Stereo-selective preparation of teneraic acid, trans-(2S,6S)-piperidine-2,6-dicarboxylic acid, via anodic oxidation and cobalt-catalyzed carbonylation

Article abstract of DOI:10.1248/cpb.c17-00391

Teneraic acid (piperidine-2,6-dicarboxylic acid) is a naturally occurring imino acid that comprises three stereoisomers due to its two asymmetric centers at C2 and C6. The configuration of natural teneraic acid is reported to correspond to trans-(2S,6S).

Full text of DOI:10.1248/cpb.c17-00391

854
Chem. Pharm. Bull. 65, 854–861 (2017)
Vol. 65, No. 9
Regular Article
Stereo-Selective Preparation of Teneraic Acid, trans-(2S,6S)-Piperidine-
2,6-dicarboxylic Acid, via Anodic Oxidation and Cobalt-Catalyzed
Carbonylation
Yusuke Amino,* Seiichi Nishi, and Kunisuke Izawa
Institute for Innovation, Ajinomoto Co., Inc.; 1–1 Suzuki-cho, Kawasaki-ku, Kawasaki 210–8681, Japan.
Received May 12, 2017; accepted May 31, 2017
Teneraic acid (piperidine-2,6-dicarboxylic acid) is a naturally occurring imino acid that comprises three
stereoisomers due to its two asymmetric centers at C2 and C6. The configuration of natural teneraic acid is
reported to correspond to trans-(2S,6S). However, a few studies are focused on the stereospecific synthesis of
trans-(2S,6S)-teneraic acid. The present study investigates a convenient synthetic method that includes regio-
specific anodic oxidation and stereospecific cobalt-catalyzed carbonylation to obtain trans-(2S,6S)-teneraic
acid. Methyl (S)-N-benzoyl-α-methoxypipecolate, the key intermediate that displays a structure that cor-
responds to an intermediate (N-α-hydroxyalkyl amide) of intramolecular amidocarbonylation, was obtained
via an anodic oxidation of methyl (S)-N-benzoylpipecolate. Subsequently, cobalt-catalyzed carbonylation
converted the methyl (S)-N-benzoyl-α-methoxypipecolate to trans-(2S,6S)-N-benzoyl-teneraic acid dimethyl
ester in good optical purity (>95% enantiomeric excess (ee)) and modest yield (63%). Finally, de-protection
occurred via acidic hydrolysis to obtain trans-(2S,6S)-teneraic acid. The stereochemistry of synthesized ten-
eraic acid was confirmed as corresponding to trans-(2S,6S) by comparing its physical properties with those
of a cis-meso-isomer and those of a trans-(2S,6S)-isomer that were reported in previous studies.
Key words teneraic acid; piperidine-2,6-dicarboxylic acid; stereoisomer; amidocarbonylation; anodic oxida-
tion; carbonylation
Teneraic acid (also termed as piperidine-2,6-dicarboxylic sive. The following two procedures have been reported for
acid) is a naturally occurring imino acid derivative that was synthesizing a racemate. Chrystal et al. synthesized racemic
isolated from a red alga, Porphyra tenera,1) which comprises teneraic acid derivative via α,α′-dibromination of pimeric acid
three stereoisomers, namely, trans-(2S,6S)-, trans-(2R,6R)-, followed by cyclization using liquid ammonia.2) Einhorn et al.
and cis–meso-isomers due to two asymmetric centers at C2 synthesized racemic N-tert-butoxycarbonyl-teneraic acid di-
and C6 (Fig. 1). A study by Kawauchi et al. reported that methyl ester via alkylation of the dianion of N-tert-butoxycar-
the stereochemistry of natural teneraic acid corresponded to bonyl iminodiacetic acid dimethyl ester with diiodopropane.10)
trans.1) Additionally, cis–meso-isomers of teneraic acid de- Additionally, Chrystal et al. separated cis–meso-isomer and
rivatives are readily obtained by platinum(IV) oxide (PtO2) racemic trans-isomer by using fractional precipitation of race-
or palladium on carbon (Pd/C) catalyzed hydrogenation of mic mixture of teneraic acid diamide.2) However, it is essential
pyridine-2,6-dicarboxylic acid derivatives, respectively.2–4) to develop additional facile and practical synthesis methods
Mitsakos et al. reported on dihydrodipipecolinate synthase with respect to trans-teneraic acid.
