Synthesis of the Nonribosomal Peptide Phevalin and Analogs

Article abstract of DOI:10.1021/acs.joc.8b03206

Phevalin, a cyclic nonribosomal peptide produced by Staphylococcus aureus, has intriguing biological properties. A synthetic route to access phevalin and similar pyrazinone natural products tyrvalin, leuvalin, phileucin, and a few synthetic analogs is described. The reaction sequence involves a one-pot carbamate deprotection/imine formation/aerobic oxidation to form the pyrazinone-containing products.

Full text of DOI:10.1021/acs.joc.8b03206

Note
pubs.acs.org/joc
Cite This: J. Org. Chem. XXXX, XXX, XXX−XXX
Synthesis of the Nonribosomal Peptide Phevalin and Analogs
†
†
‡
,†
́
Remya Ramesh, Michael T. Bovino, Yibin Zeng, and Jeffrey Aube*
†Division of Chemical Biology and Medicinal Chemistry, UNC Eshelman School of Pharmacy, University of North Carolina at
Chapel Hill, Chapel Hill, North Carolina 27599, United States
‡Department of Medicinal Chemistry, University of Kansas, Lawrence, Kansas 66045, United States
S
* Supporting Information
ABSTRACT: Phevalin, a cyclic nonribosomal peptide produced by
Staphylococcus aureus, has intriguing biological properties. A synthetic
route to access phevalin and similar pyrazinone natural products
tyrvalin, leuvalin, phileucin, and a few synthetic analogs is described.
The reaction sequence involves a one-pot carbamate deprotection/
imine formation/aerobic oxidation to form the pyrazinone-containing
products.
hevalin 1a, also known as aureusimine B, was first isolated
from a soil actinomycete (Streptomyces sp.) by Alvarez et
P
al., who described it as a calpain inhibitor,1 although this
activity was subsequently disproven.2 Related pyrazinone-
containing natural products (tyrvalin (1b), leuvalin (1c), and
phileucin (1i)) were later reported to be produced by
nonribosomal peptide synthetases in S. aureus3,4 or E. salina.5
As a class, nonribosomal peptides are structurally diverse and
exhibit a variety of biological activities.6 Since S. aureus is a
major human pathogen, several groups have attempted to
understand the biological significance of these natural
products.7−10 Studies indicate that aureusimines do not display
antibiotic activity on their own but are associated with host−
pathogen interactions.9,10 The potential use of phevalin in
quorum sensing studies has also been reported.11
Biosynthetically, aureusimines arise from a dipeptide
assembled on the protein AusA. The protein reductively
releases a dipeptidyl aldehyde, which undergoes condensation
to form a cyclic imine. This imine is then oxidized to the
aromatic natural product.3,4,12 In a previous synthesis of
phevalin, we prepared the precursor amino aldehyde toward
phevalin from the corresponding Boc-protected amine in situ
and found that it underwent both cyclization and oxidation in
trifluoroacetic acid upon heating (Figure 1c).2 Later, Aldrich
and co-workers reconstituted the activity of AusA in vitro and
confirmed that this oxidation is spontaneous.8 Interestingly,
Fischbach and co-workers have reported that dipeptidyl
aldehyde precursors to phevalin and related natural products
have some stability in biological media and activity against
cathepsins in cellular assays.13
In the above-noted synthesis, we prepared the aldehyde
through a DIBAL-mediated reduction of the corresponding
ester. We and others7,8,14 have since found that, although this
step is reproducible, the ester reduction required careful
monitoring and was often accompanied by alcohol resulting
from over-reduction of the aldehyde. Given the continued
interest in phevalin and other members of this class in
biological studies, we now report full details of a modestly
Figure 1. (a) Pyrazinone-containing natural products and (b) the
proposed biosynthesis of phevalin.8 (c) Final stage of our previously
reported synthesis of phevalin.2
modified total synthesis of phevalin and nine additional
members of the class.
Received: December 18, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.joc.8b03206
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
In modifying this route, we decided to replace the ester
reduction step with an oxidation of the corresponding alcohol,
readily prepared by coupling of Boc-Val with amino alcohol 3
(Scheme 1). Oxidation using Dess-Martin periodinane (DMP)
tyrvalin (1b), SO3·pyr (Parikh−Doering oxidation) was
employed for the oxidation of alcohol 4b, probably
necessitated by the presence of the phenolic hydroxyl group.
The aldehyde obtained was converted to tyrvalin (1b) using
the procedure adopted for the synthesis of other compounds.
All the compounds were fully characterized and prepared on
ca. 30 mg scale.
