Gennari, C.

Gude, M.

Piarulli, Umberto

Potenza, D.

Salom, B.

GUDE M.

POTENZA D.

SALOM B.

GENNARI C.

PIARULLI, UMBERTO

Archivio istituzionale della ricerca - Università dell'Insubria

Pergamon Tetndwdron Letters, Vol. 37, No. 47, pp. 8589-8592, 1996 Copyright © 1996 Elsevier Science Lid Printed in Great Britain. All rights reserved 0040-4039/96 $15.00 + 0.00 PII: S0040-4039(96)01963-6 A New Method for the Solution and Solid Phase Synthesis of Chiral [3-Sulfonopeptides under Mild Conditions Markus Gude, Umberto Piarulli, Donatella Potenza, Barbara Salom, Cesare Gennari* Dip. di Chimica Organica eIndustriale, Universita' diMilano, Centro CNR (Sost.Org.Nat.), via Venezian 21, 20133 Milano, Italy Istituto di Scienze Mat.Fis. e Chimiche, II Facolt~t diScienze dell'Universit~ di Milano, via Lucini 3, 22100 Como, Italy Abstract: Chiral I~-sulfonopeptides w re synthesized both in solution and in the solid phase using the sulfonyl chlorides derived from enantiomerically pure 2-substituted taurines under mild coupling conditions [cat. 4-dimethylaminopyridine (DMAP) and excess methyl trimethylsilyl dimethylketene acetal (MTDA) as a proton trap]. Copyright © 1996 Elsevier Science Ltd Peptides are attractive targets for drug discovery because of their affinities and specificities toward biological receptors and the simplicity with which large peptide libraries can be synthesized in a combinatorial format. However, the poor stability and bioavailability of peptides in vivo have generally limited their therapeutic application. One approach toward overcoming this obstacle has been the development of non-natural biopolymer scaffolds (carbamates, peptoids, ureas, sulfonamides, etc.) with improved pharmacological properties relative to peptides.l The ability to efficiently assemble large synthetic oligomers also provides an opportunity to generate unnatural polymers with defined secondary and tertiary structures. Such structures should provide increased insight into the relationships between monomer structure and polymer conformation and may provide new classes of folded polymers with novel properties. 2 In the field of sulfonamides, the synthesis of cc-sulfonopeptides (1)has remained an elusive goal. 1i,3 Our group at Milano has recently described the synthesis of vinylogous ulfonopeptides (2, vs-peptides) via an iterative process, both in solution 1i,4 and in the solid phase.lj In collaboration with Clark Still at Columbia University and Peter Nestler at Cold Spring Harbor Laboratory, we also described the binding of tweezer-like molecular receptors based on vs-peptides to an encoded combinatorial tripeptide library, showing not only that vs-peptide based receptors bind oligopeptides, but also that the binding selectivity is just as high as that of receptors built with ~x-aminoacids.lj [~-Sulfonopeptides (3) were synthesized by our group via 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-mediated sulfonyl chloride coupling, li and by the Liskamp group in The Netherlands via N-methylmorpholine (NMM)- mediated sulfinyl chloride coupling followed by oxidation of the resulting sulfinamides to sulfonamides, lh,5 R Ho. o .R2 H O O H H H H 1 ~ ..-., - - L  I ..'