) (+)-10b, DCC, HOBt, THF,
0 °C to room temperature.
135.1, 135.3, 135.9, 136.2, 153.7, 153.9, 169.5, 169.6. IR (KBr):
1711, 1757 cm-1. MS (ESI+): m/z 389 (M + K), 373 (M + Na),
351 (M + H). Anal. Calcd for C21H22N2O3: C, 71.98; H, 6.33; N,
7.99. Found: C, 71.79; H, 6.31; N, 7.97.
(4RS)-5-Benzyloxycarbonyl-2-[(R)-(methoxycarbonyl)(4-ben-
zyloxyphenyl)methyl]-2,5-diazaspiro[3,4]octan-1-one (9c). The
analogue of nocardicine 9c was prepared from triazine 11c (0.67
mmol) according to the general procedure described above to afford
(257 mg, 25%) of 9c as a 1:1 mixture of diastereoisomers that could
not be separated by column chromatography. Rf ) 0.20 (hexane/
1
EtOAc 1:1), white foam. H NMR (300 MHz, CDCl3): δ 1.70-
2.49 (m, 4H), 3.25-3.80 (m, 7H), 4.79-5.22 (m, 4H), 5.42, 5.68
(s, 1H), 6.80-7.49 (m, 14H). 13C NMR (75 MHz, CDCl3): δ 22.6,
23.2, 33.7, 34.9, 47.6, 48.3, 52.4, 52.5, 53.7, 56.5, 56.7, 66.8, 67.2,
69.8, 71.2, 72.0, 115.0, 115.1, 124.8, 125.0, 127.2, 127.3, 127.4,
127.7, 127.9, 128.2, 128.3, 128.4, 129.2, 129.4, 136.1, 136.3, 136.5,
153.5, 153.7, 158.8, 169.0, 169.2, 170.1, 170.4. IR (KBr): 1705,
1744, 1765 cm-1. MS (ESI+): m/z 553 (M + K), 537 (M + Na),
J. Org. Chem, Vol. 71, No. 20, 2006 7727

Mac´ıas et al.
stirred solution of â-lactam (()-3 (1.86 g, 8 mmol) in EtOAc/hexane
1:2 (30 mL) at 60 °C was added L- or D-N-Boc-phenylalanine (2.12
g, 8 mmol), and in a few minutes a white solid precipitated. To
this solution, at 60 °C, was added a mixture hexane/EtOAc 1:2
(30 mL) and portions of EtOAc until the solid was dissolved
completely. The solution was allowed to stand at room temperature
for 24 h, and the resulting white crystals were filtered off and further
recrystallized from hexane/EtOAc 1:2 and EtOAc, to afford the
corresponding diastereomeric salt, 14 (yield 34%) or 13 (yield 32%),
respectively.
Synthesis of Optically Pure â-Lactams (-)-3 and (+)-3. Salt
14 or 13 (2 mmol) was dissolved in CH2Cl2 (25 mL), and saturated
aqueous NaHCO3 (25 mL) was added. The mixture was stirred for
10 min, and the aqueous phase was extracted with CH2Cl2 (2 × 20
mL). The organic layer was dried over anhydrous Na2SO4 and
evaporated to afford the optically active â-lactams (-)-3 or (+)-3.
The enantiomeric purity of this compounds was determined by
HPLC (see SI).
FIGURE 12. Becke3LYP/6-31+G* optimized structures, 22- and 23-
syn (lengths are in Å).
515 (M + H). Anal. Calcd for C30H30N2O6: C, 70.02; H, 5.88; N,
5.44. Found: C, 70.20; H, 5.90; N, 5.42.
Modified Procedure for the Multigram Preparation of Spiro
â-Lactam 9b. (()-5-Benzyloxycarbonyl-2-(4-methoxyphenyl)-
2,5-diazaspiro[3,4]octan-1-one (9b). To a stirred solution of the
N-benzyloxycarbonyl L-proline acid chloride (20 mmol) in dry
dichloromethane (70 mL) cooled to -40 °C, was added dropwise
dry triethylamine (11.1 mL, 80 mmol). The solution became yellow
to confirm that the ketene was formed. After 25 min at low
temperature a purple solution of the triazine 11b (6.6 mmol) and
BF3‚OEt2 (2.51 mL, 20 mmol), previously mixed in CH2Cl2 (40
mL), was added dropwise. The mixture was allowed to warm slowly
to room temperature overnight and then quenched with saturated
aqueous HCl 1 N. The organic layer was separated, and the aqueous
layer was extracted with CH2Cl2 (1 × 20 mL); the combined organic
layers were washed with brine (40 mL) and dried over anhydrous
Na2SO4. The solution was then concentrated to give a red oil, which
was dried under vaccum to eliminate the traces of dichloromethane.