Scheme 1. Modified Synthesis of Phevalin
The chemical mechanism of the spontaneous aerobic
oxidation is currently unknown, although similar oxidations
to form heterocycles have been documented.16−19 The
reaction could possibly proceed via a radical pathway involving
a captodative radical at the C-3 position of the pyrazinone.20 In
such a case, a C-3 cyclopropyl group might be expected to
undergo ring opening.21,22 However, upon submission to the
standard conditions, pyrazinone 1j, which contains a cyclo-
propyl group at this position, was obtained, albeit in a slightly
lower yield (33%), but without any observable byproduct
resulting from ring opening. We also note that this sequence
proceeds in very poor yield for substrates devoid of alkyl
groups at C-3 or at C-6.
We briefly examined the use of hot hexafluoroisopropanol
(HFIP) for the deprotection of 5a (Scheme 3).23 As before, we
Scheme 3. Alternative Product Observed upon HFIP
Deprotection
reliably gave the aldehyde 5a as a single diastereomer as
expected.15 Deprotection using TFA gave phevalin in 46%
yield over two steps from alcohol 4a. Although the reaction
gave moderate yields, we did not isolate any other side
products in this reaction.
This route was used to make other naturally occurring
aureusimines and a few synthetic analogs (Scheme 2). The
required dipeptide alcohols (4b−j) were synthesized from
commercially available Boc-protected amino acids and amino
alcohols. Oxidation of alcohol followed by deprotection of
carbamate gave the natural products 1b, 1c, 1i, and six
additional analogs in moderate yields (33−55%). In the case of
a
Scheme 2. Synthesis of Pyrazinones
observed complete consumption of starting material and
phevalin (1a) was obtained in 15% yield, but under these
circumstances a new product 6 was also observed. This
product could be formed from the initially formed imine by a
[1,3]-hydride shift, indicating that there are two pathways
possible for this intermediate. The product 6 did not convert
to phevalin after stirring at room temperature for 2 days.
In summary, we have developed a convenient and reliable
synthetic route to the pyrazinone natural products exemplified
by phevalin, tyrvalin, leuvalin, and phileucin. In addition to
these natural products, six synthetic analogs were also
synthesized.
EXPERIMENTAL SECTION
■
General Information. Reactions were carried out in either round-
bottom flasks or glass sample vials (TFE-lined cap). All reagents,
starting materials, and solvents (including dry solvents) were obtained
from commercial suppliers and used without further purification.
Thin-layer chromatography (TLC) was performed using commercial
silica gel 60 F254-coated aluminum-backed sheets. Visualization was
accomplished with UV light, or by immersion in Seebach’s stain
followed by heating with a heat gun for ca. 30 s. Purification was
carried out by an automated flash chromatography/medium-pressure
liquid chromatography (MPLC) system using normal phase silica gel
a
This example used SO3·pyr instead of DMP for Step 1.
B
DOI: 10.1021/acs.joc.8b03206
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
flash columns. The infrared (IR) spectra were acquired as thin films
using a universal ATR sampling accessory on an FT-IR spectrometer;
the absorption frequencies are reported in cm−1. All NMR spectra
were recorded on a 400 MHz spectrometer with a dual carbon/proton
cryoprobe. NMR samples were recorded in deuterated chloroform
(CDCl3) or deuterated methanol (CD3OD). Chemical shifts are
reported in parts per million (ppm) and referenced to the center line
tert-Butyl ((S)-1-(((S)-1-Hydroxy-3-(4-hydroxyphenyl)-
propan-2-yl)amino)-3-methyl-1-oxobutan-2-yl)carbamate
(4b). Yield 75% (670 mg) from L-tyrosinol hydrochloride (500 mg)
and N-Boc-L-valine (533 mg). White solid, mp 115−119 °C; [α]2D2
−44.3 (c 0.80, MeOH); Rf = 0.68 (EtOAc); IR 3295 (broad peak),
1
1686, 1646 cm−1; H NMR (400 MHz, CD3OD) δ 7.05 (m, 2H),
6.68 (m, 2H), 4.04 (m, 1H), 3.79 (d, J = 6.8 Hz, 1H), 3.49 (d, J = 5.4
Hz, 2H), 2.81 (dd, J = 13.6, 6.5 Hz, 1H), 2.65 (dd, J = 13.8, 7.8 Hz,
1H), 1.94 (m, 1H), 1.45 (s, 9H), 0.86 (d, J = 6.8 Hz, 6H); 13C{1H}
NMR (100 MHz, CD3OD) δ 174.1, 156.9, 155.5, 131.3, 130.3, 116.1,
111.4, 80.6, 64.0, 61.9, 54.2, 37.0, 32.0, 28.7, 19.7, 18.4; HRMS (ESI-
TOF) m/z: [M + H]+ calcd for C19H31N2O5, 367.2233; found
367.2234.
tert-Butyl ((S)-1-(((S)-1-Hydroxy-4-methylpentan-2-yl)-
amino)-3-methyl-1-oxobutan-2-yl)carbamate (4c). Known
compound; spectroscopic data matched those in the literature.24
Yield 92% (400 mg) from L-leucinol (162 mg) and N-Boc-L-valine
(300 mg).
