A convenient method for the synthesis of β-amino acids via the Arndt-Eistert approach using p-toluenesulphonyl chloride as a carboxylic group activating agent

Article abstract of DOI:10.1081/SCC-120002414

A simple method for the synthesis of Z-/Boc-/Fmoc-protected β-amino acids by the Arndt-Eistert approach employing p-toluenesulphonyl chloride for the activation of the carboxyl group of N^α-protected amino acid is described. The method is rapid and gave good yields with opitical purity.

Full text of DOI:10.1081/SCC-120002414

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A CONVENIENT METHOD FOR THE SYNTHESIS OF β-
AMINO ACIDS VIA THE ARNDT-EISTERT APPROACH
USING p-TOLUENESULPHONYL CHLORIDE AS A
CARBOXYLIC GROUP ACTIVATING AGENT
Ganga-Ramu Vasanthakumar a & Vommina V. Suresh Babu b
a Department of Studies in Chemistry , Bangalore University , Central College Campus,
Bangalore, 560 001, India
b Department of Studies in Chemistry , Bangalore University , Central College Campus,
Bangalore, 560 001, India
Published online: 16 Aug 2006.
To cite this article: Ganga-Ramu Vasanthakumar & Vommina V. Suresh Babu (2002) A CONVENIENT METHOD FOR
THE SYNTHESIS OF β-AMINO ACIDS VIA THE ARNDT-EISTERT APPROACH USING p-TOLUENESULPHONYL CHLORIDE AS A
CARBOXYLIC GROUP ACTIVATING AGENT, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 32:4, 651-657, DOI: 10.1081/SCC-120002414
To link to this article: http://dx.doi.org/10.1081/SCC-120002414
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SYNTHETIC COMMUNICATIONS, 32(4), 651–657 (2002)
A CONVENIENT METHOD FOR
THE SYNTHESIS OF b-AMINO ACIDS
VIA THE ARNDT-EISTERT APPROACH
USING p-TOLUENESULPHONYL
CHLORIDE AS A CARBOXYLIC
GROUP ACTIVATING AGENT
Ganga-Ramu Vasanthakumarand
Vommina V. Suresh Babu*
Department of Studies in Chemistry,
Central College Campus, Bangalore University,
Bangalore 560 001, India
ABSTRACT
A simple method for the synthesis of Z-/Boc-/Fmoc-protected
b-amino acids by the Arndt-Eistert approach employing
p-toluenesulphonyl chloride for the activation of the carboxyl
group of Na-protected amino acid is described. The method is
rapid and gave good yields with opitical purity.
b-Amino acids exhibit a wide spectrum of biological importance simi-
lar to natural a-amino acids. They serve as building blocks for the synthesis
of b-peptides,1 and b-lactam antibiotics.2 Seebach3 and Gellman4,5 groups
reported that b-peptides also form stable secondary structures like their
*Corresponding author.
651
Copyright & 2002 by Marcel Dekker, Inc.
www.dekker.com

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VASANTHAKUMAR AND SURESH BABU
a-peptide counterpart. b-Peptides shared by proteinaceous side chains of
a-amino acids have been shown to be stable to common a-peptidases for
few days.6 The Arndt-Eistert method for the homologation of a-amino acids
is one of the important routes for the synthesis of b-amino acids. This route
involves the synthesis of commonly used a-aminoacyldiazomethanes from
the Na-protected amino acids. For this, the activation of carboxylic group of
a-amino acid employing the corresponding mixed anhydrides1,3,7 (using iso-
butoxycarbonyl chloride or ethoxycarbonyl chloride) or acid chlorides8 or
acid fluorides9 or activated esters10 has been used.
The use of p-toluenesulphonyl chloride (TsCl) in organic synthesis
is widely known. It has been used for the activation of carboxylic group
in the synthesis of amides and esters of simple carboxylic acids,11 for
the synthesis of peptides,12 b-lactams13 etc. This paper describes the use of
TsCl for the activation of carboxylic group in the synthesis of Na-protected
aminoacyldiazomethanes.
As shown in Scheme 1, Na-protected aminoacyldiazomethane was
prepared by addition of TsCl and diazomethane (CH2N2) in dichloro-
methane (CH2Cl2) to Na-protected amino acid in THF at 0ꢀC in presence
of pyridine. The reaction proceeds through the formation of carboxylic–
sulphonic mixed anhydride which was identified by IR spectra (1825–
1835 cmꢁ1). The reaction was complete in about 30 min. All the resulting
diazomethane derivatives were isolated as crystalline solids in good yield.
They have been analysed by using IR by the presence of characteristic band
1
at around 2105–2108 cmꢁ1 (-COCHN2 group) and H NMR. They were
Scheme 1. Synthesis of N-protected-b-homoamino acids.