This red oil was then dissolved in methanol, and after several
minutes, a white solid precipitated, and the mixture was stirred for
1 h; the solid was filtered and dried under vaccum. The product
thus obtained (4.33 g, 59%) is pure enough to be used in the next
step, or it can be purified by column chromatography over SiO2
with the appropriated mixture of ethyl acetate/hexane. Rf ) 0.35
(hexane/EtOAc 1:1), white solid, mp 95-97 °C. 1H and 13C NMR
show the presence of rotamers about the carbamate bond in 1:1
(-)-(4R)-2-(4-Methoxyphenyl)-2,5-diazaspiro[3,4]octan-1-
one (-)-3. Yield: 92%. [R]20D ) -11.4 (c 1.0, CHCl3); ee > 99%.
(+)-(4S)-2-(4-Methoxyphenyl)-2,5-diazaspiro[3,4]octan-1-
one (+)-3. Yield: 90%. [R]20D ) +11.2 (c 1.0, CHCl3); ee ) 97%.
General Procedure for the Preparation of Mosher’s Amides
I and II. To a stirred solution of â-lactam (-)-3 (0.05 mmol), in
dry CH2Cl2 (2 mL), cooled to 0 °C, was added triethylamine (0.075
mmol) and the (R)- or (S)-MTPA-Cl (0.06 mmol). The mixture
was allowed to warm to room temperature overnight and then
quenched with saturated aqueous NaHCO3. The aqueous layer was
extracted twice with CH2Cl2 (10 mL), and the combined organic
layers were washed with brine (15 mL) and dried over anhydrous
Na2SO4. The solvent was removed in vacuo to afford the diaste-
reoisomeric Mosher’s amides (I and II) which were studied by
NMR to determine the absolute configuration of compound (-)-3.
Assignment of 1H NMR Spectrum of Mosher’s Amide (R,R)-
1
I. H NMR (500 MHz, CDCl3): δ (ppm) 1.76 (m, 2H, H2), 2.13
(dt, J ) 13.1, J ) 6.5 Hz, 1H, H3â), 2.30 (dt, J ) 13.7, J ) 7.1
Hz, 1H, H3R), 2.80 (dt, J ) 11.2, J ) 6.7 Hz, 1H, H1R), 3.47 (dt,
J ) 11.5, J ) 6.4 Hz, 1H, H1â), 3.60 (d, J ) 4.8 Hz, 1H, H5R),
3.74 (s, 3H, H10), 3.80 (s, 3H, H19), 4.32 (d, J ) 4.8 Hz, 1H,
H5â), 6.89 (d, J ) 8.9 Hz, 2H, H17), 7.37 (d, J ) 8.9 Hz, 2H,
H16), 7.47 (m, 3H, H13,14), 7.63 (d, J ) 6.9 Hz, 2H, H12).
1
ratio. H NMR (300 MHz, CDCl3): δ 1.80-2.10 (m, 2H), 2.23
(m, 1H), 2.45 (m, 1H), 3.42-4.18 (m, 7H), 4.91-5.18 (m, 2H),
6.70-7.40 (m, 9H). 13C NMR (75 MHz, CDCl3): δ 22.8, 23.4,
33.7, 34.9, 47.6, 48.3, 52.5, 53.7, 55.4, 67.0, 67.7, 71.4, 72.1, 114.2,
117.6, 117.8, 127.7, 127.8, 127.9, 128.1, 131.6, 131.8, 135.1, 136.1,
153.7, 153.8, 155.9, 165.9, 166.0. IR (KBr): 1698, 1741 cm-1
.
MS (ESI+): m/z 405 (M + K), 389 (M + Na), 367 (M + H).
Anal. Calcd for C21H22N2O4: C, 68.84; H, 6.05; N, 7.65. Found:
C, 69.05; H, 6.03; N, 7.62.
General Procedure for Resolution of 9b with L- or D-N-Boc-
Phenylalanine. General Procedure for the Cleavage of Cbz and
Bn Groups. The corresponding N-Cbz or O-Bn substrate was
dissolved in a 3:1 mixture of EtOAc/methanol and transferred via
cannula to a flask under H2 (1 atm) containing 20% weight of 10%
Pd-C catalyst. The mixture was stirred overnight, the catalyst was
filtered off on Celite, and the organic layer was concentrated in
vacuo to afford the corresponding NH or OH compound.
