Design, synthesis and evaluation of poly-L-proline type-II peptide mimics based on the 3-azabicyclo[3.1.0]hexane system

Full text of DOI:10.1021/jo981814p

330
J . Org. Chem. 1999, 64, 330-331
Design , Syn th esis a n d Eva lu a tion of
P oly-L-P r olin e Typ e-II P ep tid e Mim ics Ba sed
on th e 3-Aza bicyclo[3.1.0]h exa n e System
Rui Zhang and J ose S. Madalengoitia*
Department of Chemistry, University of Vermont,
Burlington, Vermont 05405
Received September 8, 1998
F igu r e 1. Stereodrawing of an overlay of 1 with leucine in which
the ø1 angle is gauche(-) and the two ø2 angles are gauche(-)
and trans, respectively (rms ) 0.30 Å). Light bonds denote 1. Dark
bonds denote leucine.
Recently, the poly-L-proline type-II (PPII) secondary
structure has been shown to play a critical role in mediating
several cellular signaling pathways.1,2 Due to the potential
utility of agents capable of inhibiting these signaling mech-
anisms, we are developing a program focused on the design
and synthesis of mimics of the PPII secondary structure. Our
basic strategy involves the synthesis of oligopeptides com-
posed of proline-templated amino acids (PTAAs) that will
populate the PPII conformation in solution.3 Among the
PTAAs that we desire are those that arise from the
3-azabicyclo[3.1.0]hexane system since molecular modeling
studies exhibit that, for these PTAAs, the amino acid
templated on the proline skeleton will possess a ø1 angle of
approximately -60°.4 These PTAAs should prove of great
interest since NMR and X-ray crystal structures of receptor-
bound PPII helices show that for the nonprolyl amino acids
in these PPII helices a ø1 angle of approximately -60°
(gauche(-) relative to the amine nitrogen) is common. An
overlay of a leucine PTAA analogue with a leucine construct
in which the ø1 angle is gauche(-) and the two ø2 angles
are trans and gauche(-), respectively, displays the goodness
of fit between these compounds (Figure 1).5 This paper
describes the synthesis of monomeric, dimeric, and trimeric
leucine PTAA analogue based on the 3-azabicyclo[3.1.0]-
hexane system and conformational studies of these mol-
ecules in CDCl3.
Sch em e 1
The synthesis of the oligomeric PTAAs was achieved by a
modular assembly described in Scheme 1. Reaction of tricycle
* To whom correspondence should be addressed.
(1) For a recent PPII review see: Siligardi, G.; Drake, A. F. Peptide Sci.
1995, 37, 281.
(2) For recent examples of proteins that bind PPII helices see: (a) Raj,
P. A.; Marcus, E.; Edgerton, M. Biochemistry 1996, 35, 4314. (b) Lee, C.-
H.; Saksela, K.; Mirza, U. A.; Chait, B. T.; Kuriyan, J . Cell 1996, 85, 931.
(c) Peng, S.; Kasahara, C.; Rickles, R. J .; Schreiber, S. L. Proc. Natl. Acad.
Sci. U.S.A. 1995, 92, 12408. (d) J ardetzky, T. S.; Brown, J . H.; Gorga, J .
C.; Stern, L. J .; Urban, R. G.; Strominger, J . L.; Wiley: D. C. Proc. Natl.
Acad. Sci. U.S.A. 1996, 93, 734. (e) Zeile, W. L.; Purich, D. L.; Southwick,
F. S. J . Cell Biol. 1996, 133, 49.
(3) (a) Zhang, R.; Brownewell, F.; Madalengoitia, J . S. J . Am. Chem. Soc.
1998, 120, 3894. (b) Zhang, R.; Madalengoitia, J . S. Tetrahedron Lett. 1996,
37, 6235.
(4) For cyclopropane ring constraints of amino acids see: (a) Martin, S.
