Bioinspired Synthesis of the Furopyrazine Alkaloid Hyrtioseragamine A

Article abstract of DOI:10.1021/acs.joc.1c00174

A biosynthetic hypothesis proposed herein was used to guide the total synthesis of the marine-derived alkaloid hyrtioseragamine A. In the key biomimetic step, an enedione underwent acid-mediated isomerization-cyclodehydration to form the rare furopyrazine

Full text of DOI:10.1021/acs.joc.1c00174

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Bioinspired Synthesis of the Furopyrazine Alkaloid
Hyrtioseragamine A
Shi-Wei Kim, Paul A. Hume, and Jonathan Sperry*
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ABSTRACT: A biosynthetic hypothesis proposed herein was used to guide the total synthesis of the marine-derived alkaloid
hyrtioseragamine A. In the key biomimetic step, an enedione underwent acid-mediated isomerization−cyclodehydration to form the
rare furopyrazine core of the natural product. The spectroscopic data for the synthetic sample is in full agreement with that described
in the isolation report.
biomimetic alkaloid synthesis,7 an alternative biosynthetic
proposal to the furo[2,3-b]pyrazine unit present in the
INTRODUCTION
■
The Hyrtios genus of marine sponges is a rich source of
hyrtioseragamines was developed based on a reordering of the
structurally unique natural products with a wide range of
biological activities.1 In 2011, Kobayashi and co-workers
key oxidation events (Scheme 1B). We posit that debromo-
barretin (5) is a natural product present in Hyrtios sp., given its
structural similarity to the marine alkaloid barretin.8 Oxidative
cleavage of 5 would give the enedione 6 (i.e.,: higher oxidation
state than 3) that upon PK cyclization would generate the
intermediate acetal 7 possessing the correct oxidation state for a
favorable elimination of water to occur, forming the furo[2,3-
b]pyrazine and hence hyrtioseragamine A (1). The energetics of
furopyrazine formation in both biosynthetic proposals were
studied using density functional theory (DFT), the results of
which support our hypothesis (Scheme 2):9 the ΔHrxn
associated with the transformation P1 → P2 is calculated to
be +32.1 kcal/mol, providing a lower bound on the intervening
activation enthalpy. The ΔGrxn is lower (although still
significant) at +22.4 kcal/mol. However, this value includes
the entropic gain associated with the formation of water, which is
unlikely to be fully present in the transition state. In contrast, the
energy changes calculated for the transformation P3 → P5 are
ΔHrxn = +5.2 kcal/mol, and ΔGrxn = −4.1 kcal/mol, proceeding
via intermediate P4 (∼10 kcal/mol higher in energy than P3).
reported the isolation of hyrtioseragamines A (1) and B (2)
from an Okinawan marine sponge Hyrtios sp. (SS-985) (Figure
1).2 These structurally unprecedented alkaloids are the only
furo[2,3-b]pyrazines to be isolated from the nature and are
intriguing targets for total synthesis studies.
Figure 1. Hyrtioseragamines A (1) and B (2).
The furopyrazine ring was proposed by the isolation
chemists2 to arise biosynthetically by Paal−Knorr (PK)
reaction3,4 of the diketopiperazine (DKP) 3 to give the 2,3-
diaminofuran 4 that upon dehydrogenation gives hyrtioserag-
amine A (1) (Scheme 1A). We considered this proposal would
be unfavorable; the PK synthesis of furans is endergonic,3 and
the 2,3-diaminofuran 4 is electron-rich. Moreover, we are only
aware of one report describing the synthesis of furopyrazines
using the PK reaction.5,6 In continuation of our interest in
Received: January 22, 2021
Published: March 9, 2021
https://dx.doi.org/10.1021/acs.joc.1c00174
© 2021 American Chemical Society
J. Org. Chem. 2021, 86, 4779−4785
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Scheme 1. Initially Proposed (A)2 and Modified (B)
Biosynthesis of Hyrtioseragamine A
RESULTS AND DISCUSSION
A model system was established to test the modified biomimetic
hypothesis (Scheme 3). While tryptophan-derived DKPs are
■
Scheme 3. Attempted Oxidative Cleavage of DehydroDKP 8
known to undergo oxidative cleavage,10 we are not aware of any
examples of the corresponding dehydroDKPs undergoing this
reaction. Regardless, we subjected the dehydroDKP 811 to
various oxidants known to cleave the indole C2−C3 bond,
anticipating the formation of the enedione 9a/9b. In all cases,
1
TLC and H NMR spectroscopic analysis of the reaction
mixtures indicated degradation had occurred, with only traces of
3-hydroxyoxindole 10 and indole-3-carbaldehyde (11) detect-
able, the latter arising from oxidative cleavage of the
dehydroDKP double bond.12
As we were unable to oxidatively cleave the indole in
dehydroDKP 8, an alternative synthesis of the model
furopyrazine precursor 9 was developed (Scheme 4). Selenium
dioxide-mediated oxidation of o-nitroacetophenone gave the
glyoxal 12,13 which underwent condensation with the DKP 1314
Scheme 2. DFT-Calculated Energetics for the Furopyrazine
Assembly Steps Proposed in Scheme 1
Scheme 4. Model Biomimetic Furopyrazine Synthesis
This alternative reaction sequence is likely to be readily
accessible at ambient temperature.