inhibition activity as a biological activity of teneraic acid.5)
Conversely, cobalt-catalyzed amidocarbonylation was de-
However, stereoselective total synthesis of an optically veloped by Wakamatsu et al. in our laboratories as a useful
active trans-isomer was not achieved until a study by Was- method for the synthesis of N-acyl-α-amino acids (6) from
serman et al. who synthesized a trans-(2S,6S)-isomer by mul- amide (4), aldehyde (5), and carbon monoxide (CO)11–13) (Chart
tistep reactions and determined the absolute stereochemistry 1). Dicobalt octacarbonyl [Co2(CO)8] functions as the catalyst
of natural teneraic acid as corresponding to trans-(2S,6S).6) precursor. The first step of the amidocarbonylation involves
Ohtani et al. obtained optically pure trans-(2R,6R)- or trans- a nucleophilic addition of the amide (4) to the aldehyde (5)
(2S,6S)-isomers via a photocatalytic reaction with activated with the formation of an N-α-hydroxyalkyl amide (7), which
cadmium sulfide (CdS) particles by using (R,R)- or (S,S)-2,6- eliminates the hydroxide ion (OH−) to form the corresponding
diaminopimelic acid as the starting material wherein the N-acylimine cation (8a) or N-acyl-iminium ion (8b). An active
mechanism resembles a possible biosynthetic pathway.7–9) catalyst species, tetracarbonylcobaltate [Co(CO)4]−, forms a
However, optically active 2,6-diaminopimelic acid is expen- corresponding alkyl cobalt complex (9) with the N-acylimine
Fig. 1. Structures of Stereoismers of Teneraic Acid
*To whom correspondence should be addressed. e-mail: yusuke_amino@ajinomoto.com
© 2017 The Pharmaceutical Society of Japan

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Chart 1. Cobalt-Catalyzed Amidocarbonylation of Amides with Aldehydes
Reagents: (a) CO/H2 (13.7MPa), Co2(CO)8 in acetone; (b) CH2N2 in CH3OH–Et2O.
Chart 2. Synthesis of Teneraic Acid via Hydroformylation–Intramolecular Amidocarbonylation
cation (8a) in a nucleophilic substitution. CO insertion trans-
forms alkyl cobalt complex (9) into an acyl cobalt complex
Results and Discussion
Synthesis of Teneraic Acid via Cobalt-Catalyzed Hy-
(10). This product (10) forms N-acyl-amino acid (6) via an droformylation–Intramolecular Amidocarbonylation The
intermolecular nucleophilic attack of water on the acyl bond. reaction condition of amidocarbonylation is identical to that of
The intramolecular formation of an oxazolone derivative (11) a hydroformylation reaction (i.e., CO–H2=1:1, approximately
with subsequent hydrolysis also seems possible.12,13) In addi- 13.7MPa, 100°C). Thus, it is possible to use N-alkenyl amides
tion to original cobalt-catalyzed amidocarbonylation, Beller as starting materials to synthesize N-acyl-cyclic-imino acids
developed palladium-catalyzed amidocarbonylation for the via tandem carbonylation (hydroformylation of an olefin fol-
synthesis of numerous N-acyl-α-amino acid derivatives.14)
lowed by an intramolecular amidocarbonylation). First, this
The scope of application range of this reaction is signifi- method was applied to the synthesis of teneraic acid as shown
cantly broadened with its combination with other reactions in Chart 2. This synthesis constitutes a superior process be-
to yield intermediates that are similar to intermediates 7 cause the desired carbon framework is formed in a single step
or 8 in Chart 1.15–17) The synthetic application of cobalt- reaction.