tert-Butyl ((S)-1-(((S)-1-Hydroxy-3-methylbutan-2-yl)-
amino)-3-methyl-1-oxobutan-2-yl)carbamate (4d). Known
compound; spectroscopic data matched those in the literature.25,26
Yield 89% (370 mg) from L-valinol (142 mg) and N-Boc-L-valine
(300 mg).
tert-Butyl ((S)-1-(((S)-1-Hydroxypropan-2-yl)amino)-3-meth-
yl-1-oxobutan-2-yl)carbamate (4e). Known compound; spectro-
scopic data matched those in the literature.27 Yield 64% (402 mg)
from L-alaninol (170 mg) and N-Boc-L-valine (500 mg).
tert-Butyl ((S)-1-(((2S,3S)-1-Hydroxy-3-methylpentan-2-yl)-
amino)-3-methyl-1-oxobutan-2-yl)carbamate (4f). Yield 71%
(511 mg) from L-isoleucinol (280 mg) and N-Boc-L-valine (500 mg).
White solid, mp 128−131 °C; [α]2D2 −48.2 (c 1.0, CHCl3); Rf = 0.19
(40% EtOAc/hexanes); IR 3326 (broad peak), 2961, 1684, 1651
cm−1; 1H NMR (400 MHz, CDCl3) δ 6.40 (d, J = 8.4 Hz, 1H), 5.15
(d, J = 8.2 Hz, 1H), 3.84−3.61 (complex, 4H), 3.06 (t, J = 5.7 Hz,
1H), 2.12 (m, 1H), 1.65 (m, 1H), 1.45 (m, 1H, partially obscured),
1.43 (s, 9H), 1.12 (m, 1H), 0.97−0.85 (complex, 12H); 13C{1H}
NMR (100 MHz, CDCl3) δ 172.4, 156.3, 80.3, 63.5, 60.9, 56.2, 35.5,
30.4, 28.4, 25.6, 19.5, 18.2, 15.6, 11.3; HRMS (ESI-TOF) m/z: [M +
H]+ calcd for C16H33N2O4, 317.2440; found 317.2441.
tert-Butyl ((S)-1-(((S)-2-Hydroxy-1-phenylethyl)amino)-3-
methyl-1-oxobutan-2-yl)carbamate (4g). Known compound;
spectroscopic data matched those in the literature.25 Yield 82%
(400 mg) from (S)-2-phenylglycinol (200 mg) and N-Boc-L-valine
(317 mg).
1
of residual solvent (CDCl3: δ 7.26 ppm for H NMR and 77.16 for
1
13C NMR; CD3OD: δ 3.31 ppm for H NMR and 49.00 for 13C
NMR). Coupling constants are reported in hertz (Hz). HRMS data
were collected with a time-of-flight mass spectrometer and an
electrospray ion source (ESI). Melting points were determined in
open capillary tubes using an automated melting point apparatus and
are uncorrected. Optical rotations were recorded on an automatic
polarimeter at 589 nm.
General Procedure for Peptide Coupling. To an oven-dried,
round-bottomed flask equipped with a stir bar Boc-protected amino
acid (200 mg, 1.0 equiv), amino alcohol (139 mg, 1.0 equiv), EDC
(229 mg, 1.3 equiv), and HOBt (183 mg, 1.3 equiv) were added.
Anhydrous DCM (20 mL) was then added, followed by DIPEA (0.74
mL, 3.0 equiv). The reaction mixture was stirred at rt under an inert
atmosphere for 16 h. The reaction mixture was then diluted with
DCM (20 mL) and washed with 1 M HCl (10 mL) solution, followed
by saturated NaHCO3 (10 mL) and brine (10 mL). The organic
layers were then dried over anhydrous Na2SO4 and filtered, and the
solvent was removed under reduced pressure to yield the crude
residue, which was purified on a semiautomated purification system
using a normal phase silica flash column (100% hexanes to 100%
EtOAc over 20 min) to give product.
General Procedure for Conversion of Dipeptide Alcohol to
Pyrazinones. To a glass vial equipped with a stir bar, the dipeptide
alcohol (100 mg, 1.0 equiv), Dess-Martin periodinane (182 mg, 1.5
equiv) and DCM (5 mL) were added. The resulting cloudy solution
was then allowed to stir at rt for 1.5 h. A solution of saturated
Na2S2O3 (2 mL) and saturated NaHCO3 (2 mL) were added to the
reaction mixture and stirred vigorously until two clear layers
separated. The organic layer was separated, and the aqueous layer
was extracted with DCM (10 mL × 2). The combined organic layer
was then dried over anhydrous sodium sulfate and filtered, and the
solvent was removed under reduced pressure to give the crude
aldehyde. This crude material was taken forward for the next step
without further purification. To an open round-bottomed flask
equipped with a stir bar, the aldehyde and DCM (5 mL) were added.