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Scheme 5. Unsuccessful Route to Hyrtioseragamine A (1)
to give enedione 14 as a single Z-isomer.15 Acid-mediated
cleavage of the remaining acetamide, followed by reduction gave
aniline 15, the precursor for the key biomimetic furopyrazine
formation. As the PK furan synthesis is generally promoted by
strong acids,3 this was the logical place to start. Treating 15 with
a variety of acids revealed that neat sulfuric acid led to the
desired furopyrazine 16, with several other acids degrading the
substrate. Milder conditions for this bioinspired transformation
were sought; the reaction was not compatible with base and did
not proceed under neutral conditions (water, micellar catalysis,
and organic solvents) at a variety of temperatures. The overall
reaction (Z-15 → 16) involves iterative isomerization16 (Z-15
→ E-15) and PK cyclization steps, which could only be achieved
in modest yield under strongly acidic conditions. This may mean
that the biosynthesis of the furopyrazine unit in the hyrtioserag-
amines is enzyme-mediated.
Based on the unsuccessful approach outlined in Scheme 5, a
modified strategy was pursued that focused on installing the
polar guanidine moiety after furopyrazine formation (Scheme
6). Commercially available DKP 22 underwent condensation
with the protected aminopropanal 2317 to give dehydroDKP
24. Hydrogenation of 24 followed by Boc protection gave 25
that upon condensation with 12 gave enedione 26. The
reduction of 26 to the desired furopyrazine precursor 27 led
to complex mixtures arising from indiscriminate reductionan
alternative route to the natural product was needed. Pleasingly,
when 26 was subjected to the biomimetic cyclization conditions,
furopyrazine 28 was formed. This reaction is very clean, but the
low yield is attributed to poor recovery of the product from
sulfuric acid. All that now remained was to install the guanine
side chain; removal of the phthaloyl group followed by
guanylation gave unstable product 29 bearing the complete
skeleton of hyrtioseragamine A. Swift reduction of 29 followed
With the biomimetic proposal verified on a model system,
attention turned to the natural product itself (Scheme 5).
Acylation of DKP 1718 gave 18 that underwent condensation
with glyoxal 12 to give the enedione 19. Selective removal of the
Boc group from the DKP gave 20 that upon selective reduction
of the nitro group at 0 °C gave the furopyrazine precursor 21.
Subjecting 21 to the conditions discovered during the model
study was anticipated to induce the furopyrazine formation and
remove both Boc groups from the arginine side chain to give
hyrtioseragamine A (1); however, treating 21 with sulfuric acid
led to a vast amount of degradation. Major issues were
encountered when attempting to recover any polar product(s)
from the sulfuric acid (1 contains eight heteroatoms), but the
natural product was not detected by analysis of the crude
1
by Boc cleavage gave hyrtioseragamine A (1); the H and 13C
NMR spectroscopic data for synthetic 1 were in full agreement
with that described in the isolation report.2
In summary, we have proposed a modified biosynthesis of
hyrtioseragamine A that involves debromobarretin undergoing
oxidative cleavage followed by cyclodehydration, validated by
DFT calculations. This postulate ultimately provided the basis
for a bioinspired total synthesis and structural affirmation of this
structurally unprecedented marine alkaloid. Moreover, the use
of a PK reaction to assemble the furopyrazine ring means
heterocyclic analogues of hyrtioseragamine A (e.g., pyrrolopyr-
azine and thienopyrazine) should be accessible, providing an
avenue to improve the modest antimicrobial activity displayed
by the natural product.2
1
reaction mixture by H NMR spectroscopy or mass spectrom-
EXPERIMENTAL PROCEDURES
Commercially available reagents were used throughout without
purification, unless otherwise stated. Anhydrous solvents were used
etry. For the same reasons, we did not attempt to cyclize
enedione 20 as recovery of polar products would be trouble-
some.
■
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Scheme 6. Total Synthesis of Hyrtioseragamine A (1)
as supplied. Dichloromethane, acetonitrile, and dimethylformamide
(DMF) were dried using an LC Technology Solutions Inc. SP-1 solvent
purification system under an atmosphere of dry nitrogen. All reactions
were routinely carried out in oven-dried glassware under a nitrogen
atmosphere, unless otherwise stated. Powdered 4 Å molecular sieves
were activated prior to use by heating at 250 °C under vacuum
overnight. Analytical thin layer chromatography was performed using
silica plates, and compounds were visualized at 254 and/or 360 nm
ultraviolet irradiation, followed by staining ethanolic vanillin solution.
