Synthesis and Fluorescent Properties of β-Pyridyl α-Amino Acids

Article abstract of DOI:10.1021/acs.joc.9b00036

The preparation of a new class of β-pyridyl α-amino acid is described using a highly regioselective, ytterbium-catalyzed hetero-Diels-Alder reaction of enones with vinyl ethers followed by a modified Knoevenagel-Stobbe reaction as the key heterocycle form

Full text of DOI:10.1021/acs.joc.9b00036

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Article
Synthesis and Fluorescent Properties of #-Pyridyl #-Amino Acids
Alexander H. Harkiss, Jonathan D. Bell, Astrid Knuhtsen, Andrew G. Jamieson, and Andrew Sutherland
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00036 • Publication Date (Web): 06 Feb 2019
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The Journal of Organic Chemistry
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Synthesis and Fluorescent Properties of -Pyridyl -
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Amino Acids
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Alexander H. Harkiss, Jonathan D. Bell, Astrid Knuhtsen, Andrew G. Jamieson and Andrew
Sutherland*
WestCHEM, School of Chemistry, The Joseph Black Building, University of Glasgow,
Glasgow G12 8QQ, United Kingdom.
Andrew.Sutherland@glasgow.ac.uk
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
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Table of Contents Graphic:
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Abstract: The preparation of a new class of -pyridyl -amino acid is described using a highly
regioselective, ytterbium-catalyzed hetero Diels-Alder reaction of enones with vinyl ethers followed by a
modified Knoevenagel-Stobbe reaction as the key heterocycle forming steps. Investigation of the
properties and applications of these amino acids showed that they could be utilized in solid phase peptide
synthesis for the preparation of a biologically relevant hexapeptide, while pyridines bearing electron-rich
substituents exhibited strongly fluorescent properties with high quantum yields and MegaStokes shifts. A
solvatochromic study with the most fluorogenic amino acid, a p-methoxyphenyl analogue revealed that
this charge-transfer based chromophore is highly sensitive to solvent polarity with a bathochromic shift
of 115 nm on changing from THF to phosphate-buffered saline.
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The Journal of Organic Chemistry
INTRODUCTION
-Amino acids are the fundamental building blocks of life, serving as the basic structural components
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of all peptides and proteins.1 They also play a key role in enzyme mechanisms and in signal induction
pathways. With continued developments in biomedical and life sciences, proteinogenic and non-
proteinogenic -amino acids are used in a wide variety of applications, such as probes to study biological
mechanism and function, the investigation of bioactive protein conformations and in the discovery of new
pharmacologically active compounds.1,2 In the field of organic chemistry, amino acids are used as chiral
building blocks in total synthesis and as chiral ligands and auxiliaries for the development of new
asymmetric methods.3 This importance of -amino acids in a range of scientific fields continues to drive
development of new synthetic methods for their preparation.
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Recently, attention has focused on the synthesis and application of unnatural -amino acids with
heteroaryl containing side-chains.2,4,5 -Amino acids bearing pyridyl moieties are of particular interest
due to the improved medicinal chemistry properties imparted by this heterocycle. For example, the
pyridine analogue of tyrosine has potent antibiotic and antitumour properties,6 while the incorporation of
3-pyridylalanine into the gonadotropin-releasing hormone antagonist cetrorelix resulted in improved
pharmacokinetic properties and receptor binding.7 These types of properties have compelled the
development of new syntheses of -pyridyl -amino acids.8 The Brimble research group described the
asymmetric synthesis of L-2-pyridylalanine using a modified Mitsunobu-Tsunoda reaction (Figure 1a).8h
Treatment of the Belokon nickel complex9 with cyanomethylene tributylphosphorane and 2-
pyridinemethanol gave after hydrolysis, the corresponding amino acid in high overall yield and excellent
enantiomeric excess. A number of chemoenzymatic approaches have also been reported,10 such as the
Anabaena variabilis phenylalanine ammonia lyase (AvPAL) catalyzed ammonolysis of cinnamates
(Figure 1b).11 Using E. coli BL21(DE3) whole cells expressing AvPAL and a high concentration of
ammonia to drive the ammonolysis reaction gave L-2-pyridylalanine in 76% ee. A general synthesis of -
pyridyl -amino acids was recently reported by the Jui group using a photoredox catalyzed radical
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conjugate addition of halogenated pyridines with dehydroalanines (Figure 1c).12 An asymmetric version
of this process was also described using a chiral tert-butyl oxazolidinone dehydroalanine derivative.
We were interested in developing a new general approach for the synthesis of optically active -pyridyl
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-amino acids. In particular, the aim of the project was the preparation of -amino acids bearing
conjugated 4-arylpyridyl side-chains with the potential to display fluorescent properties and, that could
be easily incorporated into peptides for biological imaging applications. Here we report a two-stage
strategy utilizing a highly regioselective, lanthanide-catalyzed hetero Diels-Alder reaction of enone-
derived -amino acids with ethyl vinyl ether, followed by a modified Knoevenagel-Stobbe reaction of the
resulting dihydropyran with hydroxylamine (Figure 1d). The utility of these amino acids for solid phase
peptide synthesis (SPPS) and analysis of their photophysical properties is also described.
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Figure 1. Selective synthetic methods for the preparation of -(2-pyridyl)--amino acids.
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The Journal of Organic Chemistry
RESULTS AND DISCUSSION
The first stage of the synthesis of the -pyridyl -amino acids involved the preparation of suitably
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protected enone-derived -amino acids (Scheme 1). Initially, N-trityl protected phosphonate ester 3 was
prepared in three-steps (92% overall yield) from L-aspartic acid 1.13 Following protection of the amino
and carboxylic acid groups, the key step involved the highly regioselective reaction of the anion of
dimethyl methylphosphonate with the sterically accessible -methyl ester of 2 to give phosphonate ester
3. Horner-Wadsworth-Emmons reaction of 3 with a wide range of aldehydes under mild conditions, gave
enones 4a–j, exclusively as the E-isomers.14,15 The focus of this project was to prepare aryl substituted
pyridyl side-chains and therefore, benzaldehyde derivatives were mainly used to explore the scope of this
reaction. As expected, the majority of these gave high yields of the E-enones (66–95%), although highly
electron-rich aldehydes required longer reaction times (120 h) and gave more modest yields (e.g. 4e).
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Scheme 1. Synthesis of Enone-Derived -Amino Acids 4a–ja
aIsolated yields are shown.
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The enone-derived -amino acids were then investigated as substrates for the key two-stage pyridine-
forming process (Scheme 2). The first-step involved an inverse electron-demand hetero Diels-Alder
reaction with ethyl vinyl ether. Using N-trityl-protected enone 4b and conditions first reported by
Danishefsky and co-workers,16 with Yb(fod)3 as a Lewis acid catalyst (5 mol%) gave none of the resulting
dihydropyran cycloaddition product. Previous work by us has shown that reactions of the enone moiety
of amino acids such as 4a–j are restricted by the steric bulk of the N-trityl protecting group.5 Therefore,
having used the trityl protecting group to conduct the regioselective functionalization of the side-chain
ester of 2, this was removed and replaced with a smaller N-protecting group (Cbz) using a two-step N-
deprotection/reprotection strategy, under standard conditions. Diels-Alder reaction of the resulting Cbz-
protected enone-derived -amino acids 5a–j with ethyl vinyl ether, in the presence of Yb(fod)3 (5 mol%)
proceeded smoothly and gave dihydropyrans 6a–j, as single regioisomers in 70–93% yields. Ciufolini and
co-workers have shown that dihydropyrans of this type are latent forms of 1,5-dicarbonyl compounds and
can be converted to pyridines by reaction with hydroxylamine via a modified Knoevenagel-Stobbe
reaction.17,18 Treatment of dihydropyrans 6a–j with hydroxylamine hydrochloride at 70 C allowed clean
conversion of all structural analogues to the corresponding pyridines 7a–j. In general, this gave the
majority of products in good yields (58–71%). While the three nitro analogues 6h, 6i and 6j were also
cleanly converted to the pyridines, challenging isolation and purification of these highly polar compounds
resulted in more modest yields (36–40%). To expand the range of substituted pyridyl side-chains, two
nitro-analogues were subjected to a chemoselective reduction using tin dichloride, which allowed access
to amino-analogues 7k and 7l.19 Finally, various deprotection strategies to access the parent amino acids
were explored, however, direct acid-mediated removal of both the amine and carboxylic acid protecting
groups was found to be the most efficient approach, giving all 12 structural analogues 8a–l in high yields
(86–99%).
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The Journal of Organic Chemistry
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Scheme 2. Synthesis of -Pyridyl -Amino Acids 8a–la
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aIsolated yields are shown.
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Having developed a general and effective route to -pyridyl -amino acids, the potential of these for
various applications was explored. Initially, a study was conducted to demonstrate that these could be
easily incorporated into peptides using routine Fmoc-based SPPS methodology. A pentapeptide, Val-Pro-
Thr-Leu-Lys based on the Bax-binding domain of Ku70, a multifunctional protein involved in DNA repair
and cell-death regulation was chosen as a model substrate.20 Matsuyama and co-workers have previously
shown that this pentapeptide and similar analogues have low cytotoxicity and are highly effective at
penetrating living cells.21 The Val-Pro-Thr-Leu-Lys pentapeptide was prepared in a microwave assisted
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peptide synthesizer using Rink Amide ChemMatrix® resin as the polymer support22 and routine SPPS
methodology (Scheme 3). On coupling Fmoc-L-Lys(Boc)-OH onto the polymer support using HCTU
activation, successive rounds of piperidine-mediated N-deprotection and coupling using the next amino
acid gave the pentapeptide. Following a subsequent N-terminal Fmoc-deprotection, the Fmoc-protected
version of -pyridyl -amino acid 8d, compound 9 was coupled onto the polymer-supported
pentapeptide.23 After a final Fmoc-deprotection step, the N-terminus was acetyl capped and a TFA
cleavage cocktail that included triisopropylsilane (TIPS) as a cation scavenger was used to remove the
side-chain protecting groups and release the peptide from the polymer support. The resulting hexapeptide,
10 was purified by reverse phase-HPLC (>95% purity) and characterized by high-resolution electrospray
ionization mass spectrometry (see Supporting Information for data).
