Stereoselective Synthesis of syn -γ-Hydroxynorvaline and Related α-Amino Acids

Article abstract of DOI:10.1055/s-0039-1690705

The total syntheses of three enantiomerically pure non-proteinogenic amino acids, l -norvaline, γ-oxonorvaline, and syn -γ-hydroxynorvaline, are reported. The chromatography-free route pivoted on the construction of highly enantiomerically enriched substituted α-amino-γ-oxopentanoic acid, from which all three members were accessed divergently via chemoselective and stereoselective reductions. The rapid synthesis of this key α-amino-γ-oxopentanoic acid was achieved by a highly diastereoselective crystallisation-driven three-component Mannich reaction from the readily available building blocks acetone, glyoxylic acid monohydrate, and (S)-(4-methoxyphenyl)ethylamine. The enantiomeric purity of all target molecules was confirmed by HPLC analysis, either of the amino acids or their derivatives.

Full text of DOI:10.1055/s-0039-1690705

SYNTHESIS
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X
© Georg Thieme Verlag Stuttgart · New York
2019, 51, A–H
feature
A
en
D. Valachová et al.
Feature
Synthesis
Stereoselective Synthesis of syn--Hydroxynorvaline and Related
-Amino Acids
Dominika Valachováa
OMe
NH2
Branislav Ferkoa
Eva Puchľováa
Oľga Caletkováa
Dušan Berkeša
Andrej Kolarovičb
Pavol Jakubec*a
• one-pot
OMe
COOH
• gram scale
L-norvaline
• inexpensive reagents
• excellent selectivity
• simple protocol
• no chromatography
4 steps, 24% overall yield
O
NH2
H2N
O
COOH
γ-oxonorvaline
2 steps, 55% overall yield
O
HN
CHCl3
rt, 2 days
0
0
0
0-0
0
0
1-8
8
7
2-3
8
1
7
COOH
OH
60%
dr 98:2
a Faculty of Chemical and Food Technology, Slovak University
of Technology, Radlinského 9, 81237 Bratislava, Slovakia
pavol.jakubec@stuba.sk
b Department of Chemistry, Faculty of Education, Trnava
University, Priemyselná 4, 91843 Trnava, Slovakia
OH NH2
HO
COOH
COOH
syn-γ-hydroxynorvaline
5 steps, 33% overall yield
mix
→
stir
→
filter
Received: 09.08.2019
Accepted after revision: 17.09.2019
Published online: 16.10.2019
have always also been hugely popular model substrates for
the development of alternative methodologies. Amongst
the less frequently used, but elegant and efficient methods
of stereoselective synthesis, crystallisation-induced asym-
metric transformation (CIAT) stands out.9 This method al-
lows the formation of enantiomerically pure crystalline
compounds due to a preferential crystallisation of one iso-
mer from a solution containing a mixture of simultaneously
equilibrating species.
DOI: 10.1055/s-0039-1690705; Art ID: ss-2019-f0447-fa
Abstract The total syntheses of three enantiomerically pure non-pro-
teinogenic amino acids, L-norvaline, -oxonorvaline, and syn--hy-
droxynorvaline, are reported. The chromatography-free route pivoted
on the construction of highly enantiomerically enriched substituted -
amino--oxopentanoic acid, from which all three members were ac-
cessed divergently via chemoselective and stereoselective reductions.
The rapid synthesis of this key -amino--oxopentanoic acid was
achieved by a highly diastereoselective crystallisation-driven three-
component Mannich reaction from the readily available building blocks
acetone, glyoxylic acid monohydrate, and (S)-(4-methoxyphenyl)ethyl-
amine. The enantiomeric purity of all target molecules was confirmed
by HPLC analysis, either of the amino acids or their derivatives.
In sharp contrast to its sophisticated principles10 is the
usual technical simplicity of CIAT, making the method very
attractive compared to many traditional approaches. Typi-
cally, the process involves stirring a heterogeneous reaction
mixture at room or slightly elevated temperature followed
by simple filtration of a crystalline product. Arguably, filtra-
tion represents by far the most straightforward purification
operation in organic chemistry and has gained undeniable
popularity amongst all practising chemists. Due to such
simple workup and multiple attractive ecological and eco-
nomic features, CIAT has been frequently implemented in
various industrial processes11 and was recognised for its en-
vironmentally friendly character.12
Enantiomerically pure norvalin (1) and its -oxygenated
analogues -oxonorvaline (2) and especially syn--hy-
droxynorvaline (3) have been popular subjects of numerous
synthetic studies (Scheme 1).13 Despite its structural sim-
plicity, norvaline (1) as an arginase inhibitor has been re-
cently used in the treatment of Alzheimer disease,14 has
anti-inflammatory effects,15 and is even used as a common
food supplement. syn--Hydroxynorvaline (3), first isolated
in 1966 by Fowden from Lathyrus odoratus seed16 and later
by Eugster from the mushroom Boletus satanas Lenz,17 has
been reported to show several biological activities, includ-
ing anti-diabetic properties.18 The -amino--hydroxy acid
Key words stereoselective synthesis, -amino acids, Mannich reac-
tion, crystallisation-induced, norvaline, -hydroxynorvaline, aza-Mi-
chael addition
The immense quantity of enantiomerically pure pro-
teinogenic -amino acids found in nature forms the founda-
tion of all living organisms. Although their non-proteino-
genic counterparts are far less abundant, they play vital
roles in various biogenic processes, cell signalling, and en-
zyme inhibition1 and allow industrial exploration of biolog-
ical activities as antibiotics and metal chelators.2 Naturally,
synthesis of enantiomerically pure -amino acids has al-
ways been one of the most studied parts of modern stereo-
selective synthesis.3 A vast array of diverse traditional ap-
proaches including resolution,4 efficient organocatalysis,5
cooperative catalysis,6 metal catalysis,7 and auxiliary-based
synthesis8 has been developed. However, as the industrial
and academic demands for greener, waste-free, and time-
and cost-efficient processes never cease, -amino acids
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–H
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
D. Valachová et al.
Feature
Synthesis
Biographical Sketches
From left to right:
gree from the University of Manchester un-
der the supervision of Prof. Jonathan P.
