Selective Methylmagnesium Chloride Mediated Acetylations of Isosorbide: A Route to Powerful Nitric Oxide Donor Furoxans

Article abstract of DOI:10.1021/acs.orglett.8b01060

Isosorbide was functionalized with furoxan for the first time to give adducts that release nitric oxide up to 7.5 times faster than the commercial vasodilator, isosorbide-5-mononitrate (Is5N). The synthesis was facilitated by MeMgCl-mediated selective acetylation of isosorbide or selective deacetylation of isosorbide-2,5-diacetate, which was rationalized in terms of a more stable 5-alkoxide magnesium salt using DFT. Isosorbide-furoxans are safer to handle than Is5N due to greater thermal stability.

Full text of DOI:10.1021/acs.orglett.8b01060

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Selective Methylmagnesium Chloride Mediated Acetylations of
Isosorbide: A Route to Powerful Nitric Oxide Donor Furoxans
Patrick Kielty,† Dennis A. Smith,† Peter Cannon,‡ Michael P. Carty,§ Michael Kennedy,†
Patrick McArdle,† Richard J. Singer,∥ and Fawaz Aldabbagh*,†,⊥
†School of Chemistry, National University of Ireland Galway, University Road, Galway, H91 TK33, Ireland
‡Avara Pharmaceutical Services, Shannon Industrial Estate, Shannon, Co. Clare, V14 FX09, Ireland
§Biochemistry, School of Natural Sciences, National University of Ireland Galway, University Road, Galway, H91 TK33, Ireland
∥Department of Chemical and Pharmaceutical Sciences, School of Life Sciences, Pharmacy & Chemistry, Kingston University,
Penrhyn Road, Kingston upon Thames, KT1 2EE, U.K.
S
* Supporting Information
ABSTRACT: Isosorbide was functionalized with furoxan for the first
time to give adducts that release nitric oxide up to 7.5 times faster than
the commercial vasodilator, isosorbide-5-mononitrate (Is5N). The
synthesis was facilitated by MeMgCl-mediated selective acetylation of
isosorbide or selective deacetylation of isosorbide-2,5-diacetate, which
was rationalized in terms of a more stable 5-alkoxide magnesium salt
using DFT. Isosorbide-furoxans are safer to handle than Is5N due to
greater thermal stability.
Scheme 1. (A) Is5N and Furoxan Drugs; (B) Protection−
Deprotection of Isosorbide 1 Allowing Selective
Functionalization with Furoxan
itric oxide (NO) is a reactive free radical with a vast array
1
in cancer,2 antimicrobial
Nof physiological functions,
processes,3 wound healing,4 and most significantly, vaso-
dilation.5 Clinically, nitrate ester drugs are used to effect
vasodilation. Nitroglycerin, pentaerythritol tetranitrate, iso-
sorbide dinitrate (IsDiN), and isosorbide-5-mononitrate
(Is5N, Scheme 1A) are prescribed to patients suffering from
angina, where released NO induces relaxation of vascular
smooth muscles (vasodilation) to reduce blood pressure.6
Nitrate esters, however, are well-known explosives,7 including
IsDiN, a particularly hazardous side product in the manufacture
of Is5N.8 Furoxans (1,2,5-oxadiazole 2-oxides) are highly
energetic molecules,9 heralded as alternative NO-donors.10−15
The Cassella-Hoechst drug, CAS 1609, is a furoxan with potent
and long-lasting vasodilation10,11 and is devoid of the tolerance
associated with nitrate ester drugs.16 Isosorbide (1) is a
sustainable feed-stock industrially produced from the double
dehydration of D-sorbitol.17 Apart from the nitrate esters, there
are few valuable derivatives of 1 due to difficulties in selective
functionalization and substitution at the 2- and 5-hydroxyl
groups.18 Herein, for the first time, furoxan is combined with
isosorbide (1), and in doing so, we have developed a simple
and selective acetylation protection−deprotection protocol for
scaffold 1 (Scheme 1B).
