Efficient large-scale synthesis of Boc-L-azatyrosine

Article abstract of DOI:10.1080/00397910802406695

A rapid and convenient process for the synthesis of Boc-L-azatyrosine is described from commercially available Boc-β-iodo-Ala-OMe. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910802406695

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Efficient Large-Scale Synthesis
of Boc-L-azatyrosine
Hervé Germain a , Craig S. Harris a , Fabrice Renaud
a & Nicolas Warin a
a AstraZeneca, Centre de Recherches, Z. I. la
Pompelle, Reims , France
Published online: 08 Jan 2009.
To cite this article: Hervé Germain , Craig S. Harris , Fabrice Renaud & Nicolas
Warin (2009) Efficient Large-Scale Synthesis of Boc-L-azatyrosine, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 39:3, 523-530, DOI: 10.1080/00397910802406695
To link to this article: http://dx.doi.org/10.1080/00397910802406695
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Synthetic Communications1, 39: 523–530, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802406695
Efficient Large-Scale Synthesis
of Boc-L-azatyrosine
´
Herve Germain, Craig S. Harris, Fabrice Renaud, and Nicolas Warin
AstraZeneca, Centre de Recherches, Z. I. la Pompelle, Reims, France
Abstract: A rapid and convenient process for the synthesis of Boc-L-azatyrosine
is described from commercially available Boc-b-iodo-Ala-OMe.
Keywords: Hydroxy-deboronation, large scale, Negishi cross-coupling
L-Azatyrosine is an important naturally occurring amino acid first
isolated from Streptomyces chibaensis[1] and a known antibiotic; it has
also shown activity as an anticancer agent.[2] Recently, we became
interested in the synthesis of L-azatyrosine as a structural template in
one of our anticancer programs. Despite its relatively simple structure,
there are few efficient chemical syntheses[3–6] to provide large quantities
of optically pure L-azatyrosine, and it is very expensive to buy.
We initially considered two approaches by Myers et al., who reported a
stereoselective synthesis from 2-methyl-5-hydroxypyridine involving
five chemical steps with an overall yield of 17%,[3] and Sooper et al.,
who reported a racemic total synthesis involving a key enzymatic resolu-
tion step to afford, after five chemical steps, optically pure L-azatyrosine
in an overall yield of 12%.[4] While both syntheses had their merits,
they both involved at least five chemical steps with poor overall yields
and chromatography after each step.
Received August 5, 2008.
Address correspondence to Craig S. Harris, AstraZeneca, Centre de
Recherches, Z. I. la Pompelle, BP1050, 51689 Reims Cedex 2, France. E-mail:
craig.harris2@astrazeneca.com
523

524
H. Germain et al.
Herein, we report a very efficient and reliable formal synthesis of
Boc-L-azatyrosine (1) in just three steps from commercially available
Boc-b-iodo-Ala-OMe.
RESULTS AND DISCUSSION
While there are various asymmetric a-amino acid synthetic approaches
available,[7] even nowadays none offers the certainty of near-total chiral
transfer or generation of absolute stereochemical configuration at the
a-carbon.[7] The Negishi style alkyl-aryl cross-coupling reaction with
various halopyridine precursors using the homochiral organozinc reagent
derived from L-serine was recently reported with total retention of
absolute configuration by Jackson et al.[8,9] Therefore, we based our
synthesis on this direct Negishi cross-coupling using the organozincate
(2).[9] We envisaged two possible disconnections based on this approach:
a two-step process based on a successful direct cross-coupling using
O-protected variants of 2-bromo-5-hydroxypyridine (3) as the precursors
followed by deprotection to liberate the phenol, and a three-step process
relying on the boronation and subsequent hydroxy-deboronation of
known bromide (5) (Scheme 1).[9]
Scheme 1. Proposed retrosynthetic analysis. Our strategy concentrated on trans-
forming known intermediate 5 to 1 via hydroxy-deboronation.