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SYNTHESIS OF b-AMINO ACIDS
653
then converted to their corresponding b-amino acids by the Wolff rearran-
gement using silver benzoate/1,4-dioxane/water by refluxing the mixture
for 6 h at 70ꢀC. All the N-protected b-amino acids were obtained as crystal-
line solids and were well characterised. The comparison of the determined
optical rotations of the D- and L-isomers of Fmoc-a-Phg and that of b-Phg
revealed that this method is completely free from racemisation.
Thus the present procedure is an efficient method for the synthesis
of Na-protected aminoacyldiazomethane using TsCl for carboxylic group
activation. The advantages of this method are (a) the mild reaction
conditions (b) the use of cheaper reagent (c) the general applicability
(d) the selectivity, and (e) the good yields.
EXPERIMENTAL
All the amino acids used, unless otherwise specified has L-configura-
tion only. Melting points reported are uncorrected. Thin layer chro-
matography (TLC) was carried out using the solvent systems: (i) ethyl
acetate : hexane (35 : 65), (ii) chloroform : methanol : acetic acid (45 : 2 : 1),
(iii) chloroform : methanol (9 : 1) and the Rf values are designated as RfA,
1
RfB and RfC respectively. H NMR spectra were recorded on a Brucker
AMX-400 MHz spectrometer. IR spectra were recorded on a Nicolet model
impact 400 D FT-IR spectrometer. Elemental analyses were carried out
by Perkin-Elmer Analyser and the samples were dried for 24 h under
vacuum before analysis. Diazomethane gas was generated by heating
N-nitroso-N-methyl-p-toluene sulphonamide with alcoholic KOH.14
General Procedure for Preparation of Na-Protected Aminoacyldi-
azomethane Derivatives: TsCl (0.210 g, 1.1 mmol) and pyridine (0.08 mL,
1 mmol) were added to a solution of Na-protected amino acid (1 mmol) in
THF (5 mL) and stirred at 0ꢀC for 15 min. The reaction mixture was treated
with a saturated solution of CH2N2 in dry CH2Cl2 (20 mL) and stirred
for 1 h at 0ꢀC. The progress of the reaction was monitored by TLC and
IR. After completion of the reaction, the excess of CH2N2 was decomposed
by the dropwise addition of acetic acid. The mixture was washed with
NaHCO3 (25 mL ꢂ 3), 5% HCl (25 mL ꢂ 3) and brine, dried over anhydrous
Na2SO4 and evaporated under reduced pressure. The resulting residue
was recrystallized using CH2Cl2/hexane to yield the title compound as a
crystalline solid.
Na-Fmoc-phenylalanyldiazomethane (IIa): Yield, 94%; m.p., 136ꢀC;
RfA, 0.62; RfB, 0.8; [a]2D5 þ 16.5 (c ¼ 1, CHCl3); Anal. Calc. for
C25H23N3O3: C, 72.60; H, 5.60; N, 10.16; Found: C, 72.35; H, 5.57; N,