catalyzed amidocarbonylation to synthesize functionalized
Cobalt-catalyzed amidocarbonylation of (S)-N-benzoyl-
N-acyl-α-amino acid derivatives was extended by establishing allylglycine (12) results in a complex mixture that includes
stereo-selective synthesis of 6-substituted pipecolic acid via an N-benzoyl teneraic acid (18) as a major product. Specifically,
anodic oxidation–cobalt-catalyzed carbonylation reaction.18,19)
it is considered that the hydrogenation of olefin moiety forms
The present study provides a facile stereo-specific route N-benzoyl-norvaline (22%). The mixture was treated with
to obtain trans-(2S,6S)-teneraic acid from (S)-pipecolic acid ethereal diazomethane (CH2N2) and carefully purified by
by using regioselective anodic oxidation and stereospecific using silica gel column chromatography to obtain N-benzoyl-
cobalt-catalyzed carbonylation. Furthermore, the details of teneraic acid dimethyl ester (19) in a 62% isolated yield. The
characterization of trans-(2S,6S)-teneraic acid are presented formation of N-benzoyl-teneraic acid should initiate the for-
in the study.
mation of alkyl cobalt intermediate 13, and this is followed
by carbonylation and intramolecular amidocarbonylation via

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intermediates 14, 15, 16, and 17 as previously discussed. A reoselectivity in the amidocarbonylation step is discussed in
previous study indicated that hydroformylation of a terminal detail below. However, the chiral HPLC analysis revealed that
olefin proceeded regioselectively under these conditions to the compound 19 approximately corresponds to a 7:3 mixture
yield a straight-chain aldehyde, such as intermediate 14, which of trans-(2S,6S)-isomer (19a) and trans-(2R,6R)-isomer (19b)
subsequently produced an internal N-α-hydroxyl amide (15) in (analytical conditions 1, Fig. 2). Furthermore, approximately
situ.15) This constitutes the reason for the selective formation 1:1 ratio of trans-(2S,6S)-isomer (19a) and trans-(2R,6R)-
of the 6-membered ring selectivity.
isomer (19b) were obtained when (RS)-N-benzoyl-allylglycine
Interestingly, normal phase HPLC analysis of obtained was used as a starting material. These results indicate that
N-benzoyl-teneraic acid dimethyl ester (19) revealed that the the reaction using (S)-N-benzoyl-allylglycine (12) proceeded
trans-isomer was obtained predominantly (analytical condi- selectively to form trans-(2S,6S)-isomer as a major prod-
tions 2). The mechanism for the significant high trans-ste- uct although miner trans-(2R,6R)-isomer was produced via
epimerization at C2 asymmetric center at an early stage of
the reaction. The formation of trans-(2R,6R)-isomer (19b) can
be attributed to the facile migration of cobalt carbonyl moiety
from δ-position (13) to α-position (20) of the amide group and
to their equilibrium. This type of isomerization of an alkyl co-
balt complex was previously observed in the cobalt-catalyzed
isomerization–carbonylation reaction of N-acyl unsaturated
cyclic amines.16)
Synthesis of Teneraic Acid via Anodic Oxidation–Co-
balt-Catalyzed Carbonylation An extant study already
discussed stereoselective synthesis of methyl trans-N-benzoyl-
6-methylpipecolate.18) The synthesis consists of regioselective
anodic oxidation of N-benzoyl-6-methylpiperidine to yield
N-benzoyl-α-methoxy-6-methylpiperidine, and this is followed
by its subsequent stereospecific cobalt-catalyzed carbonyl-
ation. This synthesis was applied to methyl N-benzoyl-pipeco-
late (21) to obtain N-benzoyl-teneraic acid dimethyl ester (19)
as shown in Chart 3. This synthesis method directly passes
through to form a more stable alkyl cobalt complex, namely
23, and thus it is possible to avoid epimerization as observed
in the fore-mentioned synthesis.
The anodic oxidation of methyl (S)-N-benzoyl-pipecolate
Fig. 2. Chiral HPLC Chromatogram of trans-(2S,6S)-N-Benzoyl-ten-
eraic Acid Dimethyl Esters (19a) and trans-(2R,6R)-N-Benzoyl-teneraic
Acid Dimethyl Esters (19b) Obtained by Hydroformylation–Intramolecu-
lar Amidocarbonylation
(21) proceeded regioselectively to result in methyl (S)-N-
benzoyl-α-methoxy-pipecolate (22) in good yield.20–22) Com-
pound 22 corresponds to the N-α-hydroxyalkyl amide interme-
diate (7) in Chart 1 as well as the internal N-α-hydroxyalkyl
Reagents: (a) anodic oxidation: Et4N+OTs− in CH3OH; (b) CO/H2 (13.7MPa), Co2(CO)8 in acetone; (c) CH2N2 in CH3OH; (d) conc. HCl, 110°C; (e) NaOCH3 in CH3OH.