Optical rotations were measured using a PerkinElmer 341 polarimeter
at λ = 598 nm and are given in deg dm−1 cm3 g−1. Melting points were
recorded on an electrothermal melting point apparatus and are
uncorrected. Infrared spectra were obtained using a PerkinElmer
spectrum one Fourier transform infrared spectrometer as thin films
between sodium chloride plates. Absorption maxima are expressed in
wavenumbers (cm−1). NMR spectra were recorded on a Bruker
with the aid of COSY, NOESY, ROESY, HMBC, and edited HSQC
experiments. All experiments were conducted at 298 K. Conventional
NMR tubes (5 mm diameter, Norell) using a sample volume of 500 μL
were used. High-resolution mass spectra were obtained by electrospray
ionization in positive ion mode at a nominal accelerating voltage of 70
eV on a Bruker micrOTOF-QII mass spectrometer at a nominal
accelerating voltage of 70 eV on a microTOF mass spectrometer.
(S,Z)-1-Acetyl-6-methyl-3-(2-(2-nitrophenyl)-2-
oxoethylidene)piperazine-2,5-dione (14). Potassium tert-butox-
ide [1.0 M in tetrahydrofuran (THF), 0.68 mL] was added dropwise at
0 °C to glyoxal 1213 (80 mg, 0.44 mmol) and DKP 1314 (140 mg, 0.68
mmol) in DMF (3.2 mL). The reaction mixture was stirred at 0 °C for
10 min, and then, NH4Cl (sat. aq, 20 mL) was added. The resultant
mixture was extracted with ethyl acetate (3 × 60 mL), dried (Na2SO4),
filtered, and concentrated in vacuo. Purification by column chromatog-
raphy on silica gel eluting with ethyl acetate/petroleum ether (1:1) gave
the title compound (46 mg, 0.14 mmol, 32%) as a yellow oil; IR (neat)
νmax: 1707, 1645, 1525, 1364, 1287, 1274, 1214, 1210, 750; [α]2D1 +9.7
(c 0.33, CHCl3); 1H NMR (400 MHz, CDCl3): δ 11.23 (br s, 1H), 8.11
(dd, J = 8.3, 1.1 Hz, 1H), 7.76 (dt, J = 7.6, 1.1 Hz, 1H), 7.67 (dt, J = 8.0,
1.5 Hz, 1H), 7.54 (dd, J = 7.6, 1.5 Hz, 1H), 6.69 (s, 1H), 5.12 (dd, J =
6.4, 6.4 Hz, 1H), 2.57 (s, 3H), 1.59 (d, J = 7.2 Hz, 3H); 13C{1H} NMR
(101 MHz, CDCl3): δ 192.9 (C), 171.4 (C), 166.8 (C), 158.6 (C),
146.5 (C), 137.4 (C), 136.2 (C), 134.1 (CH), 131.7 (CH), 128.4
(CH), 124.7 (CH), 105.1 (CH), 53.0 (CH), 27.1 (Me), 21.3 (Me);
HRMS (ESI) m/z: [M + Na]+ calcd for C15H13N3O6Na, 354.0697;
found, 354.0707.
1
DRX400 spectrometer operating at 400 MHz for H nuclei and 100
MHz for 13C nuclei. Chemical shifts are reported in parts per million
(ppm) relative to the tetramethylsilane (TMS) peak recorded as δ 0.00
ppm in CDCl3/TMS solvent, or the residual chloroform (δ 7.26 ppm),
acetone (δ 2.05 ppm), or dimethyl sulfoxide (DMSO) (δ 2.50 ppm)
peaks. The 13C NMR values were referenced to the residual chloroform
(δ 77.1 ppm), acetone (δ 29.9 ppm), or DMSO (δ 39.5 ppm) peaks.