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VASANTHAKUMAR AND SURESH BABU
1
9.78; IR (gmax): 2108 cmꢁ1 (-COCHN2); H NMR (CDCl3, d): 3.1 (2H, d),
4.2 (2H, m), 4.5 (2H, d), 5.2 (1H, s), 5.4 (1H, br) and 7.2–7.7 (13H, m).
Na-Fmoc-valyldiazomethane (IIb): Yield, 85%; m.p., 124–125ꢀC; RfA,
0.69; RfB, 0.81; [a]D25 ꢁ 23.40 (c ¼ 1, CHCl3); Anal. Calc. for C21H21N3O3: C,
69.4; H, 5.81; N, 11.56; Found: C, 69.28; H, 5.92; N, 11.46; IR (gmax):
1
2105 cmꢁ1 (-COCHN2); H NMR (CDCl3, d): 0.90 (6H, d), 1.75 (1H, m),
4.10 (1H, m), 4.25 (1H, m), 4.45 (2H, m), 5.30 (1H, s), 5.4 (1H, d) and
7.3–7.9 (8H, m).
Na-Fmoc-phenylglycinyldiazomethane (IIc): Yield, 90%; m.p., 148–
149ꢀC; RfA, 0.66; RfB, 0.80; [a]D25 ꢁ 32.3 (c ¼ 1, CHCl3); Anal. Calc. for
C24H29N3O3: C, 72.50; H, 4.81; N, 10.57; Found: C, 72.68; H, 4.87; N,
1
10.6; IR (gmax): 2107 cmꢁ1 (-COCHN2); H NMR (CDCl3, d): 4.25 (2H,
m), 4.4 (2H, d), 5.1 (1H, s), 6.05 (1H, br) and 7.2–7.8 (13H, m).
Na-Fmoc-D-phenylglycinyldiazomethane (IId): Yield, 92%; m.p.,
153ꢀC; RfA, 0.7; RfB, 0.83; [a]D25 þ 32.3 (c ¼ 1, CHCl3); Anal. Calc. for
C24H29N3O3: C, 72.50; H, 4.81; N, 10.57; Found: C, 72.43; H, 4.90;
1
N, 10.52; IR (gmax): 2106 cmꢁ1 (-COCHN2); H NMR (CDCl3, d): 4.2 (2H,
m), 4.5 (2H, d), 5.1 (1H, s), 6.05 (1H, br) and 7.2–7.8 (13H, m).
Na-Boc-alanyldiazomethane (IIe): Yield, 93%; m.p., 82ꢀC; RfA, 0.72;
RfB, 0.82; [a]2D5 ꢁ 21.7 (c ¼ 1, CHCl3); Anal. Calc. for C, 50.69; H, 7.08;
N, 19.70; Found: C, 50.80; H, 7.20; N, 19.47; IR (gmax): 2106 cmꢁ1
1
(-COCHN2); H NMR (CDCl3, d): 1.05 (3H, d), 1.32 (9H, s), 4.21 (1H,
m), 5.25 (1H, s) and 5.5 (1H, br).
Na-Boc-valyldiazomethane (IIf ): Yield, 90%; m.p., 64ꢀC; RfA, 0.72;
RfB, 0.83; [a]D25 ꢁ 30.4 (c ¼ 1, CHCl3); Anal. Calc. for C11H19N3O3: C,
54.75; H, 7.93; N, 17.41; Found: C, 54.62; H, 7.78; N, 17.42; IR (gmax):
2107 cmꢁ1 (-COCHN2); 1H NMR (CDCl3, d): 0.92 (6H, d), 1.32 (9H, s), 1.75
(1H, m), 4.21 (1H, br), 5.20 (1H, s) and 5.42 (1H, br).
Na-Z-phenylalanyldiazomethane (IIg): Yield, 92%; m.p., 82–83ꢀC;
RfA, 0.68; RfB, 0.78; [a]D25 þ 48.2 (c ¼ 1, CHCl3); Anal. Calc. for
C18H17N3O3: C, 66.86; H, 5.29; N, 12.99; Found: C, 66.72; H, 5.35;
N, 12.86; IR (gmax): 2107 cmꢁ1 (-COCHN2); 1H NMR (CDCl3, d): 2.9
(2H, d), 4.45 (1H, m), 5.0 (2H, s), 5.22 (1H, s), 5.46 (1H, br) and 7.2–7.3
(10H, m).
Na-Z-alanyldiazomethane (IIh): Yield, 88%; m.p., 93–94ꢀC; RfA, 0.70;
RfB, 0.81; [a]2D5 ꢁ 50.3 (c ¼ 1, CHCl3); Anal. Calc. for C12H13N3O3: C, 58.29;
H, 5.29; N, 16.99; Found: C, 57.95; H, 5.13; N, 16.78; IR (gmax): 2107 cmꢁ1
1
(-COCHN2); H NMR (CDCl3, d): 1.2 (3H, d), 4.26 (1H, m), 5.1 (2H, s),
5.25 (1H, s), 5.5 (1H, br) and 7.32 (5H, s).
Na-Z-phenylglycyldiazomethane (IIi): Yield, 86%; m.p., 83–84ꢀC; RfA,
0.69; RfB, 0.79; [a]D25 ꢁ 36.4 (c ¼ 1, CHCl3); Anal. Calc. for C17H15N3O3:
C, 66.01; H, 4.88; N, 13.58; Found: C, 65.72; H, 4.76; N, 13.72; IR

