Synthesis, characterization, and antibacterial activity of structurally complex 2-Acylated 2,3,1-benzodiazaborines and related compounds

Article abstract of DOI:10.1002/cbdv.201400007

A set of 2-acylated 2,3,1-benzodiazaborines and some related boron heterocycles were synthesized, characterized, and tested for antibacterial activity against Escherichia coli and Mycobacterium smegmatis. By high-field solution NMR, the heretofore unknown class of 2-acyl-1-hydroxy-2,3,1-diazaborines has been found to be able to exist in several interconvertable structural forms along a continuum comprised of an open hydrazone a, a monomeric B-hydroxy diazaborine b, and an anhydro dimer c. X-Ray crystallography of one of the anhydro dimers, 17c, revealed it to have an unprecedented structure featuring a double intramolecular O→B chelation. The crystal structure of another compound, 37, showed it to be based on a new pentacyclic B heterocycle framework. Nine compounds were found to possess activities against E. coli, and two others were active against M. smegmatis. The finding that these two contain isoniazid covalently embedded in their structures suggests that they might possibly be acting as prodrugs of this well-known antituberculosis agent in vivo.

Full text of DOI:10.1002/cbdv.201400007

CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
1381
Synthesis, Characterization, and Antibacterial Activity of Structurally
Complex 2-Acylated 2,3,1-Benzodiazaborines and Related Compounds
by Divya Kanichara), Lance Roppiyakudaa), Ewa Kosmowskaa), Michelle A. Fausta), Kim P. Trana),
Felicia Chowa), Elena Bugloa), Michael P. Groziak*a), Evan A. Sarinab), Marilyn M. Olmstead*b),
Isba Silvac), and H. Howard Xu*c)
a) Department of Chemistry and Biochemistry, California State University, East Bay, 25800 Carlos Bee
Blvd., Hayward, CA 94542-3089, USA
(phone: þ1-510-8853407; fax: þ1-510-8854675; e-mail: michael.groziak@csueastbay.edu)
b) Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, CA 95616-
5292, USA (phone: þ1-530-7526668; fax: þ1-530-7528995; e-mail: mmolmstead@ucdavis.edu)
c) Department of Biological Sciences, California State University, Los Angeles, 5151 State University
Dr., Los Angeles, CA 90032-8201, USA
(phone: þ1-323-3432188; fax: þ1-323-3436451; e-mail: hxu3@calstatela.edu)
A set of 2-acylated 2,3,1-benzodiazaborines and some related boron heterocycles were synthesized,
characterized, and tested for antibacterial activity against Escherichia coli and Mycobacterium
smegmatis. By high-field solution NMR, the heretofore unknown class of 2-acyl-1-hydroxy-2,3,1-
diazaborines has been found to be able to exist in several interconvertable structural forms along a
continuum comprised of an open hydrazone a, a monomeric B-hydroxy diazaborine b, and an anhydro
dimer c. X-Ray crystallography of one of the anhydro dimers, 17c, revealed it to have an unprecedented
structure featuring a double intramolecular O!B chelation. The crystal structure of another compound,
37, showed it to be based on a new pentacyclic B heterocycle framework. Nine compounds were found to
possess activities against E. coli, and two others were active against M. smegmatis. The finding that these
two contain isoniazid covalently embedded in their structures suggests that they might possibly be acting
as prodrugs of this well-known antituberculosis agent in vivo.
Introduction. – Although there are several drugs available to treat tuberculosis, the
Mycobacterium tuberculosis organism responsible for this disease is adept at acquiring
resistance. Additional drugs are needed to combat emergent organisms that are
resistant to at least isoniazid (¼ isonictinylhydrazine) and rifampin, thereby causing
multidrug-resistant tuberculosis (MDR TB) [1]. After 60 years, isoniazid (INH) is still
one of the most effective anti-TB drugs used today. It is now well-known to interfere
with fatty acid metabolism by inhibiting the mycobacterial FabI homolog (InhA) of
enoyl-ACP reductase (ENR), an NAD-dependent redox enzyme responsible for
catalyzing a late step in fatty acid biosynthesis. Lesser known is the fact that
sulfonylated benzo- and other ring-fused 2,3,1-diazaborine heterocycles also possess
antibacterial properties, particularly against Gram-negative organisms [2]. Early
studies had indicated that these compounds affected lipopolysaccharide biosynthesis
[3]. Subsequent structural studies [4] established that their biomacromolecular target is
in fact the same ENR enzyme inhibited by the in vivo activated form of the prodrug
INH. In these studies, diazaborine inhibitors of ENR like the sulfonylated boron
ꢀ 2014 Verlag Helvetica Chimica Acta AG, Zꢁrich

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CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
heterocycle 1 were shown to form a covalent BꢀO bond with the OH group at C(2’) of
the NAD cofactorꢂs ribose unit (Scheme 1). In turn, this borate-based bisubstrate
analog species is bound tightly yet noncovalently at the active site of ENR.
Interestingly, FabI is also the biomacromolecular target of the commonly used
broad-spectrum (bacteria, fungi, viruses) bacteriostatic germicide triclosan [5].
Scheme 1
Promising results had once been reported for a handful of benzodiazaborines
prepared and tested against M. tuberculosis [6]. To follow up on this lead, we decided to
explore the possibility that replacing the sulfonyl moiety in 1 with an acyl group would
lead to a retention of antibacterial activity via FabI inhibition in some members of the
relatively unexplored class of 2-acylated 2,3,1-diazaborines. Motivated by the continu-
ing interest in fatty acid biosynthetic targets, particularly FabI [7], for developing new
antibacterial agents [8], and by the attraction of developing B-based therapeutics in
general [9], we synthesized, characterized, and tested several new acylated diazabor-
ines. Further encouraged by the reported antimalarial activity of thiosemicarbazones
[10], and especially by the antifungal activity of B heterocycle-forming ones [11], we
also included in our investigation these types of compounds and others with a structural
basis for possible antibacterial activity.
Results and Discussion. – 2-Acylated 1-Hydroxy-2,3,1-benzodiazaborines. Results
from the preparation of the simplest compound illustrate how the isolated structural
form depended on the conditions of synthesis for many of the 2-acylated diazaborine
compounds prepared. When 2-formylbenzeneboronic acid and formic acid hydrazide
were condensed in H2O or aqueous EtOH at room temperature (Scheme 2), the first-
formed open hydrazone species a apparently underwent rapid intramolecular cycliza-
tion to produce monomer 2b, isolated in 83% yield. The structure of 2b was readily
1
determined by its H-NMR spectrum, which revealed seven signals – one of which
(from the OH) exchanged upon addition of D2O. Monomer forms like b were the most
commonly encountered ones in our previous studies of 2-alkyl-1-hydroxy-2,3,1-
benzodiazaborines [12]. By contrast, the same condensation conducted in MeCN at
reflux (828), followed by azeotropic removal of H2O by distillation, gave not 2b but its
BꢀOꢀB anhydro dimer, 2c, in 57% yield. The proclivity of 1-hydroxy-2,3,1-benzodia-
zaborines for undergoing dehydrative dimerization was first observed in the parent
heterocycle some 50 years ago [13]. The 1H-NMR spectrum of 2c in CDCl3 contained
only six signals – none of which exchanged upon addition of D2O. Interestingly, its
1H-NMR spectrum in (D6)DMSO revealed that 2c had immediately undergone
hydrolysis to 2b by reacting with the propitious moisture in this NMR solvent. Based on

CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
1383
spectral data as well as the X-ray crystal structure determined of another compound
(17c, vide infra), we now know that anhydro dimers like 2c are stabilized by
intramolecular chelation of the C¼O O-atoms to the B-centers. We had once
encountered a 2-formylbenzeneboronic acid hydrazone which formed a tridehydro
trimer (a boroxine) with two intramolecular chelations [14]. Notably, chelation to form
a borate mimics the trapping of the NAD HOꢀC(2’) group within FabI by the
antibacterial boron heterocycle 1 [4b], and highlights the electrophilic character of the
B-center in many of our compounds.
Scheme 2
Further 2-acylated 2,3,1-diazaborines were prepared by condensation of 2-formyl-
or 2-acetylbenzeneboronic acid with commercially available or readily prepared acyl
hydrazides. Depending on the substitution pattern or conditions of synthesis, products
were isolated in open a, monomer b, or anhydro dimer c form (Scheme 2). Although
exceptions were encountered, preparation in 50% aqueous EtOH at ambient temper-
ature typically favored the open a form, while those in anhydrous EtOH or MeCN at
reflux, followed by azeotropic removal of H2O by distillation, favored anhydro dimer
form c. In aqueous EtOH at 238, 2-formylbenzeneboronic acid condensed with benzoic
acid hydrazide and 4-toluic acid hydrazide to give open forms 3a and 4a, respectively.
These forms are likely intramolecularly N!B chelated as the 2-formylbenzenebor-
onic acid 2,4-dinitrophenylhydrazones we studied previously [15]. When the same
condensations were conducted in refluxing EtOH (788) or MeCN (828), however, the