Boc-L-Azatyrosine Synthesis
525
Our first attempts used O-protected variants of 2-bromo-5-hydroxy-
pyridine (3) as the precursors, but we never detected any coupled pro-
duct during the reaction; instead, only reduction of the Ar-Br bond
was observed. A successful cross-coupling has already been reported
with O-protected 2-iodo-5-hydroxypyridine.[6] However, this required
preparation of the iodo pyridine precursor and two full equivalents of
the organozinc reagent, generated from the expensive Boc-b-iodo-
Ala-OMe, which would have been very costly on a large scale. The
synthesis of the corresponding 5-bromo analog (5) was reported in a
yield of 60%.[9] This and subsequent boronate ester (4) were very ver-
satile intermediates, offering useful diversification points for the synth-
esis of other potentially interesting compounds for our program.
Therefore, we decided to focus on the synthesis of 5, with the hope
of transforming the bromide to the hydroxy function by passing via
a mild oxidative hydroxy-deboronation procedure such as that recently
reported for boronic acids and esters at C-3 of pyridines by Voisin
et al. (Scheme 1).[10]
Our synthesis of Boc-L-azatyrosine (1) started from commercially
available Boc-b-iodo-Ala-OMe, which was can also be conveniently
prepared from L-serine in three steps.[11]
As mentioned previously, the 5-bromo pyridine amino acid ester (5)
has already been prepared by palladium-catalyzed cross-coupling of
serine organozinc reagents with 2,5-dibromopyridine.[9] We were able
to routinely reproduce the yields reported,[9] thus affording 5 in yields
varying between 50 and 70%.
The transformation of 5 to boronate 4 was achieved under standard
boronation conditions using a slight excess of bis(pinacolato)diboron in
the presence of Pd(dppf)Cl2 ꢀ CH2Cl2 to afford a quantitative yield of
the corresponding boronate (4), which, after a simple workup, was used
without further purification in the next step.
Oxidation of the resulting boronate (4) to phenol (1) was achieved
using aqueous hydrogen peroxide at 0 ꢁC.[10] The authors note that the
use of hydrogen peroxide was essential to avoid the formation of the
corresponding pyridine N-oxide consistently observed even using just a
slight excess of mCPBA (Scheme 2).
To check the molecular structure, crystals of L-azatyrosine were
prepared after deprotection of (1) in refluxing 2M HCl=EtOH (Fig. 1).
To our knowledge, this is the first x-ray of L-azatyrosine reported in
the literature.
In conclusion, we have developed a very efficient and reproducible
large-scale synthesis of chiral Boc-L-azatyrosine methyl ester, using just
three chemical steps and two chromatographic purifications, in an overall
yield of 43% from Boc-b-iodo-Ala-OMe.

526
H. Germain et al.
Scheme 2. Reagents and conditions: (a) (i) Zn(s), 1,2-dibromoethane, 90 ꢁC, then
TMS-Cl, rt, 1 h followed by 6, 35 ꢁC, 2 h; (ii) 2,5-dibromopyridine, Pd(Ph3P)2Cl2,
68 ꢁC, 2 h, 54%; (b) bis(pinacolato) diboron, Pd(dppf)Cl2 . CH2Cl2, KOAc,
dioxane, 85 ꢁC, 16 h, quantitative; (c) H2O2, DCM, 0 ꢁC to rt, 16 h, 80%.
Figure 1. Molecular structure of L-azatyrosine with atomic numbering.