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655
(gmax): 2108 cmꢁ1 (-COCHN2); 1H NMR (CDCl3, d): 4.5 (1H, br), 5.02
(2H, s), 5.22 (1H, s), 5.40 (1H, br) and 7.25 (10H, s).
General Procedure for Preparation of N-Protected b-Amino Acid: Silver
benzoate (5.7 mg, 2.5 ꢂ 10ꢁ2 mmol) was added to a solution of Na-protected
aminoacyldiazomethane (1 mmol) in 1,4-dioxane (10 mL) and water (5 mL)
and refluxed at 70ꢀC for 6 h. It was filtered and the solvent was evaporated
under reduced pressure. The residue was dissolved in saturated aqueous
Na2CO3 (20 mL) and stirred for 20 min. The mixture was washed with
ether (20 mL ꢂ 3). The aqueous layer was acidified to pH 2 and extracted
with ethyl acetate (20 mL ꢂ 3). The extracts were pooled, washed with water
(20 mL ꢂ 2), dried over anhydrous Na2SO4 and evaporated under reduced
pressure. The residue was precipitated using CH2Cl2/hexane to get the title
compound as a crystalline solid.
N-Fmoc-b-homophenylalanine (IIIa): Yield, 85%; m.p., 110–112ꢀC
RfB, 0.63; RfC, 0.75; [a]D25 ꢁ 26.2 (c ¼ 1, CHCl3); Anal. Calc. for C25H23NO4:
C, 75.05; H, 5.58; N, 3.70; Found: C, 75.21; H, 5.46; N, 3.80; IR (gmax):
1703 cmꢁ1 (CO of urethane); 1H NMR (CDCl3, d): 2.43 (1H, d), 2.52
(2H, m), 2.71 (1H, d), 3.6 (1H, m), 4.1 (1H, m), 4.2 (2H, m) and 7.3–7.8
(14H, m).
N-Fmoc-b-homovaline (IIIb): Yield, 80%; m.p., 153–154ꢀC; RfB, 0.60;
RfC, 0.78; [a]2D5 ꢁ 36.4 (c ¼ 1, CHCl3); Anal. Calc. for C21H23NO4: C, 70.14;
H, 5.89; N, 4.20; Found: C, 70.06; H, 5.67; N, 4.35; IR (gmax): 1705 cmꢁ1
(CO of urethane); 1H NMR (CDCl3, d): 0.85 (6H, d), 1.75 (1H, m), 2.30 (1H,
d), 2.45 (1H, d), 3.75 (1H, m), 4.30 (3H, m) and 7.3–7.8 (9H, m).
N-Fmoc-b-homophenylglycine (IIIc): Yield, 82%; m.p., 96–98ꢀC; RfB,
0.61; RfC, 0.78; [a]D25 ꢁ 22.2 (c ¼ 1, CHCl3); Anal. Calc. for C24H21NO4: C,
74.40; H, 5.45; N, 3.61; Found: C, 74.26; H, 5.23; N, 3.80; IR (gmax):
1705 cmꢁ1 (CO of urethane); 1H NMR (CDCl3, d): 2.5 (2H, d), 4.25
(2H, m), 4.4 (2H, d), 5.85 (1H, br) and 7.2–7.8 (13H, m).
N-Fmoc-b-D-homophenylglycine (IIId): Yield, 84%; m.p., 102ꢀC; RfB,
0.62; RfC, 0.79; [a]D25 þ 21.8 (c ¼ 1, CHCl3); Anal. Calc. for C24H21NO4: C,
74.40; H, 5.45; N, 3.61; Found: C, 74.34; H, 5.16; N, 3.76; IR (gmax):
1
1705 cmꢁ1 (CO of urethane); H NMR (CDCl3, d): 2.4 (2H, d), 4.25 (2H,
m), 4.3 (2H, d), 5.80 (1H, br), 7.2–7.8 (13H, m) and 8.3 (1H, br).
N-Boc-b-homoalanine (IIIe): Yield, 88%; m.p., 98–99ꢀC; RfB, 0.62;
RfC, 0.72; [a]2D5 ꢁ 18.1 (c ¼ 1, CHCl3); Anal. Calc. for C9H17NO4: C, 53.45;
H, 8.47; N, 6.92; Found: C, 53.28; H, 8.56; N, 6.56; IR (gmax): 1690 cmꢁ1
(CO of urethane); 1H NMR (CDCl3, d): 1.2 (3H, d), 1.33 (9H, s), 2.4 (2H, d),
4.01 (1H, m), 5.25 (1H, br) and 8.4 (1H, br).
N-Boc-b-homovaline (IIIf ): Yield, 86%; m.p., 65–66ꢀC; RfB, 0.61; RfC,
0.70; [a]2D5 ꢁ 20.2 (c ¼ 1, CHCl3); Anal. Calc. for C11H21NO4: C, 57.37;
H, 9.19; N, 6.08; Found: C, 56.90; H, 9.34; N, 5.82; IR (gmax): 1690 cmꢁ1

