MW-Enhanced High-Speed Deprotection of
Boc Group Using p -TsOH and
Concommitant Formation of N -Me-Amino
Acid Benzyl Ester p -TsOH Salts
Vommina V.Suresh Babu,Basanagoud S.Patil,and
Ganga-Ramu Vasanthakumar
Department of Studies in Chemistry,Bangalore University,
Bangalore,India
Abstract:A high-speed,complete deprotection of Boc group from Boc amino acids and protected peptide esters employing p -TsOH in toluene under microwave irradiation is found to be complete in 30s.The deprotection can be carried out in methanol and acetonitrile also.Under the prent conditions,C-peptide benzyl esters and O-benzyl ethers have been found to be stable.This has permitt
ed us to carry out the synthesis of [Leu]enkephalin employing the Boc /Bzl-group strategy.Further more,it has been found that both N a -Fmoc and N a -Z groups are completely stable.The prent conditions can be extended for the concomitant removal of the Boc group and the formation of C-benzyl amino acid esters as well.This has been utilized for the synthesis of N -Me amino acid benzyl esters starting from Boc-N -Me amino acids in a single step.
Keywords:Boc group,deprotection,microwave irradiation,N -Me amino acid benzyl esters
INTRODUCTION
For the chemical synthesis of peptides and small proteins in solution,the tert -butoxycarbonyl (Boc)group is the preferred urethane-type-protecting group
Received in India February 18,2005
Address correspondence to Vommina V.Suresh Babu,Department of Studies in Chemistry,Central College Campus,Bangalore University,(Dr. B.R.Ambedkar Veedhi),Bangalore 560001,India.E-mail:
Synthetic Communications w ,35:1795–1802,2005Copyright #Taylor &Francis,Inc.ISSN 0039-7911pr
int /1532-2432online DOI:
10.1081/SCC-200063953
1795
for N a -amino protection of amino acids over an equally popular benzyloxycar-bonyl (Z )group.This is primarily becau it is cleaved by relatively mild acids.Conquently,it is one of the most commonly employed amino-protect-ing groups in solution-pha peptide chemistry.[1]There is a necessity for the protection of veral functional groups prent in the side chains of the amino acids.The stability of such side chain–protecting groups during the elongation of the peptide chain and their removal at the end of the synthesis are the key factors in the lection of protecting groups.This has led to the u of combinations of Z /t Bu,Boc /Bzl,and Fmoc /t Bu strategy.[2–4]
In general,the Boc group is deprotected by using HCl /AcOH,dioxane EtoAc,anhydrous liquid HF,HBr /AcOH,10%H 2SO 4/dioxane,98%HCO 2H,and so for.[5]The u of Lewis acid BF 3.Et 2O in AcOH [
6]and veral silicon reagents such as SiCl 4-phenol,[7]TMSCl-phenol,[8]TMSI,[9]TMSOTf-TFA,[10]and TMSOTf-2,6-lutidine [11]have been explored.Boc-group deprotection was performed by using AlCl 3doped with neutral alumina under microwave (MW)irradiation for 1–2.5min (with 76–93%yield)[12]also.Removal of the Boc group using the strong acid TFA either neat or in combination with CH 2Cl 2(50%)is commonly preferred and more convenient than deprotection with mineral acids or 98%HCO 2H becau,after deprotection,excess of trifluoroacetic acid (TFA)can be easily and com-pletely removed (bp of TFA is 71–728C).The lective cleavage of the Boc group using an organic acid p -toluenesulfonic acid (p -TsOH)in ether /ethanol,1,4-dioxane,AcOH-CH 2Cl 2,or acetonitrile requires veral hours.[13–17]Over a period of time,the microwave irradiation technique found veral applications in organic synthesis [18–20]In the prent communi-cation,we describe the utility for the rapid removal of the Boc group in peptide synthesis using p -TsOH.