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Scheme 3. SPPS of a -Pyridyl -Amino Acid Containing Hexapeptide
As the -amino acids contained -deficient pyridine moieties, it was proposed that electron-rich
conjugating groups may confer fluorescent properties through a charge-transfer (push-pull) mechanism.24
Accordingly, the optical properties of -pyridyl -amino acids 8a–l were measured. As expected,
pyridines with non-conjugating or electron-deficient substituents showed weak fluorescence, while
compounds with electron-rich substituents showed strong fluorescence (Figure 2b and Table 1). In
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The Journal of Organic Chemistry
general, these amino acids showed emission maxima from 366 to 460 nm, with all compounds displaying
large Stokes shifts. Amino acids 8c and 8d displayed particularly interesting optical properties.
Naphthalene analogue 8c gave a red-shifted emission spectrum with the main band at 452 nm and a
MegaStokes shift of 193 nm. As well as the strongest fluorescence, 4-methoxyphenyl analogue 8d
displayed the highest quantum yield (0.46) and highest value of brightness (11923 cm–1 M–1).
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(a)
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8k
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Wavelength (nm)
Figure 2. (a) Absorption spectra of amino acids 8c–8f and 8k recorded at 1 × 10–5 M in methanol.
(b) Emission spectra of 8c (0.5 × 10–5 M), 8d (1 × 10–7 M) and 8e, 8f and 8g (1 × 10–5 M), recorded in
methanol.
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Table 1.
Photophysical data of pyridyl-derived -amino acids 8c–f and 8k.
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Brightness
(cm–1 M–1)
Amino acid
QY
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Abs (nm)a  (cm–1 M–1)

Em (nm)a
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8c
8d
8e
8f
259
283
34360
25920
8600
380, 452b 0.18
366c
6185
0.46
11923
1634
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11600
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Spectra were recorded in methanol at: a 1 × 10–5 M; b 0.5 × 10–5 M; c 1 × 10–7 M.
A solvatochromic study with 8d was next performed to further explore the charge transfer properties of
this -pyridyl -amino acid. The absorption maxima (278–280 nm) in a range of solvents were found to
be independent of polarity, indicating negligible intramolecular interaction between the electron-rich
methoxyphenyl and electron-deficient pyridine moieties in the ground state (see Supporting Information).
In contrast, the emission maxima were found to be highly dependent on solvent polarity (Figure 3). In
THF, a Stokes shift of 28 nm and an emission maximum of 308 nm was observed, while in phosphate-
buffered saline (PBS), the emission maximum was found at 423 nm, with a Stokes shift of 143 nm. Such
strong solvatochromism, which can be attributed to the stabilization of the highly polar excited-state
charge transfer character of 8d by polar solvents, is expected for compounds that undergo an internal
charge transfer upon excitation.25 These results show that 8d is highly sensitive to the polarity of its
surroundings, with fluorescence emission in the visible region under aqueous conditions (PBS).
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The Journal of Organic Chemistry
308
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EtOAc
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Figure 3.
Emission spectra of 8d in various solvents. All spectra were recorded using a concentration
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of 1 × 10–5 M, except for methanol which was recorded using a concentration of 1 × 10–7 M.
CONCLUSIONS
In summary, a new class of -pyridyl -amino acid has been synthesized, introducing side-chain
diversity via a Horner-Wadsworth-Emmons reaction and using a highly regioselective lanthanide-
catalyzed hetero Diels-Alder reaction and a modified Knoevenagel-Stobbe condensation to form the key
pyridine ring-system. Despite the relatively small size of the amino acid side-chain, the electron-rich 4-
arylpyridine chromophores were found to have strong charge-transfer based fluorescence, large Stokes
shifts and high quantum yields. A solvatochromic study of the highly fluorogenic amino acid 8d showed
this compound to be particularly sensitive to the polarity of its surrounding with markedly different
emission wavelengths in various solvents. Environment-sensitive amino acid 8d was also directly
incorporated into a biologically relevant peptide as part of a SPPS process. Work is underway on the
preparation of further analogues in which the optical properties of these -amino acids will be further
tuned for potential spectroscopic and microscopic applications.
EXPERIMENTAL SECTION
The synthesis of dimethyl (2S)-2-aminobutandioate hydrochloride and, compounds 2, 3, 4a–4d, 4g–4h,
4j, 5b–5d, 5h and 5j has been previously described in the literature.5,13,14a,14c,26 All reagents and starting
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materials were obtained from commercial sources and used as received unless otherwise stated. Dry
solvents were purified using a solvent purification system. Brine refers to a saturated solution of sodium
chloride. All reactions were performed in oven-dried glassware under an atmosphere of argon unless
otherwise stated. Flash column chromatography was carried out using silica gel (40–63 μm) and neutral
aluminium oxide (50–200 μm). Aluminium-backed plates pre-coated with silica gel 60 (UV254) were used
for thin layer chromatography and were visualized under ultraviolet light and by staining with KMnO4 or
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1
ninhydrin. H NMR spectra were recorded on an NMR spectrometer at 400 or 500 MHz and data are
reported as follows: chemical shift in ppm relative to tetramethylsilane or the solvent as the internal
standard (CDCl3, δ 7.26 ppm), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet
or overlap of nonequivalent resonances, integration). 13C{1H} NMR spectra were recorded on an NMR
spectrometer at 101 or 126 MHz and data are reported as follows: chemical shift in ppm relative to
tetramethylsilane or the solvent as internal standard (CDCl3, δ 77.0 ppm), multiplicity with respect to
hydrogen (deduced from DEPT experiments, C, CH, CH2 or CH3). IR spectra were recorded on a FTIR
spectrometer; wavenumbers are indicated in cm–1. Mass spectra were recorded using electron impact or
electrospray ionization techniques. HRMS spectra were recorded using a dual-focusing magnetic analyzer
mass spectrometer. Melting points are uncorrected. Optical rotations were determined as solutions
irradiating with the sodium D line (λ = 589 nm) using a polarimeter. [α]D values are given in units 10−1
deg cm2 g−1. Fluorescence spectra were recorded on a spectrofluorophotometer. Emission data were
measured using an excitation slit width of 3 nm and emission slit width of 5 nm. Quantum yield data were
measured using anthracene and ʟ-tryptophan as standard references.
Methyl (2S,5E)-6-(2’,4’-dimethoxyphenyl)-4-oxo-2-(tritylamino)hex-5-enoate (4e). Methyl (2S)-5-
(dimethoxyphosphoryl)-4-oxo-2-(tritylamino)pentanoate (3) (1.11 g, 2.24 mmol) was dissolved in
acetonitrile (24 mL) and potassium carbonate (0.370 g, 2.68 mmol) was added. The mixture was stirred
at room temperature for 0.5 h followed by addition of 2,4-dimethoxybenzaldehyde (0.743 g, 4.48 mmol).
The temperature was increased to 50 °C and the mixture was stirred for 120 h. Once the reaction was
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complete, the solution was concentrated in vacuo, redissolved in ethyl acetate (40 mL) and washed with
water (2 × 30 mL) and brine (30 mL). The organic layer was dried over MgSO4 and concentrated in vacuo.
Purification by flash column chromatography on silica gel eluting with 30% ethyl acetate in petroleum
ether (40–60) gave methyl (2S,5E)-6-(2’,4’-dimethoxyphenyl)-4-oxo-2-(tritylamino)hex-5-enoate (4e)
(0.564 g, 47%) as a yellow foam. Mp 66–70 °C; Rf 0.15 (30% ethyl acetate in petroleum ether); IR (neat)
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2948, 1737, 1599, 1266, 1210, 1160, 1028, 734, 706 cm−1; [α]D +52.3 (c 1.0, CHCl3); H NMR (500
MHz, CDCl3) δ 2.83 (dd, J = 14.9, 6.8 Hz, 1H), 2.87–2.95 (m, 2H), 3.25 (s, 3H), 3.74–3.82 (m, 1H), 3.83
(s, 3H), 3.84 (s, 3H), 6.45 (d, J = 2.3 Hz, 1H), 6.51 (dd, J = 8.6, 2.3 Hz, 1H), 6.69 (d, J = 16.3 Hz, 1H),
13
7.12–7.27 (m, 9H), 7.44–7.54 (m, 7H), 7.82 (d, J = 16.3 Hz, 1H); C{1H} NMR (126 MHz, CDCl3) δ
45.3 (CH2), 51.8 (CH3), 53.9 (CH), 55.5 (CH3), 55.5 (CH3), 71.3 (C), 98.4 (CH), 105.5 (CH), 116.5 (C),
124.8 (CH), 126.4 (3 × CH), 127.8 (6 × CH), 128.9 (6 × CH), 130.2 (CH), 138.8 (CH), 145.9 (3 × C),
160.1 (C), 163.2 (C), 174.6 (C), 198.0 (C); MS (ESI) m/z 558 (M+Na+, 100); HRMS (ESI) calcd for
C34H33NNaO5 (M+Na+) 558.2251, found 558.2227.
Methyl (2S,5E)-6-(2H-1’,3’-benzodioxol-5’-yl)-4-oxo-2-(tritylamino)hex-5-enoate (4f). The
reaction was carried out according to the above procedure for the synthesis of methyl (2S,5E)-6-(2’,4’-
dimethoxyphenyl)-4-oxo-2-(tritylamino)hex-5-enoate (4e) using methyl (2S)-5-(dimethoxyphosphoryl)-
4-oxo-2-(tritylamino)pentanoate (3) (0.215 g, 0.434 mmol) and piperonal (0.130 g, 0.868 mmol) for 120
h. Purification by flash column chromatography on silica gel eluting with 20% ethyl acetate in petroleum
ether (40–60) gave methyl (2S,5E)-6-(2H-1’,3’-benzodioxol-5’-yl)-4-oxo-2-(tritylamino)hex-5-enoate
(4f) (0.201 g, 89%) as a yellow oil. Rf 0.12 (20% ethyl acetate in petroleum ether); IR (neat) 3055, 2898,
1734, 1594, 1489, 1447, 1254, 1035, 705 cm−1; [α]D26 +49.2 (c 1.0, CHCl3); 1H NMR (400 MHz, CDCl3)
δ 2.75 (dd, J = 15.1, 6.8 Hz, 1H), 2.84–2.95 (m, 2H), 3.27 (s, 3H), 3.78 (dt, J = 9.3, 6.8 Hz, 1H), 5.98 (s,
2H), 6.52 (d, J = 16.1 Hz, 1H), 6.81 (d, J = 7.9 Hz, 1H), 6.97–7.04 (m, 2H), 7.13–7.29 (m, 9H), 7.39 (d,
J = 16.1 Hz, 1H), 7.47–7.53 (m, 6H); 13C{1H} NMR (101 MHz, CDCl3) δ 45.8 (CH2), 51.9 (CH3), 53.8
(CH), 71.3 (C), 101.6 (CH2), 106.6 (CH), 108.6 (CH), 124.5 (CH), 125.0 (CH), 126.5 (3 × CH), 127.9 (6
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× CH), 128.1 (C), 128.8 (6 × CH), 143.1 (CH), 145.8 (3 × C), 148.5 (C), 150.0 (C), 174.5 (C), 197.2 (C);
MS (ESI) m/z 542 (M+Na+, 100); HRMS (ESI) calcd for C33H29NNaO5 (M+Na+) 542.1938, found
542.1915.