To this solution TFA (0.88 mL, 40 equiv) was added, and the
reaction mixture was left to stir at rt for 16 h. The reaction mixture
was diluted with DCM (10 mL), washed with brine (5 mL), and dried
over anhydrous Na2SO4, and the solvent was removed under reduced
pressure to give a crude residue which was further purified on a
semiautomated purification system using a normal phase silica flash
column (100% hexanes to 60% ethyl acetate in hexanes over 30 min)
to give the product.
Tyrvalin (1b). Parikh−Doering oxidation was used to prepare the
aldehyde 5b. To a glass vial equipped with a stir bar was added tert-
butyl ((S)-1-(((S)-1-hydroxy-3-(4-hydroxyphenyl)propan-2-yl)-
amino)-3-methyl-1-oxobutan-2-yl)carbamate (4b, 100 mg, 1.0
equiv) and dissolved in anhydrous DMSO (2 mL). To this solution
was added triethylamine (0.23 mL, 6.0 equiv) followed by a solution
of SO3·pyr (130 mg, 3.0 equiv) in DMSO (1 mL) dropwise. The
reaction was allowed to proceed at room temperature for 2 h. The
reaction mixture was diluted with EtOAc (10 mL) and washed with
ice-cold water. The organic layer was separated, dried over sodium
sulfate, filtered, and concentrated to give the crude residue. This crude
aldehyde was then converted to the pyrazinone (tyrvalin, 1b) by
following the same procedure described above.
tert-Butyl ((S)-1-(((S)-1-Hydroxy-3-phenylpropan-2-yl)-
amino)-4-methyl-1-oxopentan-2-yl)carbamate (4h). Known
compound; spectroscopic data matched those in the literature.25,26
Yield 84% (330 mg) from L-phenylalaninol (163 mg) and N-Boc-L-
leucine (250 mg).
tert-Butyl ((2S,3S)-1-(((S)-1-Hydroxy-3-phenylpropan-2-yl)-
amino)-3-methyl-1-oxopentan-2-yl)carbamate (4i). Yield 86%
(414 mg) from L-phenylalaninol (200 mg) and N-Boc-L-isoleucine
(306 mg). White solid, mp 143−147 °C; [α]2D2 − 42.7 (c 1.0, CHCl3);
Rf = 0.19 (40% EtOAc/hexanes); IR 3338, 3294,1682, 1656 cm−1; 1H
NMR (400 MHz, CDCl3) δ 7.31−7.20 (complex, 5H), 6.16 (m, 1H),
4.86 (m, 1H), 4.19 (m, 1H), 3.86 (m, 1H), 3.70 (m, 1H), 3.57 (m,
1H), 2.93−2.81 (complex, 2H), 2.54 (t, J = 5.8 Hz, 1H), 1.85 (m,
1H), 1.44 (s, 9H), 1.33 (m, 1H), 1.01 (m, 1H), 0.88−0.83 (complex,
6H); 13C{1H} NMR (100 MHz, CDCl3) δ 172.0, 156.1, 137.8, 129.3,
128.7, 126.8, 80.4, 63.8, 60.0, 53.0, 37.1, 36.9, 28.4, 24.7, 15.7, 11.6;
HRMS (ESI-TOF) m/z: [M + H]+ calcd for C20H33N2O4, 365.2440;
found 365.2442
tert-Butyl ((S)-1-Cyclopropyl-2-(((S)-1-hydroxy-4-methyl-
pentan-2-yl)amino)-2-oxoethyl)carbamate (4j). Yield 34%
(240 mg) from L-leucinol (300 mg) and (S)-2-((tert-butoxycarbonyl)-
amino)-2-cyclopropylacetic acid (500 mg). White solid, mp 103−106
°C; [α]2D2 −22.9 (c 0.80, CHCl3); Rf = 0.19 (40% EtOAc/hexanes);
IR 3306, 2956, 1691, 1651 cm−1; 1H NMR (400 MHz, CDCl3) δ 6.09
(m, 1H), 5.17 (br s, 1H), 4.03 (m, 1H), 3.72 (m, 1H), 3.50 (m, 1H),
3.38 (m, 1H), 2.67 (br s, 1H), 1.64 (m, 1H), 1.40 (s, 9H), 1.43−1.28
(complex, 2H), 1.11 (m, 1H), 0.94−0.91 (complex, 6H), 0.68−0.40
tert-Butyl ((S)-1-(((S)-1-Hydroxy-3-phenylpropan-2-yl)-
amino)-3-methyl-1-oxobutan-2-yl)carbamate (4a). Known
compound; spectroscopic data matched those in the literature.24
Yield 87% (1.40 g) from L-phenylalaninol (0.70 g) and N-Boc-L-valine
(1.0 g).
C
DOI: 10.1021/acs.joc.8b03206
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
(complex, 4H); 13C{1H} NMR (100 MHz, CDCl3) δ 172.1, 156.1,
80.6, 66.0, 59.4, 50.4, 40.1, 28.4, 25.0, 23.2, 22.2, 13.9, 3.8, 3.3; HRMS
(ESI-TOF) m/z: [M + H]+ calcd for C16H31N2O4, 315.2283; found
315.2280.