Initial experiments in the prent studies centered on the deprotection of the Boc group from amino acids under MW irradiation.(The microwave reaction was carried out in a LG MS 194A microwave oven producing microwave radiation with a frequency of 2450MHz.The microwave oven is 1200-W oven and the reaction was specifically carried out at 60%of the total power output,which would correspond to an average power of 720W.)In a typical procedure,a solution of Boc-amino acid in tolu
梦见别人身上有血
ene and p -TsOH was expod to MW irradiation (Scheme 1).The deprotection,as monitored by TLC,was found to be complete in 30s.Further more,depro-tection was confirmed by recording IR spectra (revealed by the abnce of a urethane carbonyl stretching vibrational frequency in the
region
Scheme 1.Deprotection of Boc group under MW irradiation.
V.V.Suresh Babu,B.S.Patil,and G.-R.Vasanthakumar
1796
1690–1705cm21).After cooling,in most of the cas,the resulting amino acid p-TsOH salt parated out as a solid.Filtration,washing with toluene, and,if necessary,recrystallization using suitable solvent resulted in the isolation of the product(Table1).The deprotection has been found to be complete withi
n30s when Boc derivatives were dissolved in acetonitrile and methanol also.Becau of their low bp,it was necessary to u a greater quantity of solvent.In addition,the u of toluene led to the paration of the amino acid ester salt as a solid,which can be easily isolated byfiltration.
The studies are then extended to demonstrate its utility for the removal of the Boc group in peptide synthesis.It has been found that the u of two equivalents of p-TsOH in toluene is sufficient to achieve the complete depro-tection under MW irradiation within30s(Table2).The deprotection of the Boc group from Boc-peptide esters when carried out using acetonitrile and MeOH was also found to be smooth and complete.As reported,the deprotec-tion of the Boc group using p-TsOH in acetonitrile at ambient temperature requires about3to4h.[13,14]The deprotection carried out by refluxing the toluene using an oil bath(around105–1108C)took about2h for the complete removal of the Boc group.Thus,employing the prent conditions, the removal of the Boc group from veral dipeptides as well as all the four Boc-protected intermediates in the synthesis of pentapeptide[Leu]enkephalin have been carried out(Scheme2).Thefinal pentapeptide was obtained in 63%yield[Yield63%;mp158–598C;found:C,60.48;H,6.83;N,12.40. C28H37N5O7calcd.for C,60.53;H,6.71;N,12.60%;[a]D25222.9(c¼1, DMF);1H NMR(d,CDCl3):0.83–0.9(6H,m),1.14(2H,t),1.6(lH,m), 2.65(4H,m),2.8–3.2(4H,m),3.4(2H,br)3.6–4.0(3H,m),5.7–6.1 Table1.Physical characteristics for the amino acid derivatives
Entry Protected amino acid Product Time
(s)
Mp
(8C)
Yield
(%)
1Boc-Ala-OH TsOH.H-Ala-OH3017096 2Boc-Val-OH TsOH.H-Val-OH3066–6895 2Boc-Phe-OH TsOH.H-Phe-OH30114–1698 2Boc-Cys(Bzl)-OH TsOH.H-Cys(Bzl)-OH30156–5794 3Boc-Trp-OH TsOH.H-Trp-OH45204–0596 4Boc-Glu(OBzl)-OH TsOH.H-Glu(OBzl)-OH30114–1692 5Fmoc-Lys(Z)-OH No reaction a———6Fmoc-Ser(Bzl)-OH No reaction a—137–38—7Fmoc-Asp(OBzl)-OH No reaction a—120–30—8Fmoc-Arg(Pmc)-OH No reaction a—132–33—9Z-N Me-Leu-OH No reaction a———a MW irradiation for15min in15cycles with720W of power has resulted in no change of the reactants and the starting compounds have been isolated completely. No decomposition was noticed.
Deprotection of Boc Group1797
(4H,m),7.2–7.6(9H,m),and 8.2(lH,s).Mass:(M þH)þ556.28.].Its purity,as analyzed by HPLC,was satisfactory.The 1H NMR and mass spectra also confirmed the same.