Clayden. After returning to Slovakia she car-
ried out her PhD studies, working on the
synthesis of natural products in the group of
Prof. Gracza. In 2012 she worked as
a postdoctoral researcher in the group of
Prof. Michal Hocek at the Academy of Sci-
ences of the Czech Republic. Since 2016 she
has been working as a research assistant at
the Slovak University of Technology in
Bratislava.
During his doctoral studies he spent a period
as a visiting researcher at the Degussa-Hüls
AG labs in Hanau (Germany), under the su-
pervision of Dr. Kai Rossen. He performed
his postdoctoral research with Prof. Giulia Li-
cini (University of Padova; (pseudo)-C3-sym-
metric ligands) and Prof. Eriks Rozners
(Northeastern University, Boston; formalde-
hyde-bridged oligonucleotides; NSF-NATO
Fellowship). In 2006 he returned to his alma
mater in Bratislava as an Assistant Professor
while in parallel he further broadened his
knowledge as a postdoc in the groups of
Prof. Marko D. Mihovilovic (Vienna Universi-
ty of Technology; catalytic decarboxylations;
Ernst Mach Jubilee Fellowship) and Prof. Hel-
ma Wennemers (ETH Zürich; stereoselective
(organo)catalysis; SCIEX Fellowship). Since
2016 he has been an Associate Professor at
the Trnava University in Slovakia.
Dominika Valachová received her bache-
lor’s degree in 2018 from the Slovak Univer-
sity of Technology in Bratislava. She is now
continuing her studies under the guidance
of Dr. Jakubec in Prof. Berkeš research
group. Her undergraduate research focused
on the preparation of enantiomerically pure
amino acids by crystallisation-driven pro-
cesses.
Branislav Ferko received his PhD from the
Slovak University of Technology in 2019,
working under the supervision of Prof.
Berkeš. His work focused on the synthesis of
novel and potent fluorescence labelled ana-
logues of arylceramides for application in
PET tomography. During his PhD studies he
joined the group of Prof. Darren J. Dixon at
the University of Oxford, where he partici-
pated in two independent projects: the de-
velopment of a novel photocatalytic method
for the construction of -tertiary ethers and
a first total synthesis of a potent antibiotic.
Currently he is a postdoctoral research assis-
tant at the Slovak University of Technology,
where he is involved in the development of
novel cross-coupling reactions.
Dušan Berkeš was born in Nové Zámky
(Slovakia) in 1957. He studied chemistry and
pharmaceutical technology at the Slovak
Technical University in Bratislava, where he
received his PhD in 1987 under the supervi-
sion of Prof. J. Kovac for studies on furan de-
rivatives. He was a postdoctoral fellow in the
group of Prof. Henry-Basch at the University
Paris-Sud (1988–1989) and research fellow
at the University of Le Havre (1992–1993) in
the group of Professor Morel, on stereose-
lective synthesis of nitrogen heterocycles. In
2002 he was appointed as Associate Profes-
sor of Organic Chemistry at the Slovak Tech-
nical University in Bratislava. From 2012 he
has been the head of the detached universi-
ty laboratory in the Saneca pharmaceuticals
company. His current research is on stereo-
selective nonproteinogenic amino acid syn-
Pavol Jakubec received his PhD in 2005
from the Slovak University of Technology in
Bratislava (supervisor Prof. Dušan Berkeš). In
2006 he joined the group of Prof. Darren J.
Dixon and spent two years as a postdoctoral
researcher at the University of Manchester.
Then he followed Darren to the University of
Oxford where he worked on the total syn-
thesis of manzamine alkaloids and the devel-
opment of new methodologies. In 2014 he
moved to the US, and as a postdoctoral re-
searcher he worked for Prof. Andrew G.
Myers at Harvard University on macrolide
antibiotics. After returning to Slovakia in
2016, he became a research assistant at the
Slovak University of Technology in Bratislava.
He has co-authored 29 scientific papers, in-
Eva Puchľová studied organic chemistry at
the Slovak University of Technology in
Bratislava and obtained her MSc degree in
2016. Currently, she is a PhD student and
R&D chemist in the flavour industry, focus-
ing on stereoselective (bio)transformations
in the synthesis of aroma compounds.
thesis
and
crystallisation-induced
asymmetric transformations, with applica-
tions towards the synthesis of sphingolipid
metabolism inhibitors.
Andrej Kolarovič obtained his PhD in 2003
from the Slovak University of Technology in
Bratislava (advisor: Prof. Dušan Berkeš).
Oľga Caletková graduated in 2007 at the
Slovak Technical University of Technology in
Bratislava. In 2008 she received an MPhil de-
cluding
2 articles in Nature with Profs.
Schrock, Hoveyda, and Myers.
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–H

C
D. Valachová et al.
Feature
Synthesis
Crystallisation-driven stereoselective aza-Michael addition
NH2
Ar
∗
COOH
L-norvaline (1)
O
Ar
H2N
R1
COOH
+
∗
R1
5a
6
O
NH2
COOH
O
HN
COOH
Crystallisation-driven stereoselective Mannich reaction
γ-oxonorvaline (2)
4
O
OH
COOH
Ar
∗
common precursor for
• L-norvaline
OH NH2
+
+
HO
H2N
R1
• γ-oxonorvaline
COOH
• syn-γ-hydroxynorvaline
7
8
6
syn-γ-hydroxynorvaline (3)
Scheme 1 Retrosynthetic analysis of norvaline (1) and its -oxygenated analogues -oxonorvaline (2) and -hydroxynorvaline (3)
fragment is found in some biologically active peptides in-
cluding the antifungal agent theonellamide F.19 But perhaps
the most notable biological properties are of the cyclic form
of -hydroxynorvaline, the -amino--butyrolactones.
They form a core structural element in a series of natural
products and pharmaceutical drugs, including pesticides,
ACE inhibitors, and bacteriostatic agents.20
Our synthetic endeavour began with a critical analysis
of our previously reported reactions of amines 6a (R1 = Me)
and 6b (R1 = CH2OH) with 5a (R2 = Me) (Scheme 2). The out-
comes were scrutinised in the light of the recently emerged
successful applications of CIAT in aza-Michael and nitro-
Mannich reactions, where scarcely used chiral mediators 6c
and 6d (Table 1) were employed.24–26 The latest advance-
ments prompted us to return to the disappointing aza-
Michael addition and react acetylacrylic acid (5a) with a
broader range of enantiomerically pure amines 6 (Table 1).