The two hydroxyls of 1 are nonequivalent with an exo 2-OH
and endo 5-OH having different reactivities. The more
nucleophilic nature of the 5-OH is attributed to activation via
H-bonding with the oxygen of the adjacent cycle.19 Despite the
Received: April 4, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01060
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
difference in reactivity, base-mediated acetylations and
alkylations are unreliable and low yielding, with the only
reported selective acetylation of 1 using harmful PbO in Ac2O
to give isosorbide-5-acetate (2a).20 Acetylation with Ac2O alone
gives mixtures of isosorbide acetates with 5-acetate 2a the
major product, but in low recovered yields.21 The literature
reported syntheses of isosorbide-2-acetate (2b) are inad-
equately selective with significant draw-backs. Preparations of
2b have used DCC-mediated coupling with poor atom-
economy,22 and reduced-pressure distillation from mixtures of
isosorbide-acetates.20 More specialized enzyme-mediated ace-
tylation of 1, and enzyme-mediated hydrolysis of isosorbide-
2,5-diacetate 5 have been reported.23 Inexpensive MeMgCl has
found application as a non-nucleophilic base, including in the
Roche AG industrial deprotonation of secondary amines and α-
deprotonation of nitriles.24 Herein, the selective acetylation of
the 2-OH and 5-OH of 1 was achieved by simply altering the
number of equivalents of MeMgCl, as part of a protection−
deprotection strategy that allowed facile alkylation to give
unique furoxan NO-donors in high yields.
Scheme 2. DFT of Proposed Alkoxide Intermediates from
the Reaction of Isosorbide 1 with MeMgCl
a
a
All geometry optimizations were performed using Gaussian 16W/
GaussView 6 with DFT B3LYP functional in the gas phase and a
6311G (2d,p) basis set.26 Mg and Cl atoms depicted in blue and green,
respectively. Table S2 contains model energies in hartrees.
NaH and BuLi in combination with Ac2O gave low
conversions with little selectivity toward the 2- or 5-positions
of isosorbide 1 (Table 1). MeMgCl (1.1 equiv) and Ac2O
a
+
Table 1. Optimizing the Formation of 2a and 2b
coordination of the MgCl ion was preferred at the 5-alkoxide
6a by 11.7 kcal/mol over the strained 2-alkoxide 6c. The
greater stability of 6a arises from more effective +MgCl
coordination to the oxygen atom on the adjacent ring with
steric hindrance from the isosorbide junction 3,4-hydrogens
affecting coordination in complex 6c. The energy minimum
conformation of 1 preferentially gave 2-alkoxide complex 6b,
with ethereal +MgCl coordination within the same ring, over 2-
alkoxide 6c by 2.7 kcal/mol. Moreover, +MgCl coordination to
2-alkoxide 6c was only possible using the higher energy
alternative conformer of 1. The enhanced stability of 5-alkoxide
6a over 2-alkoxide complexes 6b and 6c was reflected in shorter
Mg+···−OR(alkoxide) and Mg+···OR2(ether) bond distances
(Table S3).
b
c
c
c
base (equiv)
1 (%)
2a (%)
2b (%)
5 (%)
NaH (1.1)
47
24
10
43
6
30
40
8
15
25
13
11
12
9
BuLi (1.1)
12
2
d
MeMgCl (1.1)
MeMgCl (2.8)
MeMgCl (2.8)
MeMgCl (1.3)
76 (73)
1
45
e
f
d
−
−
82 (78)
88 (83)
d
3
a
b
See Table S1 for full list of optimization experiments. Conversion
Upon addition of >2 equiv of MeMgCl, both the 2- and 5-
OHs of 1 were deprotonated, and acetylation occurred at the
less stable, more reactive 2-alkoxide, delivering a switch in
selectivity to give 2b. DFT modeling supported the
c
d
determined by gravimetry. Conversion determined by GC. Isolated
e
f
yield. 0 °C, AcCl (1 equiv) as acetylation agent. Ac2O (2.5 equiv),
Amberlite IR-120, 2 h, 80 °C, then filtration, evaporation, and addition
of MeMgCl, THF, reflux, 8 h.
+
greater stabilization of MgCl at the 5-alkoxide in the 2,5-
dialkoxide complex with shorter bond lengths between
Mg+···−OR(alkoxide) and Mg+···OR2(ether) compared with
+MgCl coordination at the 2-position (Figure S3). Using the
principle of forming the more thermodynamically stable +MgCl
intermediate (i.e., the 5-alkoxide, analogous to 6a), a selective
monodeacetylation of isosorbide-2,5-diacetate 5 with MeMgCl
(1.3 equiv) was carried out (Table 1). The multigram synthesis
of isosorbide-2-acetate (2b) was achieved in 83% yield without
the requirement for chromatography with diacetate 5 formed in
situ from 1 using Ac2O and Amberlite IR-120 catalyst.
