Co-Catalyzed Transannulation of Pyridotriazoles with Isothiocyanates and Xanthate Esters

Article abstract of DOI:10.1021/acs.orglett.0c03099

An efficient radical transannulation reaction of pyridotriazoles with isothiocyanates and xanthate esters was developed. This method features conversion of pyridotriazoles into two N-fused heterocyclic aromatic systems-imino-thiazolopyridines and oxo-thia

Full text of DOI:10.1021/acs.orglett.0c03099

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Letter
Co-Catalyzed Transannulation of Pyridotriazoles with
Isothiocyanates and Xanthate Esters
Ziyan Zhang and Vladimir Gevorgyan*
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ABSTRACT: An efficient radical transannulation reaction of pyridotria-
zoles with isothiocyanates and xanthate esters was developed. This method
features conversion of pyridotriazoles into two N-fused heterocyclic
aromatic systemsimino-thiazolopyridines and oxo-thiazolopyridine de-
rivativesvia one-step Co(II)-catalyzed transannulation reaction proceed-
ing via a radical mechanism. The synthetic usefulness of the developed
method was illustrated in the synthesis of amino acid derivatives and further
transformations of obtained reaction products.
Scheme 1. Concepts for Transannulation of Pyridotriazoles
eteroaromatic rings are paramount motifs of drugs and
bioactive molecules.1 Thus, not surprisingly that, in drug
H
discovery research, the development of novel synthetic
approaches for the rapid construction of new heterocyclic ring
systems from easily available precursors is of central
importance.2 Recently, transition-metal-catalyzed denitrogena-
tive transformations of pyridotriazoles have been emerging as a
powerful tool for the synthesis of diverse N-heterocyclic
frameworks.3,4 These protocols take advantage of the well-
known ring−chain tautomerism of the pyridotriazole core in
solution into the corresponding diazo tautomer A, which then
can be trapped by a transition-metal catalyst to form the reactive
pyridyl metal carbene intermediate B (see Scheme 1a). First
reported in 2007,5 the transannulation reaction of pyridotria-
zoles was then quickly extended to other transannulations,6 as
well as X−H insertions,7 cyclopropanation,8 and other
reactions.9 These protocols operate through an ionic pathway,
involving transition-metal carbene intermediate B.
Recently, metalloradical catalysis employing open-shell
metalloradical complexes has been introduced as a conceptually
new strategy featuring alternative trends in reactivity and
selectivity. Of particular importance are stable metalloradicals of
cobalt(II) porphyrins ([Co(Por)]), which form strong metal−
carbon single bonds, thus behaving as carbene radical
surrogates.10 Lately, a rich chemistry of Co(II)-based metal-
loradicals toward a variety of useful synthetic transformations
has been developed.11,12 Surprisingly, engagement of this
strategy in denitrogenative transannulation reaction is scarce.13
In continuation of our studies on application of pyridotriazoles
in synthesis of nitrogen-containing heterocycles,5a−e herein, we
report a Co-catalyzed denitrogenative transannulation of
pyridotriazoles with isothiocyanates and xanthate esters toward
the synthesis of two N-heterocyclic aromatic systems proceeding
via carbene radical intermediate C (see Scheme 1b).
First, transannulation of pyridotriazole 1a and t-butyl
isothiocyanate 2a14 en route to imino-thiazolopyridine 3a was
examined in the presence of several synthesized metal-
Received: September 15, 2020
https://dx.doi.org/10.1021/acs.orglett.0c03099
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Letter
a
a
Table 1. Optimization of Reaction Conditions
Scheme 3. Scope of Isothiocyanates
b
entry
deviation from standard conditions
yield of 3a (%)
c
1
2
3
4
5
6
none
72
Co(F20TPP)
Co[(p-OMe)4TPP]
Fe(TPP)Cl
Fe(F20TPP)Cl
Ru(TPP)CO
64
48
d
0
0
d
d
0
a
b
0.05 mmol scale; 1:2 = 1:5. Determined by gas chromatography/
mass spectroscopy (GC/MS), using pentadecane as an internal
c
standard with a catalyst loading of 8 mol %. 0.2 mmol scale, isolated
d
yields. Pyridotriazole 1a was mostly recovered.
a
Scheme 2. Scope of Pyridotriazoles
a
0.2 mmol scale, isolated yields. See the Supporting Information for
experimental details. bReaction was performed in 1 mmol scale.
cToluene (0.5 M) used as a solvent.
a
0.2 mmol scale, isolated yields. See Supporting Information for
b
experimental details. Toluene (0.5 M) used as a solvent.
porphyrin-based catalysts (Table 1). It was found that Co(TPP)
(TPP = tetraphenyl porphyrin) was a superior catalyst delivering
3 in 72% isolated yield (Table 1, entry 1). Employment of other
Co-porphyrin-based catalysts was less efficient (Table 1, entries
2 and 3), whereas use of other Fe- and Ru-porphyrin-based
catalysts was inefficient (Table 1, entries 4−6).
Next, the generality of this methodology was examined. With
regard to the pyridotriazole component, several differently
substituted heterocycles at C5 and C6 turned out to be capable
substrates for transannulation with t-butyl isothiocyanate (2a)
(Scheme 2). Thus, halogenated and methylated pyridotriazoles
reacted smoothly to give the corresponding imino-thiazolopyr-
B
https://dx.doi.org/10.1021/acs.orglett.0c03099
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a
Scheme 4. Scope of Transannulation with Xanthate Ester