Under conditions suitable for the crystallisation-driven
processes, four commercially available enantiomerically
pure chiral amines 6c–f were investigated in the reaction
monitored by reverse-phase HPLC. Selected results are re-
ported in Table 1.27 Pleasingly, already after a brief screen-
ing of the reaction conditions, several highly diastereoselec-
tive reactions were discovered when amines 6e and 6f were
employed in chlorinated solvents. The aza-Michael addition
with amine 6e afforded adduct 4e in 75% yield and excellent
diasteroselectivity (dr 99:1) when performed in chloro-
form. The most outstanding datapoint was obtained with
amine 6f when the addition was performed in chloroform
at room temperature. Without any precautions to exclude
moisture or light or special demands for solvent purity,
amino acid 4f was isolated by simple filtration in excellent
yield (88%) and dr (99:1) after five days of stirring. In good
agreement with the previous investigations, the origin of
the highly diastereoselective reaction was confirmed as
crystallisation-induced asymmetric transformation.
Intrigued by the broad spectrum of the biological activi-
ties and the frequent natural abundance of these structur-
ally similar compounds, we envisaged that a unified syn-
thetic approach featuring CIAT from a common intermedi-
ate could enhance the existing syntheses. All three target
amino acids possess the same structural backbone and dif-
fer only in the oxidation state at C-4. The structural similar-
ities led us to believe that amino ketone 4 could be the key
common intermediate (Scheme 1). Further retrosynthesis
revealed that ketone 4 could be accessed via a stereoselec-
tive aza-Michael addition from unsaturated acid 5a21 or
Mannich reaction from acetone (7), glyoxylic acid monohy-
drate (8), and a suitable chiral mediator 6 (Scheme 1). CIAT
approaches featuring stereoselective aza-Michael addition
to various related -aryl- and -alkyl-substituted -
oxobutenoic acids 5 have already been developed (Scheme
2).22 However, as we reported, the attempted CIAT in the
aza-Michael addition to unsaturated acid 5a remained elu-
sive due to gel formation or low stereoselectivity (Scheme
2).23 Therefore, our primary goal was to significantly im-
prove the stereocontrol in the aza-Michael addition to
acetylacrylic acid (5a).
Unsuccessful CIAT
Previous results
Successful CIAT
Ar
Ar
∗
Ar
∗
Ar
R1
O
H2N
R1
H2N
R1
O
HN
R1
O
HN
∗
6a,b
6
R2
COOH
R2
COOH
Me
COOH
R2 = Me
Ref. 23
>30 examples
Refs. 22, 25
4a,b
5
4 up to 92% dr 98:2
(5a R2 = Me)
4a Ar = Ph, R1 = Me dr ~2:1
4b Ar = Ph, R1 = CH2OH
dr 95:5, inseparable gel
R1 = Me, CH2OH, CONH2
R2 = Ar, hetaryl, alkyl, functionalised alkyl
Ar = Ph
Scheme 2 Crystallisation-induced asymmetric transformation of N-nucleophiles in aza-Michael addition to acylacrylic acids 5
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–H

D
D. Valachová et al.
Feature
Synthesis
Table 1 Attempted CIAT: Screening of Chiral Amines 6 in Reaction with 5aa
Ar
Ar
H2N
R1
R1
O
O
HN
6
COOH
COOH
OMe
rt
5a
4c–f
Br
H2N
CONH2
H2N
H2N
H2N
6c
6d
6e
6f
Amine
Conditions
Results
Isolated
b
b
6c (1.3 equiv)
6d (1.3 equiv)
6e (1.1 equiv)
6e (1.1 equiv)
6f (1.1 equiv)
6f (1.3 equiv)
CHCl3, 7 d
CHCl3, 7 d
CH2Cl2, 7 d
CHCl3, 6 d
CHCl3, 5 d
CHCl3, 5 d
–
–
b
b
–
–
suspension, dr 49:51
suspension, dr 97:3
suspension, dr 92:8
suspension, dr 92:8
4e, dr 49:51, 42%
4e, dr 99:1, 75%
4f, dr 99:1, 65%
4f, dr 99:1, 88%
a Reaction conditions: 5a (1 equiv), 6, solvent, rt, 5–7 d.
b The reaction mixture did not form a suspension.
Encouraged by the feasibility of the stereoselective crys-
tallisation-driven aza-Michael reaction, the accessibility of
4f via a three-component Mannich reaction from acetone
(7), amine 6f, and glyoxylic acid monohydrate (8) was as-
sessed (Scheme 3).28 Such an ambitious process would ben-
efit not only from a potentially reduced overall step-count
but also from the much easier commercial access to the
starting materials. Having already identified chloroform as
the optimal solvent for CIAT and amine 6f as the most suit-
able chiral mediator, the starting point for the three-com-
ponent Mannich reaction was well set. To our delight, there
was no need for further extensive optimisation, because the
usage of acetone as a co-solvent allowed the swift forma-
tion of 4f with an initial dr of ~1:1. Similar to the aza-
Michael reaction, the rapid epimerisation of (S,R)-4f accom-
panied by crystallisation of the desired (S,S)-adduct enabled
a smooth transformation of the three inexpensive, com-
mercially available components into enantiomerically pure
amino acid 4f. Simple filtration of the solid precipitate
formed over the course of the reaction afforded 4f in 60%
yield and 98:2 dr. The reaction was successfully performed
already on a multigram scale, but the indisputable simplici-
ty of the reaction execution (‘mix → stir → filter’) holds
promises for even more significant scale-up.
to our retrosynthetic analysis (Scheme 1), a controlled syn-
1,3-induction was required to install the stereogenic centre
at C-4. We employed a highly stereoselective method taking
advantage of a pre-organised six-membered Mn chelate
formed in situ from amino ketone 4f and manganese di-
chloride.29 After a smooth syn-stereoselective reduction
with sodium borohydride in MeOH, -amino--hydroxy
acid 9 was formed in an excellent 98:2 dr, as determined by
HPLC. Due to the highly polar nature of the molecule, after a
mild lactonization, we isolated its cyclic form, lactone 10.