. ,# / I 0 . .0  I I I I n "  "~r~ i I ,.4, k~r~ : " R O O R10 O R20 O H~ "~R1 H 'J , j~  i~ I i~ I O ' ; ' \OR2 r - -  : ~ , "  " "~ -= ~y,,', (x-sulfonopeptides (1) vinylogous sulfonopoptides (2) I]-sulfonopeptides (3) 8589 8590 A recent report by the Liskamp group on the solid phase synthesis of ~-sulfonopeptides via NMM- mediated sulfonyl chloride coupling 6 prompted us to disclose our recent results in this field. Here we report on the solution and solid phase synthesis of ~l-sulfonopeptides under mild, "neutral" conditions. Chiral, enantiomerically pure 2-substituted taurines (4) were prepared from natural ~-aminoacids following the 1989 route of Ienaga et al. (Scheme 1). 7 R a,b ,. I~ ~ c,d ,IN Ms " /~/,-, BocNH COOMe BocNH R +NH3~ S03 (4) Scheme 1. a) LiBH 4, EtOH; b) MsCI, Et3N, CH2CI2; c) HC1 in dioxane; d) Na2SO 3, H20. In selected cases, the final ion-exchange-resin purification of the betain could be avoided, and the crude betain was purified by evaporating the solvent (H20) to a small volume and precipitation (e.g. 4, R = PhCH 2, overall yield 60%). Betains 4 were then transformed into the N-Boc protected tetrabutylammonium salts by treatment with Bu4NOH-30H20 and Boc20 in water : THF (1:3) followed by dichloromethane extraction. The crude salts were then converted into sulfonyl chlorides (5) using triphosgene and cat. DMF in dichloromethane at 0 ° ' RT (Scheme 2). li,8 The sulfonyl chlorides were purified by a rapid flash chromatography and can be stored indefinitely without decomposition at +4°C (e.g. 5, R = PhCH 2, overall yield 60%). R R a • , S03 NBu4 +NH Boc-NH (4) R Boc .NH'~ SO2CI (s) Scheme 2. a)Bu4NOH-30H20 (1 mol.eq.), Boc20 (1 mol.eq.), water:THF (1:3), dilution with water and extraction with CH2C12; b) triphosgene (0.66 mol.eq.), DMF (cat.), dichloromethane, 0 ° ' RT; flash chromatography. With the activated monomers 5 in our hands, we started the coupling experiments using benzylamine as a model case. Reaction of sulfonyl chloride 5 (R=Bn, 1.5 mol.eq.) with benzylamine (1 tool. eq.) in dichloromethane in the presence of various bases and additives was investigated (Table). Bn Bn /~SO2C I BnNH2 . /~SO2NHB n Boc-NH CH2CI2 Boc-NH (6) Table Base or additive Mol.eq. % Yield excess RSO2CI decomposes within DBU 2 80 1 h DBU + DMAP 1 + 1 88 2 h DMAP 2 86 5 h MTDA 2 94 >24 h MTDA + DMAP l + 1 94 24 h 8591 It is worth noting that bases like DMAP or DBU (in particular), although they are known to catalyse the coupling process,lij are harmful to the sulfonyl chloride (see last column of the Table). Therefore it is important to find coupling conditions which make use of substoichiometric amounts of a mild base or no base at all. 9 A solution to this problem was found using methyl trimethylsilyl dimethylketene acetal (MTDA) as a proton trap. The new coupling procedure was then tested in the synthesis of a real ~-suifono dipeptide (8, Scheme 3). In this case, the sulfonyl chloride (5, R=Bn, 1.5 mol.eq.) was reacted with the ammonium chloride (7, 1 mol.eq.) in the presence of excess (4 mol.eq.) MTDA. The reaction was sluggish (38% yield) in the absence of DMAP, but became fast and clean with increasing amounts of DMAP (3% DMAP: 50% yield; 15% DMAP: 69% yield; 40% DMAP: 78% yield; 150% DMAP: 78% yield). Bn . /~SO2CI  Bn~ ,-.Bn _ Bn Boc-NH I u, ,u I 6 HCI C I+/~SO2NHBn ,. / I  X ~S~" .fin ..SO2NHBn Boc-NH v N v MeOH" H3 N CH2CI2 i Scheme 3 (7) (8) H Synthesis of a ~-sulfonopeptide on solid support was initiated by removal of the Boc group from Boc- glycine PEG-resin 91j [PEG = poly(ethylene glycol)] by treatment of the resin