5
010
C. Hemmerlin et al. / Tetrahedron Letters 42 (2001) 5009–5012
8
,9
AzPro couple (AzPro means that a nitrogen atom has
tertiary amide function Ala-AzPip in a planar arrange-
ment while the nitrogen N , substituted for CH , takes
a
a
a
been substituted for the CH in the pyrrolidine ring),
AzPro induces a bVI-like reverse turn with the preced-
ing residue when incorporated in a peptide sequence,
whereas Pro is well-known to favor a classical b-turn
with the following residue. In the past few years, we
moved to the couple Pip/AzPip, where AzPip repre-
sents the Pip (H-Pip-OH, pipecolic acid, homoproline,
a pyramid one with a S(L) nitrogen absolute configura-
10,11
a
tion.
The axial disposition of the pyramidal N
amide function and the cisoid form of the Ala-AzPip
tertiary amide can be explained by the concept of
pseudo allylic strains which favors the 1,2 diaxial rela-
tionship between the two N-substituted carbonyls over
what would be strong 1,2 diequatorial repulsions. The
10
a
piperidine 2-carboxylic acid) residue, the CH of which
12
1
has been changed for a nitrogen atom, and we have just
reported the conformational features in the solid state
of the AzPip motif included in an aza-dipeptide
sequence. To confirm the observed main conforma-
and torsion angles of Ala and AzPip residues in
1
addition with the cis-configured Ala –AzPip bond are
characteristic of the observed pseudo bVI turn, which is
doubly stabilized by the two strong intramolecular
1
0
3
tional propensities, we studied the aza-tripeptide Boc-
Ala-AzPip-Ala-NHiPr, which is more adapted to mimic
a modified aza-peptide backbone. It was prepared in
seven steps according to a classical liquid-phase synthe-
sis, but going from N- to C-terminus (Scheme 1). The
pivotal intermediate molecule was Boc-AzPip-OBzl 3,
hydrogen bonds: the expected (Boc)CꢀO···HN(Ala )
one of type i+3i in addition to the
(Boc)CꢀO···HN(iPr) of type i+4i closing a 13-mem-
bered ring and sharing the same carbonyl acceptor
group.
9
which was obtained, similarly to Boc-AzPro-OBzl, first
In solution, FTIR spectra (DCM, c=5 mM) present, in
the more informative w(NꢁH) domain, broad bands
by benzyloxycarbonylation of the commercial t-butyl-
carbazate 1, to give the N,N%-diacylhydrazine Boc-NH-
NH-Z 2, followed by bi-metallation and in situ
intramolecular cyclization after treatment with 1,4-
dibromobutane, to give the N,N% orthogonally pro-
tected hexahydropyridazine 3. After elimination of the
Boc protective group to get the hydrazide 4, and then
taking into account the poor nucleophilicity of the
involved hydrazine nitrogen, we made use of the highly
efficient coupling method DIPCDI/AtOH to react 4
with Boc-Ala-OH to form the protected aza-dipeptide
Boc-Ala-AzPip-OBzl, which was hydrogenolyzed to
obtain the hexahydropyridazide 5. This latter com-
pound was condensed with 4-nitrophenylchlorocarbon-
ate to get the mandatory ‘activated’ 4-nitrophenyl-
carbazate 6 able to react with H-Ala-NHiPr to get
the desired crude aza-tripeptide isopropylamide 7. A
final silica gel flash chromatography (AcOEt) fur-
nished the pure Boc-Ala-AzPip-Ala-NHiPr 7, mono-
crystals of which, suitable for X-ray diffraction, were
then grown from a slowly cooled AcOEt solution.
−
1
highlighting the presence of free (:3428 cm ) and
−
1
1
bonded (:3369 cm ) NꢁH vibrators. H NMR signals
(chloroform, DMSO) are split, thus evidencing the
occurrence of two populations of stereoconformers.
1
3
Isomer M, which is 90% populated in CDCl and
3
14
about 45% in DMSO, and isomer m have in common
the cis-disposition of the Ala-AzPip tertiary amide
function as attested, in NOESY spectra, by the absence
of any spatial proximity between (Ala /Ala% )CH and
(AzPip/AzPip%)CH , but by a medium range connectiv-
2
ity between (Ala )CH
case of isomer M.
1
1
a
6
o
6
1
a
3
6
and (Ala )NH
6
protons in the
If we look (Fig. 2) at the NH
6
proton chemical shift
variation as a function of the solvent composition
increasing percentages of DMSO in CDCl /DMSO
(
3
3
solution at constant concentration), both (Ala ) NH
and NHiPr amide protons of isomer M are shielded
from solvation by solvent molecules, that corroborates
the presence of the iPrNH···OꢀC(Boc), [i+4i type],
and especially the stronger (Ala )NH···OꢀC(Boc), [i+
i type], intramolecular hydrogen bonds which stabi-
6
6
6
3
3
A stereo drawing of the three-dimensional crystal
molecular structure of the aza-tripeptide 7 (deposited at
the Cambridge Crystallographic Data Center) is shown
in Fig. 1. The saturated hexahydropyridazine heterocy-
cle adopts a quasi chair conformation in which the two
nitrogen atoms deserve special attention: on the one
hand, X-ray experimental data show the N atom of the
lize the bVI-like turn. Thus, rotamers of stereoisomer
M in solution (CDCl , DMSO) are folded and resemble
3
very closely to the 3D molecular structure observed in
the crystalline state. As regards isomer m, all of its NH
6
protons are freely accessible (solvent effect, Fig. 2) and
NOESY spectrum data are in agreement on the cis-
1
form of the Ala% –AzPip% amide bond and the slow
inversion at the NMR time scale and room temperature
of the six-membered diazaheterocycle, partly driven by
3
a
the pyramidal inversion of the rather sp hybridized N .
By taking into account the previous NMR data and the
observed NOE connectivities with respect to the pseu-
doallylic constraint, which imposes the axial disposition
of the Na amide function with the largest distance
between the carbonyl oxygen atoms, we can calculate
(
CS Chem 3D Pro™, CambridgeSoft Corp.) a model
rotamer of stereoisomer m (Fig. 3). This fulfills, for the
best, all of the latter constraints and releases the huge
steric constraints between (Ala% )Me and N if a bVI-
like turn were present.
Figure 1. Stereo drawing of the crystal molecular structure of
a
1
a
Boc-L
-Ala-(N S)-AzPip-
L
-Ala-NHiPr derived from X-ray dif-
fraction.
Hemmerlin, Christine