6-Benzyl-3-isopropylpyrazin-2(1H)-one (Phevalin, 1a).
Known compound.2Yield 46% (30 mg) over two steps from 4a
(100 mg). White solid, mp 179−181 °C; Rf = 0.32 (40% EtOAc/
hexanes); IR 3360, 1647 cm−1; 1H NMR (400 MHz, CDCl3) δ 12.54
(br s, 1H), 7.35−7.24 (complex, 6H), 3.83 (s, 2H), 3.42 (septet, J =
6.8 Hz, 1H), 1.24 (d, J = 6.8 Hz, 6H); 13C{1H} NMR (100 MHz,
CDCl3) δ 162.2, 157.5, 136.8, 136.1, 129.3, 129.0, 127.5, 122.5, 36.8,
30.2, 20.2; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C14H17N2O,
229.1341; found 229.1333.
6-(4-Hydroxybenzyl)-3-isopropylpyrazin-2(1H)-one (tyrva-
lin, 1b). Known compound.3,4,8Yield 45% (30 mg) over two steps
from 4b (100 mg). Yellow solid, mp 207−210 °C; Rf = 0.68 (EtOAc);
IR 3253 (broad peak), 1640, 1615 cm−1; 1H NMR (400 MHz,
CD3OD) δ 7.09−7.05 (complex, 3H), 6.76−6.72 (complex, 2H),
3.72 (s, 2H), 3.34 (septet, J = 6.8 Hz, 1H, partially obscured), 1.18 (d,
J = 6.9 Hz, 6H); 13C{1H} NMR (100 MHz, CD3OD) δ 162.5, 157.9,
157.7, 140.0, 130.9, 128.4, 123.1, 116.6, 36.2, 31.0, 20.4; HRMS (ESI-
TOF) m/z: [M + H]+ calcd for C14H17N2O2, 245.1290; found
245.1283.
6-Isobutyl-3-isopropylpyrazin-2(1H)-one (leuvalin, 1c).
Known compound.8Yield 52% (32 mg) over two steps from 4c
(100 mg). White solid, mp 135−138 °C; Rf = 0.48 (40% EtOAc/
hexanes); IR 3364, 1645 cm−1; 1H NMR (CDCl3, 400 MHz) δ 12.89
(br s, 1H), 7.18 (s, 1H), 3.40 (septet, J = 6.8 Hz, 1H), 2.38 (d, J = 7.2
Hz, 2H), 2.05 (m, 1H), 1.25 (d, J = 6.9 Hz, 6H), 0.98 (d, J = 6.6 Hz,
6H); 13C{1H} NMR (CDCl3, 100 MHz) δ 161.3, 158.0, 137.6, 123.1,
39.6, 30.4, 28.4, 22.4, 20.1; HRMS (ESI-TOF) m/z: [M + H]+ calcd
for C11H19N2O, 195.1497; found 195.1495.
mp 150−154 °C; Rf = 0.35 (40% EtOAc/hexanes); IR 2951, 1648
cm−1; 1H NMR (400 MHz, CDCl3) δ 7.35−7.24 (complex, 6H), 3.82
(s, 2H), 2.65 (d, J = 7.0 Hz, 2H), 2.22 (m, 1H), 0.97 (d, J = 6.6 Hz,
6H); 13C{1H} NMR (100 MHz, CDCl3) δ 157.9, 136.8, 135.8, 129.3,
129.1, 127.6, 122.5, 41.8, 36.8, 26.9, 22.8; HRMS (ESI-TOF) m/z:
[M + H]+ calcd for C15H19N2O, 243.1497; found 243.1490.
(S)-6-Benzyl-3-(sec-butyl)pyrazin-2(1H)-one (1i). Known com-
pound.13Yield 53% (35 mg) over two steps from 4i (100 mg). White
solid, mp 146−150 °C; [α]D22 14.6 (c 0.70, CHCl3); Rf = 0.39 (40%
EtOAc/hexanes); IR 2962, 1633 cm−1; 1H NMR (400 MHz, CDCl3)
δ 12.35 (br s, 1H), 7.35−7.24 (complex, 6H), 3.83 (s, 2H), 3.25
(sextet, J = 6.9 Hz, 1H), 1.82 (m, 1H), 1.55 (m, 1H, partially
obscured by water signal), 1.21 (d, J = 6.9 Hz, 3H), 0.92 (t, J = 7.4
Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3) δ 161.5, 158.1, 137.0,
136.3, 129.3, 128.9, 127.4, 122.7, 36.8, 27.6, 17.8, 12.2; HRMS (ESI-
TOF) m/z: [M + H]+ calcd for C15H19N2O, 243.1497; found
243.1488.