With the 2- or 5-positions on isosorbide effectively blocked
through acetylation, functionalization of the available hydroxyl
with furoxan was now possible. Furoxan electrophiles 7a and 7b
can be readily prepared in two steps from their respective
cinnamyl alcohols (see Supporting Information).14,15 Alkylation
of isosorbides with the furoxan bromides 7a and 7b was
mediated by Ag2O. Reaction of 5-acetate 2a with bromide 7a
yielded isosorbide-5-acetate-2-furoxan 3a in 81% yield with
furoxan attachment at the exo-position confirmed by X-ray
crystallography (Scheme 3).
provided high conversion and selectivity for acetylation at the
5-position, with 5-acetate 2a isolated in 73% yield. By simply
increasing the amount of MeMgCl (>2 equiv), a switch in
selectivity of acetylation was achieved with the isomeric 2-
acetate 2b as the major product, occurring in a ratio of 45:1
over 2a. Conversion was low, however, and replacing Ac2O
with more reactive AcCl enabled near complete conversion of 1
to 2b in 78% isolated yield, with trace levels (<0.5%) of isomer
2a detected by GC (Figure S2).
DFT modeling was used to investigate this remarkable
+
selectivity of deprotonation by assessing the stability of MgCl
complex 6a relative to 6c (Scheme 2) formed through ethereal
(THF-like)25 chelation to the adjacent ring oxygen atom in the
isosorbide substrate. DFT was carried out in the gas phase,
since solvent effects (the role of THF) would not affect the
relative energies of the observed isosorbide complexes. Two
conformations of 1 were of interest: the minimum energy
model and an alternative conformer 1.0 kcal/mol higher in
energy. Starting from the minimum energy conformer 1,
B
DOI: 10.1021/acs.orglett.8b01060
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Synthesis of Isosorbide-2-furoxan 4a and X-ray
Crystal Structure of 3a (Thermal Ellipsoids Set at 40%
Probability)
Scheme 5. Synthesis and X-ray Crystal Structure of
Isosorbide-2,5-difuroxan 8 (Thermal Ellipsoids Set at 30%
Probability)
lattice only occurred in 4c, presumably due to the isosorbide
hydroxyl being deacetylated (Figure S10 and Table S5B).
NO-release from our isosorbide furoxans was quantitatively
measured along with that of the commercial vasodilator, Is5N.
Furoxans have long been known to release NO upon reaction
with thiols, including cysteine.12 DFT calculations supported
the addition of a sulfanyl radical onto the CN bond of the
furoxan followed by expulsion of NO,27 although others give
alternative thiolate addition mechanisms,12,14 which are
energetically less favorable.27 NO-release holds a reasonable
correlation with vascular tissue relaxation in a series of
furoxans.11,12 NO is, however, never measured directly due to
very rapid oxidation in aqueous and biological fluids to nitrite
Deprotection of 5-acetate 3a through basic hydrolysis
afforded isosorbide-2-furoxan 4a in 91% yield. Similarly,
synthesis of furoxan isomer 4b from 2-acetate 2b via
isosorbide-2-acetate-5-furoxan 3b occurred in an overall 90%
yield over two steps of alkylation and deprotection (Scheme 4).
Scheme 4. Synthesis of Isosorbide-5-furoxans 4b and 4c and
X-ray Crystal Structure Depicting One of the Two Molecules
in the Asymmetric Unit Cell of 4c (Thermal Ellipsoids Set at
50% Probability)
12,14
−
−
(NO2 ) and nitrate (NO3 ).