Chart 3. Synthesis of Teneraic Acid via Anodic Oxidation–Carbonylation

Vol. 65, No. 9 (2017)
Chem. Pharm. Bull.
857
amide intermediate (15) in Chart 2.
of pyridine-2,6-dicarboxylic acid (26) as shown in Chart 4.2)
The cobalt-catalyzed carbonylation (CO–H2=1:1, approxi- Subsequently, compound 3 was converted to cis–meso-N-ben-
mately 13.7MPa, 100°C) of compound 22 yielded a mixture zoyl teneraic acid dimethyl ester (19c) by methyl esterification
of trans-(2S,6S)-N-benzoyl-teneraic acid dimethyl ester (19a, that was followed by N-benzoylation. The obtained cis–meso-
36.7%), i.e., a product generated by nucleophilic opening of isomer 19c was identical to that obtained by the epimeriza-
the complex by MeOH, and its half-ester (25, 26.3%), i.e., a tion of compound 19a with respect to a normal phase HPLC
product generated by nucleophilic opening of the complex by analysis (Fig. 4).
water, with a combined yield of 63.0%. The configuration of
A Potential Mechanism for Stereoselective Carbonyl-
trans-(2S,6S) of compound 25 was confirmed by converting it ation It is possible to rationalize the cause of high selec-
to compound 19a by using methyl esterification with CH2N2. tivity in the above mentioned two syntheses by a potential
It should be noted that cobalt-catalyzed carbonylation of com- mechanism shown in Chart 5.18) Given that HCo(CO)4 (which
pound 22 resulted in a trans-(2S,6S)-isomer that possesses corresponds to the actual catalyst in the carbonylation) is as
excellent optical purity (>95% enantiomeric excess (ee)) as strongly acidic as hydrochloric acid in a polar solvent, it is
determined by a chiral HPLC analysis as shown in Fig. 3. The reasonable to assume that the reaction proceeds through the
formation of half-ester (25) is thought to be caused by the N-acyl-iminium ion intermediates 23a and/or 23b. The severe
reaction between the intermediate 24 and water contained in steric hindrance to 1,2-diequatorial substitution in 23b re-
the solvent.
quires that the methoxycarbonyl group of the N-acyl-iminium
Additionally, trans-(2S,6S)-isomer (19a) was easily epimer- intermediate should be axial in a manner similar to 23a. It
ized to yield a trans-(2R,6R)-isomer (19b) and cis–meso- is postulated that the stereo-determining step corresponds
isomer (19c) by treating it with sodium methoxide (CH3ONa) to the formation of the amidoalkyl cobalt complex 23. As
in methanol (Chart 3). The stereoisomers were used as a shown in Chart 5, an attack of [Co(CO4)]− with respect to the
mixture of standards for chiral HPLC analysis as well as for N-acyl-iminium ion 23a could occur preferentially from the
normal phase HPLC analysis to differentiate between the iso- less hindered equatorial side to exclusively yield the trans-N-
mers and to identify the stereochemistry of reaction products. benzoyl-teneraic acid dimethyl ester. This type of a process
The trans-configuration of compound 19a was reconfirmed was favored even though the cis–meso-isomer could be ther-
by both HPLC analysis conditions by comparing its retention modynamically more stable.
times with those of cis–meso-stereoisomer (19c) derived from
pyridine-2,6-dicarboxylic acid (26) as described below (Figs.
3, 4). Finally, the stereochemistry of compound 19a was es-
tablished by converting it to trans-(2S,6S)-teneraic acid (1) via
acidic hydrolysis (Chart 3).