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Methyl (2S,5E)-6-(2’,4’-dinitrophenyl)-4-oxo-2-(tritylamino)hex-5-enoate (4i). The reaction was
carried out according to the above procedure for the synthesis of methyl (2S,5E)-6-(2’,4’-
dimethoxyphenyl)-4-oxo-2-(tritylamino)hex-5-enoate (4e) using methyl (2S)-5-(dimethoxyphosphoryl)-
4-oxo-2-(tritylamino)pentanoate (3) (1.06 g, 2.15 mmol) and 2,4-dinitrobenzaldehyde (0.842 g, 4.29
mmol) for 1 h. Purification by flash column chromatography on silica gel eluting with 60% diethyl ether
in petroleum ether (40–60) gave methyl (2S,5E)-6-(2’,4’-dinitrophenyl)-4-oxo-2-(tritylamino)hex-5-
enoate (4i) (0.801 g, 66%) as an orange solid. Mp 58–61 °C; Rf 0.25 (60% diethyl ether in petroleum
ether); IR (neat) 3057, 2952, 1734, 1596, 1531, 1448, 1266, 834, 706 cm−1; [α]D26 +37.7 (c 1.0, CHCl3);
1H NMR (500 MHz, CDCl3) δ 2.79 (dd, J = 15.8, 6.8 Hz, 1H), 2.89 (dd, J = 15.8, 5.0 Hz, 1H), 2.96 (br s,
1H), 3.35 (s, 3H), 3.78–3.84 (m, 1H), 6.60 (d, J = 16.1 Hz, 1H), 7.15–7.28 (m, 9H), 7.43–7.52 (m, 6H),
7.78 (d, J = 8.6 Hz, 1H), 7.86 (d, J = 16.1 Hz, 1H), 8.46 (dd, J = 8.6, 2.3 Hz, 1H), 8.90 (d, J = 2.3 Hz,
1H); 13C{1H} NMR (126 MHz, CDCl3) δ 45.5 (CH2), 52.1 (CH3), 53.5 (CH), 71.3 (C), 120.7 (CH), 126.6
(3 × CH), 127.7 (CH), 128.0 (6 × CH), 128.8 (6 × CH), 130.6 (CH), 133.4 (CH), 136.0 (CH), 136.6 (C),
145.6 (3 × C), 148.1 (C), 148.2 (C), 174.1 (C), 196.5 (C); MS (ESI) m/z 588 (M+Na+, 100); HRMS (ESI)
calcd for C32H27N3NaO7 (M+Na+) 588.1741, found 588.1714.
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Methyl (2S,5E)-2-[(benzyloxycarbonyl)amino]-4-oxonon-5-enoate (5a). To a solution of methyl
(2S,5E)-2-(tritylamino)-4-oxonon-5-enoate (4a) (0.398 g, 0.902 mmol) in dichloromethane (10 mL) was
added trifluoroacetic acid (0.138 mL, 1.80 mmol). The reaction mixture was stirred at room temperature
for 2 h before concentrating in vacuo. The resulting residue was dissolved in chloroform (5 mL) and
petroleum ether (40–60) was added until an orange oil formed, at which point the solvent was decanted
off. The resulting oil was dissolved in dichloromethane (10 mL) and N,N’-diisopropylethylamine (0.397
mL, 2.27 mmol) was added, followed by benzyl chloroformate (0.200 mL, 1.36 mmol). The reaction
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mixture was stirred at room temperature for 1 h, before diluting with water (30 mL). The mixture was
then extracted with dichloromethane (3 × 15 mL), dried over MgSO4 and concentrated in vacuo.
Purification by flash column chromatography on silica gel eluting with 30% ethyl acetate in petroleum
ether (40–60) gave methyl (2S,5E)-2-[(benzyloxycarbonyl)amino]-4-oxonon-5-enoate (5a) (0.201 g,
67%) as a white solid. Mp 40–42 °C; Rf 0.29 (30% ethyl acetate in petroleum ether); IR (neat) 3352, 2959,
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1723, 1627, 1503, 1209, 1061, 979, 737, 698 cm−1; [α]D +23.7 (c 1.0, CHCl3); H NMR (400 MHz,
CDCl3) δ 0.94 (t, J = 7.4 Hz, 3H), 1.44–1.55 (m, 2H), 2.20 (qd, J = 7.2, 1.4 Hz, 2H), 3.11 (dd, J = 18.0,
4.2 Hz, 1H), 3.34 (dd, J = 18.0, 4.2 Hz, 1H), 3.73 (s, 3H), 4.62 (dt, J = 8.5, 4.2 Hz, 1H), 5.11 (s, 2H), 5.79
(d, J = 8.5 Hz, 1H), 6.06 (dt, J = 15.8, 1.4 Hz, 1H), 6.86 (dt, J = 15.8, 7.2 Hz, 1H), 7.28–7.39 (m, 5H);
13C{1H} NMR (101 MHz, CDCl3) δ 13.8 (CH3), 21.4 (CH2), 34.7 (CH2), 41.7 (CH2), 50.1 (CH), 52.8
(CH3), 67.2 (CH2), 128.2 (2 × CH), 128.3 (CH), 128.7 (2 × CH), 130.1 (CH), 136.4 (C), 149.4 (CH),
156.2 (C), 171.8 (C), 197.8 (C); MS (ESI) m/z 356 (M+Na+, 100); HRMS (ESI) calcd for C18H23NNaO5
(M+Na+) 356.1468, found 356.1455.
Methyl
(5e). The reaction was carried out according to the above procedure for the synthesis of methyl (2S,5E)-
2-[(benzyloxycarbonyl)amino]-4-oxonon-5-enoate (5a) using methyl (2S,5E)-6-(2’,4’-
(2S,5E)-2-[(benzyloxycarbonyl)amino]-6-(2’,4’-dimethoxyphenyl)-4-oxohex-5-enoate
dimethoxyphenyl)-4-oxo-2-(tritylamino)hex-5-enoate (4e) (0.550 g, 1.03 mmol). Purification by flash
column chromatography on silica gel eluting with 40% ethyl acetate in petroleum ether (40–60) gave
methyl (2S,5E)-2-[(benzyloxycarbonyl)amino]-6-(2’,4’-dimethoxyphenyl)-4-oxohex-5-enoate (5e)
(0.168 g, 70%) as a yellow oil. Rf 0.16 (40% ethyl acetate in petroleum ether); IR (neat) 3362, 2949, 1722,
1600, 1504, 1210, 1028 cm−1; [α]D26 +19.6 (c 1.0, CHCl3); 1H NMR (400 MHz, CDCl3) δ 3.23 (dd, J =
17.9, 4.2 Hz, 1H), 3.47 (dd, J = 17.9, 4.2 Hz, 1H), 3.73 (s, 3H), 3.83 (s, 3H), 3.85 (s, 3H), 4.67 (dt, J =
8.7, 4.2 Hz, 1H), 5.11 (s, 2H), 5.91 (d, J = 8.7 Hz, 1H), 6.44 (d, J = 2.3 Hz, 1H), 6.50 (dd, J = 8.6, 2.3 Hz,
1H), 6.66 (d, J = 16.4 Hz, 1H), 7.27–7.39 (m, 5H), 7.45 (d, J = 8.6 Hz, 1H), 7.84 (d, J = 16.4 Hz, 1H);
13C{1H} NMR (101 MHz, CDCl3) δ 41.8 (CH2), 50.2 (CH), 52.6 (CH3), 55.5 (CH3), 55.5 (CH3), 66.9
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(CH2), 98.4 (CH), 105.6 (CH), 116.1 (C), 123.6 (CH), 128.0 (2 × CH), 128.1 (CH), 128.5 (2 × CH), 130.3
(CH), 136.3 (C), 139.4 (CH), 156.1 (C), 160.1 (C), 163.4 (C), 171.8 (C), 197.8 (C); MS (ESI) m/z 450
(M+Na+, 100); HRMS (ESI) calcd for C23H25NNaO7 (M+Na+) 450.1523, found 450.1503.
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Methyl (2S,5E)-2-[(benzyloxycarbonyl)amino]-6-(2H-1’,3’-benzodioxol-5’-yl)-4-oxohex-5-enoate
(5f). The reaction was carried out according to the above procedure for the synthesis of methyl (2S,5E)-
2-[(benzyloxycarbonyl)amino]-4-oxonon-5-enoate (5a) using methyl (2S,5E)-6-(2H-1’,3’-benzodioxol-
5’-yl)-4-oxo-2-(tritylamino)hex-5-enoate (4f) (1.51 g, 2.91 mmol). Purification by flash column
chromatography on silica gel eluting with 40% ethyl acetate in petroleum ether (40–60) gave methyl
(2S,5E)-2-[(benzyloxycarbonyl)amino]-6-(2H-1’,3’-benzodioxol-5’-yl)-4-oxohex-5-enoate (5f) (0.972 g,
81%) as a yellow oil. Rf 0.20 (40% ethyl acetate in petroleum ether); IR (neat) 3369, 2953, 1721, 1503,
1490, 1447, 1253, 1035, 929, 737, 698 cm−1; [α]D29 +28.6 (c 1.0, CHCl3); 1H NMR (400 MHz, CDCl3) δ
3.21 (dd, J = 17.9, 4.2 Hz, 1H), 3.43 (dd, J = 17.9, 4.2 Hz, 1H), 3.74 (s, 3H), 4.67 (dt, J = 8.6, 4.2 Hz,
1H), 5.11 (s, 2H), 5.88 (d, J = 8.6 Hz, 1H), 6.01 (s, 2H), 6.52 (d, J = 16.1 Hz, 1H), 6.81 (d, J = 8.1 Hz,
1H), 6.99–7.05 (m, 2H), 7.28–7.38 (m, 5H), 7.46 (d, J = 16.1 Hz, 1H); 13C{1H} NMR (101 MHz, CDCl3)
δ 42.3 (CH2), 50.1 (CH), 52.7 (CH3), 67.0 (CH2), 101.7 (CH2), 106.6 (CH), 108.7 (CH), 123.5 (CH), 125.3
(CH), 128.0 (2 × CH), 128.1 (CH), 128.5 (2 × CH), 128.5 (C), 136.3 (C), 143.9 (CH), 148.5 (C), 150.2
(C), 156.1 (C), 171.7 (C), 197.2 (C); MS (ESI) m/z 434 (M+Na+, 100); HRMS (ESI) calcd for
C22H21NNaO7 (M+Na+) 434.1210, found 434.1192.