The following removal of the chiral mediator was achieved
by a mild reductive debenzylation by using trifluoroacetic
acid as the solvent and triethylsilane as the reducing
agent.30 Our synthesis of syn--hydroxynorvaline (3) was
completed by a gentle lactone opening, yielding the natural
product in 33% yield over five steps. The spectroscopic data
(1H and 13C NMR), melting point, and specific rotation of
our material were in excellent agreement with the pub-
lished data.13c HPLC analysis of the N-tosyl derivative of
OMe
OMe
6f
CHCl3
rt, 2 days
With the straightforward approach to amino acid 4f via
the Mannich reaction established, its envisaged utility in
the stereoselective synthesis of norvaline (1) and its oxy-
genated analogues was surveyed. Firstly, the synthesis of -
hydroxynorvaline (3), the most complex target, was investi-
gated, and the results are presented in Scheme 4. According
OH
H2N
+
O
O
HN
HO
COOH
COOH
60%
dr 98:2
7
8
4f
Scheme 3 Crystallisation-driven stereoselective three-component
Mannich reaction
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–H

E
D. Valachová et al.
Feature
Synthesis
OMe
OMe
OMe
HCl, H2O
40 °C, 2 h
then
MnCl2·4H2O
NaBH4, MeOH
rt, 1 h
· CF3COOH
NH2
Et3SiH, TFA
60 °C, 30 min
NaOH
H2O, rt, 1 h
HN
OH NH2
COOH
O
HN
OH HN
K2CO3
58%
(from 4f)
COOH
O
O
COOH
dr 98:2
95%
~100%
er 92:8
O
O
4f dr 98:2
9
10
11
syn-γ-hydroxynorvaline (3)
5 steps
33% overall yield
Scheme 4 Stereoselective synthesis of syn--hydroxynorvaline (3)
OMe
OMe
OMe
Raney Ni, H2
MeOH, H2O
55 °C, 24 h
HS(CH2)2SH
HCl, H2O
· HCl
HN
H2, Pd/C
rt, 48 h
HCl
O
NH2 . HCl
COOH
O
HN
NH2
COOH
HN
95 °C, 48 h
rt, 24 h
S
S
COOH
4f dr 98:2
COOH
74%
53%
(from 12)
er 97:3
92%
er 97:3
COOH
L-norvaline (1)
12
13
γ-oxonorvaline (2)
2 steps
55% overall yield
4 steps
24% overall yield
Scheme 5 Synthesis of norvaline (1) and -oxonorvaline (2) via chemoselective reductions of amino acid 4f
hydroxynorvaline confirmed a high enantiomeric purity (er
92:8).31 To the best of our knowledge, this is the first time
that the enantiomeric purity of syn--hydroxynorvaline
was determined by HPLC analysis instead of the traditional
measurement of optical rotation.
norvaline (2) confirmed again only negligible erosion of the
stereochemical purity during the synthesis; an excellent
97:3 er was detected.31
In conclusion, we have developed short and highly ste-
reoselective syntheses of three enantiomerically pure non-
proteinogenic amino acids: L-norvaline (1), -oxonorvaline
(2), and syn--hydroxynorvaline (3). The unified, chroma-
tography-free strategy pivots on a highly diastereoselective
crystallisation-driven Mannich reaction using acetone (7),
glyoxylic acid monohydrate (8), and amine 6f as inexpen-
sive, commercially available starting materials. Further che-
moselective and/or stereoselective reductive manipulation
of key intermediate 4f allowed the preparation of all three
amino acids from the same common intermediate. The high
enantiomeric purities of norvaline (1), -oxonorvaline (2),
and syn--hydroxynorvaline (3) were confirmed by HPLC
analysis, either directly of the amino acids or of their deriv-
atives.
Unlike preparation of -hydroxynorvaline (3), the syn-
thesis of norvaline (1) required complete but chemoselec-
tive removal of two latent functional groups – the N-benzyl
moiety and the oxo group in the -position (Scheme 5). Al-
though undeniable progress has been made in the deoxy-
genation of organic compounds,32 especially via advanced
Clemmensen33 or Wolff–Kishner reactions,34 epimerisa-
tion-prone substrate 4f restricted the available arsenal to
only a few options. While the classical Clemmensen reduc-
tion has met with only limited success, the two-step
Mozingo desulfuration of thioacetal 12 proved to be a high-
ly reliable alternative. Firstly, thioacetal 12, available from
ketone 4f in excellent yield, was treated with Raney nickel
under a hydrogen atmosphere at 55 °C, resulting in the for-
mation of amino acid 13 (Scheme 5). Without any purifica-
tion, the intermediate underwent Pd-catalysed debenzyla-
tion that completed the envisaged global deprotection. Nor-
valine (1) was isolated in a respectable 53% yield and 97:3
er, thus indicating a high stability of the present stereogenic
centre at C-2.31 Featuring another acid-promoted chemose-
lective debenzylation, -oxonorvaline (2) was obtained di-
rectly from key intermediate 4f in just one step in an excel-
lent 92% chemical yield (Scheme 5). HPLC analysis of -oxo-
Melting points are uncorrected. 1H and 13C NMR spectra were record-
ed on Varian VXR-300 and 600 MHz spectrometers (300 MHz, 600
MHz and 75.4 MHz, 151 MHz, respectively). High-resolution mass
spectra were recorded on an Orbitrap Velos PRO, Thermo Scientific 5
machine. Optical rotations were measured on a JASCO P-1020 or PO-
LAR L-mP (IBZ Mestechnik) polarimeter (concentration, c, is given as
g/100 mL). The reaction progress was monitored by reverse-phase
HPLC using a UV-VIS-205 detector (at 205 nm), Varian ProStar 310
pump (1.0 mL/min flow), Clarity DataApex software, and Nucleodur®
Phenyl-Hexyl C18 column (Macherey-Nagel, 250 × 4.0, 5 m). H2O–
MeCN based eluents were used for most of the analyses, with Et3N
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–H

F
D. Valachová et al.
Feature
Synthesis
and 85% aq H3PO4 (v/v 1:1) as additives (1% v/v compare to the H2O–
MeCN mixture). (E)-4-Oxopent-2-enoic acid (5a) was prepared by a
published method.21a
Yield: 1.54 g, 58% (over 2 steps from 4f), dr 98:2; white crystalline
solid; mp 69–70.4 °C; []D25 –114.3 (c 1, CHCl3).