Boc-L-Azatyrosine Synthesis
527
EXPERIMENTAL
General Methods
1H NMR spectra were recorded on a Bruker Biospin Avance 500 spectro-
meter. Chemical shifts are reported as d values downfield from internal
TMS in appropriate organic solutions. The purity and the structures of
the products were confirmed by liquid chromatography-mass spectro-
metry (LCMS) on a Waters 2690 photodiode array detector system using
the following conditions: column, Symmetry C-18; solvent A, water 0.1%
formic acid; solvent B, CH3CN; flow rate, 2.5 ml=min; run time, 4.5 min;
gradient, from 0 to 100% solvent B; and mass detector, micro mass ZMD.
Synthetic Chemistry
Methyl (2S)-3-(5-Bromopyridin-2-yl)-2-[(2-methylpropan-2-yl)oxycarbo-
nylamino]-propanoate (5)
Zinc dust (148.2 g, 2.28 M) was added to a 4-L, three necked, round-
bottomed flask, purged with nitrogen and heated with a heat gun for
10 min under vacuum. The slurry was cooled to room temperature, and
a solution of 1,2-dibromoethane (9.78 ml, 114 mmol) in N,N-dimethyl
formamide (DMF) (500 ml) was added dropwise over a period of
10 min. The resulting suspension was heated at 90 ꢁC for 30 min and
cooled to room temperature. Chlorotrimethylsilane (2.9 ml, 22.8 mmol)
was added, and the suspension was stirred for a further 30 min at room
temperature. A solution of Boc-b-iodo-Ala-OMe (125 g, 379.9 mmol) in
DMF (800 ml) was added dropwise over a period of 10 min and the
resulting suspension was heated at 35 ꢁC for 2 h. After the time, thin-layer
chromatography (TLC, silica gel, petroleum ether=AcOEt 75=25 with a
ninhydrin stain) showed complete consumption of the starting material.
The reaction mixture was cooled to room temperature and 2,5-dibromo-
pyridine (117.1 g, 494 mmol) was added followed by (Ph3P)2PdCl2 (13.3 g,
19 mmol). The resulting suspension was heated at 68 ꢁC for 2 h, cooled to
room temperature, and filtered to remove the excess of zinc. The filtrate
was partitioned between a mixture of water (2 L) and ether (1 L). The
phases were separated, and the aqueous phase was re-extracted with ether
(2 ꢂ 500 ml); the organic phases were combined, dried (MgSO4), filtered,
and concentrated to afford a yellow oil. The yellow oil was purified by
flash chromatography (pet. ether=AcOEt 75=25) to afford methyl (2S)-
3-(5-bromopyridin-2-yl)-2-[(2-methylpropan-2-yl)oxycarbonylamino]-
propanoate (5) (73.7 g, 54%) as a pale yellow solid. [a]1D7 ¼ ꢃ13.3ꢁ, c ¼ 0.9

528
H. Germain et al.
in acetone; LCMS (retention time ¼ 3.47 min. purity 100%), ESIþ m=z
1
305 (M-tBu)þ, 359, 361 (M þ H)þ; H NMR (CDCl3) d 1.42 (s, 9H),
3.19–3.35 (m, 2H), 3.70 (s, 3H), 4.64–4.74 (m, 1H), 5.67 (d, 1H,
J ¼ 7.6 Hz), 7.05 (d, 1H, J ¼ 8.3 Hz), 7.73 (dd, 1H, J ¼ 8.3 Hz,
J0 ¼ 2.1 Hz), 8.57 (d, 1H, J ¼ 2.1 Hz); 13C NMR (CDCl3) d 28.2,
38.2, 52.2, 55.1, 79.0, 118.6, 126.1, 132.4, 139.3, 150.0, 155.7, 156.7,
172,7, 176.5.
Methyl (2S)-2-[(2-Methylpropan-2-yl)oxycarbonylamino]-3-[5-(4,4,5,
5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridin-2-yl]propanoate (4)
Methyl (2S)-3-(5-bromopyridin-2-yl)-2-[(2-methylpropan-2-yl)oxycarbo-
nylamino]-propanoate (5, 7.5 g, 20.9 mmol), bis(pinacolatodiboron)
(6.90 g, 27.2 mmol), potassium acetate (6.15 g, 62.6 mmol), and Pd(dppf)
Cl2 ꢀ CH2Cl2 (1.01 g, 1.26 mmol) was suspended in 1,4-dioxane (60 ml)
and heated at 85 ꢁC for 16 h. The mixture was diluted with CH2Cl2
(50 ml), water (20 ml) was added, and the product was extracted with
CH2Cl2 (150 ml). The organic layer was washed with a saturated aqueous
solution of NaHCO3 and brine, dried over MgSO4, and concentrated to
afford methyl (2S)-2-[(2-methylpropan-2-yl)oxycarbonylamino]-3-[5-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridin-2-yl]propanoate (4)
(8.45 g, 100%) as a pale yellow gum. LCMS (retention time ¼ 3.61 min,
purity 95%), ESIþ m=z 407.3 (M þ H)þ; 1H NMR (CDCl3) d 1.34
(s, 12H), 1.42 (s, 9H), 3.23–3.38 (m, 2H), 3.68 (s, 3H), 4.64–4.70
(m, 1H), 5.90 (d, 1H, J ¼ 7.3 Hz), 7.14 (d, 1H, J ¼ 8.7 Hz), 7.98 (dd,
1H, J ¼ 7.7 Hz, J0 ¼ 1.3 Hz), 8.84 (s, 1H).
Methyl (2S)-3-(5-Hydroxypyridin-2-yl)-2-[(2-methylpropan-2-yl)oxycar-
bonylamino] propanoate or Boc-L-azatyrosine (1)
Methyl (2S)-2-[(2-methylpropan-2-yl)oxycarbonylamino]-3-[5-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)pyridin-2-yl]propanoate (4, 8.45 g,
20.9 mmol) was dissolved in CH2Cl2 (40 ml), and H2O2 (30%, 5.4 ml,
47.0 mmol) was added at 0 ꢁC. The reaction mixture was stirred at room
temperature for 16 h. After this time, an aqueous solution of sodium
thiosulfate (50 ml) was added to the mixture at 0 ꢁC. The mixture was
extracted with CH2Cl2 (2 ꢂ 50 ml); the combined organic layers were
washed with water and brine, dried with MgSO4, and concentrated.
The residue was purified by flash chromatography, eluting with
CH2Cl2=MeOH (98:2=95:5) to give (1) (4.9 g, 80%) as a white solid.
[a]17 ¼ ꢃ11.4ꢁ, c ¼ 1.0 in EtOH); LCMS (retention time ¼ 2.09 min,
D