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CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
anhydro dimers 3c and 4c, respectively, were produced instead, via the intermediacy of
their monomer b forms. Besides reaction conditions, the structural forms of the
compounds obtained were also found to depend on the substitution pattern of the
starting materials. With the slightly more electron-rich 2-acetylbenzeneboronic acid,
benzoic acid hydrazide and 4-toluic acid hydrazide condensed to form only anhydro
dimers 5c and 6c, respectively, even in aqueous EtOH at ambient temperature.
Similarly, simple alkanoic acid hydrazides such as acetic and isobutyric acid hydrazides
condensed with 2-formylbenzeneboronic acid to form anhydro dimers 7c and 8c
exclusively. Nitro-substituted benzoic acid hydrazide reagents, on the other hand, gave
only open forms 9a and 10a, but halogeno-substituted ones led only to anhydro dimers
11c–13c.
1H-NMR Spectroscopy was particularly useful in determining the various solution
structures. The open forms a exhibited an extremely downfield-shifted, D2O-
exchangeable NH signal at d(H) 12 and a 2-H D2O-exchangeable singlet for B(OH)2
at 8.5. Monomer forms b exhibited a sharp 1-H D2O-exchangeable OH singlet, almost
always at d(H) 8.5–9.5. Anhydro dimer forms c displayed no D2O-exchangeable peaks
in their NMR spectrum. In addition, it was noted that their C¼O stretching frequency
had shifted to a lower wavenumber in the FT-IR spectrum compared to that of open
forms a. Related by the removal or addition of H2O, forms a – c sometimes readily
interconverted simply by choice of NMR solvent. Loss (or gain) of H2O was observed
to be facile more often in (D6)DMSO than in CDCl3.
2-Formylbenzeneboronic acid condensed with semicarbazide to give B heterocycle
14b displaying a structural sensitivity to solvents like 2b. Heterocycle 14b existed as a
monomer in CDCl3 but formed an anhydro dimer (14c; not shown) in (D6)DMSO. In a
1:1 ratio with carbohydrazide, 2-formylbenzeneboronic acid condensed to form
anhydro dimer 15c. Using picolinic, nicotinic, and isonicotinic acid hydrazides as
reagents, 2-formylbenzeneboronic acid furnished 16b, 17c, and 18c. The 1H-NMR data
of 16b indicated that this picolinic acid-based heterocycle must have an unusual
structure, as its OH singlet resonance was uniquely found at a much higher than normal
field (d(H) 4.12), and it exchanged with D2O only very slowly1). Benzo-fused nicotinic
and isonicotinic 17c–19c were formed as anhydro dimers, but the thiophene-fused
versions 20a and 21a were obtained only in open form, even when prepared in MeCN at
828, possibly because of the different substituent bond vector angles off of a thiophene
ring.
Crystal Structure of 17c. The structure of anhydro dimer 17c was solved by single-
crystal X-ray diffraction using a crystal grown via slow evaporation from anhydrous
MeCN (Fig. 1 and Table 1). The molecule has a crystallographic center of inversion. Its
structure is unprecedented, but a molecule with the same central core has been
reported previously [16]. For steric reasons, the nicotinyl C¼O groups have chelated
from opposite sides of the BꢀOꢀB linkage plane, producing a doubly chelated molecule
1
)
Note added in proof: After this article was submitted, the crystal structure of 16b was determined,
revealing a zwitterion produced by the intramolecular addition of the picolinyl N-atom to the B-
atom acting as a Lewis acid. Besides explaining the slow D2O exchange of the OH in the 1H-NMR
spectrum, we note that this structure resembles that of diazaborine 1, while inhibiting ENR. Details
will be published elsewhere.

CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
1385
Fig. 1. Molecular structure of 17c (50% probability ellipsoids). Selected bond lengths [ꢃ] and angles [8]:
O(1)ꢀB(1), 1.4173(10); O(2)ꢀC(8), 1.2969(9); N(1)ꢀC(8), 1.3201(10); N(1)ꢀN(2), 1.4105(9);
N(1)ꢀB(1), 1.5960(11); N(2)ꢀC(1), 1.2884(11); C(1)ꢀC(2), 1.4620(12); C(7)ꢀB(1), 1.5871(12);
B(1)ꢀO(2A), 1.5722(11); C(8)ꢀC(9), 1.4862(11); B(1A)ꢀO(1)ꢀB(1), 111.62(9); C(8)ꢀO(2)ꢀB(1A),
121.03(6); C(8)ꢀN(1)ꢀN(2), 114.21(7); C(8)ꢀN(1)ꢀB(1), 119.98(7); N(2)ꢀN(1)ꢀB(1), 125.72(6);
C(1)ꢀN(2)ꢀN(1),
115.69(7);
N(2)ꢀC(1)ꢀC(2),
128.26(8);
C(7)ꢀC(2)ꢀC(1),
120.02(7);
C(2)ꢀC(7)ꢀB(1), 119.16(7); O(2)ꢀC(8)ꢀN(1), 121.28(7).
which is axially chiral and propeller-like in shape. Pairs of enantiomers are contained in
the unit cell. The nicotinyl C¼O bond length of 1.2969(9) ꢃ in 17c is significantly longer
than that in nicotinamide (1.22 ꢃ) [17], indicating a lowering of bond order due to the
chelation. The B-atoms are tetrahedrally substituted, and the OꢀB chelate bond length
of 1.5722(11) ꢃ is longer than the OꢀB bond of 1.4173(10) ꢃ in the boronic anhydride
linkage, but close to the NꢀB bond length of 1.5960(11) ꢃ and the CꢀB bond length of
1.5871(12) ꢃ. The nicotinyl amide CꢀN bond length of 1.3201(10) ꢃ is only slightly
shorter than that in nicotinamide (1.34 ꢃ) [17], but the amide CꢀC bond length of
1.4862(11) ꢃ is longer than that in the same reference (1.42 ꢃ). As expected, based on
the presence of an sp3-hybridized B-atom, the diazaborine ring is much more distorted
from planarity than the benzene ring. Compound 17c can be viewed as a double
zwitterion with the nicotinyl NꢀC¼O fragments delocalizing the positive charges which
counterbalance the negative borate anionic charges. The double chelation in 17c
resembles that found in the triphenylboroxin of the 1,1-dimethylhydrazone of 2-
formylbenzeneboronic acid [14].
Compounds from Sulfonyl Hydrazides and Thiosemicarbazides. To obtain refer-
ence compounds for potential bioactivity in our acylated B heterocycles, a set of
sulfonylated ones was prepared. Members of this class of compounds are much more
straightforward to characterize, and they have well-documented antibacterial proper-
ties [2e]. Besides the benchmark heterocycle 1, 22–32 were synthesized. Without
exception, these were obtained as monomeric 1-hydroxy-2,3,1-diazaborines exhibiting

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Table 1. Crystallographic Data of 17c and 37
17c
37
Empirical formula
Mr
C26H18B2N6O3
484.08
C15H10B2N4O2
299.89
Crystal color and habit
Crystal size [mm]
Diffractometer
Temp. [K]
Wavelength [ꢃ]
Crystal system
Space group
Z
colorless, parallelepiped
0.50ꢁ0.23ꢁ0.20
Bruker ApexII
90(2)
0.71073
monoclinic
C2/c
colorless, block
0.50ꢁ0.35ꢁ0.26
Bruker ApexII
90(2)
0.71073
monoclinic
C2/c
4
8
Unit cell parameters:
a [ꢃ]
25.5569(11)
5.7176(3)
19.1584(9)
128.852(2)
2180.17(18)
1.475
13.168(3)
13.741(3)
15.038(4)
102.980(3)
2651.5(11)
1.502
b [ꢃ]
c [ꢃ]
b [8]
V [ꢃ3]
Dx (calc.) [g cmꢀ3
]
Absorption coefficient [mmꢀ1
F(000)
]
0.099
1000
0.101
1232
q Range for data collection
Index ranges
2.73–30.038
ꢀ35ꢂhꢂ35,
ꢀ8ꢂkꢂ8, ꢀ26ꢂlꢂ26
15880
2.38–30.518
ꢀ18ꢂhꢂ18,
ꢀ17ꢂkꢂ19, ꢀ21ꢂlꢂ21
15519
Reflections collected
Independent reflections
3164 [Rint ¼0.0131]
4050 [Rint ¼0.0133]
Observed reflections (I>2s(I))
Completeness to q¼30.038 [%]
Absorption correction
3001
3797
99.4
99.8
semi-empirical from equivalents semi-empirical from equivalents
Max. and min. transmission
Solution method
0.980; 0.952
SHELXS-97
0.746; 0.712
SHELXS
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2s(I)]
R Indices (all data)
Largest diff. peak and hole [e ꢃꢀ3
SHELXL-97
3164/0/168
1.055
R1 ¼0.0358, wR2 ¼0.0998
R1 ¼0.0370, wR2 ¼0.1010
0.451; ꢀ0.224
SHELXL-2012
4050/0/209
1.057
R1 ¼0.0382, wR2 ¼0.1055
R1 ¼0.0397, wR2 ¼0.1075
0.483; ꢀ0.280
]
1
an OH resonance in the H-NMR spectrum at d(H) 7.3–7.4 in CDCl3, and a narrow,
medium-intensity OH stretching absorption in the FT-IR spectrum at 3500 cmꢀ1. A
small set of thiosemicarbazones was also prepared. While the thiophene-derived 33a
and 34a were obtained only in open a form, even from refluxing MeCN, the benzo-
fused versions 35b and 36b, both first prepared and solved crystallographically as
reported in [11], were obtained in monomer b form. None of the thiosemicarbazones
formed anhydro dimers, possibly because their S-atoms do not chelate as well as an O-
atom onto a B-center. Interestingly, monomer 36b was found by 1H-NMR to
spontaneously hydrolyze to its open form (36a, not shown) when dissolved in
(D6)DMSO due to reaction with propitious H2O in this solvent.

CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
1387
Pentacyclic Compounds from Carbohydrazides. Whereas, in a 1:1 ratio, 2-
formylbenzeneboronic acid and carbohydrazide had condensed to form the anhydro
dimer 15c, in a 2 :1 ratio, under dehydrating conditions they generated a new
1
compound with a very simple H-NMR spectrum exhibiting no D2O-exchangeable
resonances. This product was identified as the symmetric pentaheterocycle 37 by
various analyses including X-ray crystallography (Scheme 3). Apparently, due to an
extremely high effective concentration, the anhydro ꢄdimerizationꢂ of the two carbonyl-
linked 1-hydroxy-2,3,1-benzodiazaborines to form the central 1,3,5,2,6-oxadiazadibor-
inane ring of 37 was facile and exclusive. Similar 2 :1 condensations were used to obtain
pentacyclic compounds 38–41. Like 37, these exhibited absorptions in the UV/VIS
spectra at lmax ca. 330–350 nm, diagnostic of their extended p-conjugated ring system.
Interestingly, based on a C¼O signal at d(C) 150 in the 13C-NMR spectrum and C¼O
stretching frequency at 1750 cmꢀ1 in the FT-IR spectrum, B heterocycles 37, 39, and 41
appear to possess a rather ketone-like C¼O group.
Scheme 3
Crystal Structure of 37. The structure of the pentacyclic B heterocycle 37 was solved
by single-crystal X-ray diffraction using a crystal grown from Et2CO (Fig. 2 and
Table 1). Like 17c, 37 has an unprecedented structure. A planar molecule with two

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CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
Fig. 2. Molecular structure of 37 (50% probability ellipsoids). Selected bond engths [ꢃ] and angles [8]:
O(1)ꢀC(8), 1.2067(9); O(2)ꢀB(1), 1.3708(10); O(2)ꢀB(2), 1.3720(10); N(1)ꢀC(7), 1.2944(10);
N(1)ꢀN(2), 1.3938(9); N(2)ꢀC(8), 1.4140(10); N(2)ꢀB(1), 1.4450(11); N(3)ꢀN(4), 1.3979(9);
N(3)ꢀC(8), 1.4107(10); N(3)ꢀB(2), 1.4485(10); N(4)ꢀC(9), 1.2932(11); C(1)ꢀB(1), 1.5301(11);
C(6)ꢀC(7), 1.4495(11); C(9)ꢀC(10), 1.4528(11); C(15)ꢀB(2), 1.5296(12); B(1)ꢀO(2)ꢀB(2), 120.09(6);
C(7)ꢀN(1)ꢀN(2),
C(8)ꢀN(2)ꢀB(1),
C(8)ꢀN(3)ꢀB(2),
C(1)ꢀC(6)ꢀC(7),
O(1)ꢀC(8)ꢀN(2),
116.81(7);
124.33(7);
124.37(6);
118.41(7);
123.59(7);
N(1)ꢀN(2)ꢀC(8),
N(4)ꢀN(3)ꢀC(8),
C(9)ꢀN(4)ꢀN(3),
N(1)ꢀC(7)ꢀC(6),
N(3)ꢀC(8)ꢀN(2),
111.81(6);
111.81(6);
116.29(7);
127.64(8);
112.14(6);
N(1)ꢀN(2)ꢀB(1),
N(4)ꢀN(3)ꢀB(2),
C(6)ꢀC(1)ꢀB(1),
O(1)ꢀC(8)ꢀN(3),
O(2)ꢀB(1)ꢀN(2),
123.83(6);
123.82(7);
116.45(7);
124.27(7);
119.28(7);
O(2)ꢀB(2)ꢀN(3), 119.25(7).
internal symmetry planes of reflection and one twofold axis of rotation was anticipated,
but packing forces presumably caused it to deviate ever so slightly from that, making
the path of the ring fusions somewhat curved. Examining the six internal angles of the
central ring, the BꢀNꢀC ones are wide (1248), while the NꢀCꢀN angle is narrow
(1198), a geometry clearly connected to the short C(8)ꢀO(1) length of 1.2067(9) ꢃ.
While this C¼O is formally of a urea-type, it is more ketone-like both crystallo-
graphically and spectroscopically (NMR and FT-IR, vide supra).
Miscellaneous Compounds. 2-Formyl-4-methoxybenzeneboronic acid was con-
densed with the aldehyde reagent Purpaldꢅ in refluxing EtOH to give 42b, and with
methyl 3-hydrazinylthiophene-2-carboxylate in MeOH at room temperature to give 43.
When the latter reaction was conducted in refluxing MeCN instead, lactone 44 was
obtained. These three compounds were sought, hoping further stabilization of an NAD-
adduct, if one should form, by H-bonding or acylation.
Antibacterial Assay Results. Most of the compounds were tested for activities
against Escherichia coli and against Mycobacterium smegmatis as a surrogate for M.
tuberculosis (Table 2). Nine compounds, 1, 14b, 22, 24, 25, 28, 31, 35b, and 39, were
found inactive against the latter but active against the former with MIC values of 2–

CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
1389
Table 2. Antibacterial Activity (MIC values [mg mlꢀ1]) for Compounds Tested
Compound
E. coli
M. smegmatis
Compound
E. coli
M. smegmatis
1
2b
3c
4a
4c
32
>32
>32
>32
>32
>32
>32
16
>32
>32
>32
32
>32
4
>32
8
16
>32
>32
>32
>32
>32
>32
>32
ꢃ32
ꢃ32
>32
>32
>32
>32
4
28
29
30
31
32
33a
35b
37
38
39
40
41
42b
44
Amikacin
Isoniazid
Pyrazinamide
Triclosan
32
>32
>32
2
>32
>32
16
>32
>32
32
>32
>32
>32
>32
1
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
ꢂ0.25
2
5c
6c
14b
15c
16b
17c
18c
20a
22
23
24
25
26
2
>32
>32
>32
>32
>32
>32
>32
>32
ꢃ32
0.25
0.5
27
32 mg mlꢀ1. Of those, six, i.e., all except for 14b, 35b, and 39, are members of the known
class of antibacterial sulfonylated B heterocycles. Compound 1 had been reported
active against E. coli with an MIC value of 25 mg mlꢀ1 [2e]; we found 32 mg mlꢀ1
.
Compound 31 had been reported active against E. coli with an MIC value of 2.5 mg mlꢀ1
[2a]; we found 2 mg mlꢀ1
.
Also found active against E. coli (MIC values of 16 mg mlꢀ1) were the
carbamoylated and thiocarbamoylated B heterocycles 14b and 35b, respectively.
Semicarbazone 14b is new, but thiosemicarbazone 35b is one of those reported to
show appreciable antifungal activity [11], and so it seems to have crossover activity.
The pentacyclic B heterocycle 39 was weakly active against E. coli (MIC value of
32 mg mlꢀ1).
Although a handful were weakly active, two compounds, the anhydro dimer 18c and
the open semicarbazone 20a, were found to have good activities (MIC values of 2–4 mg
mlꢀ1) against M. smegmatis but not E. coli. Interestingly, both of these compounds
were prepared using isonicotinic acid hydrazide, the tuberculosis drug INH, thus
raising the possibility of an in situ hydrolysis which releases INH during the anti-
bacterial assay.

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CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
Conclusions. – Based on the facility with which the new 2-acylated 2,3,1-
diazaborines of this study undergo interconversion among open, monomer, and
anhydro dimer forms (i.e., a, b, and c, resp.), any one of these is potentially ultimately
responsible for activity detected in the E. coli or M. smegmatis antibacterial assays.
Moreover, although it was not observed in the chemical evaluation and character-
ization of these compounds, even a complete hydrolysis producing the starting
materials used in their synthesis must be considered. In this scenario, since INH itself is
a prodrug, a compound like 18c or 20a which releases it in vivo would be acting as a pro-
prodrug. This consideration of hydrolysis is much less likely to be warranted for the
more structure-stable pentacyclic or thiosemicarbazone-based B heterocycles, and, of
course, is known not to be applicable in the case of the well-investigated 2-sulfonylated
ones.
We thank Dr. Katy Korsmeyer and Mary Lou Stuart for their BTE-SV Program (Johnson &
Johnsonꢂs Bridge-to-Employment Silicon Valley), which supported highschool summer interns Alexey
Voronin, Sarah Jacobs, Queenie Ma, Kamal Homran, Genie Ma, Yutong Chen, Xiaohan Liu, Chongsuk
Jeong, and Hannah Nourie. M. P. G. thanks CSUEB for Faculty Support Grants and a Joan Sieber
Interdisciplinary Research Award, and the National Science Foundation for the 500-MHz FT-NMR
spectrometer (MRI grant 0420615). H. H. X. thanks the National Institutes of Health (NIGMS grant
SC3GM83686) and the Army Research Office (grant W911NF-12-1-0059). I. S. acknowledges scholar-
ship support from the NIH Minority Biomedical Research Support – Research Initiative for Scientific
Enhancement (MBRS-RISE) Program (5R25GM061331).
Experimental Part
General. Starting materials, reagents, and solvents were purchased from SigmaꢀAldrich or Fisher
Scientific (Acros). UV Spectra: Thermo Scientific NanoDrop 2000c spectrophotometer; lmax in nm. FT-
IR Spectra: PerkinElmer Spectrum Two spectrophotometer with a Pike Technologies MIRacle ATR
(attenuated total reflectance) accessory with a Ge crystal; ˜n in cmꢀ1. 1H-NMR Spectra: Varian 500 FT-
NMR spectrometer (500 MHz); d in ppm rel. to Me4Si as internal standard, J in Hz; Yt, pseudo-triplet.
13C-NMR Spectra: Varian 500 FT-NMR spectrometer (125 MHz); d in ppm rel. to solvent C-atom
resonances as internal standards. Recording of 13C-NMR spectra was sometimes limited by solubility
(often in CDCl3), and when the quadrupolar relaxation did not obscure the 13C-NMR signal of a C-atom
attached to a B-atom, this signal was assigned based on its broadened lineshape. HR-ESI-TOF-MS: The
Scripps Center for Mass Spectrometry, San Diego, CA, only for representative compounds; in m/z. These
were determined using MeOH solns., in which the interconversion of forms a–c and the formation of
MeOH adducts were facile. Elemental microanalyses: Atlantic Microlab, Inc., Norcross, GA; in %.
General Procedure. Using commercially available starting materials, a 0.5–1.0m soln. of a substituted
areneboronic acid was combined with a soln. of 1 equiv. of a substituted hydrazine in the same solvent.
For 37–41, a 2 :1 ratio of boronic acid to hydrazine was used. If the solvent was H2O or 50% aq. EtOH,
the mixture was allowed to evaporate slowly from an open beaker in the hood overnight, reducing its
volume by at least half. Often, the product precipitated during this process. It was isolated by suction
filtration, washed with a small amount of ice-cold H2O, and dried in vacuo. If the solvent was anh. EtOH
or MeCN, the mixture was heated at reflux overnight and then concentrated by reducing its volume by at
least half via azeotropic distillation, removing H2O. More anh. solvent was added, the mixture was heated
at reflux for an additional 4–24 h, and the concentration of the mixture via azeotropic distillation was
repeated. The product, which typically precipitated upon cooling to r.t. or 08, was isolated as described
above.
2-[(4-Methylphenyl)sulfonyl]-2,3,1-benzodiazaborinin-1(2H)-ol (1). Solvent: MeCN, 828. Yield:
65%. M.p. 159–1618 ([2e]: 162–1648). Weakly active against E. coli, to the same degree as reported in
[2e].

CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
1391
1-Hydroxy-2,3,1-benzodiazaborinine-2(1H)-carbaldehyde (2b). Solvent: 50% aq. EtOH, 238. Yield:
83%. M.p. 90–938, resolidifies to 2c with m.p. 194–1958. UV (MeOH): 287. FT-IR: 3420 (OH), 1700
(C¼O), 1615, 1288, 1078, 888, 760. 1H-NMR (CDCl3): 9.32 (s, CHꢀO); 8.44 (s, exchangeable, OH); 8.23
1
(d, J¼7.5, 1 H); 8.11 (s, HC¼N); 7.79 (Yt, 1 H); 7.71–7.65 (m, 2 H). H-NMR ((D6)DMSO): 10.00 (s,
CHꢀO); 8.23 (s, exchangeable, OH); 8.22 (d, J¼7.5, 1 H); 8.02 (s, CH¼N); 7.74–7.70 (m, 2 H); 7.60 (Yt,
1 H). 13C-NMR (CDCl3): 175.9 (CHꢀO); 145.0 (C¼N); 134.2; 133.1; 132.3; 130.8; 127.8. 13C-NMR
((D6)DMSO): 138.6; 135.8; 131.1; 130.3 (br., CꢀB); 128.4; 126.7. Dimerizes to 2c in a concentration-
dependent manner in CDCl3, by NMR.
1,1’-Oxybis(2,3,1-benzodiazaborinine-2(1H)-carbaldehyde) (2c). Solvent: MeCN, 828. Yield: 57%.
M.p. 193.5–195.08. UV (MeOH): 276. FT-IR: 1623, 1605 (C¼O), 1399, 1323, 1312, 876, 771, 736.
1H-NMR (CDCl3): 8.55 (s, CHꢀO); 7.97 (s, CH¼N); 7.86 (d, J¼7.0, 1 H); 7.56 (Yt, 1 H); 7.52–7.45 (m,
2 H).
{2-[(E)-(2-Benzoylhydrazinylidene)methyl]phenyl}boronic Acid (3a). Solvent: EtOH, 238. Yield:
1
99%. M.p. 200–2038. UV (MeOH): 299. FT-IR: 3299 (OH), 1643 (C¼O), 1546, 1382, 1287. H-NMR
((D6)DMSO): 11.92 (s, exchangeable, NH); 8.77 (s, HC¼N); 8.51 (s, exchangeable, B(OH)2); 7.89 (d, J¼
6.9, 2 H); 7.70–7.35 (m, 7 H). 13C-NMR ((D6)DMSO): 171.7 (C¼O); 163.3; 153.0; 149.7; 137.5; 137.3 (br.,
CꢀB); 133.4; 130.4; 129.7; 128.6; 127.8; 125.9.
(Oxydi-2,3,1-benzodiazaborinine-1,2-diyl)bis(phenylmethanone) (3c). Solvent: EtOH, 788. Yield:
81%. Solvent: MeCN, 828. Yield: 91%. M.p. 233–2378. UV (MeOH): 297. FT-IR: 1546 (C¼O), 1436,
1398, 1279. 1H-NMR (CDCl3): 8.07 (s, CH¼N); 7.98 (d, J¼7.5, 1 H); 7.74 (d, J¼7.0, 2 H); 7.54–7.58 (m,
1 H); 7.47–7.51 (m, 3 H); 7.38 (Yt, 2 H). 13C-NMR (CDCl3): 171.8; 153.0; 132.4; 132.0; 131.7; 131.5;
130.8; 130.7; 130.5; 129.9; 128.6; 128.2; 128.0; 127.7; 127.6.
(2-{(E)-[2-(4-Methylbenzoyl)hydrazinylidene]methyl}phenyl)boronic Acid (4a). Solvent: 50% aq.
EtOH, 238. Yield: 90%. M.p. 190–1938. UV (MeOH): 300. FT-IR: 3310 (OH), 1651 (C¼O), 1551, 1440,
1389, 1281, 831. 1H-NMR ((D6)DMSO, only slightly soluble): 11.90 (s, exchangeable, NH); 8.76 (s,
HC¼N); 8.52 (s, exchangeable, B(OH)2); 7.92 (d, J¼8.0, 1 H); 7.85 (d, J¼8.0, 2 H); 7.62 (d, J¼7.0, 1 H);
7.45 (Yt, 1 H); 7.38 (Yt, 1 H); 7.33 (d, J¼8.0, 2 H); 2.38 (s, Me). 13C-NMR ((D6)DMSO): 162.9 (C¼O);
149.2; 141.8; 137.4; 137.2 (br., CꢀB); 134.0; 130.5; 129.1; 129.0; 128.8; 127.7; 125.7; 21.1 (Me). Insoluble in
CDCl3. HR-ESI-TOF-MS: 283.1262 ([MþH]þ , C15H16BN2O3þ ; calc. 283.1248), 305.1089 ([MþNa]þ
,
C15H15BN2NaOþ3 ; calc. 305.1068).
(Oxydi-2,3,1-benzodiazaborinine-1,2-diyl)bis[(4-methylphenyl)methanone] (4c). Solvent: MeCN,
828. Yield: 63%. M.p. >2608. UV (MeOH): 299. FT-IR: 1700, 1584 (C¼O), 1549, 1438. 1H-NMR
(CDCl3): 8.06 (s, HC¼N); 7.97 (d, J¼7.5, 1 H); 7.64 (d, J¼8.5, 2 H); 7.53–7.57 (m, 1 H); 7.47 (d, J¼3.5,
2 H); 7.17 (d, J¼7.5, 2 H); 2.35 (s, Me). 1H-NMR ((D6)DMSO): 8.28 (s, CH¼N); 7.74 (d, J¼5.9, 1 H);
7.66 (d, J¼8.0, 1 H); 7.59 (d, J¼8.5, 2 H); 7.53–7.57 (m, 2 H); 7.28 (d, J¼8.5, 2 H); 2.34 (s, Me).
13C-NMR ((D6)DMSO): 171.0 (C¼O); 152.7 (C¼N); 143.2; 131.5; 131.4; 130.4; 130.2; 128.6; 128.5; 128.4;
126.6; 21.2 (Me). HR-ESI-TOF-MS: 511.2123 ([4cþH]þ , C30H25B2N4O3þ ; calc. 511.2107), 265.1161
([4bþH]þ , C15H14BN2Oþ2 ; calc. 265.1143).
[Oxybis(4-methyl-2,3,1-benzodiazaborinine-1,2-diyl)]bis(phenylmethanone) (5c). Solvent: 50% aq.
EtOH, 238. Yield: 95%. M.p. 248–2508. UV (MeOH): 297. FT-IR: 1587, 1560, 1546, 1451, 1427, 1283, 798,
731. 1H-NMR ((D6)DMSO): 7.79 (dd, J¼5.0, 3.5, 1 H); 7.75 (dd, J¼5.0, 3.5, 1 H); 7.68 (d, J¼8.0, 2 H);
7.60 (Yt, 1 H); 7.55 (Yt, 1 H); 7.47 (Yt, 1 H); 2.53 (s, Me). 13C-NMR ((D6)DMSO, 408): 169.6 (C¼O);
158.1 (C¼N); 140.4 (br., CꢀB); 132.4; 131.1; 130.9; 130.8; 130.1; 129.5; 128.3; 127.7; 126.1; 20.7 (Me).
[Oxybis(4-methyl-2,3,1-benzodiazaborinine-1,2-diyl)]bis[(4-methylphenyl)methanone] (6c). Sol-
vent: 50% aq. EtOH, 238. Yield: 96%. M.p. >2608. UV (MeOH): 297. FT-IR: 1581, 1544, 1510, 1428,
1283, 805, 783, 744. 1H-NMR (CDCl3): 7.78 (d, J¼3.0, 1 H); 7.74 (d, J¼3.0, 1 H); 7.58 (d, J¼8.0, 2 H);
7.52–7.56 (m, 2 H); 7.27 (d, J¼8.0, 2 H); 2.54 (s, Me); 2.35 (s, Me). 13C-NMR (CDCl3): 169.8 (C¼O);
158.0; 142.9; 132.1; 131.5; 131.3; 130.7; 128.2; 127.0; 125.6; 21.6; 21.1.
1,1’-(Oxydi-2,3,1-benzodiazaborinine-1,2-diyl)diethanone (7c). Solvent: 50% aq. EtOH, 238. Yield:
99%. M.p. 216–2188. UV (MeOH): 290. FT-IR: 1574, 1555, 1476, 884. 1H-NMR (CDCl3): 7.92 (s,
CH¼N); 7.86 (d, J¼7.5, 1 H); 7.53 (Yt, 1 H); 7.47–7.43 (m, 2 H); 1.63 (s, Me). 1H-NMR ((D6)DMSO):
8.15 (s, CH¼N); 7.63 (dd, J¼6.5, 2.5, 1 H); 7.60 (dd, J¼6.5, 2.5, 1 H); 7.54–7.48 (m, 2 H); 2.39 (s, Me).
13C-NMR (CDCl3): 177.1 (C¼O); 151.7; 131.8; 130.8; 130.4; 128.5; 128.3; 19.0.