1H NMR (300 MHz, CDCl3):  = 7.31–7.26 (m, 2 H), 6.89–6.85 (m, 2 H),
4.62 (dqd, J = 7.7, 6.5, 3.5 Hz, 1 H), 4.07 (q, J = 6.6 Hz, 1 H), 3.81 (s, 3 H),
3.46 (t, J = 8.5 Hz, 1 H), 1.94 (ddd, J = 13.0, 8.6, 7.7 Hz, 1 H), 1.71 (ddd,
J = 13.0, 8.4, 3.5 Hz, 1 H), 1.37 (d, J = 6.6 Hz, 3 H), 1.26 (d, J = 6.5 Hz, 3
H).
13C NMR (300 MHz, CDCl3):  = 177.8, 158.9, 137.0, 128.2, 113.9, 74.8,
57.2, 55.4, 54.7, 37.8, 24.8, 21.3.
2-[1-(4-Bromophenyl)ethylamino]-4-oxopentanoic Acid (4e)
To a solution of acid 5a (0.228 g, 2.00 mmol) in CHCl3 (10 mL) was
added amine (S)-6e (0.440 g, 2.20 mol, 1.1 equiv) and the resulting
mixture was stirred at rt and monitored by HPLC. After 6 d, the insol-
uble solid was collected by filtration, washed (Et2O), and dried, yield-
ing amino acid 4e.
Yield: 0.471 g, 75%, dr 99:1; white solid; mp 128 °C; []D25 –18.6 (c 1,
MeOH–5% aq HCl, 9:1).
HRMS (HESI): m/z [M + H]+ calcd for C14H20NO3: 250.14377; found:
250.14358.
1H NMR (300 MHz, DMSO-d6 + DCl):  = 7.63 (d, J = 8.5 Hz, 2 H), 7.54
(d, J = 8.5 Hz, 2 H), 4.54 (q, J = 6.5 Hz, 1 H), 3.72 (t, J = 4.9 Hz, 1 H),
3.25–3.03 (m, 2 H), 2.10 (s, 3 H), 1.59 (d, J = 6.8 Hz, 3 H).
13C NMR (75 MHz, DMSO-d6 + DCl):  = 203.8, 169.4, 135.8, 132.1,
130.8, 122.7, 57.0, 52.0, 42.7, 29.7, 19.9.
Lactone 11
To a mixture of lactone 10 (2.92 g, 11.7 mmol) and Et3SiH (1.4 g, 1.9
mL, 12 mmol) was added TFA (9 mL) and the resulting mixture was
stirred at 60 °C. After 30 min, the mixture was concentrated in vacuo
and the crude product was triturated with cold Et2O (30 mL). The pre-
cipitated white solid was collected by filtration, washed with Et2O (5
mL), and dried, yielding lactone 11.
HRMS (HESI): m/z [M + H]+ calcd for C13H17BrNO3: 314.03863; found:
314.03849.
Yield: 2.56 g, 95%, dr 98:2; white solid; mp 132–134 °C; []D25 –11.8 (c
1, MeOH).
2-[1-(4-Methoxyphenyl)ethylamino]-4-oxopentanoic Acid (4f)
1H NMR (600 MHz, DMSO-d6):  = 8.76 (br s, 3 H), 4.86–4.81 (m, 1 H),
4.49 (t, J = 9.7 Hz, 1 H), 2.39 (ddd, J = 12.8, 9.9, 8.7 Hz, 1 H), 2.27 (ddd,
J = 12.8, 9.3, 2.2 Hz, 1 H), 1.34 (d, J = 6.5 Hz, 3 H).
13C NMR (151 MHz, DMSO-d6):  = 172.9, 158.4 (q, J = 31.1 Hz), 117.2
(q, J = 301.2 Hz), 75.2, 47.2, 32.3, 20.6.
By Michael addition: To a solution of acid 5a (4.0 g, 35 mmol) in CHCl3
(175 mL) was added amine (S)-6f (6.3 g, 6.2 mL, 42 mmol, 1.2 equiv)
and the resulting mixture was stirred at rt and monitored by HPLC.
After 5 d, the insoluble solid was collected by filtration, washed
(Et2O), and dried, yielding amino acid 4f.
HRMS (HESI): m/z [M + H]+ calcd for C5H10NO2: 116.07061; found:
Yield: 8.3 g, 88%, dr 99:1; white solid.
116.07069.
By Mannich reaction: glyoxylic acid monohydrate (8; 4.6 g, 50 mmol)
was suspended in a mixture of CHCl3 (200 mL) and acetone (74 mL),
which was cooled to 0 °C. Amine (S)-6f (9.1 g, 8.9 mL, 60 mmol, 1.2
equiv) was added dropwise within 10 min to this solution, and the re-
action mixture was stirred at 0 °C. After 30 min, the mixture was
warmed to rt. The reaction was monitored by HPLC and after 48 h the
precipitate was collected by filtration, washed with Et2O (15 mL), and
dried, yielding amino acid 4f.
syn--Hydroxynorvaline (3)
To a mixture of lactone 11 (229 mg, 1.00 mmol) and H2O (2.3 mL) was
added 2 M aq NaOH (1.4 mL) at rt. The reaction mixture was stirred
for 1 h and passed through a column of Dowex 50WX8 ion exchange
resin (H+ form, 3 g). The column was washed thoroughly with H2O (50
mL) and the amino acid was eluted with NH4OH (25–27% NH3 in H2O,
20 mL). The ammoniacal eluate was lyophilized to dryness in vacuo
affording syn--hydroxynorvaline (3).
Yield: 7.9 g, 60%, dr 98:2; white solid; mp 120 °C (dec.); []D25 –27.8 (c
1, MeOH–5% aq HCl, 9:1).