Synthesis of cis–meso-Teneraic Acid Furthermore,
cis–meso-isomer of teneraic acid (3) was prepared according
to the reported procedure by PtO2 catalyzed hydrogenation
Fig. 3. Chiral HPLC Chromatogram of trans-(2S,6S)-N-Benzoyl-ten-
eraic Acid Dimethyl Ester (19a) Obtained by Anodic Oxidation–Cobalt-
Catalyzed Carbonylation
Fig. 4. Normal Phase HPLC Chromatogram of Stereoisomers of N-
Benzoyl-teneraic Acid Dimethyl Ester (19)
(a) trans-(2S,6S)-Isomer (19a) is initially obtained from the reaction. (b) trans-
(2S,6S)-Isomer (19a) is obtained by methyl esterification of a half-ester of trans-
(2S,6S)-isomer (25).
(a) Mixture of cis–meso-isomer (19c), trans-(2S,6S)-isomer (19a), and trans-
(2R,6R)-isomer (19b) obtained by epimerization of trans-(2S,6S)-isomer (19a). (b)
trans-(2S,6S)-Isomer (19a).
Reagents: (a) PtO2 in 50% acetic acid, H2 (10MPa); (b) SOCl2 in CH3OH; (c) benzoyl chloride, Et3N in THF.
Chart 4. Synthesis of cis–meso-Teneraic Acid

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Chart 5. A Possible Mechanism for Stereoselective Carbonylation
Fig. 5. NMR Spectra of trans-(2S,6S)-Teneraic Acid (1) and cis–meso-Teneraic Acid (3)
(a) 1H-NMR spectra of trans-(2S,6S)-isomer (1), (b) 13C-NMR spectra of trans-(2S,6S)-isomer (1), (c) 1H-NMR spectra of cis–meso-isomer (3), (d) 13C-NMR spectra of
cis–meso-isomer (3).
Characterization of Teneraic Acid The results indi- dicarboxylic acid, which corresponds to a 5-membered-ring
cated that physical property values (NMR spectrum and mass homolog of teneraic acid, is also known as trans-(2S,5S).23–25)
spectrum) of synthesized teneraic acid were in reasonable Conversely, it has been reported that the configuration of the
agreement with those of a natural trans-isomer.1) Additionally, naturally occurring liner (ring opened) α-iminodicarboxylic
the NMR spectrum of the synthesized teneraic acid were not acid, alanopine, is of the meso-(RS)-form.26,27)
consistent with those of cis–meso-isomer that was synthesized
independently2) (Fig. 5). The melting point of the synthesized
Conclusion
teneraic acid (235–236°C) did not coincide with that of the
In this study, stereoselective synthesis of trans-(2S,6S)-ten-
reported value (217–219°C), and the discrepancy of the melt- eraic acid was successfully achieved with high efficiency. The
ing points of both compounds can be attributed to the low synthesis consists of a regioselective anodic oxidation at the
purity of the natural product relative to the synthesized one. α-position of methyl N-benzoyl-pipecolate. This was followed
Moreover, the circular dichroism (CD) spectra of the synthetic by stereospecific cobalt-catalyzed carbonylation. Additionally,
trans-(2S,6S)-teneraic acid (1) and natural product exhibited trans-(2S,6S)-stereochemistry of natural teneraic acid was re-
the same positive Cotton effect, and thus the natural product is confirmed by comparing its physical properties with those of
considered to possess the same absolute configuration as that synthetic trans-(2S,6S)-teneraic acid.
of the trans-(2S,6S)-isomer (Fig. 6). It should be noted that the
absolute configuration of naturally occurring pyrrolidin-2,5-

Vol. 65, No. 9 (2017)
Chem. Pharm. Bull.