(()-2-(4-Methoxyphenyl)-2,5-diazaspiro[3,4]octan-1-one (()-
3. This compound was prepared according to the general procedure
described above to remove the N-benzyloxycarbonyl group, using
10 mmol of the N-Cbz-â-lactam 9b, to afford 2.28 g of (()-3, in
Assignment of 1H NMR Spectrum of Mosher’s Amide (R,S)-
II. 1H NMR (500 MHz, CDCl3): 1.63 (m, 1H, H2â), 1.89 (m, 1H,
H2R), 2.11 (ddd, J ) 10.2, J ) 6.1, J ) 4.4 Hz, 1H, H3â), 2.36
(ddd, J ) 13.1, J ) 10.5, J ) 6.4 Hz, 1H, H3R), 2.77 (ddd, J )
11.1, J ) 7.2, J ) 3.8 Hz, 1H, H1â), 3.48 (ddd, J ) 11.1, J ) 9.1,
J ) 6.4 Hz, 1H, H1R), 3.54 (d, J ) 4.7 Hz, 1H, H5R), 3.78 (s, 3H,
H10), 3.82 (s, 3H, H19), 4.38 (d, J ) 4.7 Hz, 1H, H5â), 6.91 (d,
J ) 9.0 Hz, 2H, H17), 7.36 (d, J ) 9.0 Hz, 2H, H16), 7.43 (m,
3H, H13,14), 7.56 (m, 2H, H12).
1
98% yield as a white solid: mp 78-80 °C. H NMR (300 MHz,
CDCl3): δ 1.82-2.18 (m, 3H), 2.25 (m, 1H), 2.44 (broad s, 1H),
2.98 (m, 1H), 3.23 (m, 1H), 3.59 (d, J ) 5.4 Hz, 1H), 3.67 (d, J )
5.4 Hz, 1H), 3.80 (s, 3H), 6.87 (m, 2H), 7.30 (m, 2H). 13C NMR
(75 MHz, CDCl3): δ 25.6, 32.5, 47.3, 55.4, 56.2, 74.1, 114.2, 117.7,
131.8, 156.0, 169.3. IR (KBr): 1731, 3263 cm-1. MS (ESI+): m/z
255 (M + Na), 233 (M + H). Anal. Calcd for C13H16N2O2: C,
67.22; H, 6.94; N, 12.06. Found: C, 67.30; H, 6.97; N, 12.03.
Resolution of (()-3 with L- or D-N-Boc-Phenylalanine. To a
Synthesis of â-Peptides 4. (-)-(4R)-5-Benzyloxycarbonyl-2-
(4-methoxyphenyl)-2,5-diazaspiro[3,4]octan-1-one (-)-9b. To a
stirred solution of the â-lactam (-)-3 (4 mmol) in EtOAc/H2O 1:1
(20 mL) at room temperature, NaHCO3 (0.672 g, 8 mmol) and
benzyl chloroformate (0.67 mL, 4.4 mmol) were added. The reaction
was stirred vigorously for 2 h, and the aqueous layer was extracted
with EtOAc (2 × 15 mL). The combined organic layers were
washed with brine (20 mL) and dried over anhydrous Na2SO4. The
7728 J. Org. Chem., Vol. 71, No. 20, 2006

Peptidomimetics and Oligo-â-peptides
1
(EtOAc). H and 13C NMR show the presence of rotamers about
1
the carbamate bond in ≈ 3:1 ratio. H NMR (300 MHz, CDCl3):
δ 1.46 (s, 9H), 1.90-2.27 (m, 4H), 3.46-3.85 (m, 4H), 5.09-
5.20 (m, 2H), 5.38 (m, 1H), 7.35 (m, 5H), 9.00 (broad s, 1H). 13
C
NMR (75 MHz, CDCl3): δ 22.5, 23.1, 28.2, 34.1, 35.2, 42.9, 43.2,
48.2, 48.9, 67.2, 67.4, 67.9, 68.7, 79.4, 80.8, 127.6, 127.8, 127.9,
128.3, 135.8, 136.2, 154.1, 155.1, 156.3, 156.5, 177.3, 178.1. IR
(KBr): 1715, 3398 cm-1. MS (ESI+): m/z 417 (M + K), 401
(M + Na), 379 (M + H). Anal. Calcd for C19H26N2O6: C, 60.30;
H, 6.93; N, 7.40. Found: C, 60.11; H, 6.92; N, 7.43.