1H NMR (300 MHz, DMSO-d6 + DCl):  = 7.41 (d, J = 8.7 Hz, 2 H), 6.93
(d, J = 8.7 Hz, 2 H), 4.44 (q, J = 6.8 Hz, 1 H), 3.71 (s, 3 H), 3.65 (m, 1 H),
3.15–3.00 (m, 2 H), 2.05 (s, 3 H), 1.55 (d, J = 6.8 Hz, 3 H).
Yield: 133 mg, ~100%, er 92:831; white solid; mp 213 °C (dec.) (Lit.13c
228–230 °C, EtOH–H2O); []D +18 (c 1.3, H2O) [Lit.13c []D +21 (c
25
25
1.3, H2O)].
13C NMR (75 MHz, DMSO-d6 + DCl):  = 204.4, 169.7, 160.3, 130.2,
128.2, 114.9, 57.5, 55.8, 52.1, 43.0, 30.0, 20.4.
HRMS (HESI): m/z [M + H]+ calcd for C14H20NO4: 266.13868; found:
266.13868.
1H NMR (300 MHz, D2O):  = 4.11–4.01 (m, 1 H), 3.74 (dd, J = 9.1, 4.3
Hz, 1 H), 2.06 (dt, J = 15.0, 4.3 Hz, 1 H), 1.77 (dt, J = 15.0, 9.1 Hz, 1 H),
1.24 (d, J = 6.2 Hz, 3 H).
13C NMR (75 MHz, D2O):  = 175.3, 66.6, 54.2, 38.9, 22.7.
All data are in good agreement with previously published data.13c
Lactone 10
Amino acid 4f (2.84 g, 10.7 mmol) and MnCl2·4H2O (424 mg, 2.14
mmol, 20 mol%) were suspended in MeOH (100 mL), and the mixture
was cooled to 0 °C. NaBH4 (605 mg, 16.1 mmol, 1.5 equiv) was added
portionwise during 30 min, and the reaction mixture was allowed to
warm to rt. The reaction was monitored by HPLC, and after 1 h it was
concentrated in vacuo, affording crude hydroxy acid 9. To the crude
acid was added 8 M aq HCl (55 mL) and the reaction mixture was
stirred at 40 °C. After 2 h, the mixture was concentrated in vacuo. The
residue was diluted with 10% aq K2CO3 (55 mL) and extracted with
CH2Cl2 (3 × 50 mL). The combined organic layers were dried (MgSO4)
and concentrated in vacuo. The crude product was recrystallised
(Et2O–hexane), yielding lactone 10.
Thioacetal 12
To a mixture of amino acid 4f (2.65 g, 10.0 mmol) and 12 M aq HCl
(10 mL) was added dropwise ethane-1,2-dithiol (1.2 g, 1.1 mL, 0.013
mol); the resulting mixture was stirred at rt. After 24 h the insoluble
solid was collected by filtration and washed with Et2O (3 × 20 mL),
yielding thioacetal 12.
Yield: 2.8 g, 74%; white solid; mp 182–183 °C; []D25 –29.4 (c 1.0, DM-
SO).
1H NMR (300 MHz, DMSO-d6):  = 10.26 (br s, 1 H), 7.57 (d, J = 8.7 Hz,
2 H), 6.96 (d, J = 8.7 Hz, 2 H), 4.26 (q, J = 6.5 Hz, 1 H), 3.76 (s, 3 H), 3.40
(dd, J = 9.2, 2.5 Hz, 1 H), 3.34–3.06 (m, 4 H), 2.77 (dd, J = 14.4, 2.0 Hz, 1
H), 2.46 (m, 1 H), 1.61 (m, 6 H).
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–H

G
D. Valachová et al.
Feature
Synthesis
13C NMR (75 MHz, DMSO-d6):  = 169.7, 159.6, 129.9, 128.0, 114.1,
64.0, 56.6, 56.3, 55.2, 44.1, 39.6, 30.9, 20.8.
References
(1) Neumann-Staubitz, P.; Neumann, H. Curr. Opin. Struct. Biol.
2016, 38, 119.
(2) Ohfune, Y. Acc. Chem. Res. 1992, 25, 360.
HRMS (HESI): m/z [M + H]+ calcd for C16H24NO3S2: 342.11921; found:
342.11935
(3) (a) Wagner, I.; Musso, H. Angew. Chem. Int. Ed. 1983, 22, 816.
(b) Zarándi, M.; Szolomájer, J. In Amino Acids, Peptides and Pro-
teins, Vol. 42; Ryadnov, M.; Hudecz, F., Ed.; The Royal Society of
Chemistry: Cambridge, 2018, 1.
(4) (a) Chenault, H. K.; Dahmer, J.; Whitesides, G. M. J. Am. Chem.
Soc. 1989, 111, 6355. (b) Wang, B.; Liu, Y.; Zhang, D.; Feng, Y.; Li,
J. Tetrahedron: Asymmetry 2012, 23, 1338.
(5) (a) Corey, E. J. J. Am. Chem. Soc. 1997, 119, 12414. (b) Kang, Q.-
K.; Selvakumar, S.; Maruoka, K. Org. Lett. 2019, 21, 2294.
(c) Bøgevig, A.; Juhl, K.; Kumaragurubaran, N.; Zhuang, W.;
Jørgensen, K. A. Angew. Chem. Int. Ed. 2002, 41, 1790. (d) List, B.
Synlett 2001, 1675.
Norvaline (1)
To a mixture of thioacetal 12 (0.754 g, 2.00 mmol) in H2O (10 mL) and
MeOH (10 mL) was added Raney Ni (W-2, 3.8 g) and the heteroge-
neous mixture was stirred under H2 (balloon) at 55 °C. After 1 h, the
mixture was cooled to rt, Raney Ni (W-2, 3.9 g) was added, and the
mixture was stirred again under H2 (balloon) at 55 °C. After 24 h, the
mixture was filtered through a pad of Celite, the filter cake was
washed with boiling MeOH (5 × 20 mL), and the filtrate was concen-
trated in vacuo. H2O (10 mL) was added and the insoluble solid was
collected by filtration, washed (Et2O), and dried, affording a white sol-
id. This residue was dissolved in MeOH (20 mL), and 10% Pd/C (100
mg) was added under argon. The mixture was degassed and stirred
under H2 (balloon) at rt. After 48 h (complete conversion by LCMS),
the mixture was filtered through a pad of Celite, the filter cake was
washed with MeOH (2 × 10 mL), and the combined filtrates were con-
centrated in vacuo, affording norvaline (1).
