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Ammonium Formate Catalytic Transfer Hydrogenation:A Convenient Method for Removal of Halogenated Benzyloxycarbonyl and Benzyl Protecting Groups in Peptide Synthesis

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Ammonium Formate Catalytic Transfer Hydrogenation:A Convenient Method for Removal of Halogenated Benzyloxycarbonyl and Benzyl Protecting Groups in Peptide Synthesis Indian Journal of Chemistry Vol. 39B, July 2000, pp. 504- 508 Ammonium formate catalytic transfer hydrogenation:A convenient method for removal of halogenated benzyloxycarbonyl and benzyl protecting groups in peptide synthesis D Channe G o wda·, B R ajesh & Shankare Gowd a Department of Studies in Chemistry, Uni versit y of Mysore, Manasagangotri , Mysore 570006, India Received 10 August ! 998; accepted (revised) 28 January 2000 A new appli cati on of ammonium fo rmate catalyti c transfer hydrogenolysis, in the presence of pall adised carbon, for removal of 2-chl orobenzyloxycarbony l (2-ClZ), 2,6-di chl orobenzyl (2,6-C 12Bzl), bro mobenzylo xycarbonyl (B rZ) and phenacyl ester (OPa) from amino acids, pepti des and hi gh molecul ar weight polymers is reported. Rapid and selecti ve removal of protecting groups un­ ti on and subsequent W:-branching were enhanced by der moderate, neutral and ambient conditions is often removal of the Boc group during each cycle by treat­ a necessary step in the a, rea of the peptide chemistry. ment for I hr with 50% TFA-CH2Ch when the Z A number of reagents have been developed for this group was used for N £-protection of lysine. purpose. The utility of ammonium formate in the Subsequently these problems were solved by em­ presence of I 0% palladium on carbon as reducin g pl oying more acid sta(?le 2-chlorobenzyloxycarbonyl agent for vari ous functi onal groups has been reviewed (2-CIZ) 11 group for W:-protection of lys in e and 2,6- 12 by Ram and Ehrenkanfer in 198i. It has been suc­ dichl orobenzy l (2,6-CI 2 Bzl) group for 0-protection cessfull y employed for the removal of some com­ of Tyr. However, removal of these groups by hydro­ monl y used protecting groups like benzy loxycarbonyl genation, even after several days, was incomplete. (Z), benzy l ester, benzy l ether and nitro groups from This necessitates the use of even more vi gorous 2 peptides . Further, it has been applied to the release acidic condition during the fin al stages of the synthe­ of peptides from the Merrifield polystyrene resin un­ sis. As a result, these groups were commonly de­ 13 der moderate reacti on condition and pressure in a bl ocked by low HF treatment for 2 hr • The HF 3 neutral medium .4. Several further applicati ons of thi s treatment of longer peptides always results in low system have since been described; reducti on of aro­ yield . Our own experi ence of synthesis of protein­ mati c of aldehydes and ketones to methylene deri va­ based hi gh molecul ar weight polymers, HF treatment 6 ti ves5, reduction of nitroketals to aminoketals , the produces low yield and low molecul ar weight poly­ 14 selecti ve deprotecti on of the 2-hydroxy group in mers . 7 meth yl 2,3-d i-O-benzy lidene-a-D-gluco-pyranoside , We report herein th at ammonium fo rmate catalytic the selective reducti on of the heterocycli c rings in transfer hydrogenati on can be conveniently employed 8 quinolines and isoquinolines , reducti on and cyclisa­ for the removal of 2-chlorobenzyloxycarbonyl groups 9 ti on of orthonitro-omega-nitrostyrenes to indoles and of Lys(2-CIZ), 2,6-dichlorobenzyl group of Tyr(2,6- 10 deoxygenati on of heteroaromatic N-oxides . Ch Bzl) and bromobenzyloxycarbonyl group of The protecti on of Lys residues as Lys(Z) or the Tyr(BrZ) which are commonly used in peptide syn­ masking of Tyr residues as Tyr(Bzl) would not pre­ thesis and debl ocked by HF treatment. Further, vent signi ficant side chain branching when tert­ phenacyl esters of carboxyl function can al so be de­ butyloxycarbonyl (Boc) group is used for Na-protec­ bl ocked. Typi cal examples of the applicati on of this ti on. The benzyl group is unsati sfactory for 0 - procedure are li sted in Table I. To address the issue protection of Tyr not onl y because of its rapid re­ of its applicability to biologicall y ac tive peptides, moval in 50% TFA but also because of its partial re­ leucine-enkephalin, Boc-Tyr (2,6-C I2Bzl) - Gly - Gl y­ arrangement to give 3-benzyltyrosine. W:- Deprotec- Phe-Leu-OC4H9-t was deblocked. The debl ocking was GOWDA eta/.: AMMONIUM FORMATE CATALYTIC TRANSFER HYDROGENATION 505 Table 1-Products obtained after removal of 2-CIZ, 2,6-Cl2Bzl, 2-BrZ and OPa from Lys(2-CIZ), Tyr(2,6-CJ 2Bzl), Tyr(Brz) and Boc­ Pro-OPa by ammonium formate catalytic transfer hydrogenation Substrate Product Reaction Yield mp (°C) [ a]p Specific Rotation period (%) Found Reported 17 Found Reported 17 (min) Boc-Lys(2-CIZ) Boc-Lys 10 95 202-203 202-203 - 10. 