1
13C NMR values are reported as chemical shift δ and assignment. H
NMR shift values are reported as chemical shift δ, relative integral,
multiplicity (s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet),
coupling constant (J in Hz) and assignment. Assignments are made
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(S,Z)-3-(2-(2-Aminophenyl)-2-oxoethylidene)-6-methylpi-
perazine-2,5-dione (15). Hydrochloric acid (conc. aq, 1 mL) was
added dropwise to DKP 14 (372 mg, 1.12 mmol) in dioxane (5 mL),
and the reaction mixture was stirred at room temperature for 4 h, after
which NaHCO3 (sat. aq, 20 mL) was added dropwise at 0 °C. The
resultant aqueous phase was extracted with ethyl acetate (3 × 30 mL),
dried (Na2SO4), filtered, and concentrated in vacuo to give the (S,Z)-3-
methyl-6-(2-(2-nitrophenyl)-2-oxoethylidene)piperazine-2,5-dione
(S1) (300 mg, 1.04 mmol, 93%) as a pale pink solid; mp 170.4−171.8
°C; IR (neat) νmax: 2923, 2854, 1707, 1643, 1525, 1457, 1341, 1232,
791, 703; [α]2D1 -1.1 (c 0.33, DMF); 1H NMR (400 MHz, (CD3)2SO): δ
11.21 (br s, 1H), 9.08 (br s, 1H), 8.12 (d, J = 7.9 Hz, 1H), 7.90 (t, J = 7.5
Hz, 1H), 7.84−7.80 (m, 2H), 6.57 (s, 1H), 4.46−4.40 (m, 1H), 1.44 (d,
J = 7.0 Hz, 3H); 13C{1H} NMR (101 MHz, (CD3)2SO): δ 192.6 (C),
167.7 (C), 155.7 (C), 146.8 (C), 142.7 (C), 134.8 (C), 134.1 (CH),
132.3 (CH), 128.7 (CH), 124.5 (CH), 100.3 (CH), 50.8 (CH), 19.4
(Me); HRMS (ESI) m/z: [M + Na]+ calcd for C13H11N3O5Na,
312.0591; found, 312.0586.
DKP S1 (20 mg, 0.070 mmol) and palladium (10% on carbon, 10
mg) in ethanol (2 mL) were stirred vigorously at 0 °C under an
atmosphere of hydrogen for 20 min. The reaction mixture was filtered
and concentrated in vacuo to give the title compound (18 mg, 0.070
mmol, quant.) as a red solid; mp 215.8−220.0 °C; IR (neat) νmax: 1693,
1619, 1454, 1383, 1228, 1161, 779, 748; [α]2D1 +1.6 (c 0.19, DMF); 1H
NMR (400 MHz, (CD3)2SO): δ 11.68 (br s, 1H), 10.06 (br s, 1H), 7.75
(d, J = 7.6 Hz, 1H), 7.40 (br s, 2H), 7.28 (t, J = 7.8 Hz, 1H), 7.02 (s,
1H), 8.80 (d, J = 8.2 Hz, 1H), 6.58 (t, J = 7.7 Hz, 1H), 4.39−4.33 (m,
1H), 1.41 (d, J = 7.0 Hz, 3H); 13C{1H} NMR (101 MHz, (CD3)2SO): δ
193.1 (C), 167.3 (C), 156.7 (C), 152.1 (C), 140.2 (C), 134.8 (CH),
130.2 (CH), 117.5 (C), 117.4 (CH), 114.9 (CH), 99.6 (CH), 50.6
(CH), 19.7 (Me); HRMS (ESI) m/z: [M + Na]+ calcd for
C13H13N3O3Na, 282.0849; found, 282.0842.
(Me), 28.2 (Me), 27.9 (Me), 24.8 (CH2); HRMS (ESI) m/z: [M + H]+
calcd for C31H43N6O11, 675.2984; found, 675.2973.
(Z)-2-(3-((E)-2,3-Bis(tert-butoxycarbonyl)guanidino)propyl)-
5-(2-(2-aminophenyl)-2-oxoethylidene)-3,6-dioxopiperazine-
1-carboxylate (21). DKP 19 (30 mg, 0.045 mmol) was stirred in
TFA/CH2Cl2 (1:1, 0.6 mL) at 0 °C for 4 h. The solvent was removed
under a stream of nitrogen, and the crude residue was dissolved in
ethanol (1 mL). Palladium (10% on carbon, 10 mg) was added, and the
resultant mixture was stirred vigorously under an atmosphere of
hydrogen at 0 °C for 2 h. The reaction mixture was filtered and washed
with ethyl acetate (20 mL) and concentrated in vacuo. Purification by
column chromatography on silica gel eluting with ethyl acetate/
petroleum ether (3:1) gave the title compound (19 mg, 0.036 mmol,
80%) as an orange solid; mp 39.0−41.3 °C; IR (neat) νmax: 3327, 2919,
2850, 1700, 1627, 1572, 1366, 1322, 1224, 1130, 748; 1H NMR (400
MHz, (CD3)2SO): δ 11.69 (br s, 1H), 11.49 (br s, 1H), 8.96 (br s, 1H),
8.34 (t, J = 6.1 Hz, 1H), 8.40 (d, J = 8.4 Hz, 1H), 7.39 (br s, 2H), 7.28 (t,
J = 7.6 Hz, 1H), 7.01 (s, 1H), 6.80 (d, J = 8.5 Hz, 1H), 7.49 (t, J = 7.5
Hz, 1H), 4.38−4.35 (m, 1H), 1.86−1.75 (m, 2H), 1.64−1.58 (m, 2H),
1.46 (s, 9H), 1.38 (s, 9H), 1 × CH2 under residual solvent; 13C{1H}
NMR (101 MHz, (CD3)2SO): δ 193.0 (C), 166.5 (C), 163.1 (C),
157.0 (C), 155.3 (C), 152.1 (C), 152.0 (C), 139.7 (C), 134.9 (CH),
130.2 (CH), 117.5 (C), 117.4 (CH), 114.9 (CH), 99.9 (CH), 82.2 (C),
78.1 (C), 54.4 (CH), 30.9 (CH2), 28.0 (Me), 27.6 (Me), 23.5 (CH2), 1
× CH2 under residual solvent; HRMS (ESI) m/z: [M + H]+
C26H37N6O7 requires, 545.2714; found, 545.2718.