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Methyl (2S,5E)-2-[(benzyloxycarbonyl)amino]-6-(4’-fluorophenyl)-4-oxohex-5-enoate (5g). The
reaction was carried out according to the above procedure for the synthesis of methyl (2S,5E)-2-
[(benzyloxycarbonyl)amino]-4-oxonon-5-enoate (5a) using methyl (2S,5E)-6-(4-fluorophenyl)-4-oxo-2-
(tritylamino)hex-5-enoate (4g) (0.367 g, 0.744 mmol). Purification by flash column chromatography on
silica gel eluting with 30% ethyl acetate in petroleum ether (40–60) gave methyl (2S,5E)-2-
[(benzyloxycarbonyl)amino]-6-(4’-fluorophenyl)-4-oxohex-5-enoate (5g) (0.174 g, 57%) as a white
solid. Mp 66–69 °C; Rf 0.20 (30% ethyl acetate in petroleum ether); IR (neat) 3333, 2951, 1748, 1730,
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1685, 1508, 1236, 1161, 981, 829, 698 cm−1; [α]D27 +26.5 (c 1.0, CHCl3); 1H NMR (400 MHz, CDCl3) δ
3.24 (dd, J = 17.9, 4.2 Hz, 1H), 3.46 (dd, J = 17.9, 4.2 Hz, 1H), 3.75 (s, 3H), 4.68 (dt, J = 8.5, 4.2 Hz,
1H), 5.12 (s, 2H), 5.84 (d, J = 8.5 Hz, 1H), 6.62 (d, J = 16.2 Hz, 1H), 7.06–7.13 (m, 2H), 7.28–7.39 (m,
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5H), 7.49–7.58 (m, 3H); C{1H} NMR (101 MHz, CDCl3) δ 42.5 (CH2), 50.2 (CH), 52.9 (CH3), 67.2
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(CH2), 116.4 (d, 2JC-F 22.0 Hz, 2 × CH), 125.3 (CH), 128.2 (2 × CH), 128.3 (CH), 128.7 (2 × CH), 130.5
(C), 130.5 (d, 3JC-F 8.5 Hz, 2 × CH), 136.4 (C), 142.9 (CH), 156.2 (C), 164.4 (d, 1JC-F 252.6 Hz, C), 171.7
(C), 197.4 (C); MS (ESI) m/z 408 (M+Na+, 100); HRMS (ESI) calcd for C21H20FNNaO5 (M+Na+)
408.1218, found 408.1201.
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Methyl (2S,5E)-2-[(benzyloxycarbonyl)amino]-6-(2’,4’-dinitrophenyl)-4-oxohex-5-enoate (5i).
The reaction was carried out according to the above procedure for the synthesis of methyl (2S,5E)-2-
[(benzyloxycarbonyl)amino]-4-oxonon-5-enoate (5a) using methyl (2S,5E)-6-(2’,4’-dinitrophenyl)-4-
oxo-2-(tritylamino)hex-5-enoate (4i) (0.620 g, 1.10 mmol). Purification by flash column chromatography
on silica gel eluting with 40% ethyl acetate in petroleum ether (40–60) gave methyl (2S,5E)-2-
[(benzyloxycarbonyl)amino]-6-(2’,4’-dinitrophenyl)-4-oxohex-5-enoate (5i) (0.293 g, 59%) as a brown
solid. Mp 88–91 °C; Rf 0.15 (40% ethyl acetate in petroleum ether); IR (neat) 3382, 2955, 1720, 1597,
1528, 1343, 1211, 835, 736 cm−1; [α]D27 +19.7 (c 1.0, CHCl3); 1H NMR (500 MHz, CDCl3) δ 3.34 (dd, J
= 18.1, 4.3 Hz, 1H), 3.49 (dd, J = 18.1, 4.3 Hz, 1H), 3.76 (s, 3H), 4.74 (dt, J = 8.5, 4.3 Hz, 1H), 5.10 (s,
2H), 5.91 (d, J = 8.5 Hz, 1H), 6.70 (d, J = 16.1 Hz, 1H), 7.26–7.36 (m, 5H), 7.85 (d, J = 8.6 Hz, 1H), 8.01
(d, J = 16.1 Hz, 1H), 8.49 (dd, J = 8.6, 2.3 Hz, 1H), 8.89 (d, J = 2.3 Hz, 1H); 13C{1H} NMR (126 MHz,
CDCl3) δ 42.7 (CH2), 49.9 (CH), 52.9 (CH3), 67.0 (CH2), 120.7 (CH), 127.8 (CH), 128.0 (2 × CH), 128.2
(CH), 128.5 (2 × CH), 130.7 (CH), 132.4 (CH), 136.1 (C), 136.4 (C), 136.9 (CH), 148.1 (C), 148.2 (C),
156.0 (C), 171.3 (C), 196.5 (C); MS (ESI) m/z 480 (M+Na+, 100); HRMS (ESI) calcd for C21H19N3NaO9
(M+Na+) 480.1014, found 480.0997.
Methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-(4’-phenylpyridin-2’-yl)propanoate (7b). To a
solution of methyl (2S,5E)-2-[(benzyloxycarbonyl)amino]-4-oxo-6-phenylhex-5-enoate (5b) (0.186 g,
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0.506 mmol) in ethyl vinyl ether (5 mL) was added tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-
octanedionato)ytterbium (0.0269 g, 0.0253 mmol). The reaction tube was then purged with argon, sealed
and the reaction mixture was stirred at 110 °C for 96 h. The mixture was then allowed to cool to room
temperature and concentrated in vacuo. The reaction mixture was washed through a silica plug eluting
with 20% ethyl acetate in petroleum ether (40–60) and gave dihydropyran 6b (0.208 g, 93%).
Dihydropyran 6b (0.208 g, 0.476 mmol) was then added to a solution of hydroxylamine hydrochloride
(0.166 g, 2.38 mmol) in acetonitrile (5 mL). The reaction mixture was stirred at 70 °C for 16 h and then
concentrated in vacuo. Purification by flash column chromatography on neutral alumina (Brockmann V
grade) eluting with 1% methanol in dichloromethane gave methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-
(4’-phenylpyridin-2’-yl)propanoate (7b) (0.131 g, 71%) as a yellow oil. Rf 0.79 (1% methanol in
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dichloromethane); IR (neat) 3346, 2951, 1719, 1506, 1212, 1061, 764, 697 cm−1; [α]D +27.6 (c 1.1,
CHCl3); 1H NMR (500 MHz, CDCl3) δ 3.36 (dd, J = 14.9, 5.2 Hz, 1H), 3.44 (dd, J = 14.9, 5.2 Hz, 1H),
3.70 (s, 3H), 4.81 (dt, J = 8.3, 5.2 Hz, 1H), 5.10 (d, J = 12.4 Hz, 1H), 5.13 (d, J = 12.4 Hz, 1H), 6.35 (d,
J = 8.3 Hz, 1H), 7.29–7.37 (m, 7H), 7.42–7.50 (m, 3H), 7.56–7.63 (m, 2H), 8.51 (d, J = 5.1 Hz, 1H);
13C{1H} NMR (126 MHz, CDCl3) δ 39.2 (CH2), 52.5 (CH3), 53.6 (CH), 67.0 (CH2), 120.1 (CH), 121.8
(CH), 127.1 (2 × CH), 128.2 (3 × CH), 128.6 (2 × CH), 129.2 (2 × CH), 129.2 (CH), 136.6 (C), 138.1 (C),
149.3 (C), 149.7 (CH), 156.2 (C), 157.6 (C), 172.2 (C); MS (ESI) m/z 413 (M+Na+, 100); HRMS (ESI)
calcd for C23H22N2NaO4 (M+Na+) 413.1472, found 413.1461.
Methyl
(2S)-2-[(benzyloxycarbonyl)amino]-3-(4’-propylpyridin-2’-yl)propanoate
(7a).
Dihydropyran formation was carried out according to the above procedure for the synthesis of methyl
(2S)-2-[(benzyloxycarbonyl)amino]-3-(4’-phenylpyridin-2’-yl)propanoate (7b) using methyl (2S,5E)-2-
[(benzyloxycarbonyl)amino]-4-oxonon-5-enoate (5a) (0.103 g, 0.309 mmol) for 96 h. This gave
dihydropyran 6a (0.108 g, 86%). Dihydropyran 6a (0.107 g, 0.264 mmol) was then treated with
hydroxylamine hydrochloride (0.0917 g, 1.32 mmol) in acetonitrile (3 mL) at 70 °C for 16 h. Purification
by flash column chromatography on neutral alumina (Brockmann V grade) eluting with 1% methanol in
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dichloromethane gave methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-(4’-propylpyridin-2’-yl)propanoate
(7a) (0.054 g, 58%) as a pale yellow oil. Rf 0.85 (1% methanol in dichloromethane on alumina); IR (neat)
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3344, 2959, 1721, 1606, 1506, 1209, 1055, 738, 697 cm−1; [α]D +18.0 (c 0.7, CHCl3); H NMR (500
MHz, CDCl3) δ 0.92 (t, J = 7.3 Hz, 3H), 1.57–1.67 (m, 2H), 2.52 (t, J = 7.3 Hz, 2H), 3.23 (dd, J = 14.8,
5.2 Hz, 1H), 3.32 (dd, J = 14.8, 5.2 Hz, 1H), 3.67 (s, 3H), 4.74 (dt, J = 8.2, 5.2 Hz, 1H), 5.08 (d, J = 12.4
Hz, 1H), 5.12 (d, J = 12.4 Hz, 1H), 6.38 (d, J = 8.2 Hz, 1H), 6.91–6.97 (m, 2H), 7.28–7.37 (m, 5H), 8.33
(d, J = 5.0 Hz, 1H); 13C{1H} NMR (126 MHz, CDCl3) δ 13.8 (CH3), 23.5 (CH2), 37.3 (CH2), 38.9 (CH2),
52.4 (CH3), 53.6 (CH), 66.9 (CH2), 122.3 (CH), 124.0 (CH), 128.2 (2 × CH), 128.2 (CH), 128.6 (2 × CH),
136.6 (C), 149.1 (CH), 152.3 (C), 156.2 (C), 156.9 (C), 172.2 (C); MS (EI) m/z 356 (M+, 46), 297 (36),
253 (22), 189 (48), 135 (95), 91 (100); HRMS (EI) calcd for C20H24N2O4 (M+) 356.1736, found 356.1729.