3-Cyclopropyl-6-isobutylpyrazin-2(1H)-one (1j). Yield 33%
(20 mg) over two steps from 4j (100 mg). White solid, mp 173−176
°C; Rf = 0.39 (40% EtOAc/hexanes); IR 2957, 1648 cm−1; 1H NMR
(400 MHz, CDCl3) δ 13.05 (br s, 1H), 7.07 (s, 1H), 2.57 (m, 1H),
2.38 (d, J = 7.2 Hz, 2H), 2.05 (m, 1H), 1.07−0.95 (complex, 10H);
13C{1H} NMR (100 MHz, CDCl3) δ 158.7 157.9, 136.2, 123.3, 39.5,
28.4, 22.3, 11.4, 10.1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for
C11H17N2O, 193.1341; found 193.1334.
6-Benzyl-3-isopropyl-5,6-dihydropyrazin-2(1H)-one (6). To
a microwave vial equipped with a stir bar, the compound 5a (130 mg,
0.37 mmol) was added, followed by HFIP (3 mL) and sealed. This
reaction mixture was then heated in a microwave reactor at 140 °C for
2h. The solvent was removed under reduced pressure and the crude
was then purified on a semiautomated purification system using a
normal phase silica flash column (100% hexanes to 60% ethyl acetate
over 18 min). The product 6 was obtained was an oil (28 mg, 33%
yield) along with 13 mg of 1a (15% yield). [α]2D2 −16.5 (c 1.0,
CHCl3); Rf = 0.24 (40% EtOAc/hexanes); IR 3200, 1683, 1633 cm−1;
1H NMR (400 MHz, CDCl3) δ 7.37−7.27 (complex, 3H), 7.18−7.16
(complex, 2H), 5.76 (br s, 1H), 3.97 (m, 1H), 3.76 (m, 1H), 3.51
(ddd, J = 16.3, 10.2, 1.6 Hz, 1H), 3.27 (septet, J = 6.8 Hz, 1H), 2.94
(dd, J = 13.7, 5.0 Hz, 1H), 2.65 (dd, J = 13.7, 9.5 Hz, 1H), 1.14 (d, J
= 6.8 Hz, 3H), 1.09 (d, J = 6.9 Hz, 3H); 13C{1H} NMR (100 MHz,
CDCl3) δ 170.4, 157.7, 135.8, 129.27, 129.23, 127.5, 53.1, 51.1, 39.6,
30.7, 20.2, 19.9; HRMS (ESI-TOF) m/z: [M + H]+ calcd for
C14H19N2O, 231.1497; found 231.1490.
3,6-Diisopropylpyrazin-2(1H)-one (1d). Known compoun-
d.28Yield 50% (30 mg) over two steps from 4d (100 mg). White
solid, mp 131−134 °C; Rf = 0.64 (40% EtOAc/hexanes); IR 2965,
1
2870, 1636, 1607 cm−1; H NMR (400 MHz, CDCl3) δ 13.10 (br s,
1H), 7.22 (s, 1H), 3.41 (septet, J = 6.9 Hz, 1H), 2.84 (septet, J = 7.0
Hz, 1H), 1.33 (d, J = 7.0 Hz, 6H), 1.24 (d, J = 6.8 Hz, 6H); 13C{1H}
NMR (100 MHz, CDCl3) δ 161.7, 157.6, 143.4, 120.4, 30.3, 30.2,
21.3, 20.1; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C10H17N2O,
181.1341; found 181.1331.
3-Isopropyl-6-methylpyrazin-2(1H)-one (1e). Yield 43% (24
mg) over two steps from 4e (100 mg). White solid, mp 145−148 °C;
Rf = 0.16 (40% EtOAc/hexanes); IR 2962, 1640 cm−1; 1H NMR (400
MHz, CDCl3) δ 13.1 (br s, 1H), 7.21 (s, 1H), 3.41 (septet, J = 6.8
Hz, 1H), 2.28 (s, 3H), 1.22 (d, J = 6.8 Hz, 6H); 13C{1H} NMR (100
MHz, CDCl3) δ 161.1, 158.0, 134.4, 123.0, 30.0, 20.2, 16.0; HRMS
(ESI-TOF) m/z: [M + H]+ calcd for C8H13N2O, 153.1028; found
153.1021.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.8b03206.
(S)-6-(sec-Butyl)-3-isopropylpyrazin-2(1H)-one (1f). Yield
55% (34 mg) over two steps from 4f (100 mg). White solid, mp
145−148 °C; [α]2D2 35.5 (c 0.80, CHCl3); Rf = 0.64 (40% EtOAc/
1
Copies of H and 13C NMR spectra for compounds
prepared by the present method (PDF)
1
hexanes); IR 2965, 1638 cm−1; H NMR (400 MHz, CDCl3) δ 7.29
(s, 1H), 3.41 (septet, J = 6.7 Hz, 1H), 2.57 (sextet, J = 7.1 Hz, 1H),
1.80−1.60 (complex, 2H), 1.32 (d, J = 7.0 Hz, 3H), 1.25 (d, J = 6.8
Hz, 6H), 0.90 (t, J = 7.4 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3)
δ 161.3, 157.7, 142.7, 122.2, 37.4, 30.4, 28.6, 20.1, 19.0, 12.0; HRMS
(ESI-TOF) m/z: [M + H]+ calcd for C11H19N2O, 195.1497; found
195.1488.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jaube@unc.edu.