During this work,a systematic study regarding the stability of various protecting groups normally employed for carboxyl protection of amino acids and side chains of veral bifunctional amino acids has been carried out.It is found that amino acid benzyl,methyl,and ethyl esters are completely stable.The exposure of amino acid benzyl ester [Fmoc-Asp(OBzl)-OH]or peptide benzyl ester (H-Phe-Leu-OBzl)to MW irradiation for 15min in 15cycles with 720W of power has resulted in no change of the reactants (as analyzed by TLC as well as by IR).Further more,it has been found
that
Scheme 2.Synthesis of Leu-enkephalin;(i)coupling employing HBTU using stan-dard conditions,(ii)deprotection of Boc group using p -TsOH under MW irradiation,and (iii)CTH using 98%HCOOH /Pd-C /MeOH as solvent.
Table 2.List of peptides deportected using p -TsOH under MW irradiation Entry Protected peptide Product
a
Time (s)Mp (8C)Yield (%)1Boc-His-Pro-OMe H-His-Pro-OMe 30145892Boc-Ile-Gly-OMe H-Ile-Gly-OMe 30182–83923Boc-Phe-Ile-OMe H-Phe-Ile-OMe 30102964Boc-Pro-Phe-OMe H-Pro-Phe-OMe 30177–78885Boc-Val-Tyr-OEt H-Val-Tyr-OEt 30113–15866Boc-Ser-Gly-OBzl H-Ser-Gly-OBzl 30170907Boc-Arg(NO 2)-Ala-OBzl H-Arg(NO 2)-Ala-OBzl 30170–71828Boc-Gly-Leu-Val-OMe H-Gly-Leu-Val-OMe 30115–17889Boc-Val-Tyr-Pro-OBzl
H-Val-Tyr-Pro-OBzl
3050–60810Boc-Phe-Leu-OBzl H-Phe-Leu-OBzl 30174–768411Boc-Gly-Phe-Leu-OBzl
H-Gly-Phe-Leu-OBzl 30168–707812
Boc-Gly-Gly-Phe-Leu-OBzl
H-Gly-Gly-Phe-Leu-OBzl
新生代员工30
154–56
76
a
Mass and 1H NMR spectra are satisfactory.
V.V.Suresh Babu,B.S.Patil,and G.-R.Vasanthakumar
1798
关爱老人的作文N a -Fmoc and Z groups,Asp(OBzl),Ser(Bzl),and Arg(Pmc)have been found to be completely stable (Table 1).
Our group is half-way to the solution synthesis of cyclosporine O and its analogues in few gram quantities by employing Boc chemistry with the gment condensation approach.For this work,we needed to make large quan-tities of N -Me-Leu-OBzl and N -Me-Val-OBzl.In general,the preparation of N -Me amino acid benzyl esters has been carried out in two steps.In the first step,N a -protected (Boc,Z ,or Fmoc)N -Me amino acids have been converted to methyl or benzyl esters.Then,N a -protecting group was removed to obtain N -Me amino acid esters.[21–25]Conquently,we made initially Boc-N -Me-Leu-OBzl employing Boc-N -Me-Leu-OH,Cs 2CO 3,and benzyl bromide following the reported procedure.[26]After the deprotection of Boc group using TFA from the resulting Boc-N -Me-Leu-OBzl,N -Me-Leu-OBzl was obtained in 47%yield.Conquently,we attempted to deblock the Boc group using p -TsOH under MW irradiation.The deprotection was found to be clean and complete.Bad on the results as well as our recently published method for the synthesis of amino acid benzyl ester p -TsOH salts,[27]we reasoned that both the deprotection of the Boc group and simultaneous formation of the benzyl ester can be accomplished in a single step (Scheme 3).It is found that the u of two equivalents of p -TsOH along with BOC-N -Me-Leu-OH and benzyl alcohol in toluene and exposure of the mixture to MW irradiation for 30–45s has resulted in the direct formation of N -Me amino acid benzyl ester p -TsOH salt in one pot.Under the conditions,we have prepared few other benzyl esters required for our synthesis (Table 3).