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Methyl
(2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(naphthalen-2’’-yl)pyridin-2’-yl]propanoate
(7c). Dihydropyran formation was carried out according to the above procedure for the synthesis of methyl
(2S)-2-[(benzyloxycarbonyl)amino]-3-(4’-phenylpyridin-2’-yl)propanoate (7b) using methyl (2S,5E)-2-
[(benzyloxycarbonyl)amino]-6-(naphthalen-2’-yl)-4-oxohex-5-enoate (5c) (0.110 g, 0.264 mmol) for 72
h. This gave dihydropyran 6c (0.129 g, 83%). Dihydropyran 6c (0.129 g, 0.264 mmol) was then reacted
with hydroxylamine hydrochloride (0.0917 g, 1.32 mmol) in acetonitrile (3 mL) at 70 °C for 16 h.
Purification by flash column chromatography on neutral alumina (Brockmann V grade) eluting with 1%
methanol in dichloromethane gave methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(naphthalen-2’’-
yl)pyridin-2’-yl]propanoate (7c) (0.056 g, 48%) as a yellow solid. Mp 56–60 °C; Rf 0.84 (1% methanol
in dichloromethane); IR (neat) 3346, 3054, 2951, 1717, 1597, 1504, 1437, 1210, 1056, 1028, 816, 734
cm−1; [α]D25 +26.1 (c 0.9, CHCl3); 1H NMR (500 MHz, CDCl3) δ 3.39 (dd, J = 15.0, 5.2 Hz, 1H), 3.48
(dd, J = 15.0, 5.2 Hz, 1H), 3.71 (s, 3H), 4.84 (dt, J = 8.4, 5.2 Hz, 1H), 5.10 (d, J = 12.4 Hz, 1H), 5.13 (d,
J = 12.4 Hz, 1H), 6.38 (d, J = 8.4 Hz, 1H), 7.27–7.37 (m, 5H), 7.45–7.49 (m, 2H), 7.52–7.57 (m, 2H),
7.70 (dd, J = 8.6, 1.5 Hz, 1H), 7.86–7.92 (m, 2H), 7.94 (d, J = 8.6 Hz, 1H), 8.07 (br s, 1H), 8.55 (d, J =
5.1 Hz, 1H); 13C{1H} NMR (126 MHz, CDCl3) δ 39.3 (CH2), 52.6 (CH3), 53.6 (CH), 67.1 (CH2), 120.3
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(CH), 121.9 (CH), 124.7 (CH), 126.6 (CH), 126.9 (CH), 127.0 (CH), 127.9 (CH), 128.2 (3 × CH), 128.6
(CH), 128.6 (2 × CH), 129.1 (CH), 133.6 (C), 133.6 (C), 135.4 (C), 136.5 (C), 149.2 (C), 149.8 (CH),
156.3 (C), 157.7 (C), 172.3 (C); MS (EI) m/z 440 (M+, 38), 381 (17), 332 (45), 305 (43), 273 (76), 245
(96), 219 (98), 189 (24), 91 (100); HRMS (EI) calcd for C27H24N2O4 (M+) 440.1736, found 440.1738.
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Methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(4’’-methoxyphenyl)pyridin-2’-yl]propanoate
(7d). Dihydropyran formation was carried out according to the above procedure for the synthesis of
methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-(4’-phenylpyridin-2’-yl)propanoate (7b) using methyl
(2S,5E)-2-[(benzyloxycarbonyl)amino]-6-(4’-methoxyphenyl)-4-oxohex-5-enoate (5d) (0.602 g, 1.52
mmol) for 168 h. This gave dihydropyran 6d (0.595 g, 83%). Dihydropyran 6d (0.563 g, 1.20 mmol) was
then reacted with hydroxylamine hydrochloride (0.416 g, 6.00 mmol) in acetonitrile (12 mL) at 70 °C for
16 h. Purification by flash column chromatography on neutral alumina (Brockmann V grade), eluting with
1% methanol in dichloromethane gave methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(4’’-
methoxyphenyl)pyridin-2’-yl]propanoate (7d) (0.242 g, 48%) as a yellow oil. Rf 0.81 (1% methanol in
dichloromethane); IR (neat) 3341, 2953, 1718, 1603, 1516, 1251, 1180, 1026, 826, 698 cm−1; [α]D25 +23.0
(c 0.8, CHCl3); 1H NMR (500 MHz, CDCl3) δ 3.33 (dd, J 14.9, 5.2 Hz, 1H), 3.42 (dd, J = 14.9, 5.2 Hz,
1H), 3.70 (s, 3H), 3.86 (s, 3H), 4.80 (dt, J = 8.2, 5.2 Hz, 1H), 5.09 (d, J = 12.3 Hz, 1H), 5.13 (d, J = 12.3
Hz, 1H), 6.36 (d, J = 8.2 Hz, 1H), 6.99 (d, J = 8.7 Hz, 2H), 7.29–7.35 (m, 7H), 7.55 (d, J = 8.7 Hz, 2H),
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8.47 (d, J = 5.2 Hz, 1H); C{1H} NMR (126 MHz, CDCl3) δ 39.2 (CH2), 52.5 (CH3), 53.6 (CH), 55.6
(CH3), 67.0 (CH2), 114.7 (2 × CH2), 119.6 (CH), 121.2 (CH), 128.2 (3 × CH), 128.3 (2 × CH), 128.6 (2
× CH), 130.4 (C), 136.6 (C), 148.8 (C), 149.6 (CH), 156.3 (C), 157.5 (C), 160.8 (C), 172.3 (C); MS (ESI)
m/z 443 (M+Na+, 100); HRMS (ESI) calcd for C24H24N2NaO5 (M+Na+) 443.1577, found 443.1556.
Methyl
(2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(2’’,4’’-dimethoxyphenyl)pyridin-2’-
yl]propanoate (7e). The reaction was carried out according to the above procedure for the synthesis of
methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-(4’-phenylpyridin-2’-yl)propanoate (7b) using methyl
(2S,5E)-2-[(benzyloxycarbonyl)amino]-6-(2’,4’-dimethoxyphenyl)-4-oxohex-5-enoate (5e) (0.140 g,
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0.327 mmol) for 192 h. This gave dihydropyran 6e (0.136 g, 83%). Dihydropyran 6e (0.126 g, 0.252
mmol) was then treated with hydroxylamine hydrochloride (0.0876 g, 1.26 mmol) in acetonitrile (5 mL)
at 70 °C for 16 h. Purification by flash column chromatography on neutral alumina (Brockmann V grade)
eluting with 1% methanol in dichloromethane gave methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-
(2’’,4’’-dimethoxyphenyl)pyridin-2’-yl]propanoate (7e) (0.065 g, 57%) as a pale yellow oil. Rf 0.78 (1%
methanol in dichloromethane on alumina); IR (neat) 3343, 2951, 1719, 1604, 1508, 1207, 1028, 737, 698
cm−1; [α]D27 +22.9 (c 1.0, CHCl3); 1H NMR (500 MHz, CDCl3) δ 3.30 (dd, J = 14.8, 5.2 Hz, 1H), 3.40
(dd, J = 14.8, 5.2 Hz, 1H), 3.70 (s, 3H), 3.79 (s, 3H), 3.85 (s, 3H), 4.77 (dt, J = 8.2, 5.2 Hz, 1H), 5.09 (d,
J = 12.6 Hz, 1H), 5.12 (d, J = 12.6 Hz, 1H), 6.39 (d, J = 8.2 Hz, 1H), 6.52–6.60 (m, 2H), 7.22–7.38 (m,
8H), 8.44 (d, J = 5.2 Hz, 1H); 13C{1H} NMR (126 MHz, CDCl3) δ 39.0 (CH2), 52.3 (CH3), 53.5 (CH),
55.5 (CH3), 55.5 (CH3), 66.9 (CH2), 99.1 (CH), 105.0 (CH), 120.3 (C), 122.4 (CH), 124.0 (CH), 128.0
(CH), 128.1 (2 × CH), 128.5 (2 × CH), 131.1 (CH), 136.4 (C), 146.8 (C), 148.8 (CH), 156.1 (C), 156.4
(C), 157.7 (C), 161.5 (C), 172.2 (C); MS (ESI) m/z 451 (M+H+, 100); HRMS (ESI) calcd for C25H27N2O6
(M+H+) 451.1840, found 451.1846.