with 25% TFA (TFA = trifluoroacetic acid) in CH2CI 2 for 30 min (3 cycles), followed by washings with CH2C12, 10% Et3N in CH2C12, and again CH2CI 2. The first monomer (5, R=Bn, 2 mol.eq.) was then coupled to the free glycine amine in CH2C12 (1 mol.eq., 0.028 M) in the presence of cat. DMAP (0.4 mol.eq.) and excess MTDA (4 mol.eq.). One coupling was enough to achieve ssentially complete conversion of the amines on the support (> 95%). loa Two more coupling cycles were routinely run to obtain a "combinatorial grade" conversion (>99%). The sequence was repeated (Boc removal, coupling with $, R=Bn), and finally the I]-sulfonopeptide was cleaved by treatment ofthe resin with 10% Et3N in MeOH (24 h) to give the corresponding methyl ester (11, Scheme 4) in 70% yield 10h as a single compound, whose identity was proved by IR, NMR, and MS analysis, and by comparison with an authentic sample synthesized in solution. Scheme 4BocHN'~oO" ~ 9 1. 25% TFA in CH2CI2 2. 10% Et3N in CH2CI 2 3. 5 (R=Bn, 2 mol.eq.), MTDA (4 mol.eq.), DMAP (0.4 mol.eq.), CH2CI2 O H O 10 1. 25% TFA in CH2Cl 2 10% EtaN Bn Q O Bn O O 2. 10% EtaN in CH2CI 2 ,. in MeOH S-.. N S~ Me 3. 5 (R=Bn, 2 mol.eq.), Boc-NH ~ ~ O MTDA (4 mol.eq.), H H DMAP (0.4 mol.eq.), CH2Cl2 11 We have investigated the conformational preferences of [~-sulfonopeptides 8 and 11 in chloroform solution by variable-temperature 1H-NMR spectroscopy, FT-IR spectroscopy, and N.O.E. (N.O.E. = nuclear Overhauser ffect) experiments. 11 The experimental results are complemented by computer modeling, 11 and show a strong tendency for 13-sulfonopeptides to form well defined folded structures via intramolecular hydrogen-bonding (dotted lines, Scheme 5). 8592 J'v N-H 3238 cm 1 v C=O 1693cm 1 S[$  NH 5.52 ppm tBuq /--Ph )'=O'-- H-N,, ×O f~H- -N , , .  /S~ /I~NH4.62 ppm ~Bn ~.  0 [v N-H 3427cm" <,, />,.,Bo O:~ -Nx  [8NH506 ppm O H Iv N-H ,~68 cm "I 8 v C=O 1696 cm 1 vc=o  uoh ;I '"46° 1741 cm 1 ~ .  [ v N-H 3437 era 1.0 [ ,H6.92 ppm i \ H" hl~><"~" O 'r I I \  I JSNHS.92 O 11 ~Bn Iv N'H 3220 em1 Scheme 5.1R and NMR data shown here were obtained from chloroform solutions (0.002 M) at 297 K. Chemical shifts are independent of concentration at or below 0.005 M. Acknowledgements. We thank Professor Rob Liskamp (Utrecht University, The Netherlands) for helpful discussions, comments and sharing of ideas. We thank the Commission of the European Union (Fixed Contribution Contract for Training through Research ERB FMB ICT 950328) for financial support and for a postdoctoral research fellowship (to M.G.), NATO (Collaborative Research Grant to C.G. and H.P.Nestler, Cold Spring Harbor Laboratory, NY, USA), Merck (Merck's Academic Development Program gift to C.G.) for financial support, Pharmacia&Upjohn for a postdoctoral research fellowship (to B.S.). REFERENCES AND NOTES 1. (a) For a recent review, see: Liskamp, R.M.J. Angew, Chem., Int.Ed.Engl. 1994, 33, 633; (b) Kim, J.-M.; Bi, Y.; Paikoff, S.J.; Schultz, P.G. Tetrahedron Lett. 1996, 37, 5305; (c) Kim, J.