(6) Franchino, A.; Jakubec, P.; Dixon, D. J. Org. Biomol. Chem. 2016,
14, 93.
(7) (a) Fryzuk, M. D.; Bosnich, B. J. Am. Chem. Soc. 1977, 99, 6262.
(b) Federsel, H.-J. Nat. Rev. Drug Discovery 2005, 4, 685.
(8) (a) Schöllkopf, U.; Groth, U.; Deng, C. Angew. Chem. Int. Ed. 1981,
20, 798. (b) Oppolzer, W.; Moretti, R.; Thomi, S. Tetrahedron
Lett. 1989, 30, 6009. (c) Basso, A.; Banfi, L.; Riva, R.; Guanti, G.
J. Org. Chem. 2005, 70, 575.
Yield: 0.123 g, 53%, er 97:331; white solid; mp 290 °C (dec.) (Lit.35 297
25
25
°C); []D +4.9 (c 1, H2O) [Lit.35 []D +6.5 (c 2, H2O); Lit.4b (91% ee)
[]D20 +4.6 (c 1.0, H2O)].
(9) (a) Brands, K. M. J.; Davies, A. J. Chem. Rev. 2006, 106, 2711.
(b) Anderson, N. G. Org. Process Res. Dev. 2005, 9, 800.
(c) Ebbers, E. J.; Ariaans, G. J. A.; Houbiers, J. P. M.; Bruggink, A.;
Zwanenburg, B. Tetrahedron 1997, 53, 9417. (d) Yoshioka, R. Top.
Curr. Chem. 2007, 269, 83.
1H NMR (300 MHz, D2O):  = 3.82 (t, J = 5.9 Hz, 1 H), 1.97–1.82 (m, 2
H), 1.44 (m, 2 H), 0.99 (t, J = 7.3 Hz, 3 H).
13C NMR (75 MHz, D2O):  = 175.1, 54.7, 32.5, 17.8, 12.9.
MS (ESI): m/z [M + H]+ calcd for C5H12NO2: 118.1; found: 118.2.
(10) (a) Kuhn, R.; Jochims, J. C. Justus Liebigs Ann. Chem. 1961, 641,
143. (b) Hassan, N. A.; Bayer, E.; Jochims, J. C. J. Chem. Soc.,
Perkin Trans. 1 1998, 3747.
-Oxonorvaline Hydrochloride (2)
A mixture of amino acid 4f (1.33 g, 500 mmol) and 6 M aq HCl (100
mL) was stirred at 95 °C. After 48 h, the mixture was cooled to rt and
extracted with Et2O (3 × 100 mL). The combined organics were dis-
carded. The aqueous layer was concentrated in vacuo, yielding
a yellow solid. To this residue was added Et2O (100 mL) and the mix-
ture was stirred at rt. After 1 h, the insoluble solid was collected by
filtration, washed with Et2O (3 × 50 mL), and dried, yielding -oxonor-
valine hydrochloride (2).
(11) (a) Belanger, F.; Chase, C. E.; Endo, A.; Fang, F. G.; Li, J.; Mathieu,
S. R.; Wilcoxen, A. Z.; Zhang, H. Angew. Chem. Int. Ed. 2015, 54,
5108. (b) Menis, J.; Twelves, C. Breast Cancer 2011, 3, 101.
(c) Brands, K. M. J.; Payack, J. F.; Rosen, J. D.; Nelson, T. D.;
Candelario, A.; Huffman, M. A.; Zhao, M. M.; Li, J.; Craig, B.;
Song, Z. J.; Tschaen, D. M.; Hansen, K.; Devine, P. N.; Pye, P. J.;
Rossen, K.; Dormer, P. G.; Reamer, R. A.; Welch, C. J.; Mathre, D.
J.; Tsou, N. N.; McNamara, J. M.; Reider, P. J. J. Am. Chem. Soc.
2003, 125, 2129.
(12) Nelson, T. D. In Strategies and Tactics in Organic Synthesis, Vol. 6;
Harmata, M., Ed.; Elsevier: Amsterdam, 2005, Chap. 10, 321.
(13) (a) Ariza, J.; Font, J.; Ortuño, R. M. Tetrahedron 1990, 46, 1931.
(b) Schmeck, C.; Hegedus, L. S. J. Am. Chem. Soc. 1994, 116, 9927.
(c) Jacob, M.; Roumestant, M. L.; Viallefont, P.; Martinez, J.
Synlett 1997, 691. (d) Drummond, L. J.; Sutherland, A. Tetrahe-
dron 2010, 66, 5349. (e) Simon, R. C.; Busto, E.; Schrittwieser, J.
H.; Sattler, J. H.; Pietruszka, J.; Faber, K.; Kroutil, W. Chem.
Commun. 2014, 50, 15669.
Yield: 0.60 g, 92%, er 97:331; yellow solid; mp 147 °C (Lit.36 135–137
°C); []D25 +8.9 (c 1, H2O) [Lit.36 []D25 +8 (c 1, H2O)].
1H NMR (300 MHz, D2O):  = 4.34–4.30 (m, 1 H), 3.43–3.28 (m, 2 H),
2.27 (s, 3 H).
13C NMR (75 MHz, D2O):  = 209.6, 171.4, 48.5, 42.1, 29.0.
HRMS (HESI): m/z [M + H]+ calcd for C5H10NO3: 132.06552; found:
132.06571.
Funding Information
(14) Polis, B.; Srikanth, K. D.; Gurevich, V.; Gil-Henn, H.; Samson, A.
O. Neural Regen. Res. 2019, 14, 1562.
(15) Ming, X.-F.; Rajapakse, A. G.; Carvas, J. M.; Ruffieux, J.; Yang, Z.
BMC Cardiovasc. Disord. 2009, 9, 12.
This work was supported by the Agentúra na Podporu Výskumu a
Vývoja (Slovak Research and Development Agency) under contract no.
APVV-16-0258.