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VASANTHAKUMAR AND SURESH BABU
(CO of urethane); 1H NMR (CDCl3, d): 0.94 (6H, d), 1.32 (9H, s), 1.72 (1H,
m), 2.5 (2H, d), 4.20 (1H, br), 5.4 (1H, br) and 8.3 (1H, br).
N-Z-b-homophenylalanine (IIIg): Yield, 88%; m.p., 84–85ꢀC; RfB,
0.60; RfC, 0.68; [a]D25 ꢁ 36.2 (c ¼ 1, CHCl3); Anal. Calc. for C18H19NO4:
C, 69.21; H, 6.13; N, 4.48; Found: C, 69.63; H, 6.47; N, 4.32; IR (gmax.):
1
1698 cmꢁ1 (CO of urethane): H NMR (CDCl3, d): 2.36 (2H, d), 2.71 (2H,
d), 3.8 (1H, m), 5.01 (2H, s), 5.3 (1H, m), 7.07 (5H, s), 7.28 (5H, s) and
9.4 (1H, br).
N-Z-b-homoalanine (IIIh): Yield, 84%; m.p., 122ꢀC; RfB, 0.60; RfC,
0.69; [a]2D5 ꢁ 19.8 (c ¼ 1, CHCl3); Anal. Calc. for C12H15NO4: C, 61.00;
H, 6.39; N, 5.92; Found: C, 61.27; H, 6.23; N, 5.87; IR (gmax): 1700 cmꢁ1
(CO of urethane); 1H NMR (CDCl3, d): 1.2 (3H, d), 2.4 (2H, d), 3.6 (1H, m),
5.1 (2H, s), 5.4 (1H, br), 7.31 (5H, s) and 8.4 (1H, br).
N-Z-b-homophenylglycine (IIIi): Yield, 82%; m.p., 86–88ꢀC; RfB, 0.63;
RfC, 0.74; [a]2D5 ꢁ 18.5 (c ¼ 1, CHCl3); Anal. Calc. for C17H17NO4: C, 68.44;
H, 5.74; N, 4.95; Found: C, 68.23; H, 5.68; N, 4.67; IR (max): 1702 cmꢁ1
(CO of urethane); 1H NMR (CDCl3, d): 2.5 (2H, d), 4.2 (1H, m), 5.01 (2H, s),
5.46 (1H, br), 7.25 (10H, s) and 8.6 (1H, br).
ACKNOWLEDGMENTS
We are grateful to Prof. K. M. Sivanandaiah and B. S. Sheshadri for
usefull discussions, to Department of Science and Technology, Government
of India for financial support. GRVK thanks KSVN Trust for their
kind help.
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