The amount of NO-released
−
is commonly evaluated through NO2 measurement using the
Griess assay, in the presence of excess cysteine with the
presence of NO3− deemed less significant. Using this approach,
NO2− production from isosorbide-furoxans 3a, 4a-4c and 8 was
quantitatively monitored by regular sampling (Figure S4),
which indicated a 3−7.5 fold greater rate of release than Is5N
(kobs, Table 2). After 12 h a significant slowdown for most
Table 2. Extent and Rate of NO Release
−
−
−
NO
donor
NO2 produced
NO2 + NO3 produced
k
a
b
obs c
(mol %)
(mol %)
(h−1
)
3a
43.8 1.7
56.0 1.0
47.3 1.4
60.9 1.8
80.3 1.1
19.0 0.6
45.2 1.6
61.7 3.5
47.4 0.7
67.5 1.6
94.5 4.0
19.0 1.2
0.081
0.098
0.079
0.195
0.136
0.026
4a
4b
4c
8
Is5N
It is known that NO-production can be increased by
substituting the furoxan ring with electron-withdrawing groups,
such as nitrobenzene, which effectively increase susceptibility to
addition of activating thiols (such as cysteine).13,14 Thus, in a
quest to increase the activity of isosorbide-5-furoxan, the
preparation of p-nitro derivative 4c was carried out from
isosorbide-2-acetate 2b and bromide 7b (Scheme 4). p-
Nitrophenyl-substituted furoxan 4c was isolated in 85% yield
without the requirement for isolation of the intermediate
isosorbide-2-acetate-5-furoxan.
To further increase the level of NO-release, isosorbide was
functionalized with two furoxan moieties. The preparation of
isosorbide-2,5-difuroxan 8 was achieved in 77% isolated yield
by facile alkylation of 1 with 7a in the presence of Ag2O
(Scheme 5). The X-ray crystal structure of bis-adduct 8 was
obtained, and like the other two crystal structures (of 3a and
4c) provided a clear visual representation of the 2-exo and 5-
endo isosorbide attachments (Schemes 3−5). H-bonding in the
a
Determined by Griess assay after 24 h of incubation of 2.0 mM
b
solutions with excess L-cysteine at 37 °C. Samples of solutions after
24 h incubated with nitrate reductase prior to performing Griess assay.
c
−
Rate constant (kobs) for NO2 production calculated by linear
regression analysis of 0−4 h data points for 4c and 0−8 h data points
for all other compounds in Figure S4.
furoxans was observed, apart from isosorbide-2,5-difuroxan 8,
which exhibited a longer sustained NO-release. This retardation
in NO release allowed measurement of the extent of NO2
production after 24 h (Table 2). A thorough approach was used
−
−
−
with combined NO2 and NO3 measured. Nowadays this is
possible using a commercially available colorimetric assay that
measures total available NO2− after reduction of NO3− to NO2
−
by NADPH in the presence of the enzyme, nitrate reductase.28
Moreover, Table 2 shows NO3− levels were significant for some
C
DOI: 10.1021/acs.orglett.8b01060
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Accession Codes
furoxans, especially the most reactive compounds highlighting
the need for analysis.
CCDC 1830483−1830485 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
The p-nitrophenyl-substituted furoxan derivative 4c had a
significantly higher rate of NO-release, with this compound
being the most active, with 2−2.5 times greater rate of NO2
−
production compared to isosorbide-2- and 5-furoxans 4a and
4b. High concentrations of NO have been related to anticancer
activity for some furoxans.29 The acetate group of 3a appeared
to hinder NO-release with a slower rate, as well as a smaller
total amount of NO produced compared to its hydrolyzed
derivative 4a. As expected, substituting both available hydroxyls
on isosorbide with furoxan in compound 8 almost doubled the
extent of NO production, but the rate of NO-release was below
that of the most reactive monosubstituted isosorbide furoxan
4c. Isosorbide-2-furoxan 4a gave higher levels of NO-release
compared to its isomer 4b, and pharmacologically they would
be expected to behave differently as shown by the drug Is5N,
which is less potent than isosorbide-2-mononitrate (Is2N), as a
vasodilator.30
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: f.aldabbagh@kingston.ac.uk.
ORCID
Fawaz Aldabbagh: 0000-0001-8356-5258
Present Address
⊥Department of Pharmacy, School of Life Sciences, Pharmacy
and Chemistry, Kingston University, Penrhyn Road, Kingston
upon Thames, KT1 2EE, U.K.
Differential Scanning Calorimetry (DSC) was used for
assessment of thermally induced energy release,31 which allows
deductions regarding safe handling in comparison with Is5N,
which is classified as an explosive.32 The onset temperature for
thermal degradation of phenyl furoxans (4a, 4b, and 8) is less
accessible at 66−94 °C above that of Is5N (Table 3). Similarly,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Irish Research Council (IRC) for awarding
Patrick Kielty an Enterprise Partnership Postgraduate Scholar-
ship in association with Avara Pharmaceutical Services, and
MaryRose Kelleher (Avara Pharmaceutical Services, Shannon,
Co. Clare, Ireland), Mary Kennedy (SPDS, Tarbert, Co. Kerry,
Ireland), Gerard Fahy, John O’Reilly, and Seamus Collier (all of
the School of Chemistry, National University of Ireland
Galway, Ireland) for technical assistance.