ORCID
3-Isopropyl-6-phenylpyrazin-2(1H)-one (1g). Yield 52% (33
mg) over two steps from 4g (100 mg). White solid, mp 197−200 °C;
́
Jeffrey Aube: 0000-0003-1049-5767
Notes
1
Rf = 0.42 (40% EtOAc/hexanes); IR 2960, 1636, 1574 cm−1; H
The authors declare no competing financial interest.
NMR (400 MHz, CDCl3) δ 12.93 (br s, 1H), 7.80−7.70 (complex,
3H), 7.54−7.46 (complex, 3H), 3.47 (septet, J = 6.8 Hz, 1H), 1.30 (d,
J = 6.8 Hz, 6H); 13C{1H} NMR (100 MHz, CDCl3) δ 162.6, 157.6,
136.4, 131.2, 130.3, 129.3, 126.7, 121.8, 30.7, 20.1; HRMS (ESI-
TOF) m/z: [M + H]+ calcd for C13H15N2O, 215.1184; found
215.1175.
ACKNOWLEDGMENTS
■
We acknowledge financial support from the University of
Kansas and the University of North Carolina at Chapel Hill.
We thank Dr. Brandie Ehrmann (UNC Chapel Hill) for
HRMS analysis.
6-Benzyl-3-isobutylpyrazin-2(1H)-one (1h). Known compoun-
d.13Yield 53% (35 mg) over two steps from 4h (100 mg). White solid,
D
DOI: 10.1021/acs.joc.8b03206
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
(18) Kumar, P. M.; Kumar, K. S.; Mohakhud, P. K.; Mukkanti, K.;
Kapavarapu, R.; Parsa, K. V.; Pal, M. Construction of a six-membered
fused N-heterocyclic ring via a new 3-component reaction: Synthesis
of (pyrazolo)pyrimidines/pyridines. Chem. Commun. 2012, 48, 431−
433.
(19) Osborne, D.; Stevenson, P. J. A concise formal synthesis of
luotonin A. Tetrahedron Lett. 2002, 43, 5469−5470.
REFERENCES
■
(1) Alvarez, M. E.; White, C. B.; Gregory, J.; Kydd, G. C.; Harris, A.;
Sun, H. H.; Gillum, A. M.; Cooper, R. Phevalin, a new calpain
inhibitor, from a Streptomyces sp. J. Antibiot. 1995, 48, 1165−1167.
́
(2) Zeng, Y.; Li, Q.; Hanzlik, R. P.; Aube, J. Synthesis of a small
library of diketopiperazines as potential inhibitors of calpain. Bioorg.
Med. Chem. Lett. 2005, 15, 3034−3038.
(20) Viehe, H. G.; Janousek, Z.; Merenyi, R.; Stella, L. The
captodative effect. Acc. Chem. Res. 1985, 18, 148−154.
(21) Griller, D.; Ingold, K. U. Free-radical clocks. Acc. Chem. Res.
1980, 13, 317−323.
(3) Zimmermann, M.; Fischbach, M. A. A family of pyrazinone
natural products from a conserved nonribosomal peptide synthetase
in Staphylococcus aureus. Chem. Biol. 2010, 17, 925−930.
(4) Wyatt, M. A.; Wang, W.; Roux, C. M.; Beasley, F. C.; Heinrichs,
D. E.; Dunman, P. M.; Magarvey, N. A. Staphylococcus aureus
nonribosomal peptide secondary metabolites regulate virulence.
Science 2010, 329, 294−296.
(22) Nonhebel, D. C. The chemistry of cyclopropylmethyl and
related radicals. Chem. Soc. Rev. 1993, 22, 347−359.
(23) Choy, J.; Jaime-Figueroa, S.; Jiang, L.; Wagner, P. Novel
practical deprotection of N-Boc compounds using fluorinated
alcohols. Synth. Commun. 2008, 38, 3840−3853.
(5) Bohringer, N.; Gutschow, M.; Konig, G. M.; Schaberle, T. F.
Phileucin - a cyclic dipeptide similar to phevalin (aureusimine B) from
Streptomyces coelicolor m1146. Nat. Prod. Commun. 2017, 12, 107−
109.
(6) Sussmuth, R. D.; Mainz, A. Nonribosomal peptide synthesis-
principles and prospects. Angew. Chem., Int. Ed. 2017, 56, 3770−3821.
(7) Secor, P. R.; Jennings, L. K.; James, G. A.; Kirker, K. R.; Pulcini,
E. D.; McInnerney, K.; Gerlach, R.; Livinghouse, T.; Hilmer, J. K.;
Bothner, B.; Fleckman, P.; Olerud, J. E.; Stewart, P. S. Phevalin
(aureusimine B) production by Staphylococcus aureus biofilm and
impacts on human keratinocyte gene expression. PLoS One 2012, 7,
No. e40973.