In conclusion,the studies demonstrate that the Boc group can be depro-tected very rapidly in solution synthesis of peptides.The u of two equivalents of p -TsOH is sufficient for the complete removal of the Boc group from peptides in 30s.On the other hand,deprotection of the Boc group at ambient temperature needs veral hours.After the deprotection,the resulting peptide ester p -TsOH salts can be easily converted to the corresponding amino free amino acid ester and employed for the further extension of the chain.In “green chemistry,”which aims to reduce the u of toxic chemicals,the u of p -TsOH in place of TFA (regularly ud in peptide synthesis)may find wide acceptability.The utility of the prent procedure for the removal of the Boc group in solid-pha synthesis of [Leu]enkephalinamide is in progress.
Further,the synthesis of N -Me amino acid benzyl ester is usually accom-plished in a two-step process involving the conversion of Boc or Fmoc-N
赞美的四字成语
-Me
Scheme 3.Concomitant deprotection of Boc group and formation of N -Me amino acid benzylester TsOH salt.
Deprotection of Boc Group 1799
amino acid to the corresponding benzyl ester and then the removal of protec-ting group.It is now demonstrated that the synthesis of N -Me amino acid benzyl esters in a single operation rapidly and with high yields under MW irradiation technique can be accomplished.
药店实习周记EXPERIMENTAL
Melting points were determined by the capillary method and are uncorrected.IR spectra were recorded on a Nicolet model impact 400D FT-IR spectrometer (KBr pellets,3cm 21resolution).1H and 13C NMR spectra were recorded on a Bruker AMX 400-MHz spectrometer.Mass spectra were recorded on PE-SCIEX 150EX LCMS.Specific rotations were recorded on a Rudolf Rearch Autopol IV automatic polarimeter.The TLC analysis was carried out on precoated silica-gel plates using solvent systems (a)ethyl acetate–petroleum ether (35:65v /v)and (b)chloroform–methanol–acetic acid (40:2:1v /v).All solvents were freshly distilled prior to u.General Procedure for Boc-Group Deprotection
A mixture of Boc amino acid or peptide ester (1mmol)and p -TsOH (2mmol)in toluene (5mL)in a beaker was expod to microwave irradiation until the completion of the deprotection.On cooling to rt,the amino acid p -TsOH salt has been parated as a solid,which was filtered and washed with toluene (2mL).
In the ca of peptides,after the deprotection of the Boc group,the product was isolated as follows:the resulting peptide ester p -TsOH salt was
Table 3.p -TsOH-included MW-irradiated concomitant deprotection of the Boc group and formation of N -Me amino acid benzyl esters Entry Boc-N -Me amino acid Product a Time (s)Mp (8C)Yield (%)[a ]D 20(c ¼1,CHCl 3)241Boc-N -Me-Leu-OH H-N -Me-Leu-OBzl 30gum 9626.98(26.90,c ¼1,CHCl 3)2Boc-N -Me-D -Leu-OH H-N -Me-D-Leu-OBzl 30—95þ6.42(þ6.06,
夫妻约定财产c ¼0.90,CHCl 3)3Boc-N -Me-Val-OH H-N -Me-Val-OBzl 30—9225.13(25.20,c ¼1.18,CHCl 3)4
Boc-N -Me-D -Val-OH克罗地亚世界杯
H-N -Me-D-Val-OBzl
30
—推销和营销的区别
93
þ5.28(þ5.10,c ¼1,CHCl 3)
a
The IR analysis confirms the prence of benzyl ester and the abnce of the peak corresponds to urethane carbonyl vibrational stretching.Spectral data is given in the experimental ction.
V.V.Suresh Babu,B.S.Patil,and G.-R.Vasanthakumar
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