a
Table 3. Exothermic Events
b
c
d
NO donor onset temp (°C)
peak temp (°C)
energy release (J/g)
4a
231.0
202.9
168.3
217.9
136.7
271.8
261.4
241.7
273.7
200.4
1122.0
1474.5
1636.9
1649.5
2735.8
4b
4c
8
Is5N
REFERENCES
■
a
DSC was performed using 1.7−4.6 mg of samples in a high pressure
(1) Carpenter, A. W.; Schoenfisch, M. H. Chem. Soc. Rev. 2012, 41,
3742−3752.
b
crucible, heating at 5 °C/min. Temperature at which the exothermic
event begins. Temperature of maximal heat release. Calculated by
integration.
c
d
(2) (a) Cheng, H.; Wang, L.; Mollica, M.; Re, A. T.; Wu, S.; Zuo, L.
Cancer Lett. 2014, 353, 1−7. (b) Rizi, B. S.; Achreja, A.; Nagrath, D.
Trends Cancer 2017, 3, 659−672.
́
(3) Jones, M. L.; Ganopolsky, J. G.; Labbe, A.; Wahl, C.; Prakash, S.
the degradation exotherms peaked 61−73 °C above that of
Is5N. The energy released from such exothermic events was
lower for the phenyl furoxans by 1086−1614 J/g. The most
active NO-releasing furoxan, p-nitrophenyl furoxan 4c, is safer
than Is5N with a significantly higher onset and peak
temperature by almost 32 and 41 °C respectively, and with
an energy release that is 1099 J/g lower.
In summary, inexpensive MeMgCl-mediated acetylation has
afforded an effective and simple protection−deprotection for
both hydroxyls of isosorbide. Subsequent functionalization with
furoxan gave powerful nitric oxide donors with vastly higher
rates and amounts of NOX produced compared to the
commercial vasodilator, Is5N. Isosorbide-furoxans are safer to
handle than Is5N. Future studies will focus on the biological
evaluation of this new class of NO-donors.
Appl. Microbiol. Biotechnol. 2010, 88, 401−407.
(4) Witte, M. B.; Barbul, A. Am. J. Surg. 2002, 183, 406−412.
(5) (a) Lundberg, J. O.; Weitzberg, E.; Gladwin, M. T. Nat. Rev. Drug
Discovery 2008, 7, 156−167. (b) Bohlen, H. G. Compr. Physiol. 2015,
5, 803−828.
(6) (a) Thatcher, G. R. J. Chem. Soc. Rev. 1998, 27, 331−337.
(b) Ignarro, L. J.; Napoli, C.; Loscalzo, J. Circ. Res. 2002, 90, 21−28.
(7) Boschan, R.; Merrow, R. T.; van Dolah, R. W. Chem. Rev. 1955,
55, 485−510.
(8) Reddy, G. O.; Rao, A. S. J. Hazard. Mater. 1992, 32, 87−104.
(9) (a) Klapotke, T. M.; Piercey, D. G.; Stierstorfer, J. Propellants,
̈
Explos., Pyrotech. 2011, 36, 160−167. (b) He, C.; Shreeve, J. M. Angew.
Chem., Int. Ed. 2016, 55, 772−775.
(10) Bohn, H.; Brendel, J.; Martorana, P. A.; Schonafinger, K. Br. J.
̈
Pharmacol. 1995, 114, 1605−1612.
(11) Gasco, A.; Fruttero, R.; Sorba, G.; Di Stilo, A.; Calvino, R. Pure
Appl. Chem. 2004, 76, 973−981.
(12) (a) Medana, C.; Ermondi, G.; Fruttero, R.; Di Stilo, A.; Ferretti,
C.; Gasco, A. J. Med. Chem. 1994, 37, 4412−4416. (b) Sorba, G.;
Medana, C.; Fruttero, R.; Cena, C.; Di Stilo, A.; Galli, U.; Gasco, A. J.
Med. Chem. 1997, 40, 463−469.
(13) Ferioli, R.; Folco, G. C.; Ferretti, C.; Gasco, A. M.; Medana, C.;
Fruttero, R.; Civelli, M.; Gasco, A. Br. J. Pharmacol. 1995, 114, 816−
820.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01060.