solution was then concentrated and purified through column
Synthesis of (+)-(R,R)-â-Dipeptide (+)-4a. To a solution of
the N-Boc â-aminoester (+)-10a (0.392 g, 1 mmol) in CH2Cl2 (5
mL) cooled to 0 °C was added TFA (1 mL), and the mixture was
stirred until total disappearance of the starting material (2 h,
monitored by TLC). The solution was evaporated and the residue
was dissolved in CH2Cl2 (15 mL). Saturated aqueous NaHCO3 (15
mL) was added, and the aqueous layer was extracted with CH2Cl2
(2 × 15 mL). The combined organic layers were washed with brine
(25 mL) and dried over anhydrous Na2SO4 to afford the NH2
â-aminoester which was employed in the next step without further
purification. To a stirred solution of this compound in dry THF (5
mL) cooled to 0 °C were added HOBt (153 mg, 1 mmol), â-amino
acid (+)-10b (378 mg, 1 mmol), and dicyclohexylcarbodiimide
(DCC) (206 mg, 1.05 mmol). The mixture was allowed to warm
to room temperature overnight. After this time, the solution was
filtered and the solid (N,N-dicyclohexylurea) discarded. The solvent
was evaporated, and the residue was dissolved in EtOAc (20 mL).
The organic layer was washed with a saturated aqueous solution
of NaHCO3 (15 mL), HCl (aq) 1 N, and brine (15 mL). The organic
layer was dried over anhydrous Na2SO4. The solution was then
concentrated and purified through column chromatography (hexane/
EtOAc 1:1) to afford 0.529 g of â-dipeptide (+)-4a in 81% yield:
chromatography (hexane/EtOAc 2:1), to afford 1.42 g of (-)-9b,
in 97% yield: white solid; [R]20 ) -5.2 (c 1.0, CHCl3).
D
Synthesis of the N-Boc-â-Lactam (-)-21. (-)-(4R)-5-Benzyl-
oxycarbonyl-2-(tert-butoxycarbonyl)-2,5-diazaspiro[3,4]octan-
1-one (-)-21. Oxidative removal of p-methoxyphenyl group of
â-lactam (-)-9b (1.39 g, 3.8 mmol) according to the general
procedure described above afforded the N-unsubstituted derivative
9e (see Scheme S1) which was employed in the next step without
further purification. To a stirred solution of the N-unsubstituted-
â-lactam 9e (2.6 mmol), in acetonitrile (20 mL) cooled to 0 °C,
di-tertbutydicarbonate (5.2 mmol) and a catalytic amount of DMAP
were added, and the mixture was stirred at room temperature
overnight. Then, methylene chloride (40 mL) was added, and the
mixture was washed with 1 M KHSO4 (20 mL) and brine. The
organic layer was dried over Na2SO4, and the solvent was removed
in vacuo. The resulting crude was purified by column chromatog-
raphy (hexane/EtOAc 2:1) to afford 0.652 g of (-)-21 in 66% yield
(two steps): colorless oil; [R]20D ) -5.9 (c 0.5, CHCl3); Rf ) 0.20
(hexane/EOAc 3:1). 1H and 13C NMR show the presence of
rotamers about the carbamate bond in ≈ 1.5:1 ratio. 1H NMR (300
MHz, CDCl3): δ 1.39, 1.49 (s, 9H), 1.75-2.00 (m, 2H), 2.09-
2.22 (m, 1H), 2.28-2.41 (m, 1H), 3.40-3.58 (m, 3H), 3.75, 4.03
(d, J ) 6.3; J ) 6.5 Hz, 1H), 4.99-5.19 (m, 2H), 7.21-7.35 (m,
5H). 13C NMR (75 MHz, CDCl3): δ 22.7, 23.4, 27.7, 27.8, 33.9,
35.2, 47.4, 48.2, 51.7, 53.1, 67.1, 67.8, 71.3, 72.1, 82.9, 127.7,
127.8, 127.9, 128.0, 128.3, 135.2, 135.8, 147.7, 148.0, 153.3, 153.6,
166.9, 167.1. IR (KBr): 1694, 1731, 1822 cm-1. MS (ESI+): m/z
399 (M + K), 383 (M + Na). Anal. Calcd for C19H24N2O5: C,
63.32; H, 6.71; N, 7.77. Found: C, 63.43; H, 6.69; N, 7.80.