(Z)-2-(3-(4-Acetyl-3,6-dioxopiperazin-2-ylidene)propyl)-
isoindoline-1,3-dione (24). Cesium carbonate (664 mg, 2.04 mmol)
was added to DKP 22 (388 mg, 1.96 mmol) and aldehyde 2317 (400
mg, 1.96 mmol) DMF (6.0 mL), and the reaction mixture stirred at
room temperature for 2 h. The reaction mixture was diluted with ethyl
acetate (50 mL) and then washed with water (50 mL) and brine (50
mL). The organic phase was dried (Na2SO4), filtered, and concentrated
in vacuo. The crude residue was washed with ethyl acetate (3 × 20 mL)
to give the title compound (367 mg, 1.08 mmol, 55%) as a colorless solid;
mp 105.5−110.0 °C; IR (neat) νmax: 1682, 1634, 1412, 1370, 1270,
1234, 990, 980, 718; 1H NMR (400 MHz, CDCl3): δ 8.84 (br s, 1H),
7.86 (dd, J = 5.3, 2.9 Hz, 2H), 7.74 (dd, J = 5.5, 3.0 Hz, 2H), 7.87 (t, J =
7.9 Hz, 1H), 4.39 (s, 2H), 3.84 (t, J = 7.2 Hz, 2H), 2.65−2.60 (m, 5H);
13C{1H} NMR (101 MHz, CDCl3): δ 172.5 (C), 168.3 (C), 163.7 (C),
159.5 (C), 134.3 (CH), 131.9 (C), 128.6 (C), 123.6 (CH), 118.4
(CH), 46.0 (CH2), 36.2 (CH2), 27.2 (Me), 25.3 (CH2); HRMS (ESI)
m/z: [M + Na]+ calcd for C17H15N3O5Na, 364.0904; found, 364.0908.
2-(3-(4-Acetyl-3,6-dioxopiperazin-2-yl)propyl)isoindoline-
1,3-dione (25). Palladium (10% on carbon, 53% water, 300 mg) and
DKP 24 (590 mg, 1.73 mmol) in ethyl acetate (6.0 mL) were stirred
vigorously under reflux on a heating mantle and an atmosphere of
hydrogen for 18 h. The reaction mixture was filtered over a pad of
Celite, washed with dichloromethane (60 mL), and concentrated in
vacuo to give 2-(3-(4-acetyl-3,6-dioxopiperazin-2-yl)propyl)-
isoindoline-1,3-dione (S2) (530 mg, 1.55 mmol, 89%) as a colorless
solid; mp 154.3−157.2 °C; IR (neat) νmax: 2924, 1699, 1681, 1364,
1257, 1214, 1018, 798, 720; 1H NMR (400 MHz, CDCl3): δ 7.90 (br s,
1H), 7.80 (dd, J = 5.4, 3.0 Hz, 2H), 7.70 (dd, J = 5.5, 3.0 Hz, 2H), 4.29
(s, 2H), 4.15−4.12 (m, 1H), 3.72 (t, J = 6.8 Hz, 2H), 2.55 (s, 3H),
1.97−1.80 (m, 4H); 13C{1H} NMR (101 MHz, CDCl3): δ 171.9 (C),
168.6 (C), 168.4 (C), 167.2 (C), 134.1 (CH), 131.9 (C), 123.3 (CH),
56.1 (CH), 45.6 (CH2), 37.0 (CH2), 30.0 (CH2), 27.2 (Me), 24.2
(CH2); HRMS (ESI) m/z: [M + Na]+ calcd for C17H17N3O5Na,
366.1066; found, 366.1068.