859
carbon monoxide and hydrogen. The removal of the solvent
under reduced pressure was followed by dissolving the residue
in AcOEt (50mL) and filtering the insoluble material. The
filtrate was extracted with 10% aqueous NaHCO3 (100mL),
and the extract was washed with AcOEt (50mL). The aqueous
phase was acidified at pH 1 with concentrated HCl and then
extracted with AcOEt (50mL, repeated twice). The organic
layer was washed with brine (50mL), dried over MgSO4, and
concentrated to obtain crude trans-N-benzoyl-teneraic acid
(18) as a viscous oil (4.2g). The residue was esterified with
ethereal CH2N2 and subsequently purified by silica gel col-
umn chromatography (eluted with n-hexane–AcOEt) to yield a
trans-N-benzoyl-teneraic acid dimethyl ester (19) as a viscous
1
oil (5.19g, 17.0mmol, 62.0%); H-NMR (400MHz, CDCl3) δ:
1.30–2.20 (6H, m), 3.67 (6H, brs), 4.26–4.50 (2H, m), 7.33
(5H, m). 13C-NMR (CDCl3) δ: 18.0, 25.5, 26.3, 52.3, 55.1, 58.1,
127.1, 128.6, 130.3, 135.3, 172.1, 172.5, 174.7. IR (neat): 1740,
1650 (cm−1). Field desorption (FD)-MS m/z: 305.0 (M)+.
Synthesis of trans-(2S,6S)-Teneraic Acid (1) via Anodic
Oxidation–Cobalt Catalyzed Carbonylation
Fig. 6. CD Spectra of trans-(2S,6S)-Teneraic Acid (1)
(a) Natural authentic.1) (b) Synthetic compound.
Methyl (S)-N-Benzoyl-α-methoxy-pipecolate (22)
In the study, methyl (S)-N-benzoyl-pipecolate (21) (5.50g,
Experimental
General 1H-NMR spectra were recorded on a Brucker
Avance 400 spectrometer (400MHz) or Varian XL-300 spec- 22.2mmol), tetraethylammonium tosylate (Et4N+OTs−, 1.40g,
trometer (300MHz), and MS spectra were measured using 4.64mmol) as an electrolyte and MeOH (150mL) as a solvent
a Thermo Quest TSQ 700 spectrometer. Melting point (mp) were placed into a 200-mL electrolysis cell equipped with
measurements were performed using a Micro Melting Point two graphite-rod anodes and two graphite-rod cathodes. A
Apparatus obtained from Yanaco (Japan). Optical rotary constant current (1A, 32V) was passed through a cell that
power measurements were performed using a DIP-370 Digital was externally cooled with ice water for 9h. The solvent
Polarimeter that was manufactured by Nippon bunko (JASCO was removed under reduced pressure, and the residue was
Corporation, Japan). The CD spectra were measured using a dissolved in a mixed solvent of ether and AcOEt (200mL,
JASCO J-500A spectro polarimeter manufactured by JASCO 1:1, v/v). The organic layer was successively washed with
Corporation (Japan). The chiral column, CHIRALCEL OK, 5% NaHCO3 (50mL) and brine (50mL), dried over MgSO4,
was purchased from DAICEL CHEMICAL INDUSTRIES, and then concentrated. The residue was purified by silica gel
Ltd. (Japan). The normal phase column, YMC-Pack A-012 column chromatography (eluted with n-hexane–AcOEt) to
(S-5, 120A, SIL), was purchased from YMC Co., Ltd. (Japan). obtain methyl (S)-N-benzoyl-α-methoxy-pipecolate (22) as a
Analytical Conditions Analytical conditions 1; Chiral viscous oil (3.57g, 12.9mmol, 58.8%); 1H-NMR (300MHz,
HPLC analytical conditions for N-benzoyl-teneraic acid di- CDCl3) δ: 1.50–2.15 (5H, m), 2.17–2.28 (0.5H, brd), 2.40–2.50
methyl ester stereoisomers:
(0.5H, brd), 3.07 (1.5H, brs), 3.43 (1.5H, brs), 3.75 (3H, brs),
Column: CHIRALCEL OK 4.5×250mm. (10µm). Detec- 4.30–4.45 (0.5H, m), 4.86 (0.5H, brs), 5.40 (0.5H, brs), 5.87
tion: UV 254nm. Eluent: n-hexane–iso-propanol=8:2 (v/v). (0.5H, brs), 7.40 (5H, m). IR (neat): 2950, 1735, 1640, 1440,
Flow rate: 1.0mL/min. Temperature: 50°C.