A
g
e
ntúra
n
a
P
o
d
p
oru
V
ýsk
u
m
u
a
V
ý
v
o
j
a
(A
P
V
V-16-0
2
5
8)
(16) Fowden, L. Nature 1966, 209, 807.
(17) Matzinger, P.; Catalfomo, P.; Eugster, C. H. Helv. Chim. Acta 1972,
55, 1478.
Supporting Information
(18) (a) Ribes, G.; Broca, C.; Petit, P.; Jacob, M.; Baissac, Y.;
Manteghatti, M.; Roye, M.; Sauvaire, Y. Diabetologia 1996, 39,
A234. (b) Broca, C.; Manteghetti, M.; Gross, R.; Baissac, Y.; Jacob,
M.; Petit, P.; Sauvaire, Y.; Ribes, G. Eur. J. Pharmacol. 2000, 390,
339.
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0039-1690705.
S
u
p
p
orti
n
gInformati
o
n
S
u
p
p
orti
n
gInformati
o
n
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–H

H
D. Valachová et al.
Feature
Synthesis
(19) Matsunaga, S.; Fusetani, N.; Hashimoto, K.; Wälchli, M. Am.
Chem. Soc. 1989, 11, 2582.
Martin, H. J.; List, B. Synlett 2003, 1903. (c) Enders, D.; Grondal,
C.; Vrettou, M.; Raabe, G. Angew. Chem. Int. Ed. 2005, 44, 4079.
(d) De Lamo Marin, S.; Catala, C.; Kumar, S. R.; Valleix, A.;
Wagner, A.; Mioskowski, C. Eur. J. Org. Chem. 2010, 3985.
(29) Berkeš, D.; Kolarovič, A.; Považanec, F. Tetrahedron Lett. 2000,
41, 5257.
(20) (a) Schaper, W.; Beckmann, M.; Doller, U.; Krautstrunk, G.; Jans,
D.; Hempel, W.; Waibel, J. M. US2004/6047 A1, 2004.
(b) Nielsen, J.; Givskov, M. WO03106445 (A1), 2003.
(c) Hasegawa, H.; Shiori, N.; Narita, T.; Katori, T. US4876359 A1,
1989.
(21) (a) Scheffold, R.; Dubs, P. Helv. Chim. Acta 1967, 50, 798. (b) Xu,
C.; Bai, X.; Xu, J.; Ren, J.; Xing, Y.; Li, Z.; Wang, J.; Shi, J.; Yu, L.;
Wang, Y. RSC Adv. 2017, 7, 4763.
(22) (a) Yamada, M.; Nagashima, N.; Hasegawa, J.; Takahashi, S. Tet-
rahedron Lett. 1998, 39, 9019. (b) Kolarovič, A.; Berkeš, D.;
Baran, P.; Považanec, F. Tetrahedron Lett. 2001, 42, 2579.
(c) Kolarovič, A.; Berkeš, D.; Baran, P.; Považanec, F. Tetrahedron
Lett. 2005, 46, 975. (d) Jakubec, P.; Berkeš, D.; Šiška, R.;
Gardianová, M.; Považanec, F. Tetrahedron: Asymmetry 2006, 17,
1629. (e) Berkeš, D.; Jakubec, P.; Winklerová, D.; Považanec, F.;
Daich, A. Org. Biomol. Chem. 2007, 5, 121.
(30) Chen, C.; Genapathy, S.; Fischer, P.; Chan, W. Org. Biomol. Chem.
2014, 12, 9764.
(31) (a) For further details, see the Supporting Information. (b) The
observed negligible difference in the dr of adduct 4f (98:2), the
er of norvaline (1) (er 97:3), and the er of -oxonorvaline (2)
(97:3) could be attributed to the precision limits/accuracy of
the HPLC measurements. The apparent erosion of the enantio-
meric purity of syn--hydroxynorvaline (3) (er 92:8) could be
caused by a partial epimerisation of the intermediates during
the transformations of 4f to syn--hydroxynorvaline (3).
(32) Volkov, A.; Gustafson, K. P. J.; Tai, C.-W.; Verho, O.; Bäckvall,
J.-E.; Adolfsson, H. Angew. Chem. Int. Ed. 2015, 54, 5122.
(33) (a) Clemmensen, E. Chem. Ber. 1913, 46, 1837. (b) Vedejs, E. Org.
React. 1975, 22, 401. (c) Motherwell, W. B.; Nutley, C. J. Con-
temp. Org. Synth. 1994, 1, 219.
(34) (a) Kishner, N. J. J. Russ. Phys. Chem. Soc. 1911, 43, 582. (b) Wolff,
L. Liebigs Ann. Chem. 1912, 394, 23. (c) Furrow, M. E.; Myers, A.
G. J. Am. Chem. Soc. 2004, 126, 5436.
(35) Kisumi, M.; Sugiura, M.; Kato, I.; Chibata, I. J. Biochem. 1976, 79,
1021.
(36) Osipov, S. N.; Lange, T.; Tsouker, P.; Spengler, J.; Hennig, L.;
Koksch, B.; Berger, S.; El-Kousy, S. M.; Burger, K. Synthesis 2004,
1821.
(23) Jakubec, P.; Berkeš, D.; Kolarovič, A.; Považanec, F. Synthesis
2006, 23, 4032.
(24) Resnick, L.; Galante, R. J. Tetrahedron: Asymmetry 2006, 17, 846.
(25) Sivák, I.; Toběrný, M.; Kyselicová, A.; Caletková, O.; Berkeš, D.;
Jakubec, P.; Kolarovič, A. J. Org. Chem. 2018, 83, 15541.
(26) Marčeková, M.; Gerža, P.; Šoral, M.; Moncol, J.; Berkeš, D.;
Kolarovič, A.; Jakubec, P. Org. Lett. 2019, 21, 4580.
(27) For more comprehensive screening results, see the Supporting
Information.
(28) (a) Jakubec, P.; Petráš, P.; Duriš, A.; Berkeš, D. Tetrahedron:
Asymmetry 2010, 21, 69. For selected examples of organocata-
lysed Mannich reactions, see: (b) Pojarliev, P.; Biller, W. T.;
© 2019. Thieme. All rights reserved. Synthesis 2019, 51, A–H