Simple conversion of fully protected amino acids to zwitterions

Article abstract of DOI:10.1016/j.tetlet.2012.01.033

An operationally simple and efficient method under mild acidic conditions was developed to convert fully protected amino acids to the corresponding zwitterions without either isoelectric precipitation or ion exchange chromatography.

Full text of DOI:10.1016/j.tetlet.2012.01.033

Tetrahedron Letters 53 (2012) 1433–1434
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Simple conversion of fully protected amino acids to zwitterions
⇑
Junliang Hao , Matt Reinhard, Steven S. Henry, Eric P. Seest, Matthew D. Belvo, James A. Monn
Discovery Chemistry Research and Technologies, Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, IN 46285, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
An operationally simple and efficient method under mild acidic conditions was developed to convert fully
protected amino acids to the corresponding zwitterions without either isoelectric precipitation or ion
exchange chromatography.
Received 28 November 2011
Revised 6 January 2012
Accepted 9 January 2012
Available online 20 January 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
mGlu receptors
Amino acids deprotection
Zwitterions
Microwave-assisted synthesis
Protecting groups are widely used in multi-step synthetic
organic chemistry.1 Judicious selection of protecting groups and
subsequent development of efficient methods for their removal
can greatly enhance the overall synthetic efficiency. Synthesis of
unnatural amino acids frequently involves multiple protecting
groups, and final manipulations of synthetic amino acids into their
zwitterionic form typically involve isoelectric precipitation or
laborious ion exchange chromatography.2 Reported here is a novel
and efficient method to convert fully protected amino acids to their
corresponding zwitterionic forms in an operationally simple and
high-yielding manner.
Recent work pertaining to the preparation of conformationally
constrained glutamic acid analogs from our laboratories has
led to the discovery of several group II selective metabotropic
glutamate receptor (mGlu) agonists, such as (1R,2S,5S,6S)-2-amin-
obicyclo[3.1.0]hexane-2,6-dicarboxylic acid (1, LY354740, Fig. 1),3
and a mixed mGlu2 agonist/mGlu3 antagonist, (lR,2S,4R,5S,6S)-2-
amino-4-methyl-bicyclo[3.1.0]hexane-2,6-dicarboxylic acid (2,
LY541850).4 To obtain the desired final products in the zwitter-
ionic form, the fully protected amino acids were hydrolyzed under
prolonged (24–48 h) strongly acidic (48% HBr)3 or strongly basic
(3 N NaOH) conditions4 at an elevated temperature (100 °C)
followed by either isoelectric precipitation3 or anion exchange
column chromatography (AG1-X8 resin, acetate form, eluents:
10% AcOH in water).4 These procedures are both time consuming
and labor intensive. To overcome these operational issues, we have
redesigned the protecting groups on these amino acids, and envi-
R
H
H
O
O
HO
OH
NH2
1, R = H (LY354740)
, R = Me (LY541850)
2
Figure 1.
sioned a simple and universal deprotection–isolation methodology
that does not require laborious ion exchange chromatography.
We reasoned that, since protecting groups, such as t-butyl ester,
and t-butyloxycarbonyl (Boc) carbamate, can each be removed un-
der relatively mild acidic conditions, fully protected compound 35
might be cleanly converted to its corresponding deprotected zwit-
terion with a mild acid followed by isoelectric precipitation. Fol-
lowing several preliminary trials, we found that in the presence
of excess MeSO3H in aqueous acetone at 60 °C for 18 h, all the pro-
tecting groups on 3 were cleanly cleaved (Scheme 1).6 Next we
tried to obtain the zwitterion through isoelectric precipitation,
but were disappointed to find that the methanesulfonic acid salt
of 2 was maintained throughout this process. Therefore, we
resorted to cation exchange chromatography (Dowex 50X8-100 re-
sin, H+ form, eluents: 10% pyridine in water) to convert the salt to
the zwitterionic form. Although the overall process with MeSO3H
led to a high yield (94%) of zwitterionic 2, lengthy and laborious
ion exchange chromatography was still involved, greatly impeding
access to compounds of this type.
We have previously used a 10% aqueous AcOH solution as an
eluent in the aforementioned anion exchange chromatography4
and have found that evaporation of this eluent under reduced
⇑
Corresponding author.
E-mail address: haoju@lilly.com (J. Hao).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.01.033