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1,1’-(Oxydi-2,3,1-benzodiazaborinine-1,2-diyl)bis(2-methylpropan-1-one) (8c). Solvent: MeCN, 828.
Yield: 72%. M.p. 176–1788. UV (MeOH): 290. FT-IR: 1567 (C¼O), 1551, 1479, 1438, 1086, 864.
1H-NMR (CDCl3): 7.90 (s, CH¼N); 7.82 (d, J¼7.3, 1 H); 7.50 (Yt, 1 H); 7.46–7.40 (m, 2 H); 3.88 (sept.,
J¼6.8, Me2CH); 1.11 (d, J¼6.8, Me); 1.05 (d, J¼6.8, Me). 13C-NMR (CDCl3): 182.5 (C¼O); 151.3;
131.5; 130.6; 128.4; 28.7; 18.7; 18.3.
(2-{(E)-[2-(4-Methyl-3-nitrobenzoyl)hydrazinylidene]methyl}phenyl)boronic Acid (9a). Solvent:
EtOH, 788. Yield: 96%. M.p. 222–2248. UV (MeOH): 304. FT-IR: 3230 (OH), 3064 (NH), 1650
(C¼O), 1524, 1344, 1311. 1H-NMR ((D6)DMSO): 12.17 (br. s, exchangeable, NH); 8.80 (s, CH¼N); 8.56
(d, J¼1.5, 1 H); 8.52 (s, exchangeable, B(OH)2); 8.19 (dd, J¼1.5, 8.3, 1 H); 7.96 (d, J¼7.8, 1 H); 7.69 (d,
J¼8.3, 1 H); 7.62 (d, J¼8.3, 1 H); 7.46 (Yt, 1 H); 7.40 (Yt, 1 H); 2.60 (s, Me). 13C-NMR ((D6)DMSO):
160.9 (C¼O); 150.1; 148.8; 137.2; 136.6; 133.9; 133.2; 132.2; 129.2; 129.0; 125.5; 123.6; 56.1.
(2-{(E)-[2-(4-Nitrobenzoyl)hydrazinylidene]methyl}phenyl)boronic Acid (10a). Solvent: EtOH,
238. Yield: 81%. M.p. 205–2088. UV (MeOH): 308. FT-IR: 3232 (OH), 1650 (C¼O), 1602, 1563, 1518,
1346, 1327, 1290. 1H-NMR ((D6)DMSO): 12.25 (s, exchangeable, NH); 8.82 (s, HC¼N); 8.51 (s,
exchangeable, B(OH)2); 8.37 (d, J¼8.5, 2 H); 8.17 (d, J¼9.0, 2 H); 7.97 (d, J¼8.0, 1 H); 7.62 (d, J¼7.5,
1 H); 7.46 (Yt, 1 H); 7.41 (Yt, 1 H). 13C-NMR ((D6)DMSO): 161.5 (C¼O); 150.5; 149.3; 139.1; 137.1;
133.8; 129.3; 125.5; 123.6.
(Oxydi-2,3,1-benzodiazaborinine-1,2-diyl)bis[(4-fluorophenyl)methanone] (11c). Solvent: MeCN,
828. Yield: 82%. M.p. >2258. UV (MeOH): 299. FT-IR: 1603 (C¼O), 1566, 1553, 1452, 1441, 1230.
1H-NMR ((D6)DMSO): 8.29 (s, CH¼N); 7.81 (Yt, 2 H); 7.75 (d, J¼6.5, 1 H); 7.66 (d, J¼6.5, 1 H); 7.60–
7.52 (m, 2 H); 7.31 (Yt, 2 H). 13C-NMR ((D6)DMSO): 170.3; 163.3; 152.9; 139.0 (br., CꢀB); 134.3; 131.6;
130.3; 128.7; 126.0; 115.1.
(Oxydi-2,3,1-benzodiazaborinine-1,2-diyl)bis[(4-chlorophenyl)methanone] (12c). Solvent: EtOH,
238. Yield: 53%. M.p. >2408. UV (MeOH): 304. FT-IR: 1585, 1551, 1451, 1439, 1091, 1017, 878, 839, 817,
781, 759, 750, 729. 1H-NMR (CDCl3): 8.06 (s, CH¼N); 7.95 (d, J¼7.5, 1 H); 7.72 (d, J¼9.0, 2 H); 7.55–
7.59 (m, 1 H); 7.48–7.72 (m, 1 H); 7.36 (d, J¼9.0, 1 H). 13C-NMR (CDCl3): 170.6 (C¼O); 153.3 (C¼N);
133.2; 132.1; 131.7; 130.7; 130.4; 128.7; 128.3; 128.0. Insoluble in (D6)DMSO.
(Oxydi-2,3,1-benzodiazaborinine-1,2-diyl)bis[(2-chlorophenyl)methanone] (13c). Solvent: MeCN,
828. Yield: 71%. M.p. >2308. UV (MeOH): 295. FT-IR: 1599 (C¼O), 1570, 1434, 1053, 877, 772.
1H-NMR (CDCl3): 8.01 (d, J¼7.5, 1 H); 7.90 (s, HC¼N); 7.47 (Yt, 1 H); 7.48–7.30 (m, 6 H). 13C-NMR
(CDCl3): 172.9 (C¼O); 153.3; 131.9; 131.4; 130.5; 129.9; 129.4; 129.0; 128.6; 126.7; 126.1.
1-Hydroxy-2,3,1-benzodiazaborinine-2(1H)-carboxamide (14b). Solvent: MeCN, 828. Yield: 87%.
M.p. >2558. UV (MeOH): 304. FT-IR: 3416 (OH), 3193 (NH), 1687 (C¼O), 1417, 1323, 1298, 1154, 892,
811, 742. 1H-NMR (CDCl3): 9.31 (s, exchangeable, OH); 8.24 (d, J¼7.0, 1 H); 7.96 (s, CH¼N); 7.74 (Yt,
1 H); 7.68–7.63 (m, 2 H); 7.44 (br. s, exchangeable, NH); 5.17 (br. s, exchangeable, NH). Immediately
forms anhydro dimer 14c, releasing H2O, when dissolved in (D6)DMSO, by NMR: 1H-NMR
((D6)DMSO): 7.84 (s, exchangeable, NH); 7.83 (s, CH¼N); 7.78 (s, exchangeable, NH); 7.62 (d, J¼7.0,
1 H); 7.48 (d, J¼7.0, 1 H); 7.46–7.40 (m, 2 H). 13C-NMR ((D6)DMSO): 160.7 (C¼O); 145.5 (C¼N); 135
(br., CꢀB); 131.2; 130.2; 130.0; 127.7; 127.3. Attempts to obtain 14c via crystallization from MeCN gave
only 14b, likely due to the presence of propitious H2O. Active against E. coli.
1,1’-Oxybis(2,3,1-benzodiazaborinine-2(1H)-carbohydrazide) (15c). Solvent: MeCN, 828. Yield:
64%. M.p. 204–2058. UV (MeOH): 294, 326. FT-IR: 3343 (NH), 1699 (C¼O), 1556, 1416, 1282, 782.
1H-NMR (CDCl3, only slightly soluble): 8.59 (s, CH¼N); 8.56 (d, J¼8.0, 1 H); 7.93 (Yt, 1 H); 7.90–7.80
(m, 3 H); 6.40 (br. s, exchangeable, NH); 3.68 (br. s, exchangeable, NH2). 1H-NMR ((D6)DMSO): 8.14
(s, HC¼N); 8.13 (d, J¼8.0, 1 H); 7.70–7.61 (m, 3 H); 7.47 (br. s, exchangeable, NH); 5.20 (br. s,
exchangeable, NH2). 13C-NMR ((D6)DMSO): 159.9 (C¼O); 154.4 (C¼N); 143.4; 133.4; 130.5; 130.3 (br.,
CꢀB); 129.7; 126.6.
(1-Hydroxy-2,3,1-benzodiazaborinin-2(1H)-yl)(pyridin-2-yl)methanone (16b). Picolinic acid hydra-
zide was prepared from the ethyl ester as described in [18]. Solvent: 50% aq. EtOH, 238. Yield: 82%.
M.p. >2608. FT-IR: 3436 (OH), 1680 (C¼O), 1622, 1364, 1107, 1075, 1056, 846, 778. UV (MeOH): 292,
1
338. UV (MeOH, pH 2): 291, 340. UV (MeOH, pH 12): 299. H-NMR ((D6)DMSO): 9.37 (d, J¼5.4,
1 H); 8.55 (Yt, 1 H); 8.21 (d, J¼7.8, 1 H); 8.11 (Yt, 1 H); 8.05 (s, CH¼N); 7.80 (d, J¼7.3, 1 H); 7.48 (d,
J¼7.3, 1 H); 7.41–7.45 (m, 2 H); 4.12 (s, exchangeable very slowly, OH). 13C-NMR ((D6)DMSO): 157.6;

CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
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153.5; 146.9; 144.8; 143.5; 143 (br., CꢀB); 131.9; 130.4; 129.0; 128.9; 128.3; 127.4; 121.7. Insoluble in
CDCl3.
(Oxydi-2,3,1-benzodiazaborinine-1,2-diyl)bis(pyridin-3-ylmethanone) (17c). Solvent: MeCN, 828.
Yield: 72%. M.p. 274–2758 (dec.). UV (MeOH): 299. UV (MeOH, pH 2): 299. UV (MeOH, pH 12):
1
299. FT-IR: 1622, 1595, 1554, 1407. H-NMR ((D6)DMSO): 8.90 (d, J¼1.0, 1 H); 8.72 (d, J¼4.9, 1 H);
8.27 (s, CH¼N); 8.10 (d, J¼8.0, 1 H); 7.75 (d, J¼6.5, 1 H); 7.66 (d, J¼7.5, 1 H); 7.58–7.49 (m, 3 H).
13C-NMR ((D6)DMSO): 170.4 (C¼O); 153.0; 152.5; 151.2; 138.7; 131.7; 130.3; 128.8; 126.2; 123.0. HR-
ESI-TOF-MS: 270.1074 ([MþH]þ , C13H13BN3O3þ ; calc. 270.1044). X-Ray crystal structure determined.
(Oxydi-2,3,1-benzodiazaborinine-1,2-diyl)bis(pyridin-4-ylmethanone) (18c). Solvent: MeCN, 828.
Yield: 69%. M.p. 276–2788 (dec.). UV (MeOH): 304. UV (MeOH, pH 2): 303. UV (MeOH, pH 12):
297. FT-IR: 1682, 1604, 1580, 1408. 1H-NMR ((D6)DMSO): 8.73 (d, J¼5.1, 2 H); 8.24 (s, HC¼N); 7.76 (d,
J¼6.8, 1 H); 7.69–7.64 (m, 3 H); 7.60–7.53 (m, 2 H). 13C-NMR ((D6)DMSO): 170.8 (C¼O); 153.1; 149.5;
137.5; 131.8; 130.4; 130.2; 129.0; 128.9; 124.1. Active against M. smegmatis.
[Oxybis(4-methyl-2,3,1-benzodiazaborinine-1,2-diyl)]bis(pyridin-4-ylmethanone) (19c). Solvent:
50% aq. EtOH, 238. Yield: 78%. M.p. >2458. UV (MeOH): 283. FT-IR: 1580, 1540, 1433, 1284.
1H-NMR (CDCl3, only slightly soluble): 8.77 (d, J¼8.7, 1 H); 7.96 (d, J¼8.3, 1 H); 7.70 (d, J¼7.8, 1 H);
7.63 (d, J¼6.1, 2 H); 7.58 (Yt, 1 H); 7.51 (Yt, 1 H); 2.58 (s, Me). 1H-NMR ((D6)DMSO): 8.73 (d, J¼6.0,
2 H); 7.79 (d, J¼8.5, 1 H); 7.76 (d, J¼6.0, 1 H); 7.66 (d, J¼6.0, 2 H); 7.56 (Yt, 1 H); 7.56 (Yt, 1 H); 2.50
(s, Me).
(2-{(E)-[2-(Pyridin-4-ylcarbonyl)hydrazinylidene]methyl}thiophen-3-yl)boronic Acid (20a). Sol-
vent: MeCN, 828. Yield: 95%. Solvent: 50% aq. EtOH, 238. Yield: 79%. M.p. 205–2068. UV (MeOH):
329. FT-IR: 3313 (OH), 3009 (NH), 1682 (C¼O), 1579, 1394, 1374, 1290, 697, 685. 1H-NMR
((D6)DMSO): 12.21 (s, exchangeable, NH); 9.14 (s, CH¼N); 8.77 (d, J¼4.5, 2 H); 8.47 (s, exchangeable,
B(OH)2); 7.81 (d, J¼4.5, 2 H); 7.61 (d, J¼4.6, 1 H); 7.44 (s, J¼4.6, 1 H). 13C-NMR ((D6)DMSO): 161.5
(C¼O); 150.3 (HC¼N); 146.2; 145.8; 140.5; 139.7 (br., CꢀB); 134.4; 127.8; 121.6. HR-ESI-TOF-MS:
258.0501 ([MþH]þ , C11H9BN3O2Sþ ; calc. 258.0503), 276.0634 ([MþH]þ , C11H11BN3O3Sþ ; calc.
276.0609). Active against M. smegmatis.
(2-{(E)-[2-(Pyridin-3-ylcarbonyl)hydrazinylidene]methyl}thiophen-3-yl)boronic Acid (21a). Sol-
vent: EtOH, 238. Yield: 76%. M.p. 199–2038. UV (MeOH): 327. FT-IR: 3258 (OH), 3074, 1648
1
(C¼O), 1556, 1411, 1387. H-NMR ((D6)DMSO): 12.15 (s, exchangeable, NH); 9.12 (s, 1 H); 9.06 (s,
CH¼N); 8.76 (d, J¼6.0, 1 H); 8.47 (s, exchangeable, B(OH)2); 8.26 (d, J¼8.5, 1 H); 7.60 (d, J¼5.0, 1 H);
7.54–7.58 (m, 1 H); 7.44 (d, J¼5.0, 1 H). 13C-NMR ((D6)DMSO): 161.6 (C¼O); 152.3; 148.7; 146.3;
145.2; 139.3 (br., CꢀB); 135.5; 134.3; 129.2; 127.6; 123.6.
2-(Phenylsulfonyl)-2,3,1-benzodiazaborinin-1(2H)-ol (22). Solvent: EtOH, 788. Yield: 73%. M.p.
1
160–1618. UV (MeOH): 292. FT-IR: 3509 (OH), 1548, 1446, 1284. H-NMR (CDCl3): 8.23 (d, J¼7.5,
1 H); 8.02–8.06 (m, 3 H); 7.73 (Yt, 1 H); 7.67 (Yt, 1 H); 7.64 (Yt, 1 H); 7.55–7.59 (m, 3 H); 7.42 (s,
exchangeable, OH). 13C-NMR (CDCl3): 143.8; 137.7; 134.0; 133.9; 132.8; 132.1; 130.8; 129.1; 128.3; 127.6.
Active against E. coli.
2-[(2,4,6-Trimethylphenyl)sulfonyl]-2,3,1-benzodiazaborinin-1(2H)-ol (23). Solvent: EtOH, 788.
Yield: 76%. M.p. 151–1528. UV (MeOH): 301. FT-IR: 3521 (OH), 1402, 1336, 1289, 985, 673. 1H-NMR
(CDCl3): 8.24 (d, J¼6.0, 1 H); 7.95 (s, CH¼N); 7.73 (Yt, 1 H); 7.67 (Yt, 1 H); 7.56 (d, J¼8.0, 1 H); 7.40 (s,
exchangeable, OH); 7.26 (s, 1 H); 6.99 (s, 2 H); 2.67 (s, 2 o-Me); 2.31 (s, p-Me). 13C-NMR (CDCl3): 143.9
(C¼O); 143.1; 141.0; 134.0; 132.7; 132.1; 132.0; 131.8; 130.6; 127.5; 22.9; 21.1.
6-Methoxy-2-(phenylsulfonyl)-2,3,1-benzodiazaborinin-1(2H)-ol (24). Solvent: MeCN, 828. Yield:
71%. M.p. 170–1728. UV (MeOH): 287. FT-IR: 3508 (OH), 1550, 1446, 1421, 1284. 1H-NMR (CDCl3):
8.13 (d, J¼8.5, 1 H); 8.03 (d, J¼7.0, 2 H); 7.96 (s, CH¼N); 7.63 (Yt, 1 H); 7.54 (Yt, 2 H); 7.29 (s,
exchangeable, OH); 7.21 (dd, J¼8.5, 2.0, 1 H); 6.98 (d, J¼2.5, 1 H); 3.89 (s, MeO). 13C-NMR (CDCl3):
163.2; 143.6; 137.9; 136.1; 134.0; 129.1; 128.2; 119.0; 110.3; 77.0; 55.5. HR-ESI-TOF-MS: 317.0771 ([Mþ
H]þ , C14H14BN2O4Sþ ; calc. 317.0762). Active against E. coli.
6-Methoxy-2-[(4-methylphenyl)sulfonyl]-2,3,1-benzodiazaborinin-1(2H)-ol (25). Solvent: EtOH,
788. Yield: 90%. M.p. 175–1778. UV (MeOH): 289. FT-IR: 3500 (OH), 1596, 1399, 1347, 1264. 1H-NMR
(CDCl3): 8.12 (d, J¼8.3, 1 H); 7.95 (s, CH¼N); 7.92 (d, J¼8.3, 2 H); 7.33 (d, J¼8.1, 2 H); 7.31
(s, exchangeable, OH); 7.21 (dd, J¼8.3, 2.4, 1 H); 6.96 (d, J¼2.4, 1 H); 3.89 (s, MeO); 2.42