1405, 1205 (cm−1). FD-MS m/z: 277.0 (M)+. Electrospray ion-
Analytical conditions 2; Normal phase HPLC analytical ization (ESI)-MS m/z: 277.0 (M)+.
conditions for N-benzoyl-teneraic acid dimethyl ester stereo-
isomers:
trans-(2S,6S)-N-Benzoyl-teneraic Acid Dimethyl Ester
(Dimethyl trans-(2S,6S)-N-Benzoyl-piperidine-2.6-di-
Column: YMC-Pack A-012 (S-5, 120A, SIL) 4.6×150mm. carboxylate, 19a) Specifically, methyl (S)-N-benzoyl-α-
(5 µm). Detection: UV 254nm. Eluent: n-hexane–AcOEt=1:1 methoxy-2-piperidinecarboxylate (22) (3.57g, 12.9mmol) and
(v/v), Flow rate: 1.0mL/min. Temperature : 25°C.
CD spectrum measurement conditions:
Co2(CO)8 (800mg, 2.34mmol) in acetone (50mL) were heated
in a 100-mL stainless-steel autoclave with a 1:1 mixture of
Solvent: H2O (pH 5.9), Sample concentration: 2.0mg/mL, carbon monoxide and hydrogen (total pressure corresponded
Cell length: 1.0mm, Wavelength range: 290–190nm, Tempera- to 13.7MPa and was measured at room temperature) at 100°C
ture: 25°C.
for 3.5h. The autoclave was then cooled to ambient tem-
Synthesis of trans-N-Benzoyl-teneraic Acid Dimethyl perature, and carbon monoxide and hydrogen were purged out.
Ester 18 via Cobalt Catalyzed Hydroformylation–Intramo- The removal of the solvent under reduced pressure was fol-
lecular Amidocarbonylation of (S)-N-Benzoyl-Allylglycine lowed by dissolving the residue in AcOEt (50mL) and filter-
(12) Specifically, (S)-N-benzoyl-allylglycine (12) (6.00g, ing off the insoluble material. The filtrate was extracted with
27.4mmol) and Co2(CO)8 (600mg, 1.75mmol) in acetone 10% aqueous NaHCO3 (100mL), and the extract was washed
(50mL) were heated in a 100-mL stainless-steel autoclave with AcOEt (50mL). The combined organic phase was washed
with a 1:1 mixture of carbon monoxide and hydrogen (total with brine (50mL), dried over MgSO4, and then concentrated.
pressure corresponding to 13.7MPa that was measured at The residue was purified by silica gel column chromatography
room temperature) at 100°C for 4h. The autoclave was cooled (eluted with n-hexane–AcOEt) to obtain trans-(2S,6S)-N-ben-
to ambient temperature, and this was followed by purging out zoyl-teneraic acid dimethyl ester (19a) as a viscous oil (1.45g,

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Chem. Pharm. Bull.
Vol. 65, No. 9 (2017)
4.73 mmol, 36.7%).
(3.0mL, 42.0mmol) in MeOH (30mL) at 0°C. The mixture
The aqueous phase was acidified at pH 1 with concentrated was stirred for 30min at 0°C, then warmed to room tem-
HCl and then extracted with AcOEt (50mL, repeated twice). perature, and was stirred overnight. The solvent was evapo-
The organic layer was washed with brine (50mL), dried over rated in vacuo to yield crude cis–meso-teneraic acid dimethyl
MgSO4, and then concentrated to yield a crude half-ester (25). ester·HCl (692mg, 2.92mmol, 51.0%) that is used in the next
The residue was esterified with ethereal CH2N2 and purified step without further purification; IR (KBr): 1750 (cm−1).
by silica gel column chromatography (eluted with n-hexane–
Additionally, Et3N (0.44mL, 3.20mmol) and benzoyl chlo-
AcOEt) to obtain trans-(2S,6S)-N-benzoyl-teneraic acid di- ride (0.20mL, 1.76mmol) were added to a solution of the fore-
methyl ester (19a) as a viscous oil (1.04g, 3.39mmol, 26.3%); mentioned crude cis–meso-teneraic acid dimethyl ester·HCl
1H-NMR (300MHz, CDCl3) δ: 1.35–2.30 (6H, m), 3.73 (380mg, 1.60mmol) in tetrahydrofuran (8.0mL) at 0°C. The
(6H, brs), 4.33 (1H, brs), 4.56 (1H, brs), 7.35–7.50 (5H, m). reaction mixture was stirred overnight at room temperature.