Detailed experimental, synthetic procedures, character-
ization data, NMR spectra, DFT, DSC and crystallo-
graphic data (PDF)
(14) Schiefer, I. T.; VandeVrede, L.; Fa’, M.; Arancio, O.; Thatcher,
G. R. J. J. Med. Chem. 2012, 55, 3076−3087.
D
DOI: 10.1021/acs.orglett.8b01060
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(15) Tang, W.; Xie, J.; Xu, S.; Lv, H.; Lin, M.; Yuan, S.; Bai, J.; Hou,
Q.; Yu, S. J. Med. Chem. 2014, 57, 7600−7612.
(16) Mayer, B.; Beretta, M. Br. J. Pharmacol. 2008, 155, 170−184.
(17) Dussenne, C.; Delaunay, T.; Wiatz, V.; Wyart, H.; Suisse, I.;
Sauthier, M. Green Chem. 2017, 19, 5332−5344.
(18) Rose, M.; Palkovits, R. ChemSusChem 2012, 5, 167−176.
(19) (a) Szeja, W. J. Chem. Soc., Chem. Commun. 1981, 215−216.
(b) Che, P.; Lu, F.; Nie, X.; Huang, Y.; Yang, Y.; Wang, F.; Xu, J.
Chem. Commun. 2015, 51, 1077−1080.
(20) (a) Stoss, P.; Merrath, P.; Schluter, G. Synthesis 1987, 1987,
̈
174−176. (b) Abenhaïm, D.; Loupy, A.; Munnier, L.; Tamion, R.;
́
Marsais, F.; Queguiner, G. Carbohydr. Res. 1994, 261, 255−266.
(21) Lavergne, A.; Moity, L.; Molinier, V.; Aubry, J.-M. RSC Adv.
2013, 3, 5997−6007.
̌
̌
(22) Cekovic, Z.; Tokic, Z. Synthesis 1989, 1989, 610−612.
́
́
(23) Seemayer, R.; Bar, N.; Schneider, M. P. Tetrahedron: Asymmetry
1992, 3, 1123−1126.
(24) Harnett, G. J.; Hayes, J.; Reents, R.; Smith, D. A.; Walsh, A. U.S.
Patent US20120071683, Sept 16, 2010.
(25) (a) Guggenberger, L. J.; Rundle, R. E. J. Am. Chem. Soc. 1968,
90, 5375−5378. (b) Peltzer, R. M.; Eisenstein, O.; Nova, A.; Cascella,
M. J. Phys. Chem. B 2017, 121, 4226−4237.
(26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G.
A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.; Bloino, J.;
Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J.
V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.;
Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson,
T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.;
Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.;
Bearpark, M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov,
V. N.; Keith, T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.;
Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.;
Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J.
Gaussian 16W, Revision A.03; Gaussian, Inc.: Wallingford, CT, 2016.
(27) Burov, O. N.; Kletskii, M. E.; Fedik, N. S.; Lisovin, A. V.;
Kurbatov, S. V. Chem. Heterocycl. Compd. 2015, 51, 951−960.
(28) Hevel, J. M.; Marletta, M. A. Nitric-oxide Synthase Assays. In
Methods in Enzymology; Packer, L., Ed.; Academic Press: San Diego,
1994; Vol. 233, pp 250−258.
(29) (a) Han, C.; Huang, Z.; Zheng, C.; Wan, L.; Zhang, L.; Peng, S.;
Ding, K.; Ji, H.; Tian, J.; Zhang, Y. J. Med. Chem. 2013, 56, 4738−
4748. (b) Liu, M.-M.; Chen, X.-Y.; Huang, Y.-Q.; Feng, P.; Guo, Y.-L.;
Yang, G.; Chen, Y. J. Med. Chem. 2014, 57, 9343−9356.
(30) Wendt, R. L. J. Pharmacol. Exp. Ther. 1972, 180, 732−742.
(31) Frurip, D. J.; Elwell, T. Process Saf. Prog. 2007, 26, 51−58.
(32) Butler, A. R.; Pearson, R. J. Vasodilators for Biological Research.
In Nitric Oxide Donors; Wang, P. G., Cai, T. B., Taniguchi, N., Eds.;
Wiley-VCH: Weinheim, 2005; pp 201−231.
E
DOI: 10.1021/acs.orglett.8b01060
Org. Lett. XXXX, XXX, XXX−XXX