1434
J. Hao et al. / Tetrahedron Letters 53 (2012) 1433–1434
1) MeSO3H
We further applied these AcOH conditions to the deprotection
2) isoelectric precipitation
3) cation exchange
94%
of select natural amino acids to demonstrate the operational sim-
plicity and usefulness of this reaction to access these amino acids
in their zwitterionic form. Following the typical procedure, most
protected amino acids9 were cleanly converted to the zwitterionic
form in moderate to high yields (Table 1).10 Varying degrees of epi-
merization were observed with these amino acids,11 which might
constitute a limitation for this method.
H
H
H
O
O
O
O
O
or
HO
O
OH
H
AcOH/H2O (1:1)
160 oC, microwave
98%
NHBoc
NH2
2
3
In summary, an operationally simple and efficient method was
developed to convert appropriately fully protected amino acids to
their corresponding zwitterionic forms in high yields without
involving either isoelectric precipitation or ion exchange chroma-
tography. This method has been applied in the ongoing amino acid
based research efforts in order to access these unnatural amino
acids quickly and efficiently.
Scheme 1.
Table 1
O
AcOH/H2O
O
160 oC, 5 min
R
R
O
OH
NHBoc
NH2
References and notes
R
R
Yield (%)
e (%)
1. Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic Synthesis; John Wiley
& Sons: New York, 1999.
2. Moloney, M. G. Nat. Prod. Rep. 2002, 19, 597–616.
3. Monn, J. A.; Valli, M. J.; Massey, S. M.; Wright, R. A.; Salhoff, C. R.; Johnson, B. G.;
Howe, T.; Alt, C. A.; Rhodes, G. A.; Robey, R. L.; Griffey, K. R.; Tizzano, J. P.;
Kallman, M. J.; Helton, D. R.; Schoepp, D. D. J. Med. Chem. 1997, 40, 528–537.
4. Dominguez, C.; Prieto, L.; Valli, M. J.; Massey, S. M.; Bures, M.; Wright, R. A.;
Johnson, B. G.; Andis, S. L.; Kingston, A.; Schoepp, D. D.; Monn, J. A. J. Med. Chem.
2005, 48, 3605–3612.
84a
84
93
71
BocHN
H2N
HO
HO
73
81
88
91
5. Synthesis of this compound 3 will be reported in due course.
6. Experimental: A solution of 3 (1.70 g, 4.13 mmol) in acetone (0.23 M, 18 mL)
and water (4 M, 1.0 mL) was treated with methanesulfonic acid (21.69 mmol,
1.42 mL). The reaction was heated to 60 °C for 18 h with stirring. Cooled to
room temperature and concentrated in vacuo. The pH was adjusted to
HO
HO
HN
82
96
N
approximately
2 with 1 N aqueous NaOH solution. The solution was
Boc
concentrated to reduce the aqueous volume. The pH was further adjusted to
3 and the solid was collected via vacuum filtration. The filter cake was washed
with isopropyl alcohol. The solid was dried overnight at 50 °C under vacuum.
Proton NMR showed the material to be the methanesulfonic acid salt. It was
combined with the filtrate, and adjusted to pH ꢀ2. This mixture was loaded on
a cation exchange column (Dowex 50X8-100, H+ form). The resin was then
washed with water, 50% aq THF, and water. Finally the product was eluted from
the column with 10% aqueous pyridine. Eluents containing products were
concentrated to a white crystalline solid. Additional water was added and the
mixture was concentrated in vacuo (3 times, to remove any residual pyridine)
to get the product 2 (0.82 g, 94%).
a
Acetic acid salt was obtained.
pressure consistently produces the final amino acids in its
zwitterionic form. We reasoned that if AcOH could be employed
to simultaneously remove both t-butyl ester and Boc protecting
groups, we might be able to avoid ion exchange chromatography
altogether and obtain zwitterionic amino acids after solvent re-
moval. Thus, after adjusting both the reaction temperature and
AcOH/H2O ratio, we were delighted to find that, by heating the
mixture of 3 in AcOH/H2O (1:1, v/v) at 160 °C in a microwave for
only 5 min, this fully protected glutamate analog 3 was cleanly
converted to zwitterion 2 in a 98% yield after simple evaporation
of solvents (AcOH and water) without any further purification or
manipulation!7 Although this reaction worked equally well under
conventional heating at lower temperatures, the reaction time
was much longer. This straightforward and fast microwave proce-
dure greatly reduced operational tediousness for this conversion,
and enabled us to rapidly prepare and isolate additional unnatural
amino acids of this type.8 Finally, it is interesting to mention that
the widely used trifluoroacetic acid (TFA) worked comparably
under identical conditions. However, it yielded the TFA salt of 2
7. A typical procedure is shown here: A mixture of 3 (0.8 g, 1.94 mmol) in acetic
acid (0.2 M, 9.7 mL) and water (0.2 M; 9.7 mL) was heated to 160 °C in a
Biotage Initiator microwave for 5 min. After cooled to rt, the reaction mixture
was concentrated in vacuo. Water was added and removed in vacuo (twice) to
remove the residual acetic acid. The residual was washed with iPrOH to afford 2
(0.44 g, 98%) as a white solid after drying.
8. Application of these conditions to the syntheses of other [3.1.0]-bicyclic amino
acid analogs for the group II metabotropic glutamate receptor will be published
in due course.
9. These fully protected amino acids were either commercially available or
synthesized through the standard protecting group manipulations.
10. The deprotection followed the procedure for 3. Isolation of final amino acids
was not optimized for best recovery.
11. The enantiomeric purity of the zwitterionic amino acids after deprotection was
determined by a chiral HPLC (column: 4.6 Â 100 mm Chirobiotic T, solvent: 70/
30 (v/v) 3A EtOH/H2O containing 0.2% formic acid, flow rate: 1 mL/min,
isocratic, detection: 205 nm).
(
19F NMR spectroscopy) after the evaporation of volatiles.