Communications
Our strategy for the synthesis of the target molecule
Fmoc-X is outlined in Scheme 1. As a key step, we intended to
use a ruthenium-catalyzed ring-closing metathesis reaction to
form the central seven-membered ring. The required dipep-
tide intermediate A would in turn be derived from appropri-
ately protected vinylproline building blocks B and C; the
synthesis of these is however not trivial.[13,14]
Scheme 3. Synthesis of the type C building block 11: a) HClO4,
tBuOAc, RT, 15 h, 72%; b) Boc2O, DMAP, MeCN, RT, 15 h, 84%;
c) DIBALH, CH2Cl2, À788C, 2 h, 93%; d) PPTS, EtOH, RT, 15 h, 96%;
=
e) CH2 CHCH2SiMe3, BF3·OEt2, CH2Cl2, À788C, 30 min, 77%; f) O3,
Scheme 1. Retrosynthesis of Fmoc-X. Fmoc=fluorenylmethoxycar-
bonyl.
À788C, then NaBH4, CH2Cl2/MeOH, RT, 12 h, 87%; g) (o-
NO2Ph)SeCN, P(nBu)3, Py, THF, RT, 30 min, 92%; h) O3, CH2Cl2, D,
30 min, 90%; i) TMSOTf, CH2Cl2, 08C, 5 min, then chromatographic
separation of diastereomers (SiO2, CH2Cl2/MeOH 25:1), 51%.
DIBALH=diisobutylaluminum hydride, PPTS=pyridinium p-tolylsulfo-
nate, py=pyridine, OTf=trifluoromethanesulfonate.
As building block B, the Boc-protected trans-3-vinylpro-
line 6 was synthesized starting from l-pyroglutamic acid (1)
by the sequence shown in Scheme 2.[15] First, conversion of the
carboxylic acid into a TPS-protected alcohol followed by N-
manipulation. Ionic allylation of 7 (allyl-TMS, BF3·Et2O) then
proceeded in a cis-selective manner to afford 8 (d.r. ca. 3:1).[16]
Without separation, this mixture was ozonolyzed to give the
alcohol 9 after reductive workup. Elimination of the primary
OH functionality was then achieved through ozone oxidation
of an intermediate o-nitrophenylselenide.[17] Treatment of 10
with TMSOTf allowed the selective deprotection (Boc
cleavage) of the secondary amino group without affecting
the tert-butyl ester function.[18] Fortunately, the separation of
the cis/trans diastereomers was possible at this stage with
standard flash column chromatography on silica gel, and the
desired pure cis isomer 11 was obtained in 15% overall yield
from 1 (9 steps).
Scheme 2. Synthesis of the type B building block 6: a) SOCl2, EtOH,
RT, 15 h, 99%; b) NaBH4, LiCl, THF, RT, 15 h, 73%; c) TPSCl,
imidazole, DMF, RT, 15 h, 87%; d) Boc2O, DMAP, MeCN, RT, 15 h,
89%; e) LiN(TMS)2, PhSeCl, THF, À788C, 2 h, 78%; f) O3, CH2Cl2,
The connection of the two vinylproline building blocks 6
and 11 was then achieved by peptide coupling using PyBOP in
the presence of DIPEA.[18] Cyclization of the resulting
dipeptide 12 by ring-closing metathesis proceeded smoothly
in the presence of 5 mol% of a Grubbs II catalyst.[18] Finally,
the N-Boc and the tert-butyl ester groups were cleaved by
treatment of 13 with TFA to give the amino acid X, which was
directly converted into the target compound Fmoc-X
(Scheme 4).
=
À788C, 15 h, 78%; g) CH2 CHMgBr, CuBr·SMe2, TMSCl, THF/Et2O,
À788C, 2 h, 59%; h) LiBHEt3, THF, À788C, 1 h; i) Et3SiH, BF3·OEt2,
CH2Cl2, À788C, 2 h, 65% (2 steps); j) TBAF, THF, RT, 15 h, 89%;
k) Jones reagent, acetone, RT, 2 h, 74%. Boc=tert-butoxycarbonyl,
DMAP=4-(N,N-dimethylamino)pyridine, TBAF=tri-n-butylammonium
fluoride, TMS=trimethylsilyl, TPS=tert-butyldiphenylsilyl.
Boc protection afforded 2, which was subsequently trans-
formed to the a,b-unsaturated derivative 3 through a-
selenylation and oxidation-induced elimination using ozone.
The vinyl group was then introduced by 1,4-addition of a
cuprate reagent to give the trans-configurated pyrrolidone 4
as a pure diastereomer. Reduction of 4 with LiEt3BH gave the
corresponding “lactamol”, which on ionic hydrogenation
(Et3SiH, BF3·Et2O) afforded the deoxygenated compound 5
in 65% yield (two steps). Finally, fluoride-mediated cleavage
of the silyl protecting group and Jones oxidation gave rise to 6
in 9% overall yield (11 steps).
Using standard solid-phase Fmoc chemistry,[19] the scaf-
fold X was then incorporated as a Pro-Pro substitute into two
peptides (WT and LL), which had been previously identified
as ligands that bind to the SH3 domain (Table 1).[20]
The binding properties of the X-containing peptides (X-
WTand X-LL) to the Fyn SH3 domain were then investigated
(in comparison to the parent ligands WTand LL) by means of
NMR spectroscopy (15N HSQC[21a] and 15N SOFAST-
HMQC[21b]) and isothermal titration calorimetry (ITC).[22]
For all of the ligands, NMR spectra of 15N-Fyn-SH3 were
recorded in the absence or presence of the ligands and at
different ligand concentrations using a protein (15N-Fyn-SH3)
concentration of 0.8 mm and 0.4 mm (for WT and X-WT,
respectively) or 0.1 mm (for LL and X-LL). The low solubility
of LL and X-LL required the use of DMSO as a co-solvent.
The synthesis of the cis-5-vinylproline ester 11 (as building
block C) was performed as shown in Scheme 3. Starting again
from l-pyroglutamic acid (1), intermediate 7 was prepared as
a mixture of diastereomers by common functional-group
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 7111 –7115