6-(2-Aminophenyl)-3-methylfuro[2,3-b]pyrazin-2(1H)-one
(16). DKP 15 (20 mg, 0.070 mmol) in sulfuric acid (0.1 mL) was stirred
in a capped vial at 65 °C in a sand bath for 24 h. The reaction mixture
was quenched with NaHCO3 (10 mL) at 0 °C, and the resultant
aqueous phase extracted with ethyl acetate (4 × 15 mL). The combined
organic phases were dried (Na2SO4), filtered, and concentrated in
vacuo. Purification by column chromatography on silica gel eluting with
ethyl acetate/petroleum ether (3:1) gave the title compound (8 mg,
0.032 mmol, 47%) as an orange solid; mp 106.2−108.0 °C; IR (neat)
1
νmax: 2918, 2850, 1627, 1568, 1519, 1247, 1223, 995, 738, 704; H
NMR (400 MHz, (CD3)2SO): δ 11.92 (br s, 1H), 7.57 (dd, J = 7.8, 1.5
Hz, 1H), 7.13 (ddd, J = 8.0, 7.1, 1.5 Hz, 1H), 7.15 (s, 1H), 6.85 (dd, J =
8.3, 1.2 Hz, 1H), 6.70−6.67 (m, 1H), 5.58 (br s, 2H), 2.37 (s, 3H);
13C{1H} NMR (101 MHz, (CD3)2SO): δ 157.1 (C), 155.9 (C), 147.1
(C), 146.3 (C), 141.0 (C), 130.7 (CH), 129.6 (C), 127.9 (CH), 117.3
(CH), 114.1 (CH), 113.3 (C), 100.3 (CH), 20.1 (Me); HRMS (ESI)
m/z: [M + H]+ calcd for C13H12N3O2, 242.0924; found, 242.0922.
tert-Butyl (Z)-2-(3-((E)-2,3-Bis(tert-butoxycarbonyl)-
guanidino)propyl)-5-(2-(2-nitrophenyl)-2-oxoethylidene)-3,6-
dioxopiperazine-1-carboxylate (19). Potassium tert-butoxide (1.0
M in THF, 0.11 mL) was added dropwise to glyoxal 1213 (20 mg, 0.108
mmol) and DKP 1818 (40 mg, 0.072 mmol) in DMF (0.5 mL) at −78
°C. The reaction mixture was stirred at −78 °C for 30 min, and then,
NH4Cl (sat. aq, 5 mL) was added. The resultant mixture was extracted
with ethyl acetate (3 × 20 mL), dried (Na2SO4), filtered, and
concentrated in vacuo. Purification by column chromatography on silica
gel eluting with ethyl acetate/petroleum ether (2:3) gave the title
compound (46 mg, 0.14 mmol, 33%) as a yellow solid; mp 79.8−82.3
°C; IR (neat) νmax: 1715, 1638, 1611, 1366, 1244, 1130, 1048, 774; 1H
NMR (400 MHz, CDCl3): δ 11.47 (br s, 1H), 11.22 (br s, 1H), 8.37 (t,
J = 5.6 Hz, 1H), 8.08 (d, J = 8.1 Hz, 1H), 7.74 (t, J = 8.1 Hz, 1H), 7.65
(t, J = 7.8 Hz, 1H), 7.55 (d, J = 7.6 Hz, 1H), 6.69 (s, 1H), 4.85 (dd, J =
7.5, 5.5 Hz, 1H), 3.46 (dd, J = 13.0, 6.7 Hz, 2H), 2.05−1.90 (m, 2H),
1.75−1.63 (m, 2H), 1.55 (s, 9H), 1.48 (s, 9H), 1.47 (s, 9H); 13C{1H}
NMR (101 MHz, CDCl3): δ 192.9 (C), 163.6 (C), 163.5 (C), 156.6
(C), 156.2 (C), 153.2 (C), 150.0 (C), 146.5 (C), 139.5 (C), 136.2 (C),
134.0 (CH), 131.5 (CH), 128.3 (CH), 124.6 (CH), 105.2 (CH), 85.7
(C), 83.2 (C), 79.4 (C), 58.5 (CH), 39.8 (CH2), 33.0 (CH2), 28.3
Di-tert-butyl dicarbonate (354 mg, 1.55 mmol) and DMAP (19.0
mg, 0.155 mmoL) were added to DKP S2 (530 mg, 1.55 mmol) in
acetonitrile (9.0 mL), and the resultant mixture was stirred at room
temperature for 1 h. The reaction mixture was quenched with NH4Cl
(sat. aq, 20 mL) was added and extracted with ethyl acetate (3 × 30
mL). The combined organic phases were dried (Na2SO4), filtered, and
concentrated in vacuo to give the title compound (680 mg, 1.53 mmol,
99%; 88% from 24) as a colorless foam; mp 28.5−30.5 °C; IR (neat)
1
νmax: 1779, 1702, 1366, 1269, 1248, 1142, 719; H NMR (400 MHz,
CDCl3): δ 7.83 (dd, J = 5.4, 3.0 Hz, 2H), 7.71 (dd, J = 5.4, 3.0 Hz, 2H),
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Article
5.04 (d, J = 19.1 Hz, 1H), 4.83 (t, J = 7.2 Hz, 1H), 3.94 (d, J = 18.6 Hz,
1H), 3.77−3.70 (m, 2H), 2.54 (s, 3H), 1.89−1.86 (m, 4H), 1.51 (s,
9H); 13C{1H} NMR (100 MHz, CDCl3): δ 171.2 (C), 168.3 (C), 167.7
(C), 163.8 (C), 150.0 (C), 134.1 (CH), 132.0 (C), 123.4 (CH), 85.3
(C), 60.0 (CH), 46.4 (CH2), 37.1 (CH2), 30.0 (CH2), 27.9 (Me), 27.0
(Me), 25.2 (CH2); HRMS (ESI) m/z: [M + Na]+ calcd for
C22H25N3O7Na, 466.1585; found, 466.1576.