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Methyl
(2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(2H-1’’,3’’-benzodioxol-5’’-yl)pyridin-2’-
yl]propanoate (7f). The reaction was carried out according to the above procedure for the synthesis of
methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-(4’-phenylpyridin-2’-yl)propanoate (7b) using methyl
(2S,5E)-2-[(benzyloxycarbonyl)amino]-6-(2H-1’,3’-benzodioxol-5’-yl)-4-oxohex-5-enoate (5f) (0.972 g,
2.36 mmol) for 168 h. This gave dihydropyran 6f (0.990 g, 87%). Dihydropyran 6f (0.990 g, 2.05 mmol)
was then treated with hydroxylamine hydrochloride (0.711 g, 10.2 mmol) in acetonitrile (20 mL) at 70 °C
for 16 h. Purification by flash column chromatography on neutral alumina (Brockmann V grade) eluting
with 1% methanol in dichloromethane gave methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(2H-
1’’,3’’-benzodioxol-5’’-yl)pyridin-2’-yl]propanoate (7f) (0.530 g, 59%) as an orange oil. Rf 0.80 (1%
methanol in dichloromethane on alumina); IR (neat) 3358, 2953, 1721, 1603, 1506, 1476, 1236, 1040,
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808 cm−1; [α]D +28.0 (c 1.1, CHCl3); H NMR (400 MHz, CDCl3) δ 3.35 (dd, J = 15.0, 5.1 Hz, 1H),
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3.44 (dd, J = 15.0, 5.1 Hz, 1H), 3.72 (s, 3H), 4.82 (dt, J = 8.2, 5.1 Hz, 1H), 5.11 (d, J = 12.5 Hz, 1H), 5.15
(d, J = 12.5 Hz, 1H), 6.05 (s, 2H), 6.35 (d, J = 8.2 Hz, 1H), 6.92 (d, J = 8.0 Hz, 1H), 7.09 (d, J = 2.0 Hz,
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MHz, CDCl3) δ 39.1 (CH2), 52.4 (CH3), 53.4 (CH), 66.9 (CH2), 101.5 (CH2), 107.2 (CH), 108.9 (CH),
119.6 (CH), 121.0 (CH), 121.2 (CH), 128.1 (2 × CH), 128.5 (3 × CH), 132.1 (C), 136.4 (C), 148.5 (C),
148.6 (C), 148.7 (C), 149.5 (CH), 156.1 (C), 157.4 (C), 172.1 (C); MS (ESI) m/z 457 (M+Na+, 100);
HRMS (ESI) calcd for C24H22N2NaO6 (M+Na+) 457.1370, found 457.1355.
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Methyl
(2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(4’’-fluorophenyl)pyridin-2’-yl]propanoate
(7g). Dihydropyran formation was carried out according to the above procedure for the synthesis of methyl
(2S)-2-[(benzyloxycarbonyl)amino]-3-(4’-phenylpyridin-2’-yl)propanoate (7b) using methyl (2S,5E)-2-
[(benzyloxycarbonyl)amino]-4-oxo-6-phenylhex-5-enoate (5g) (0.126 g, 0.310 mmol) for 96 h. This gave
dihydropyran 6g (0.0980 g, 70%). Dihydropyran 6g (0.0872 g, 0.191 mmol) was then treated with
hydroxylamine hydrochloride (0.0616 g, 0.955 mmol) in acetonitrile (2 mL) at 70 °C for 16 h. Purification
by flash column chromatography on neutral alumina (Brockmann V grade) eluting with 1% methanol in
dichloromethane gave methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(4’’-fluorophenyl)pyridin-2’-
yl]propanoate (7g) (0.046 g, 60%) as a white solid. Mp 48–52 °C; Rf 0.79 (1% methanol in
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dichloromethane); IR (neat) 3346, 2952, 1719, 1606, 1513, 1223, 1059, 827 cm−1; [α]D +25.5 (c 0.6,
CHCl3); 1H NMR (500 MHz, CDCl3) δ 3.35 (dd, J = 15.0, 5.2 Hz, 1H), 3.44 (dd, J = 15.0, 5.2 Hz, 1H),
3.70 (s, 3H), 4.81 (dt, J = 8.3, 5.2 Hz, 1H), 5.09 (d, J = 12.3 Hz, 1H), 5.13 (d, J = 12.3 Hz, 1H), 6.31 (d,
J = 8.3 Hz, 1H), 7.13–7.19 (m, 2H), 7.28–7.37 (m, 7H), 7.53–7.60 (m, 2H), 8.51 (d, J = 5.1 Hz, 1H);
13C{1H} NMR (126 MHz, CDCl3) δ 39.2 (CH2), 52.5 (CH3), 53.5 (CH), 67.1 (CH2), 116.3 (d, 2JC-F = 21.8
Hz, 2 × CH), 120.0 (CH), 121.6 (CH), 128.2 (3 × CH), 128.6 (2 × CH), 128.9 (d, 3JC-F = 8.3 Hz, 2 × CH),
134.3 (d, 4JC-F = 3.2 Hz, C), 136.5 (C), 148.2 (C), 149.8 (CH), 156.2 (C), 157.8 (C), 163.6 (d, 1JC-F = 249.4
Hz, C), 172.2 (C); MS (ESI) m/z 431 (M+Na+, 100); HRMS (ESI) calcd for C23H21FN2NaO4 (M+Na+)
431.1378, found 431.1364.
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Methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(4’’-nitrophenyl)pyridin-2’-yl]propanoate (7h).
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Dihydropyran formation was carried out according to the above procedure for the synthesis of methyl
(2S)-2-[(benzyloxycarbonyl)amino]-3-(4’-phenylpyridin-2’-yl)propanoate (7b) using methyl (2S,5E)-2-
[(benzyloxycarbonyl)amino]-6-(4’-nitrophenyl)-4-oxohex-5-enoate (5h) (0.104 g, 0.252 mmol) for 96 h.
This gave dihydropyran 6h (0.097 g, 79%). Dihydropyran 6h (0.0968 g, 0.199 mmol) was then reacted
with hydroxylamine hydrochloride (0.0691 g, 0.995 mmol) in acetonitrile (2 mL) at 70 °C for 16 h.
Purification by flash column chromatography on neutral alumina (Brockmann V grade) eluting with 1%
methanol in dichloromethane gave methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(4’’-
nitrophenyl)pyridin-2’-yl]propanoate (7h) (0.035 g, 40%) as a yellow solid. Mp 79–82 °C; Rf 0.79 (1%
methanol in dichloromethane); IR (neat) 3350, 2952, 1719, 1595, 1346, 1265, 1211, 1176, 1108, 1051,
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735 cm−1; [α]D +25.7 (c 1.0, CHCl3); H NMR (500 MHz, CDCl3) δ 3.40 (dd, J = 15.1, 5.1 Hz, 1H),
3.48 (dd, J = 15.1, 5.1 Hz, 1H), 3.72 (s, 3H), 4.84 (dt, J = 8.4, 5.1 Hz, 1H), 5.09 (d, J = 12.3 Hz, 1H), 5.13
(d, J = 12.3 Hz, 1H), 6.21 (d, J = 8.4 Hz, 1H), 7.28–7.39 (m, 7H), 7.74 (d, J = 8.7 Hz, 2H), 8.33 (d, J =
8.7 Hz, 2H), 8.59 (d, J = 5.1 Hz, 1H); 13C{1H} NMR (126 MHz, CDCl3) δ 39.2 (CH2), 52.5 (CH3), 53.3
(CH), 67.0 (CH2), 120.0 (CH), 121.8 (CH), 124.4 (2 × CH), 128.1 (2 × CH), 128.1 (2 × CH), 128.2 (CH),
128.5 (2 × CH), 136.3 (C), 144.4 (C), 146.7 (C), 148.3 (C), 150.0 (CH), 156.0 (C), 158.2 (C), 172.0 (C);
MS (ESI) m/z 434 ([M−H]−, 100); HRMS (ESI) calcd for C23H20N3O6 ([M−H]−) 434.1358, found
434.1347.
Methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(2’’,4’’-dinitrophenyl)pyridin-2’-yl]propanoate
(7i). Dihydropyran formation was carried out according to the above procedure for the synthesis of methyl
(2S)-2-[(benzyloxycarbonyl)amino]-3-(4’-phenylpyridin-2’-yl)propanoate (7b) using methyl (2S,5E)-2-
[(benzyloxycarbonyl)amino]-6-(2’,4’-dinitrophenyl)-4-oxohex-5-enoate (5i) (0.198 g, 0.432 mmol) for
168 h. This gave dihydropyran 6i (0.193 g, 84%). Dihydropyran 6i (0.193 g, 0.365 mmol) was then treated
with hydroxylamine hydrochloride (0.127 g, 1.83 mmol) in acetonitrile (5 mL) at 70 °C for 16 h.
Purification by flash column chromatography on neutral alumina (Brockmann V grade) eluting with 1%
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methanol in dichloromethane gave methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(2’’,4’’-
dinitrophenyl)pyridin-2’-yl]propanoate (7i) (0.068 g, 38%) as an orange oil. Rf 0.87 (1% methanol in
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dichloromethane on alumina); IR (neat) 3364, 2953, 1721, 1599, 1532, 1346, 1211, 833, 741 cm−1; [α]D
+19.5 (c 1.0, CHCl3); 1H NMR (500 MHz, CDCl3) δ 3.38 (dd, J = 15.1, 5.2 Hz, 1H), 3.45 (dd, J = 15.1,
5.2 Hz, 1H), 3.72 (s, 3H), 4.79–4.86 (m, 1H), 5.10 (d, J = 12.3 Hz, 1H), 5.13 (d, J = 12.3 Hz, 1H), 6.16
(d, J = 8.3 Hz, 1H), 7.07–7.11 (m, 2H), 7.27–7.39 (m, 5H), 7.59 (d, J = 8.4 Hz, 1H), 8.50 (dd, J = 8.4, 2.1
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Hz, 1H), 8.60 (d, J = 5.7 Hz, 1H), 8.83 (d, J = 2.1 Hz, 1H); C{1H} NMR (126 MHz, CDCl3) δ 39.2
(CH2), 52.6 (CH3), 53.1 (CH), 67.0 (CH2), 120.2 (CH), 120.4 (CH), 122.2 (CH), 127.1 (CH), 128.2 (2 ×
CH), 128.2 (CH), 128.5 (2 × CH), 132.9 (CH), 136.3 (C), 139.6 (C), 144.2 (C), 147.8 (C), 148.5 (C),
149.7 (CH), 156.0 (C), 158.1 (C), 171.8 (C); MS (ESI) m/z 503 (M+Na+, 100); HRMS (ESI) calcd for
C23H20N4NaO8 (M+Na+) 503.1173, found 503.1154.