Synthesis of r,r-disubstituted â-amino ester (+)-10a. Methyl
(+)--(2R)-2-[1-benzyloxycarbonylpyrrolidin-2-yl]-3-(N-tert-bu-
toxycarbonylamino)propaneate (+)-10a. Prepared according to
the published procedure,5 using 1.2 mmol of the N-Boc-â-lactam
(-)-21, to afford 461 mg of (+)-10a in 98% yield as a colorless
oil: [R]20D ) +45.8 (c 0.5, CHCl3); Rf ) 0.70 (hexane/EtOAc 1:1).
1H and 13C NMR show the presence of rotamers about the
white solid; mp 54-57 °C; [R]20 ) +7.4 (c 0.7, CHCl3); Rf )
D
0.20 (hexane/EtOAc 1:1). H and 13C NMR show the presence of
rotamers (see Figure S10). H NMR (300 MHz, CDCl3): δ 1.42
1
1
(s, 9H), 1.60-2.31 (m, 8H), 3.30-3.92 (m, 11H), 4.90-5.35 (m,
4H), 5.58 (m, 1H), 7.12, 7.49 (d, J ) 7.1; J ) 6.3 Hz, 1H), 7.22-
7.40 (m, 10H). 13C NMR (75 MHz, CDCl3): δ 22.5, 23.1, 24.8,
25.5, 28.2, 34.4, 34.9, 41.9, 42.4, 43.1, 43.4, 43.6, 48.1, 48.3, 48.8,
48.9, 51.9, 52.3, 66.8, 66.9, 68.3, 68.6, 69.6, 78.9, 79.0, 127.4,
127.6, 127.9, 128.1, 128.3, 136.0, 136.2, 136.4, 153.7, 154.3, 154.4,
154.9, 155.1, 156.4, 156.8, 173.7, 173.8. IR (KBr): 1705, 3330,
3437 cm-1. MS (ESI+): m/z 675 (M + Na), 653 (M + H). Anal.
Calcd for C34H44N4O9: C, 62.56; H, 6.79; N, 8.58. Found: C, 62.69;
H, 6.77; N, 7.45.
1
carbamate bond in ≈ 1.5:1 ratio. H NMR (300 MHz, CDCl3): δ
(-)-(R,R,R)-â-Tripeptide (-)-4b. The tert-butoxycarbonyl group
of â-dipeptide (-)-4a (196 mg, 0.3 mmol) was removed according
to the general procedure to afford the corresponding NH2 â-dipep-
tide, which was employed in the next step without further
purification. The coupling reaction of this compound with â-amino
acid (+)-10b (113 mg, 0.3 mmol) according to the general
procedure described above afforded, after column chromatography,
178 mg of â-tripeptide (-)-4b in 65% yield: white foam;
[R]20D ) -19.9 (c 0.3, CHCl3); Rf ) 0.20 (hexane/EtOAc 1:2). 1H
and 13C NMR show the presence of rotamers (see Figure S11). 1H
NMR (300 MHz, CDCl3): δ 1.05-2.30 (m, 21H), 3.28-3.90 (m,
15H), 4.91-5.25 (m, 6H), 5.30, 5.58 (m, 1H), 6.92, 7.65 (m, 1H),
7.05-7.50 (m, 16H). 13C NMR (75 MHz, CDCl3): δ 22.7, 23.0,
23.3, 25.0, 25.6, 28.4, 29.4, 29.7, 34.0, 34.7, 35.2, 35.4, 36.5, 42.0,
42.2, 42.8, 43.6, 43.9, 48.2, 48.4, 48.8, 49.1, 52.1, 52.5, 67.1, 67.6,
67.9, 68.4, 68.5, 68.6, 68.9, 69.7, 79.0, 127.6, 127.8, 128.0, 128.5,
136.4, 136.6, 153.8, 154.3, 154.4, 155.1, 156.6, 156.9, 173.9, 174.6,
175.0. IR (KBr): 1701, 3417 cm-1. MS (EI): m/z 913 (M). Anal.
Calcd for C48H60N6O12: C, 63.14; H, 6.62; N, 9.20. Found: C,
63.00; H, 6.79; N, 7.43.