8° (c 0.9, AcOH) -10.7° (c 0.87) AcOH) Boc-Tyr(2,6-C 12 Bzl) Boc-Tyr IS 90 136-37 136-37 +3.1 o (c 2.1, CH30H) + 3.9°(c 2.04, AcOH) Boc-Tyr(2-BrZ) Boc- Tyr 10 92 136-37 136-37 +3.1 o (c 2.1, CH30H) + 3.9°(C 2.04, AcOH) Boc-Pro-OPa Boc-Pro 10 95 135-36 134-36 - 68 .0° (c I, AcOH) - 68.5° (c I, AcOH) completed in less than 15 min (as monitored by TLC) amino acids, peptides and polymers. This system be­ with a good yield of about 92%. The resulting pep­ ing readily available and easy to operate provides tide, Boc-Tyr-Gly-Gly-Phe-Leu-OC4H9-t was re­ products in good yields, and in certain cases in high markably homogeneous product which corresponded purity without necessity for further purification. 2 in all respects to a highly purified authentic sample • To further address the issue of its applicability to Experimental Section large and more complex peptides and proteins, high General. All the amino acids used were of molecular weight elastic protein - based polymers are L-configuration unless otherwise indicated. All Boc­ deblocked. This method of deblocking gave a yield of amino acid derivatives were purchased from Ad­ about 87% of poly[9(GVGVP), (GYGVP)] as against vanced Chern Tech (Louisville, Kentucky, USA). 52% of yield obtained when HF was used for de­ Ammonium formate was purchased from E Merck blocking15. The yields of poly(GKGIP) and and 10% palladium on carbon was obtained from Al- poly[4(GVGIP), (GKGIP)] are 89% and 87% as drich Chemical Company (USA). All solvents and 6 compared to 67% and 68%, respectivel/ . The pro­ reagents were of analytical grade or were purified tected peptides and polymers used as substrates were according to procedures recommended for peptide 17 obtained in our laboratory as intermediates in the synthesis . Thin layer chromatography (TLC) was synthesis of analogues of elastic protein - based carried out on silica gel plates obtained from What­ polymers. man, Inc., using the following solvent systems: chlo­ The procedure is simple. A solution of protected roform-methanol-acetic acid (95:5:3), chloroform­ amino acid or peptide or polymer in methanol or methanol-acetic acid (90: I 0:3) and chloroform­ DMF containing 10% palladium on carbon (1/10 to methanol-acetic acid (85: 15 :3) to check the homoge­ 115 the weight of peptide) and ammonium formate (2 neity and completion of deblocking of the peptides to 4 equivalent) is stirred at room temperature. After and amino acid derivatives. The compounds on TLC completion of hydrogenolysis, which cau be moni­ plates were detected by UV light, after spraying with tored by TLC or carbon-13 NMR spectra, the catalyst ninhydrin or by chlorine/toluidine. Melting points is filtered off and the filtrate evaporated to dryness. were determined on a Thomas Hoover melting point To remove excess ammonium formate, the product is apparatus and are uncorrected. Optical rotations were either dialyzed and lyophilized or dissolved in an or­ determined on a Perkin-Elmer 241 MC polarimeter. ganic solvent and washed with saturated NaCI solu­ Carbon-13 NMR spectra were obtained on an FX-100 tion. The yields are virtually quantitative and the instrument (Jeol, Japan). Amino acid analysis was products are analytically pure. Verification of the performed on a water HPLC Pico-Tag analyzer. Hy­ products is achieved either by comparing with drolysis of the sample was carried out using 6N HCI authentic sample or by amino acid analysis and also containing I% phenol by volume at I I 0 oc for 72 hr by NMR data. The presence of all requisite peaks and in a sealed tube under vacuum from which the air had the absence of extraneous peaks verifies the de­ been removed using nitrogen. blocking satisfactorily. Hydrogenolysis of amino acid derivatives. A In conclusion, we describe a new application of solution of the protected amino acid derivative (0.5g) ammonium formate/palladium on carbon for removal in methanol (5 mL) containing I 0% palladium on of 2-chlorobenzyloxycarbonyl, 2,6-dichlorobenzyl, 2- carbon (0. I g) and amonium formate (0.4g) was stirred bromobenzyloxycarbonyl 'and phenacyl ester from at room temperature. After completion of the hydro- 506 INDIAN J CHEM, SEC 8, JULY 2000 genolysis (monitored by TLC), the mixture was fil­ lution of poly[GK{2-CIZ}GIP] (0.5g, 0.8 mrnole) in tered through Celite and the catalyst washed with DMF (30 mlL) containing I 0% Pd-C (0.2g) and am­ meth~nol. The combined washings and filtrate was monium formate (0.5g) was stirred at room tempera­ evaporated in vacuo to an oil, which was extracted ture for 2 hr. The catalyst was filtered off and the sol­ into chloroform (2x25 mL), the extract was washed vent removed under reduced pressure.
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