(8) Wilson, D. J.; Shi, C.; Teitelbaum, A. M.; Gulick, A. M.; Aldrich,
C. C. Characterization of AusA: A dimodular nonribosomal peptide
synthetase responsible for the production of aureusimine pyrazinones.
Biochemistry 2013, 52, 926−937.
(9) Wyatt, M. A.; Magarvey, N. A. Optimizing dimodular
nonribosomal peptide synthetases and natural dipeptides in an
Escherichia coli heterologous host. Biochem. Cell Biol. 2013, 91, 203−
208.
(24) Katritzky, A. R.; Abo-Dya, N. E.; Tala, S. R.; Gyanda, K.; Abdel-
Samii, Z. K. An efficient method for the preparation of peptide
alcohols. Org. Biomol. Chem. 2009, 7, 4444−4447.
(25) Jana, A. K.; Das, S. K.; Panda, G. An efficient entry to highly
substituted chiral 2-oxopiperazines from α-amino acids via iodocyc-
lization. Tetrahedron 2012, 68, 10114−10121.
(26) Pastor, I. M.; Vastila, P.; Adolfsson, H. Employing the structural
diversity of nature: Development of modular dipeptide-analogue
ligands for ruthenium-catalyzed enantioselective transfer hydro-
genation of ketones. Chem. - Eur. J. 2003, 9, 4031−4045.
(27) Pirrung, M. C.; Zhang, F.; Ambadi, S.; Ibarra-Rivera, T. R.
Reactive esters in amide ligation with β-hydroxyamines. Eur. J. Org.
Chem. 2012, 2012, 4283−4286.
(28) Ohta, A.; Shimazaki, M.; Tamamura, H.; Mamiya, Y.;
Watanabe, T. 2-Acyloxypyrazines. Convenient acylating agents for
amines. J. Heterocycl. Chem. 1983, 20, 951−955.
(10) Blattner, S.; Das, S.; Paprotka, K.; Eilers, U.; Krischke, M.;
Kretschmer, D.; Remmele, C. W.; Dittrich, M.; Muller, T.; Schuelein-
Voelk, C.; Hertlein, T.; Mueller, M. J.; Huettel, B.; Reinhardt, R.;
Ohlsen, K.; Rudel, T.; Fraunholz, M. J. Staphylococcus aureus exploits a
non-ribosomal cyclic dipeptide to modulate survival within epithelial
cells and phagocytes. PLoS Pathog. 2016, 12, No. e1005857.
(11) Martin, N.; Meschwitz, S., Synthesis and biological evaluation
of phevalin and related pyrazinones as potential quorum sensing
inhibitors. In Abstracts of Papers of the American Chemical Society,
2018; Vol. 256.
(12) Wyatt, M. A.; Mok, M. C.; Junop, M.; Magarvey, N. A.
Heterologous expression and structural characterisation of a
pyrazinone natural product assembly line. ChemBioChem 2012, 13,
2408−2415.
(13) Guo, C. J.; Chang, F. Y.; Wyche, T. P.; Backus, K. M.; Acker, T.
M.; Funabashi, M.; Taketani, M.; Donia, M. S.; Nayfach, S.; Pollard,
K. S.; Craik, C. S.; Cravatt, B. F.; Clardy, J.; Voigt, C. A.; Fischbach,
M. A. Discovery of reactive microbiota-derived metabolites that
inhibit host proteases. Cell 2017, 168, 517−526.e518.
(14) Sun, F.; Cho, H.; Jeong, D.-W.; Li, C.; He, C.; Bae, T.
Aureusimines in Staphylococcus aureus are not involved in virulence.
PLoS One 2010, 5, No. e15703.
(15) Myers, A. G.; Zhong, B.; Movassaghi, M.; Kung, D. W.;
Lanman, B. A.; Kwon, S. Synthesis of highly epimerizable N-protected
α-amino aldehydes of high enantiomeric excess. Tetrahedron Lett.
2000, 41, 1359−1362.
(16) Scarry, S. M.; Lovell, K. M.; Frankowski, K. J.; Bohn, L. M.;
́
Aube, J. Synthesis of kappa opioid antagonists based on pyrrolo[1,2-
α]quinoxalinones using an N-arylation/condensation/oxidation re-
action sequence. J. Org. Chem. 2016, 81, 10538−10550.
(17) Shukla, G.; Srivastava, A.; Singh, M. S. Metal- and catalyst-free,
formal [4 + 1] annulation via tandem C = C/C = S functionalization:
One-pot access to 3,5-disubstituted/annulated isothiazoles. Org. Lett.
2016, 18, 2451−2454.
E
DOI: 10.1021/acs.joc.8b03206
J. Org. Chem. XXXX, XXX, XXX−XXX