1.42 (s, 9H), 1.83-2.25 (m, 4H), 3.36-3.74 (m, 7H), 4.90-5.19
(m, 2H), 5.36 (m, 1H), 7.30 (m, 5H). 13C NMR (75 MHz, CDCl3):
δ 22.6, 23.1, 28.2, 34.1, 35.0, 43.2, 43.4, 48.0, 48.8, 52.1, 52.3,
66.8, 67.2, 67.9, 68.9, 79.1, 79.3, 127.6, 127.9, 128.1, 128.3, 128.4,
135.8, 136.4, 153.8, 154.7, 156.2, 156.4, 173.7, 174.0. IR (KBr):
1694, 1730, 3362 cm-1. MS (ESI+): m/z 393 (M + H). Anal. Calcd
for C20H28N2O6: C, 61.21; H, 7.19; N, 7.14. Found: C, 61.30; H,
7.18; N, 7.16.
Synthesis of r,r-Disubstituted â-Amino Acid (+)-10b. (+)-
(2R)-2-[1-Benzyloxycarbonylpyrrolidin-2-yl]-3-(N-tert-butoxy-
carbonylamino)propanoic acid (+)-10b. To a stirred solution of
N-Boc â-lactam (-)-21 (1.2 mmol) in THF (3 mL), was added a
2 N aqueous solution of NaOH (15 mL), and the mixture was stirred
overnight. The tetrahydrofuran was removed in vacuo and the
aqueous phase was acidified (pH 2) with 1 N HCl (aq) and extracted
twice with CH2Cl2 (20 mL). The combined organic layers were
washed with brine (20 mL) and dried over anhydrous Na2SO4. The
solution was then concentrated and purified through column
chromatography to afford 395 mg of (+)-10b in 87% yield: white
solid; mp 112-114 °C; [R]20D ) +43.4 (c 0.5, CHCl3); Rf ) 0.20
J. Org. Chem, Vol. 71, No. 20, 2006 7729

Mac´ıas et al.
Synthesis of (-)-(R,R,R,R)-â-Tetrapeptide (-)-4c. The tert-
butoxycarbonyl group of â-tripeptide (-)-4b (183 mg, 0.2 mmol)
was removed according to the general procedure to afford the
corresponding NH2 â-tripeptide, which was employed in the next
step without further purification. The coupling reaction of this
compound with â-amino acid (+)-10b (76 mg, 0.2 mmol) according
to the general procedure described above afforded, after column
chromatography, 183 mg of â-tetrapeptide (-)-4c in 78% yield:
156.6, 173.3, 173.9, 174.7, 175.0. IR (KBr): 1705, 3416 cm-1
.
MS (ESI+): m/z 1196 (M + Na), 1174 (M + H). Anal. Calcd for
C62H76N8O15: C, 63.47; H, 6.53; N, 9.55. Found: C, 63.59; H,
6.51; N, 9.52.
Acknowledgment. We thank the financial support of MEC-
04-CTQ2004-03591. Also, the CIEMAT (Ministerio de Edu-
cacio´n y Ciencia, Spain) is thanked for allocating computer time.
A.M. Rabanal and A.M. Ramallal express their thanks for a
doctoral fellowship to the Asturias regional Government and
C. P. for a Ramo´n y Cajal contract.
white foam; [R]20 ) -12.6 (c 0.3, CHCl3); Rf ) 0.20 (hexane/
D
EtOAc 1:2). 1H and 13C NMR show the presence of rotamers (see
1
Figure S12). H NMR (300 MHz, CDCl3): δ 1.22, 1.40 (s, 9H),
1.50-2.33 (m, 16H), 3.25-3.87 (m, 19H), 4.80-5.29 (m, 8H),
5.31, 5.58 (m, 1H), 6.90-7.80 (m, 23H). 13C NMR (75 MHz,
CDCl3): δ 22.2, 22.3, 22.7, 23.0, 23.3, 28.4, 29.7, 34.7, 35.0, 35.2,
35.4, 36.5, 36.8, 42.1, 42.2, 42.8, 43.1, 43.6, 44.0, 48.2, 48.4, 49.0,
52.1, 52.3, 52.5, 67.1, 67.2, 67.5, 67.6, 67.9, 68.1, 68.3, 68.4, 68.9,
70.1, 78.9, 127.6, 127.7, 127.8, 128.0, 128.4, 128.5, 128.6, 135.9,
136.1, 136.2, 136.4, 136.5, 136.6, 153.8, 154.1, 154.2, 154.4, 155.2,
Supporting Information Available: Experimental procedures
and computational details. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO061189L
7730 J. Org. Chem., Vol. 71, No. 20, 2006