13C-NMR (CDCl3) δ: 18.0, 25.5, 26.3, 52.3, 55.1, 58.1, 127.1, The solvent was evaporated in vacuo, and the residue was
128.6, 130.3, 135.3, 172.1, 172.5, 174.7. IR (neat): 2950, 1735, partitioned between water (10mL) and EtOAc (30mL). The
1650, 1440, 1380, 1325, 1200, 1130, 1095, 1070, 980 (cm−1). aqueous phase was further extracted with EtOAc (30mL). The
FD-MS m/z: 305.0 (M)+. ESI-MS m/z: 274.0 (M−31). FAB-MS combined organic layers were washed with water (30mL),
m/z: 306.1347 (M+H) (Calcd for C16H20NO5: 306.1342). Spe- aqueous citric acid (10%, 30mL), and brine (30mL) and sub-
cific rotation: [α]D20 −123.8 (c=1.0, CH3OH).
trans-(2S,6S)-Teneraic Acid (trans-(2S,6S)-Piper- The crude residue was purified by silica gel column chroma-
sequently dried over anhydrous MgSO4 and then concentrated.
idine-2.6-dicarboxylic Acid, 1) Furthermore, trans- tography (eluted with n-hexane–AcOEt) to obtain cis–meso-N-
(2S,6S)-N-benzoyl-teneraic acid dimethyl ester (19a) (1.10g, benzoyl-teneraic acid dimethyl ester (19c) as a viscous oil
3.60mmol) was dissolved in 6N HCl (50mL) and heated at (250mg, 8.20mmol, 51.0%).
110°C for 9h. The resulting solution was successively washed
1H-NMR (400MHz, CDCl3) δ: 1.60–2.34 (6H, m), 3.72 (6H,
with ether (50mL) and AcOEt (50mL) and then concentrated s), 4.50–5.40 (2H, m), 7.35–7.45 (5H, m). 13C-NMR (CDCl3)
under reduced pressure to obtain crude trans-(2S,6S)-piperi- δ: 16.9, 25.9, 52.0, 55.0, 52.1, 126.7, 128.6, 129.9, 135.9,
dine-2.6-dicarboxylic acid (1) as a solid (750mg). A solution 171.3, 172.9. IR (neat): 1730, 1650 (cm−1). FD-MS m/z: 305.0
of crude 1·HCl (750mg) in ion-exchange water (10mL) was (M)+. FAB-MS m/z: 306.1347 (M+H) (Calcd for C16H20NO5:
applied to a cation-exchange resin (DOWEX™ 1×8 in the 306.1342).
acetate form) column (100mL) for purification. The column
was washed with ion-exchange water (200mL). Subsequently,
Acknowledgments The authors thank honorary Professor
compound 1 was eluted with aqueous 2N acetic acid (300mL). Minoru Sato for providing natural teneraic acid. The authors
The eluates containing 1 were pooled and concentrated in thank honorary Professor Tatsuya Shono and for his encour-
vacuo. The residue was dissolved in water (5mL), and acti- agement and continued support of this study. They express
vated carbon (200mg) was added to the solution. The solution their sincere gratitude to Mr. Shoichi Asada and Dr. Hiroyuki
was stirred for 1h and was then filtered. The filtrate was con- Izawa for their helpful discussions and assistance.
centrated in vacuo to obtain trans-(2S,6S)-teneraic acid (1) as
1
a white solid (465mg, 2.70mmol, 74.7%); H-NMR (400MHz,
Conflict of Interest All authors were employees of Aji-
D2O) δ: 1.57 (2H, brquint, J=6.0Hz), 1.85–2.00 (4H, m), 4.11 nomoto Co., Inc. when the study was conducted and have no
(2H, dd, J=4.8, 7.2Hz). 13C-NMR (D2O) δ: 19.0, 25.0, 55.3, further conflicts of interest to declare.
172.5. IR (KBr): 3460, 3050, 2950, 1660, 1585, 1440, 1380,
1260, 1115, 895, 830, 775 (cm−1). FD-MS m/z: 174.0 (M+H)+.
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