F.; Austin, R. E.; Oalmann, C. J .; Baker, W. R. Condon, S. L. deLara, E.;
Rosenberg, S. H.; Spina, K. P.; Stein, H. H.; Cohen, J .; Kleinert, H. D. J .
Med. Chem. 1992, 35, 1710. (b) Burgess, K.; Ho, K.-K.; Pettitt, B. M. J .
Am. Chem. Soc. 1994, 116, 799.
(5) (a) For the leucine construct: gauche (-) ) -60° and trans ) 180°.
CR, Câ, Cγ, and Cδ as well as the nitrogen and carbonyl carbon of leucine
were chosen to be overlaid with the corresponding atoms on the PTAA. (b)
For PPII helices in which leucine residues have a ø1 angle gauche(-), and
ø2 angles, gauche(-) and ∼ trans, see: Feng, S.; Chen, J . K. Yu, H.; Simon,
J . A.; Schreiber, S. L. Science 1994, 266, 1241. (c) Yu, H.; Chen, J . K.; Feng,
S.; Dalgarno, D. C.; Brauer, A. W.; Schreiber, S. L. Cell 1994, 76, 933.
2 with LAH in refluxing THF effected reduction of the
lactam and oxazolidine functions to afford the N-benzy-
lamino alcohol, which was debenzylated (H2, Pd-C, 50 psi)
and reprotected with Boc2O in CH2Cl2 to afford the alcohol
3 (85% from 2).3a,6 J ones oxidation of the alcohol 3 (70%)
and protection of the resulting acid (76%) gave the benzyl
ester 5. Boc deprotection and reprotection of the resultant
amine afforded the trimethylacetamide 6. Debenzylation of
the ester gave the carboxylic acid 7. The dimer 9 was
obtained by diisopropylcarbodiimide (DIC)-hydroxybenzo-
triazole (HOBt) coupling of the acid 7 and amine 8 fragments
in 75% yield. The trimer 10 was obtained by debenzylation
of 9 followed by DIC-HOBt coupling with the amine
fragment 8 in 72% yield.
(6) For the synthesis of 2: Zhang, R.; Madalengoitia, J . S. J . Org. Chem.
1999, 64, 547-555.
10.1021/jo981814p CCC: $18.00 © 1999 American Chemical Society
Published on Web 12/30/1998

Communications
J . Org. Chem., Vol. 64, No. 2, 1999 331
F igu r e 3. Selected NOEs for 9 and 10.
rotamers. A NOESY spectrum of dimer 9 shows NOEs
between 2H(i)-4cH(i + 1) and 2H(i)-4tH(i + 1). Although
these NOEs are not diagnostic for the PPII conformation,
there are no additional inter-PTAA NOEs of equal or greater
magnitude. That the 2H(i)-4cH(i + 1) and 2H(i)-4tH(i +
1) NOEs are the strongest inter-PTAA NOEs is indicative
of the preferential population of a conformation in which ψ
≈ 145°, as required for the PPII conformation. It should be
noted that the NOE between 2H(i)-4tH(i + 1) is greater
than the NOE between 2H(i)-4cH(i + 1). Since both PTAAs
populate the flattened boat conformation as evidenced by
J 5H(i)-4cH(i) ) J 5H(i+1)-4cH(i+1) ) 5.2 Hz, the unequal NOE
intensities are analogous to those found with Ac-PTAA-OBn
(11).
The trimer 10 is an important compound for our studies
since three residues define one turn of a PPII helix. A
NOESY spectrum of trimer 10 also reveals the key NOEs
between 2H(i)-4H(i + 1) and 2H(i + 1)-4H(i + 2). Fur-
thermore, these NOEs are the strongest inter-PTAA NOEs,
indicating a population in which ψ ≈ 145°. A statement
about the relative NOE intensities between 2H(i)-4cH(i +
1) and 2H(i)-4tH(i + 1) cannot be made in this case because
4cH(i + 1) and 4tH(i + 1) are not resolved. However, the
5H-4cH coupling constants are 5.0, 5.2, and 5.2 Hz for the
i, i + 1, and i + 2 PTAAs, respectively, indicating that even
in the trimer, the boat conformation is maintained by the
individual PTAAs.