Methyl
(2S)-2-[(benzyloxycarbonyl)amino]-3-{4’-[4’’-(3’’’-nitrophenyl)phenyl]pyridin-2’-
yl}propanoate (7j). Dihydropyran formation was carried out according to the above procedure for the
synthesis of methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-(4’-phenylpyridin-2’-yl)propanoate (7b) using
methyl (2S,5E)-2-[(benzyloxycarbonyl)amino]-6-(3’’-nitrobiphen-4’-yl)-4-oxohex-5-enoate (5j) (0.103
g, 0.211 mmol) for 96 h. This gave dihydropyran 6j (0.095 g, 86%). Dihydropyran 6j (0.100 g, 0.191
mmol) was then treated with hydroxylamine hydrochloride (0.0664 g, 0.955 mmol) in acetonitrile (2 mL)
at 70 °C for 16 h. Purification by flash column chromatography on neutral alumina (Brockmann V grade)
eluting with 1% methanol in dichloromethane gave methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-{4’-
[4’’-(3’’’-nitrophenyl)phenyl]pyridin-2’-yl}propanoate (7j) (0.035 g, 36%) as an off-white solid. Mp 48–
50 °C; Rf 0.70 (1% methanol in dichloromethane on alumina); IR (neat) 3340, 2951, 1719, 1602, 1519,
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1349, 1212, 1062, 730 cm−1; [α]D +22.1 (c 0.6, CHCl3); H NMR (500 MHz, CDCl3) δ 3.38 (dd, J =
14.9, 5.2 Hz, 1H), 3.47 (dd, J = 14.9, 5.2 Hz, 1H), 3.72 (s, 3H), 4.83 (dt, J = 8.3, 5.2 Hz, 1H), 5.10 (d, J
= 12.3 Hz, 1H), 5.13 (d, J = 12.3 Hz, 1H), 6.34 (d, J = 8.3 Hz, 1H), 7.27–7.43 (m, 7H), 7.65 (t, J = 8.0
Hz, 1H), 7.71–7.78 (m, 4H), 7.96 (ddd, J = 8.0, 1.9, 1.0 Hz, 1H), 8.24 (ddd, J = 8.0, 1.9, 1.0 Hz, 1H), 8.49
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(t, J = 1.9 Hz, 1H), 8.55 (d, J = 5.1 Hz, 1H); 13C{1H} NMR (126 MHz, CDCl3) δ 39.2 (CH2), 52.6 (CH3),
53.5 (CH), 67.0 (CH2), 120.0 (CH), 121.6 (CH), 122.0 (CH), 122.6 (CH), 127.9 (2 × CH), 128.0 (2 ×
CH), 128.2 (2 × CH), 128.2 (2 × CH), 128.6 (CH), 130.0 (CH), 133.0 (CH), 136.5 (C), 138.3 (C), 139.5
(C), 142.0 (C), 148.2 (C), 149.0 (CH), 149.9 (C), 156.2 (C), 157.8 (C), 172.2 (C); MS (EI) m/z 511 (M+,
61), 452 (18), 376 (25), 344 (45), 290 (73), 279 (100), 244 (16), 169 (14), 108 (71), 79 (62); HRMS (EI)
calcd for C29H25N3O6 (M+) 511.1743, found 511.1747.
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Methyl
(2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(4’’-aminophenyl)pyridin-2’-yl]propanoate
(7k). Methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(4’’-nitrophenyl)pyridin-2’-yl]propanoate (7h)
(0.0206 g, 0.0473 mmol) was dissolved in anhydrous methanol (2 mL) and tin(II) chloride dihydrate
(0.0530 g, 0.235 mmol) was added. The reaction mixture was stirred at room temperature for 2 h. Once
the reaction was complete, the solution was diluted with ethyl acetate (15 mL) and washed with saturated
potassium fluoride solution (3 × 10 mL) and brine (10 mL). The organic layer was dried over MgSO4 and
concentrated in vacuo. Purification by flash column chromatography on silica gel eluting with 40% ethyl
acetate/1% triethylamine in dichloromethane gave methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(4’’-
aminophenyl)pyridin-2’-yl]propanoate (7k) (0.0127 g, 65%) as a yellow oil. Rf 0.24 (40% ethyl
acetate/1% triethylamine in dichloromethane); IR (neat) 3364, 2947, 1721, 1597, 1520, 1211, 1057, 826
cm−1; [α]D29 +25.6 (c 0.5, CHCl3); 1H NMR (500 MHz, CDCl3) δ 3.31 (dd, J = 14.8, 5.2 Hz, 1H), 3.41
(dd, J = 14.8, 5.2 Hz, 1H), 3.69 (s, 3H), 3.89 (br s, 2H), 4.78 (dt, J = 8.3, 5.2 Hz, 1H), 5.09 (d, J = 12.4
Hz, 1H), 5.12 (d, J = 12.4 Hz, 1H), 6.40 (d, J = 8.3 Hz, 1H), 6.74 (d, J = 8.6 Hz, 2H), 7.27–7.37 (m, 7H),
7.44 (d, J = 8.6 Hz, 2H), 8.42 (d, J = 5.2 Hz, 1H); 13C{1H} NMR (126 MHz, CDCl3) δ 38.7 (CH2), 52.4
(CH3), 53.5 (CH), 66.9 (CH2), 115.3 (2 × CH), 119.0 (CH), 120.6 (CH), 127.2 (C), 128.1 (2 × CH), 128.1
(CH), 128.1 (2 × CH), 128.5 (2 × CH), 136.4 (C), 147.9 (C), 148.7 (CH), 149.4 (C), 156.2 (C), 156.8 (C),
172.1 (C); MS (ESI) m/z 406 (M+H+, 100); HRMS (ESI) calcd for C23H24N3O4 (M+H+) 406.1761, found
406.1745.
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Methyl
(2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(2’’,4’’-diaminophenyl)pyridin-2’-
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yl]propanoate (7l). The reaction was carried out according to the above procedure for the synthesis of
methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(4’’-aminophenyl)pyridin-2’-yl]propanoate (7k) using
methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-[4’-(2’’,4’’-dinitrophenyl)pyridin-2’-yl]propanoate (7i)
(0.359 g, 0.0747 mmol). Purification by flash column chromatography on silica gel eluting with 50%
ethyl acetate/1% triethylamine in dichloromethane gave methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-
[4’-(2’’,4’’-diaminophenyl)pyridin-2’-yl]propanoate (7l) (0.0198 g, 63%) as a yellow oil. Rf 0.13 (50%
ethyl acetate/1% triethylamine in dichloromethane); IR (neat) 3365, 2952, 1713, 1599, 1515, 1266, 1213,
1055, 698 cm−1; [α]D26 +15.8 (c 1.0, CHCl3); 1H NMR (500 MHz, CDCl3) δ 3.30 (dd, J = 14.7, 5.3 Hz,
1H), 3.35 (dd, J = 14.7, 5.3 Hz, 1H), 3.71 (s, 3H), 4.78 (dt, J = 8.3, 5.3 Hz, 1H), 5.07 (d, J = 12.3 Hz, 1H),
5.11 (d, J = 12.3 Hz, 1H), 6.06 (d, J = 2.2 Hz, 1H), 6.18 (dd, J = 8.2, 2.2 Hz, 1H), 6.30 (d, J = 8.3 Hz,
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1H), 6.91 (d, J = 8.2 Hz, 1H), 7.20–7.36 (m, 7H), 8.45 (d, J = 5.1 Hz, 1H); C{1H} NMR (126 MHz,
CDCl3) δ 39.1 (CH2), 52.4 (CH3), 53.5 (CH), 66.9 (CH2), 101.8 (CH), 106.6 (CH), 115.2 (C), 122.0 (CH),
123.6 (CH), 128.1 (2 × CH), 128.1 (CH), 128.5 (2 × CH), 131.3 (CH), 136.4 (C), 144.7 (C), 148.2 (C),
148.6 (C), 149.5 (CH), 156.1 (C), 157.1 (C), 172.2 (C); MS (ESI) m/z 421 (M+H+, 100); HRMS (ESI)
calcd for C23H25N4O4 (M+H+) 421.1870, found 421.1853.
(2S)-2-Amino-3-(4’-phenylpyridin-2’-yl)propanoic acid hydrochloride (8b). Methyl (2S)-2-
[(benzyloxycarbonyl)amino]-3-(4’-phenylpyridin-2’-yl)propanoate (7b) (0.064 g, 0.16 mmol) was
suspended in 6 M hydrochloric acid (5 mL) and heated under reflux for 48 h. The mixture was cooled to
room temperature and concentrated in vacuo. Trituration with diethyl ether (5 mL) gave a brown solid.
Purification by recrystallization from a mixture of methanol and diethyl ether gave (2S)-2-amino-3-(4’-
phenylpyridin-2’-yl)propanoic acid hydrochloride (8b) (0.045 g, 99%) as an off-white solid. Mp 110–113
°C; IR (neat) 3376, 2932, 1634, 1479, 1393, 1024, 844, 766 cm−1; [α]D22 +28.6 (c 1.1, MeOH); 1H NMR
(500 MHz, CD3OD) δ 3.73–3.84 (m, 2H), 4.76 (dd, J = 8.4, 6.5 Hz, 1H), 7.62–7.67 (m, 3H), 8.00–8.07
(m, 2H), 8.32 (dd, J = 6.3, 1.7 Hz, 1H), 8.50 (d, J = 1.7 Hz, 1H), 8.82 (d, J = 6.3 Hz, 1H); 13C{1H} NMR
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(126 MHz, CD3OD) δ 35.0 (CH2), 52.9 (CH), 124.3 (CH), 126.6 (CH), 129.3 (2 × CH), 130.9 (2 × CH),
133.3 (CH), 135.8 (C), 143.1 (CH), 152.3 (C), 159.4 (C), 169.8 (C); MS (ESI) m/z 277 ([M−H]−, 100);
HRMS (ESI) calcd for C14H1435ClN2O2 ([M−H]−) 277.0749, found 277.0744.
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(2S)-2-Amino-3-(4’-propylpyridin-2’-yl]propanoic acid hydrochloride (8a). The reaction was
carried out according to the above procedure for the synthesis of (2S)-2-amino-3-(4’-phenylpyridin-2’-
yl)propanoic acid hydrochloride (8b) using methyl (2S)-2-[(benzyloxycarbonyl)amino]-3-(4’-
propylpyridin-2’-yl]propanoate (7a) (0.023 g, 0.065 mmol) for 48 h. Purification by recrystallization from
a mixture of methanol and diethyl ether gave (2S)-2-amino-3-(4’-propylpyridin-2’-yl]propanoic acid
hydrochloride (8a) (0.015 g, 97%) as an off-white solid. Mp 112–116 °C; IR (neat) 3360, 2924, 1636,
1506, 1227, 1063, 820 cm−1; [α]D22 +27.5 (c 0.9, MeOH); 1H NMR (500 MHz, CD3OD) δ 1.03 (t, J = 7.3
Hz, 3H), 1.77–1.86 (m, 2H), 2.94 (t, J = 7.3 Hz, 2H), 3.66 (dd, J = 14.8, 8.9 Hz, 1H), 3.72 (dd, J = 14.8,
6.2 Hz, 1H), 4.65 (dd, J = 8.9, 6.2 Hz, 1H), 7.89 (d, J = 5.9 Hz, 1H), 8.05 (s, 1H), 8.69 (d, J = 5.9 Hz,
1H); 13C{1H} NMR (126 MHz, CD3OD) δ 13.9 (CH3), 24.1 (CH2), 34.7 (CH2), 38.9 (CH2), 52.8 (CH),
127.3 (CH), 129.5 (CH), 142.1 (CH), 151.4 (C), 166.6 (C), 169.7 (C); MS (ESI) m/z 243 ([M−H]−, 100);
HRMS (ESI) calcd for C11H1635ClN2O2 ([M−H]−) 243.0906, found 243.0902.