Boc-L-Azatyrosine Synthesis
529
purity 100%), MS ESIþ m=z 297.2 (M þ H)þ; 1H NMR (CDCl3) d 1.40 (s,
9H), 3.17–3.28 (m, 2H), 3.71 (s, 3H), 4.62–4.69 (m, 1H), 5.74 (d, 1H,
J ¼ 5.5 Hz), 7.07 (d, 1H, J ¼ 7.7 Hz), 7.19 (d, 1H, J ¼ 7.7 Hz), 8.17 (s,
1H); 13C NMR (CDCl3) d 28.2, 38.1, 52.1, 54.2, 78.6, 122.8, 124.4,
137.5, 147.7, 152.5, 155.7, 173.1.
A sample of (1) was hydrolyzed in refluxing EtOH in the presence
of 2N HCl and purified by preparative LCMS using a Water Xterra
reverse-phase column (5 microns silica, 30 mm in diameter, 250 mm long)
and decreasingly polar mixtures of water (containing 0.2% ammonium
carbonate) and acetonitrile as eluent. The fractions were evaporated to
dryness to afford the desired compound as a white amorphous solid.
Crystals were grown from the recuperated solid from hot water.
[a]17 ¼ þ59.8ꢁ, c ¼ 1.0 in HCl); LCMS (retention time ¼ 0.45 min, purity
D
100%), MS ESIþ m=z 181.09 (M-H)þ; H NMR (DMSO-d6 þ TFA) d
1
3.29–3.43 (m, 2H), 4.58–4.43 (m, 1H), 7.64 (d, 1H, J ¼ 7.7 Hz), 7.75 (d,
1H, J ¼ 7.7 Hz), 8.34 (s, 1H), 8.49 (ls, 2H); 13C NMR (DMSO-d6 þ TFA)
d 34.6, 52.7, 128.1, 130.0, 132.1, 143.5, 156.6, 170.2.
ACKNOWLEDGMENTS
´ ´
´
The authors thank Dr. Geoff Bird, Dr. Benedicte Delouvrie, and senior
analysts Dr. Delphine Dorison-Duval, Christian Delvare, and Dr. Anne
Ertan for their assistance.
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