F igu r e 2. Selected NOEs for 11 and expanded region of NOESY
spectrum showing 4cH and 4tH cross-peaks with the acetamide
methyl group.
Previous studies have shown that the bicyclo[3.1.0]hexane
system can adopt a flattened boat conformation.7 Since a
chair or boat pucker will result in different dihedral angles
φ, the conformation of the pyrrolidine ring was first inves-
tigated. An NOE experiment on the model compound Ac-
PTAA-OBn 11 (Figure 2) was utilized to determine the ring
pucker since a populated boat or chair conformation would
result in unequal distances from the acyl methyl group to
4cH and 4tH (c and t denote a cis and trans relationship
relative to the ester carbonyl).8 Indeed, a NOESY spectrum
of compound 11 shows that the acyl methyl group exhibits
a higher NOE intensity with 4tH than 4cH (Figure 2). This
information is thus consistent with a populated boat con-
formation for the monomer unit 11. The validity of this
experiment is dependent on a planar geometry for the amide
bond. For this reason, the trimethylacetyl-substituted mono-
mer 6 was not used, since it might be argued that the
significant sterics in this system could impart a nonplanar
nature to the amide bond. However, a comparison of J 5H-4cH
in both 11 and 6 reveals an identical value of 5.2 Hz. Since
J 5H-4cH is a function of the pyrrolidine pucker, it may be
extrapolated that structure 6 also populates a boat confor-
mation. Molecular modeling of acetamide 11 reproduces the
slight pucker and gives a value for φ ) -63° well within
-75° ( 30° required for the PPII conformation.9,10
Our preliminary studies show that PTAAs based on the
3-azabicyclo[3.1.0]hexane system preferentially populate the
PPII conformation in CDCl3. These PTAAs will be critical
for the synthesis of conformationally constrained PPII
mimics that possess residues with ø1 ≈ -60° and a ø2 ≈
-155°. Work is ongoing to synthesize and evaluate water-
soluble PTAAs based on this framework.
We previously have argued that the minimization of
pseudo-allylic strain between adjacent prolines can define
ψ ≈ 145° as required in the PPII conformation.3a For the
dimer 9, the minimization of pseudo-allylic strain should
place 4cH(i + 1) and 4tH(i + 1) in close spatial relationship
to 2H(i) (Figure 3) when the PTAA-PTAA amide bond is in
the trans conformation.11 For both the dimer 9 and the
trimer 10 we were able to observe only trans amide bond
Ack n ow led gm en t. Support for this work has been
provided by grants NSF Vermont EPSCoR OSR9350540
and R29 CA75009 from the NIH.
Su p p or tin g In for m a tion Ava ila ble: Experimental proce-
dures for 4 and 7 and characterization data for 3, 5, 6, and 9-11,
1H and 13C NMR spectra of 3, 5, 6, and 9-11, and NOESY spectra
of 9-11 (23 pages).
(7) (a) Fujimoto, Y.; Irreverre, F.; Karle, J . M.; Karle, I. L.; Witkop, B. J .
Am. Chem. Soc. 1971, 93, 3471. (b) Abraham, R. J .; Gatti, G. Org. Magn.
Reson. 1970, 2, 173.
(8) Compound 11 exists as 7:3 mixture of trans/cis amide bond rotamers,
respectively, while a cis rotamer is not detected in 6.
(9) Energy minimization of the trans rotamer of 11 was performed with
MMX version 6.0, Serena Software, Box 3076, Bloomington, IN 47402.
(10) The -75° ( 30° tolerance for φ in the PPII conformation is according
to: Adzhubei, A. A.; Sternberg, M. J . Mol. Biol. 1993, 229, 472.
J O981814P
(11) The designation i denotes the N-terminal residue.