(CH2), 29.8 (CH2), 27.1 (Me), 26.8 (Me), 25.7 (CH2), 1 × C not
visible; HRMS (ESI) m/z: [M + Na]+ calcd for C26H32N6O8Na,
579.2174; found, 579.2182.
Hyrtioseragamine A (1). Furopyrazine 29 (12 mg, 0.22 mmol)
and palladium (10% on carbon, 6 mg) in methanol (1 mL) were stirred
vigorously under an atmosphere of hydrogen for 2 h at room
temperature. The reaction mixture was filtered and washed with ethyl
acetate (20 mL), and the filtrate was concentrated in vacuo. The crude
residue was dissolved in dichloromethane (0.1 mL), and TFA (0.1 mL)
was added at 0 °C. The resultant mixture was stirred at 0 °C for 1 h, after
which the solvent was removed under a stream of nitrogen. Purification
by C18 SPE chromatography eluting with methanol/water/TFA
(30:70:0.1) gave the title compound (4 mg, 0.012 mmol, 56% over
two steps) as a pale-orange solid; IR (neat) νmax: 3188, 2921, 1661,
1623, 1473; HRMS (ESI) m/z: [M + H]+ calcd for C16H19N6O2,
tert-Butyl (Z)-2-(3-(1,3-dioxoisoindolin-2-yl)propyl)-5-(2-(2-
nitrophenyl)-2-oxoethylidene)-3,6-dioxopiperazine-1-carbox-
ylate (26). Potassium tert-butoxide (1.0 M in THF, 0.17 mL) was
added dropwise to glyoxal 1213 (30 mg, 0.17 mmol) and DKP 25 (50
mg, 0.11 mmol) in THF (0.7 mL) at −78 °C. The reaction mixture was
stirred at -78 °C for 10 min, and then, NH4Cl (sat. aq, 5 mL) was added.
The resultant mixture was extracted with ethyl acetate (3 × 20 mL),
dried (Na2SO4), filtered, and concentrated in vacuo. Purification by
column chromatography on silica gel eluting with ethyl acetate/
petroleum ether (3:1) gave the title compound (30 mg, 0.053 mmol,
31%) as a colorless foam; mp 48.2−49.8 °C; IR (neat) νmax: 1706, 1646,
1526, 1270, 1241, 1141, 718; 1H NMR (400 MHz, (CD3)2SO): δ 11.14
(br s, 1H), 8.11 (dd, J = 8.5, 1.3 Hz, 1H), 7.88−7.80 (m, 7H), 6.71 (s,
1H), 4.73 (t, J = 5.9 Hz, 1H), 3.59 (dt, J = 6.8, 2.0 Hz, 2H), 1.91 (dd, J =
15.5, 7.7 Hz, 2H), 1.72−1.60 (m, 2H), 1.41 (s, 9H); 13C{1H} NMR
(101 MHz, (CD3)2SO): δ 192.4 (C), 168.0 (C), 165.5 (C), 156.0 (C),
149.8 (C), 146.7 (C), 140.2 (C), 134.7 (C), 134.4 (CH), 134.2 (CH),
132.4 (C), 131.6 (CH), 128.9 (CH), 124.5 (CH), 123.0 (CH), 103.5
(CH), 84.4 (C), 58.2 (CH), 36.8 (CH2), 31.6 (CH2), 27.3 (Me), 23.6
(CH2); HRMS (ESI) m/z: [M + Na]+ calcd for C28H26N4O9Na,
585.1605; found, 585.1588.