Scheme 7. Proposed Radical Activation Mechanism
a
0.2 mmol scale, isolated yields.
Scheme 5. Product Transformations
Encouraged by the successful transannulation of pyridotria-
zoles with isothiocyanates, we examined reactivity of carbonyl
sulfide (OCS) in this transformation (Scheme 4). Because of its
gaseous nature, carbonyl sulfides are experimentally difficult to
handle. Hence, xanthate ester, which is a stable precursor of
carbonyl sulfide that is produced via Chugaev elimination, was
examined.15 Gratifyingly, xanthate ester successfully underwent
transannulation reactions with pyridotriazoles in the presence of
12 mol % Co(TPP) catalyst, providing oxo-thiazolopyridine
derivatives 5a−5f in moderate yields.
Scheme 6. Radical Scavenging Experiment Used to Trap the
Co(III)-Carbene Radical Intermediate
It was also shown that the tert-butyl group at imino-
thiazolopyridine 3a can easily be removed (under nonoptimized
conditions) to access 6 possessing an N−H moiety, which can
routinely be further functionalized, for instance, into benzoy-
lated derivative 7 (see Scheme 5). Note that the latter cannot be
accessed directly via the transannulation reaction of acylated
isothiocyanates under the reaction conditions tested. It is
believed that 6 can serve as a convenient synthon for a modular
one-step synthesis of library of N-substituted imino-thiazolopyr-
idines.
The radical nature of this transformation was validated by the
following radical trapping experiment (see Scheme 6). The
reaction in the presence of dibenzoyl peroxide 8 resulted in the
formation of benzoyloxy-containing product 9, thus supporting
involvement of the carbene radical intermediate C.
On the basis of the above study and literature reports, the
following plausible mechanism for this transannulation reaction
is proposed. Xanthate ester 4, which is obtained via Chugaev
elimination, acts as a carbonyl sulfide (12) surrogate in this
transformation (see Scheme 7a). Pyridotriazole 1 exist in
equilibrium with its open diazo tautomer A, which, upon
denitrogenative reaction with Co(II)-based metalloradical
catalyst D, produces a key α-Co(III)-pyridyl radical inter-
mediate C (Scheme 7b). A subsequent trapping of this
eletrophilc radical with isothiocyanate 216 or the in-situ-
generated carbonyl sulfide 12 produces radical intermediate E,
idine products 3b−3f in good to excellent yields. Employment
of methyl ester-containing substrate 3g and thiophene-
containing pyridotriazole 3i posed no problem. Notably, this
reaction was equally efficient with N-fused pyrazinotriazole to
deliver imino-thiazolopyrazine derivative 3h in high yield. This
reaction also appeared to be very general for isothiocyanates (see
Scheme 3). A variety of functional groups was tolerated at the
para-position of arylisothiocyanates, providing the correspond-
ing products 3k−3p in good to excellent yields. Similarly,
transannulation of isothiocyanates possessing substituents at the
meta and ortho positions of the arene proceeded uneventfully
(3q−3u). Benzodioxolyl- and naphthyl-containing isothiocya-
nates smoothly underwent transannulation, affording 3v and 3w
in good yields. In addition, it was found that alkyl
isothiocyanates also are capable partners for this transformation.
Thus, isothiocyanates possessing various benzyl and alkyl groups
all reacted well, providing products 3x−3z in good to high yields.
Moreover, this reaction chemoselectively gave transannulation
products 3aa with allyl-containing isothiocyanate, the double
bond moiety of which was not compromised. Notably, chiral
isothiocyanates could also be efficiently employed in this
reaction. Thus, isothiocyanates derived from amino acid esters
smoothly underwent transannulation reaction to produce
derivatives 3ab−3ad in moderate yields.
C
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which, upon radical cyclization, leads to imino-thiazolopyridine
3 or oxo-thiazolopyridines 5 and regenerates the Co catalyst.
In summary, we have developed general and efficient Co(II)-
catalyzed radical transannulation reactions of pyridotriazoles
with isothiocyanates and carbonyl sulfide. This operationally
simple protocol exhibits wide functional-group tolerance,
efficiently producing N-fused imino-thiazolopyridines and oxo-
thiazolopyridines.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03099.
Experimental procedures, optimization process, charac-
terization data, and 1H and 13C NMR spectra of all new
compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Vladimir Gevorgyan − Department of Chemistry and
Biochemistry, University of Texas at Dallas, Richardson, Texas
75080, United States; orcid.org/0000-0002-7836-7596;
Email: vlad@utdallas.edu
Author
Ziyan Zhang − Department of Chemistry and Biochemistry,
University of Texas at Dallas, Richardson, Texas 75080, United
States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03099
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Institute of Health (No. GM120281),
National Science Foundation (No. CHE-1955663), and Welch
Foundation (Chair, No. AT-0041) for financial support. We are
also thankful to Prof. Sheena D’Arcy for help with HRMS
analysis.
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(16) For selectivity of addition of electrophilic and nucleophilic
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