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CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
(s, Me). 1H-NMR ((D6)DMSO): 8.97 (s, exchangeable, OH); 8.14 (d, J¼7.8, 1 H); 8.13 (s, HC¼N); 7.89
(d, J¼8.3, 2 H); 7.43 (d, J¼8.1, 2 H); 7.28–7.32 (m, 2 H); 3.63 (s, MeO); 2.38 (s, Me). 13C-NMR
((D6)DMSO): 162.9; 144.8; 142.5; 136.4; 134.0; 133.9; 130.0; 127.9; 119.1; 110.9; 55.7; 21.3. Anal. calc. for
C15H15BN2O4S: C 54.57, H 4.58, N 8.48; found: C 54.80, H 4.72, N 8.50. Active against E. coli.
2-[(4-Methylphenyl)sulfonyl]-2,3,1-benzodiazaborinine-1,6(2H)-diol (26). Yield: 44% (via BBr3
demethylation of 25 in CH2Cl2). M.p. 172–1748. UV (MeOH): 275. FT-IR: 3438 (OH), 1681, 1604,
1582, 1542, 1408. 1H-NMR (CDCl3): 8.67 (br. s, exchangeable, OH); 8.04 (d, J¼8.0, 1 H); 7.91 (s,
CH¼N); 7.88 (d, J¼4.0, 2 H); 7.32 (d, J¼8.5, 2 H); 7.28 (s, exchangeable, OH); 7.14 (dd, J¼8.0, 2.0, 1 H);
6.95 (d, J¼2.5, 1 H); 2.42 (s, Me). 13C-NMR (CDCl3): 161.0; 145.0; 143.6; 136.2; 134.9; 134.1; 129.7;
128.2; 119.8; 112.9; 21.6. HR-ESI-TOF-MS: 317.0770 ([MþH]þ , C14H14BN2O4Sþ ; calc. 317.0762).
6-Methoxy-2-[(2,4,6-trimethylphenyl)sulfonyl]-2,3,1-benzodiazaborinin-1(2H)-ol (27). Solvent:
EtOH, 788. Yield: 91%. M.p. 161–1628. UV (MeOH): 286. FT-IR: 3470 (OH), 1610, 1593, 1397, 1332,
1267, 1161, 990, 762, 677. 1H-NMR (CDCl3): 8.13 (d, J¼6.5, 1 H); 7.87 (s, CH¼N); 7.28 (s, exchangeable,
OH); 7.22 (dd, J¼8.5, 2.5, 1 H); 6.98 (s, 2 H); 6.95 (d, J¼2.5, 1 H); 3.90 (s, MeO); 2.67 (s, 2 o-Me); 2.32
(s, p-Me). 13C-NMR (CDCl3): 163.2 (C¼O); 143.8; 142.9; 140.9; 136.1; 134.0; 132.0; 118.8; 110.2; 55.4;
22.9; 21.1.
4-Methyl-2-(phenylsulfonyl)-2,3,1-benzodiazaborinin-1(2H)-ol (28). Solvent: 50% aq. EtOH, 238.
Yield: 88%. M.p. 149–1528. UV (MeOH): 308. FT-IR: 3530 (OH), 1400, 1294, 1248, 1150, 937. 1H-NMR
(CDCl3): 8.24 (d, J¼7.5, 1 H); 8.07 (d, J¼6.0, 2 H); 7.71–7.75 (m, 2 H); 7.62–7.66 (m, 2 H); 7.55 (Yt,
2 H); 7.36 (s, exchangeable, OH); 2.50 (s, Me). 13C-NMR (CDCl3): 148.6; 137.9; 134.6; 133.8; 132.7; 132.2;
130.3; 129.9; 128.4; 125.7; 20.5. Active against E. coli.
4-Methyl-2-[(2,4,6-trimethylphenyl)sulfonyl]-2,3,1-benzodiazaborinin-1(2H)-ol (29). Solvent:
MeCN, 828. Yield: 80%. M.p. 153–1548. UV (MeOH): 288. FT-IR: 3502 (OH), 1605, 1402, 1336,
1
1292, 1254, 1172, 1020, 933, 666. H-NMR (CDCl3): 8.24 (d, J¼7.5, 1 H); 7.69–7.72 (m, 2 H); 7.64 (Yt,
1 H); 7.32 (s, exchangeable, OH); 6.98 (s, 2 H); 2.71 (s, 2 o-Me); 2.42 (s, Me); 2.31 (s, p-Me). 13C-NMR
(CDCl3): 147.7 (C¼O); 143.6; 141.1; 134.6; 132.6; 132.3; 132.1; 131.9; 130.1; 125.6; 23.1; 21.1; 20.5.
4-Methyl-2-[(4-methylphenyl)sulfonyl]-2,3,1-benzodiazaborinin-1(2H)-ol (30). Solvent: MeCN,
828. Yield: 75%. M.p. 168–1718 ([2e]: 179–1808). UV (MeOH): 286. FT-IR: 3504 (OH), 1398, 1348,
1
1298, 1248, 1150, 939, 671. H-NMR (CDCl3): 8.22 (d, J¼7.5, 1 H); 7.94 (d, J¼8.0, 2 H); 7.59–7.63 (m,
2 H); 7.61–7.65 (m, 1 H); 7.37 (s, exchangeable, OH); 7.33 (d, J¼8.0, 2 H); 2.49 (s, Me); 2.43 (s, Me).
13C-NMR (CDCl3): 148.4 (C¼O); 144.9; 135.0; 134.6; 132.6; 132.2; 130.2; 129.5; 128.4; 125.6; 21.7; 20.5.
2-(Phenylsulfonyl)thieno[3,2-d][1,2,3]diazaborinin-1(2H)-ol (31). Solvent: MeCN, 828. Yield:
58%. M.p. 165–1678 ([2a]: 168–1728). UV (MeOH): 299. FT-IR: 3499 (OH), 1513, 1419, 1366.
1H-NMR (CDCl3): 8.14 (s, CH¼N); 8.05 (d, J¼8.5, 2 H); 7.62–7.66 (m, 3 H); 7.56 (Yt, 2 H); 7.36 (s,
exchangeable, OH). 13C-NMR (CDCl3): 144.7; 137.8; 136.5; 134.1; 130.9; 130.0; 129.1; 128.3. HR-ESI-
TOF-MS: 293.0219 ([MþH]þ
,
C11H10BN2O3Sþ2 ; calc. 293.0220), 315.0048 ([MþNa]þ
,
C11H9BN2NaO3Sþ2 ; calc. 315.0040). Active against E. coli, to the same degree as reported in [2a].
2-[(2,4,6-Trimethylphenyl)sulfonyl]thieno[3,2-d][1,2,3]diazaborinin-1(2H)-ol (32). Solvent: EtOH,
788. Yield: 40%. M.p. 158–1598. UV (MeOH): 310. FT-IR: 3499 (OH), 1420, 1359, 1334, 1254, 1151,
1001, 915, 660. 1H-NMR (CDCl3): 8.06 (s, CH¼N); 7.64 (s, 2 H); 7.39 (s, exchangeable, OH); 6.99 (s, 2 H);
2.67 (s, 2 o-Me); 2.31 (s, p-Me). 13C-NMR (CDCl3): 144.7 (C¼O); 144.1; 141.1; 135.8; 132.0; 130.5; 130.0;
23.0; 21.1.
{2-[(E)-(2-Carbamothioylhydrazinylidene)methyl]thiophen-3-yl}boronic Acid (33a). Solvent: 50%
aq. EtOH, 238. Yield: 95%. Solvent: MeCN, 828. Yield: 64%. M.p. >2508. UV (MeOH): 336. FT-IR:
3432 (OH), 3243 (NH), 3056, 1608, 1547, 1504, 1377, 1310, 1185, 1070, 686. 1H-NMR ((D6)DMSO): 11.55
(s, exchangeable, NH); 8.77 (s, CH¼N); 8.29 (s, exchangeable, B(OH)2); 8.17 (s, exchangeable, NH);
7.52 (d, J¼5.0, 1 H); 7.40 (s, exchangeable, NH); 7.35 (d, J¼5.0, 1 H). 13C-NMR ((D6)DMSO): 177.6
(C¼S); 146.1 (HC¼N); 140.2; 139.4 (br., CꢀB); 133.9; 127.1.
(2-{(E)-[2-(Phenylcarbamothioyl)hydrazinylidene]methyl}thiophen-3-yl)boronic Acid (34a). Sol-
vent: EtOH; 238. Yield: 84%. M.p. 153–1558. UV (MeOH): 346. FT-IR: 3335 (OH), 3218 (NH), 3041
(NH), 1548 (C¼S), 1531, 1391, 1310, 1201, 1039. 1H-NMR ((D6)DMSO): 11.96 (s, exchangeable, NH);
9.69 (s, exchangeable, NH); 8.89 (s, CH¼N); 8.35 (br. s, exchangeable, B(OH)2); 7.60 (d, J¼7.5, 2 H); 7.57

CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
1395
(d, J¼5.0, 1 H); 7.39 (d, J¼5.0, 1 H); 7.35 (Yt, 2 H); 7.17–7.21 (m, 1 H). 13C-NMR ((D6)DMSO): 175.4;
145.7; 140.6; 139.0; 134.0; 128.2; 127.4; 125.1; 125.0.
1-Hydroxy-2,3,1-benzodiazaborinine-2(1H)-carbothioamide (35b). Solvent: MeCN; yield: 68%.
Solvent: 50% aq. EtOH; yield: 82%. M.p. 175–1768 ([11]: 172–1748). UV (MeOH): 317. FT-IR: 3398
(OH), 3266 (NH), 3044 (NH), 1587 (C¼S), 1408, 1299, 1255, 1131, 1023, 724. 1H-NMR (CDCl3): 10.46
(s, exchangeable, OH); 8.94 (s, exchangeable, NH); 8.27 (d, J¼7.0, 1 H); 7.97 (s, CH¼N); 7.74 (Yt, 1 H);
7.69 (Yt, 1 H); 7.63 (d, J¼8.0, 1 H); 6.87 (s, exchangeable, NH). 1H-NMR ((CD3)2CO): data in
accordance with those in [11] (compound solved by X-ray). 1H-NMR ((D6)DMSO): 10.77 (s,
exchangeable, OH); 9.72 (s, exchangeable, NH); 9.56 (s, exchangeable, NH); 8.27 (s, CH¼N); 8.11 (d,
J¼7.5, 1 H); 7.89 (d, J¼8.0, 1 H); 7.84 (Yt, 1 H); 7.73 (Yt, 1 H). Forms varying amounts of 35a by
1
reacting with the H2O in (D6)DMSO, by NMR: H-NMR ((D6)DMSO): 11.55 (s, exchangeable, NH);
8.28 (s, exchangeable, NH); 8.16 (s, CH¼N); 8.12 (s, exchangeable, B(OH)2); 7.63–7.61 (m, 1 H); 7.62 (s,
exchangeable, NH); 7.37–7.34 (m, 3 H). Active against E. coli.
1-Hydroxy-N-phenyl-2,3,1-benzodiazaborinine-2(1H)-carbothioamide (36b). Solvent: 50% aq.
EtOH, 238. Yield: 75%. M.p. 149–1518 ([11]: 149–1508). UV (MeOH): 326. FT-IR: 3382 (OH), 3223
1
(NH), 3047 (NH), 2928, 1557, 1504, 1477, 1417, 1293, 1142, 751, 711, 692. H-NMR (CDCl3): 11.24 (s,
exchangeable, NH); 10.68 (s, exchangeable, OH); 8.31 (d, J¼7.0, 1 H); 8.05 (s, CH¼N); 7.76 (Yt, 1 H);
7.67–7.71 (m, 2 H); 7.47 (d, J¼7.0, 2 H); 7.45 (Yt, 2 H); 7.34 (Yt, 1 H). Immediately forms varying
amounts of 36a when dissolved in (D6)DMSO, by NMR: 1H-NMR ((D6)DMSO): 11.99 (s, exchangeable,
NH); 9.89 (s, very slowly exchangeable, NH); 8.26 (s, exchangeable, B(OH)2); 8.23 (s, CH¼N); 7.86 (d,
J¼8.0, 2 H); 7.61–7.66 (m, 1 H); 7.36–7.42 (m, 5 H, 1 exchangeable); 7.17 (Yt, 1 H); not in accordance
with data in [11]. 13C-NMR ((D6)DMSO): 175.1 (C¼S); 144.3; 139.1; 136.1; 131.8; 129.5; 128.7; 125.4;
124.0; 123.8; in accordance with data in [11] (compound solved by X-ray).
[2,3,1]Benzodiazaborinino[2’,1’:5,6][1,3,5,2,6]oxadiazadiborinino[2,3-a][2,3,1]benzodiazaborinin-
8-one (37). Solvent: MeCN, 828. Yield: 66%. M.p. >2908 (Et2CO). UV (MeOH): 287 (sh), 296, 329. FT-
IR: 1751 (C¼O), 1435, 1402, 1279, 782. 1H-NMR (CDCl3): 8.58 (s, CH¼N); 8.26 (d, J¼7.5, 2 H); 7.92 (Yt,
1
2 H); 7.87–7.82 (m, 4 H). H-NMR ((D6)DMSO): 8.31 (s, HC¼N); 8.26 (d, J¼7.5, 2 H); 7.83–7.72 (m,
6 H). 13C-NMR ((D6)DMSO): 153.8 (C¼O); 143.8 (C¼N); 133.7; 133 (br., CꢀB); 131.2; 130.5; 130.1;
127.2. X-Ray crystal structure determined.
[2,3,1]Benzodiazaborinino[2’,1’:5,6][1,3,5,2,6]oxadiazadiborinino[2,3-a][2,3,1]benzodiazaborinine-
8-thione (38). Solvent: MeCN, 828. Yield: 88%. M.p. >2608. UV (MeOH): 284, 345. FT-IR: 1484, 1422,
1
1395 (C¼S), 1226, 1127, 899, 771. H-NMR ((D6)DMSO): 8.33 (s, CH¼N); 8.17 (d, J¼6.5, 2 H); 7.75–
7.65 (m, 6 H). 13C-NMR ((D6)DMSO): 185.8 (C¼S); 145.7; 135 (br., CꢀB); 132.8; 130.6; 130.5; 130.3;
127.0.
5,11-Dimethyl[2,3,1]benzodiazaborinino[2’,1’:5,6][1,3,5,2,6]oxadiazadiborinino[2,3-a][2,3,1]benzo-
diazaborinin-8-one (39). Solvent: MeCN, 828. Yield: 82%. M.p. >2608. UV (MeOH): 288(sh), 296, 326.
FT-IR: 1748 (C¼O), 1486, 1456, 1425, 1405, 1365, 1321, 1129, 783, 732. 1H-NMR (CDCl3): 8.51 (d, J¼7.5,
2 H); 7.96 (d, J¼7.5, 2 H); 7.88 (Yt, 2 H); 7.78 (Yt, 2 H); 2.83 (s, Me). 1H-NMR ((D6)DMSO): 8.29 (d,
J¼7.5, 2 H); 7.90 (d, J¼8.0, 2 H); 7.80–7.72 (m, 4 H); 2.61 (s, Me). 13C-NMR ((D6)DMSO): 151.2
(C¼O); 133.5; 132.2; 130.2; 126.2; 21.0. Weakly active against E. coli.
5,11-Dimethyl[2,3,1]benzodiazaborinino[2’,1’:5,6][1,3,5,2,6]oxadiazadiborinino[2,3-a][2,3,1]benzo-
diazaborinine-8-thione (40). Solvent: 50% aq. EtOH, 238. Yield: 91%. M.p. >2608. UV (MeOH): 283,
345. FT-IR: 1494, 1443, 1354, 1277 (C¼S), 1018, 823, 773. 1H-NMR ((D6)DMSO): 8.16 (d, J¼6.5, 2 H);
7.79 (d, J¼7.5, 2 H); 7.65–7.68 (m, 4 H); 2.58 (s, Me). 13C-NMR ((D6)DMSO): 183.5 (C¼S); 151.7; 137.5
(br., CꢀB); 132.7; 130.6; 130.2; 125.8; 20.3. HR-ESI-TOF-MS: 345.1159 ([MþH]þ , C17H15B2N4OSþ
calc. 345.1147), 367.0992 ([MþNa]þ , C17H14B2N4NaOSþ, calc. 367.0967).
;
Bisthieno[3’,2’:4,5][1,2,3]diazaborinino[3,2-b:2’,3’-e][1,3,5,2,6]oxadiazadiborinin-7-one (41). Sol-
vent: EtOH, 238. Yield: 99%. M.p. >2608. UV (MeOH): 299, 350. FT-IR: 1755 (C¼O), 1494, 1416,
1364, 1334. 1H-NMR (CDCl3): 8.71 (s, 2 H); 7.89 (d, J¼4.8, 2 H); 7.83 (d, J¼4.8, 2 H). 1H-NMR
((D6)DMSO): 8.62 (s, 2 H); 8.05 (d, J¼4.8, 2 H); 7.73 (d, J¼4.8, 2 H). 13C-NMR ((D6)DMSO): 153.5
(C¼O); 144.2; 139.7 (br., CꢀB); 137.3; 131.9; 129.1.
2-(4-Amino-5-sulfanyl-4H-1,2,4-triazol-3-yl)-6-methoxy-2,3,1-benzodiazaborinin-1(2H)-ol (42b).
Solvent: EtOH, 788. Yield: 84%. M.p. >3008. UV (MeOH): 271. FT-IR: 3313, 3196, 1614, 1580, 1553,

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CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
1373, 1282, 1083, 956. 1H-NMR ((D6)DMSO): 13.8 (s, exchangeable, SH); 9.22 (s, exchangeable, OH);
8.24 (d, J¼8.3, 1 H); 8.16 (s, CH¼N); 7.39 (d, J¼2.0, 1 H); 7.25 (dd, J¼8.3, 2.0, 1 H); 5.44 (s,
exchangeable, NH); 3.90 (s, MeO). 13C-NMR ((D6)DMSO): 165.9; 162.2; 149.7; 141.1; 137.1; 133.7; 122
(br., CꢀB); 118.4; 110.2; 55.4 (Me). HR-ESI-TOF-MS: 291.0833 ([MþH]þ , C10H12BN6O2Sþ ; calc.
291.0830).
Methyl 3-(1-hydroxy-6-methoxy-2,3,1-benzodiazaborinin-2(1H)-yl)thiophene-2-carboxylate (43).
Solvent: MeOH, 238. Yield: 27%. M.p. 127–1298. UV (MeOH): 357. FT-IR: 1364 (OH), 1705 (C¼O),
1395, 1262, 1250. 1H-NMR ((D6)DMSO): 8.73 (s, exchangeable, OH); 8.24 (d, J¼8.5, 1 H); 8.06 (s,
HC¼N); 7.85 (d, J¼5.5, 1 H); 7.32 (s, 1 H); 7.21–7.25 (m, 2 H); 3.89 (s, MeO); 3.34 (s, MeOCO).
8-Methoxy-13H-thieno[3’,2’:4,5][1,3,2]oxazaborinino[2,3-a][2,3,1]benzodiazaborinin-13-one (44).
Solvent: MeCN, 828. Yield: 58%. M.p. 251–2538. UV (MeOH): 271. FT-IR: 1708 (C¼O), 1559, 1496,
1348, 1176, 1025, 774. 1H-NMR (CDCl3): 8.33 (s, CH¼N); 8.31 (d, J¼8.5, 1 H); 7.84 (d, J¼5.4, 1 H); 7.71
(d, J¼5.4, 1 H); 7.34 (dd, J¼8.3, 2.2, 1 H); 7.22 (d, J¼2.2, 1 H); 3.98 (s, MeO). HR-ESI-TOF-MS:
285.0509 ([MþH]þ , C13H10BN2O3Sþ ; calc. 285.0500), 307.0418 ([MþNa]þ , C13H9BN2NaO3Sþ ; calc.
307.0319).
Antimicrobial Susceptibility Testing. Minimal inhibitory concentrations (MICs) for E. coli
ATCC#25922 strain (ATCC, Manassas, VA) were determined by broth microdilution procedures based
on CLSI standards [19], with modifications [20]. To determine MICs against M. smegmatis
ATCC#700084 (ATCC, Manassas, VA), the broth microdilution method was further modified according
to an accepted protocol [21]. Briefly, the M. smegmatis stock was streaked onto sheep blood agar plates
(Hardy Diagnostics, Santa Maria, CA), which were incubated for 2 d at 358 to obtain discrete colonies. To
prepare inoculum for MIC assays, 3–5 similar colonies were taken with a sterile disposable inoculation
loop and inoculated into a 125-ml Erlenmeyer flask containing 10 ml of tryptic soy broth and incubated at
358 in a shaker/incubator for 18–24 h until its OD600 reached 0.08–0.1. Compounds were initially
dissolved in 100% DMSO and subsequently diluted into a series of two-fold dilutions using 5% DMSO.
After the inoculum with appropriate 5·105 CFU mlꢀ1 was combined with serial dilutions of compound in
microplates, the microplates were incubated at 358, and MIC values were read at 72 h. The MIC was the
lowest concentration of a given compound which prevented visible growth of M. smegmatis at the
incubation time point.
X-Ray Crystallography. Crystals suitable for data collection were mounted in the N2 cold stream
provided by Cryo Industries low-temp. apparatus on the goniometer head of a Bruker ApexII
diffractometer. Data were collected with the use of a MoKa sealed tube. A multi-scan absorption
correction was applied using the program SADABS 2008/2 [22]. The structures were solved by direct
methods (SHELXS-97) [23] and refined by full-matrix least-squares on F2 (SHELXL-97 (17c) [23] or
SHELXL-2012 (37) [24]). Anisotropic thermal parameters were used for all non-H atoms. The H-atoms
were treated as riding on their parent C-atoms. Crystal data are compiled in Table 1. CCDC-973490 and
-973491 contain the supplementary crystallographic data for this article. These data can be obtained free
of charge via www.ccdc.cam.ac.uk.
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Received January 4, 2014