(Z)-2-(3-(5-(2-(2-Aminophenyl)-2-oxoethylidene)-3,6-dioxo-
piperazin-2-yl)propyl)isoindoline-1,3-dione (28). DKP 26 (50
mg, 0.088 mmol) in sulfuric acid (0.1 mL) was stirred in a capped vial in
a sand bath at 65 °C for 1 h. The reaction mixture was cooled to room
temperature, diluted with water (20 mL), and extracted with ethyl
acetate (6 × 20 mL). The combined organic phases were dried
(Na2SO4), filtered, and concentrated in vacuo. Purification by flash
chromatography on silica gel eluting with ethyl acetate/petroleum ether
(3:1) gave the title compound (8 mg, 0.018 mmol, 20%) as a brown solid,
and residual ethyl acetate remained (NMR) despite prolonged drying
under high vacuum; mp 94.8−98.8 °C; IR (neat) νmax: 2923, 1702,
1638, 1526, 1396, 1350, 715; 1H NMR (400 MHz, (CD3)2SO): δ 12.18
(br s, 1H), 8.00−7.96 (m, 2H), 7.84−7.75 (m, 5H), 7.70 (ddd, J = 8.7,
7.4, 1.6 Hz, 1H), 7.20 (s, 1H), 3.68 (t, J = 7.2 Hz, 2H), 2.28 (t, J = 7.8
Hz, 2H), 2.05 (quin., J = 7.2 Hz, 2H); 13C{1H} NMR (101 MHz,
(CD3)2SO): δ 168.3 (C), 156.8 (C), 151.0 (C), 147.7 (2 × C), 134.7
(CH), 133.3 (CH), 132.0 (C), 131.2 (CH), 130.5 (CH), 129.1 (C),
124.8 (CH), 123.3 (CH), 122.5 (C), 103.4 (CH), 37.7 (CH2), 30.1
(CH2), 25.1 (CH2), 1 × C not visible; HRMS (ESI) m/z: [M + Na]+
calcd for C23H16N4O6Na, 467.0955; found, 467.0995.
1
327.1564; found, 327.1566; H NMR and 13C{1H} NMR see Table
S1.9
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.joc.1c00174.
■
sı
Full experimental procedures, NMR spectra for all new
compounds, and DFT calculations (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Jonathan Sperry − School of Chemical Sciences, University of
Auckland, Auckland 1010, New Zealand; orcid.org/0000-
0001-7288-3939; Email: j.sperry@auckland.ac.nz
Authors
Shi-Wei Kim − School of Chemical Sciences, University of
Auckland, Auckland 1010, New Zealand
Paul A. Hume − School of Chemical and Physical Sciences,
Victoria University of Wellington, and MacDiarmid Institute
for Advanced Materials and Nanotechnology, Wellington
6010, New Zealand
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.joc.1c00174
Author Contributions
All authors have given approval to the final version of the
manuscript.
1-(3-(6-(2-Nitrophenyl)-2-oxo-1,2-dihydrofuro[2,3-b]-
pyrazin-3-yl)propyl)-2,3-bis(tert-butoxycarbonyl)guanidine
(29). Furopyrazine 28 (30 mg, 0.068 mmol) in hydrazine hydrate (50−
60 wt%, 0.44 μL) in methanol (1.2 mL) was stirred in a capped vial in a
sand bath at 65 °C for 1 h, after which the solvent was removed under a
stream of nitrogen gas and residual hydrazine azeotropically removed
with toluene. DMAP (0.014 mL, 0.10 mmol) was added to the crude
dry powder, followed by a solution of 1,3-di-t-butylcarbamate-2-
methylisothiourea (39 mg, 0.14 mmol) in DMF (0.5 mL). The
resulting suspension was stirred at room temperature for 18 h. The
reaction mixture was dissolved in methanol (20 mL) and adsorbed onto
silica gel. Purification by column chromatography on silica gel eluting
with ethyl acetate/petroleum ether (3:2) gave the title compound (12
mg, 0.022 mmol, 32% over two steps) as a beige solid; mp 98.6−100.0
°C; IR (neat) νmax: 2923, 1720, 1613, 1532, 1365, 1130, 746; 1H NMR
(400 MHz, CD3OD): δ 7.92−7.87 (m, 2H), 7.76 (ddd, J = 15.1, 7.7, 1.5
Hz, 1H), 7.65 (ddd, J = 15.6, 7.4, 1.5 Hz, 1H), 6.93 (s, 1H), 3.48 (t, J =
6.7 Hz, 2H), 2.91 (t, J = 7.1 Hz, 2H), 2.08 (quin., J = 7.0 Hz, 2H), 1.49
(s, 9H), 1.45 (s, 9H), 3 × NH not visible; 13C{1H} NMR (101 MHz,
CD3OD): δ 163.1 (C), 157.0 (C), 156.2 (C), 152.7 (C), 151.1 (C),
149.2 (C), 148.0 (C), 147.4 (C), 132.2 (CH), 130.3 (CH), 129.7
(CH), 124.0 (CH), 122.7 (C), 101.4 (CH), 82.9 (C), 78.9 (C), 40.0
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the University of Auckland for a doctoral scholarship
(S.W.K.). All calculations were performed using the VUW High
■
Performance Computing Cluster “Rapoi”.
̅
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