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(2S)-2-Amino-3-[4’-(naphthalen-2’’-yl)pyridin-2’-yl]propanoic acid hydrochloride (8c). The
reaction was carried out according to the above procedure for the synthesis of (2S)-2-amino-3-(4’-
phenylpyridin-2’-yl)propanoic
acid
hydrochloride
(8b)
using
methyl
(2S)-2-
[(benzyloxycarbonyl)amino]-3-[4’-(naphthalen-2’’-yl)pyridin-2’-yl]propanoate (7c) (0.026 g, 0.059
mmol) for 48 h. Purification by recrystallization from a mixture of methanol and diethyl ether gave (2S)-2-
amino-3-[4’-(naphthalen-2’’-yl)pyridin-2’-yl]propanoic acid hydrochloride (8c) (0.019 g, 94%) as a light
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yellow solid. Mp 120–124 °C; IR (neat) 3338, 2924, 1740, 1622, 1456, 1366, 1229, 818, 754 cm−1; [α]D
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+35.9 (c 0.9, MeOH); H NMR (500 MHz, CD3OD) δ 3.72–3.82 (m, 2H), 4.76 (br t, J = 7.3 Hz, 1H),
7.59–7.68 (m, 2H), 7.97 (d, J = 7.8 Hz, 1H), 8.03–8.13 (m, 3H), 8.38 (br d, J = 4.8 Hz, 1H), 8.56 (br s,
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1H), 8.61 (br s, 1H), 8.81 (d, J = 4.8 Hz, 1H); C{1H} NMR (126 MHz, CD3OD) δ 35.3 (CH2), 53.0
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(CH), 124.1 (CH), 125.1 (CH), 126.3 (CH), 128.4 (CH), 128.9 (CH), 129.6 (CH), 130.2 (CH), 130.3 (CH),
130.8 (CH), 133.1 (C), 134.8 (C), 136.2 (C), 143.8 (CH), 152.8 (C), 158.4 (C), 170.0 (C); MS (ESI) m/z
327 ([M−H]−, 100); HRMS (ESI) calcd for C18H1635ClN2O2 ([M−H]−) 327.0906, found 327.0898.
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(2S)-2-Amino-3-[4’-(4’’-methoxyphenyl)pyridin-2’-yl]propanoic acid hydrochloride (8d). The
reaction was carried out according to the above procedure for the synthesis of (2S)-2-amino-3-(4’-
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phenylpyridin-2’-yl)propanoic
acid
hydrochloride
(8b)
using
methyl
(2S)-2-
[(benzyloxycarbonyl)amino]-3-[4’-(4’’-methoxyphenyl)pyridin-2’-yl]propanoate (7d) (0.061 g, 0.15
mmol) for 48 h. Purification by recrystallization from a mixture of methanol and diethyl ether gave (2S)-2-
amino-3-[4’-(4’’-methoxyphenyl)pyridin-2’-yl]propanoic acid hydrochloride (8d) (0.046 g, 99%) as a
colorless solid. Mp 165–167 °C; IR (neat) 3340, 2926, 1738, 1370, 1225, 824 cm−1; [α]D21 +34.0 (c 0.8,
MeOH); 1H NMR (500 MHz, CD3OD) δ 3.70 (dd, J = 15.0, 9.5 Hz, 1H), 3.74 (dd, J = 15.0, 7.3 Hz, 1H),
3.91 (s, 3H), 4.68–4.74 (m, 1H), 7.17 (d, J = 8.9 Hz, 2H), 8.03 (d, J = 8.9 Hz, 2H), 8.22 (dd, J = 6.3, 1.3
Hz, 1H), 8.39 (d, J = 1.3 Hz, 1H), 8.70 (d, J = 6.3 Hz, 1H); 13C{1H} NMR (126 MHz, CD3OD) δ 35.1
(CH2), 52.9 (CH), 56.2 (CH3), 116.4 (2 × CH), 122.8 (CH), 125.0 (CH), 127.6 (C), 131.1 (2 × CH), 143.1
(CH), 152.1 (C), 158.3 (C), 164.8 (C), 170.0 (C); MS (ESI) m/z 307 ([M−H]−, 100); HRMS (ESI) calcd
for C15H1635ClN2O3 ([M−H]−) 307.0855, found 307.0847.
(2S)-2-Amino-3-[4’-(2’’,4’’-dimethoxyphenyl)pyridin-2’-yl]propanoic acid hydrochloride (8e).
The reaction was carried out according to the above procedure for the synthesis of (2S)-2-amino-3-(4’-
phenylpyridin-2’-yl)propanoic
acid
hydrochloride
(8b)
using
methyl
(2S)-2-
[(benzyloxycarbonyl)amino]-3-[4’-(2’’,4’’-dimethoxyphenyl)pyridin-2’-yl]propanoate (7e) (0.047 g,
0.11 mmol) for 16 h. Purification by recrystallization from a mixture of methanol and diethyl ether gave
(2S)-2-amino-3-[4’-(2’’,4’’-dimethoxyphenyl)pyridin-2’-yl]propanoic acid hydrochloride (8e) (0.038 g,
95%) as a light yellow solid. Mp 180–183 °C; IR (neat) 3410, 2932, 1744, 1597, 1211, 1018, 833 cm−1;
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[α]D +10.1 (c 0.1, MeOH); H NMR (400 MHz, CD3OD) δ 3.67–3.81 (m, 2H), 3.91 (s, 3H), 3.95 (s,
3H), 4.63–4.74 (m, 1H), 6.71–6.82 (m, 2H), 7.71 (d, J = 8.3 Hz, 1H), 8.26 (d, J = 6.1 Hz, 1H), 8.36 (br s,
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1H), 8.67 (d, J = 6.1 Hz, 1H); 13C{1H} NMR (101 MHz, CD3OD) δ 34.9 (CH2), 53.1 (CH), 56.4 (CH3),
56.6 (CH3), 100.0 (CH), 108.0 (CH), 117.4 (C), 126.1 (CH), 128.0 (CH), 133.8 (CH), 141.7 (CH), 150.7
(C), 158.1 (C), 160.9 (C), 166.1 (C), 169.9 (C); MS (ESI) m/z 303 (M+H+, 100); HRMS (ESI) calcd for
C16H19N2O4 (M+H+) 303.1339, found 303.1338.
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(2S)-2-Amino-3-[4’-(2H-1’’,3’’-benzodioxol-5’’-yl)pyridin-2’-yl]propanoic acid hydrochloride
(8f). The reaction was carried out according to the above procedure for the synthesis of (2S)-2-amino-3-
(4’-phenylpyridin-2’-yl)propanoic
acid
hydrochloride
(8b)
using
methyl
(2S)-2-
[(benzyloxycarbonyl)amino]-3-[4’-(2H-1’’,3’’-benzodioxol-5’’-yl)pyridin-2’-yl]propanoate (7f) (0.24 g,
0.56 mmol) for 16 h. Purification by recrystallization from a mixture of methanol and diethyl ether gave
(2S)-2-amino-3-[4’-(2H-1’’,3’’-benzodioxol-5’’-yl)pyridin-2’-yl]propanoic acid hydrochloride (8f) (0.18
g, 99%) as a light yellow solid. Mp 197–200 °C; IR (neat) 3410, 2808, 1736, 1612, 1474, 1242, 1034,
810 cm−1; [α]D27 +38.5 (c 0.1, MeOH); 1H NMR (400 MHz, CD3OD) δ 3.69 (dd, J = 12.8, 5.7 Hz, 1H),
3.74 (dd, J = 12.8, 5.7 Hz, 1H), 4.71 (t, J = 5.7 Hz, 1H), 6.12 (s, 2H), 7.07 (d, J = 8.2 Hz, 1H), 7.55 (d, J
= 1.9 Hz, 1H), 7.63 (dd, J = 8.2, 1.9 Hz, 1H), 8.20 (dd, J = 6.4, 1.9 Hz, 1H), 8.35 (d, J = 1.9 Hz, 1H), 8.71
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(d, J = 6.4 Hz, 1H); C{1H} NMR (101 MHz, CD3OD) δ 35.1 (CH2), 52.9 (CH), 103.9 (CH2), 108.8
(CH), 110.4 (CH), 123.4 (CH), 125.0 (CH), 125.5 (CH), 129.5 (C), 142.9 (CH), 150.9 (C), 152.0 (C),
153.1 (C), 158.7 (C), 169.9 (C); MS (ESI) m/z 287 (M+H+, 100); HRMS (ESI) calcd for C15H15N2O4
(M+H+) 287.1026, found 287.1029.
(2S)-2-Amino-3-[4’-(4’’-fluorophenyl)pyridin-2’-yl]propanoic acid hydrochloride (8g). The
reaction was carried out according to the above procedure for the synthesis of (2S)-2-amino-3-(4’-
phenylpyridin-2’-yl)propanoic
acid
hydrochloride
(8b)
using
methyl
(2S)-2-
[(benzyloxycarbonyl)amino]-3-[4’-(4’’-fluorophenyl)pyridin-2’-yl]propanoate (7g) (0.014 g, 0.034
mmol) for 48 h. Purification by recrystallization from a mixture of methanol and diethyl ether gave (2S)-2-
amino-3-[4’-(4’’-fluorophenyl)pyridin-2’-yl]propanoic acid hydrochloride (8g) (0.010 g, 99%) as a
yellow solid. Mp 80–84 °C; IR (neat) 3360, 2920, 1603, 1516, 1236, 1163, 827 cm−1; [α]D25 +30.8 (c 1.0,
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