-M.; Wilson, T.E.; Norman, T.C.; Schultz, P.G. Tetrahedron Lett. 1996, 37, 5309; (d) Zuckermann, R.N.; Kerr, J.M.; Kent, S.B.H.; Moos, W.H. J. Am. Chem. Soc. 1992, 114, 10646; (e) Cho, C.Y.; Moran, E.J.; Cherry, S.R.; Stephans, J.C.; Fodor, S.P.A.; Adams, C.L.; Sundaram, A.; Jacobs, J.W.; Schultz, P.G. Science 1993, 261, 1303; (f) Burgess, K.; Linthicum, D.S.; Shin, H. Angew.Chem., Int.Ed.Engl. 1995, 34, 907; (g) Han, H.; Janda, K.D.J. Am. Chem. Soc. 1996, 118, 2539; (h) Moree, W.J.; van der Marel, G.A.; Liskamp, R.M.J.J. Org. Chem. 1995, 60, 5157; (i) Gennari, C.; Salom, B.; Potenza, D.; Williams, A. Angew.Chem., lnt.Ed.Engl. 1994, 33, 2067; (j) Gennari, C.; Nestler, H.P.; Salom, B.; Still, W.C. Angew.Chem., lnt.Ed.Engl. 1995, 34, 1763 and 1765. (a) Gennari, C.; Salom, B.; Potenza, D.; Longari, C.; Fioravanzo, E.; Carugo, O.; Sardone, N. Chem.Eur.J. 1996, 2, 644; (b) Moree, W.J.; Schouten, A.; Kroon, J.; Liskamp, R.M.J. Int.J.Peptide Protein Res. 1995, 45, 501; (c) Calcagni, A.; Gavuzzo, E.; Mazza, F.; Pinnen, F.; Pochetti, G.; Rossi, D. Gazz.Chim.h. 1992, 122, 17. Dujols, F.J.M.; Mulliez, M.E.J. Org. Chem. 1996, 61, 5648. C. Gennari, B. Salom, D. Potenza (Universit~ di Milano) IT Pat. Appl. MI94 A 000989 (17 May 1994); (Pharmacia &Upjohn) Intern. Appl. PCT/EP 95/01788 (11 May 1995). (a) Moree, W.J.; van der Marel, G.A.; Liskamp, R.M.J.Tetrahedron Lett. 1992, 33, 6389; (b) Moree, WJ.; van der Marel, G.A.; Liskamp, R.M.J. ibid. 1991,32, 409; (c) Moree, W.J.; van Gent, L.C.; van der Marel, G.A.; Liskamp, R.MJ. Tetrahedron 1993, 49, 1133; (d) de Bont, D.B.A.; Moree, W.J.; Liskamp, R.M.J. Biorganic&Medicinal Chemistry 1996, 4, 667. l lth International Conference on Organic Synthesis (ICOS-II), June 30-July 4, 1996, Amsterdam, The Netherlands, Book of Abstracts, (a) Loewik, D.W.P.M.; Mulders, SJ.E.; Cheng, Y.; Still, W.C.; Liskamp, R.M.J. PO-033, page 169, (b) de Bont, D.B.A.; Moree, W.J.; Liskamp, R.M.J. PO-303, page 443. Higashiura, K.; Morino, H.; Matsuura, H.; Toyomaki, Y.; Ienaga, K. J.Chem.Soc. Perkin Trans 1 1989, 1479. This synthetic route was recently "rediscovered" by Di Bella et al. : (a) Braghiroli, D.; Di Bella, M. Tetrahedron: Asymmetry 1996, 7, 2145; (b) Braghiroli, D.; Mussati, E.; Di Bella, M.; Saladini, M. Tetrahedron: Asymmetry 1996, 7, 831. Reynolds, R.C.; Crooks, P.A.; Maddry, J.A.; Akhtar, M.S.; Montgomery, J.A.; Secrist II], J.A.J.Org.Chem. 1992, 57, 2983-2985. For a solution to this problem using TMS-silylated amines and no extra base, see : (a) Paik, S.; White, E.H. Tetrahedron 1996, 52, 5303; (b) Paik, S.; White, E.H. Tetrahedron Lett. 1996, 37, 4663. The progress of the coupling reaction to give 10 was monitored using bromophenol blue as a simple acid- base indicator to test for the presence of free amines on the support, see: Krchfi~ik, V.; V:4gner, J.; Saffir, P.; Lebl, M. Collect. Czech. Chem. Comm. 1988, 53, 2542-2548. Alternatively, the residual free amines were capped with Ac20, the compound cleaved from the resin (MeOH, Et3N), and the crude cleavage mixture analysed by 1H-NMR spectroscopy for the presence of the CH3CON signal. The yield of 11 was calculated based on the loading of resin 9, which was determined by the picric acid method (Stewart, J.F.; Young, J.D. Solid Phase Peptide Synthesis, Pierce Chemical: Rockford, Illinois, 1984). See references l i and 2a for conformational studies on vinylogous ulfonopeptides 2. 2. 3. 4. 5. 6. 7. 8. 9. 10a. 10b. 11. (Received in UK 4 September 1996; revised 1 October 1996; accepted 4 October 1996) 

A New Method for the Solution and Solid Phase Synthesis of Chiral b-Sulfonopeptides Under Mild Conditions.

A New Method for the Solution and Solid Phase Synthesis of Chiral b-Sulfonopeptides Under Mild Conditions.

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