Metal-Free Diaryl Etherification of Tertiary Amines by Ortho-C(sp2)-H Functionalization for Synthesis of Dibenzoxazepines and -ones

Article abstract of DOI:10.1021/acs.orglett.7b03328

A phenyliodine(III) diacetate mediated umpolung reactivity of the tertiary amines with suitably substituted o-hydroxybenzyl and phenyl groups is exploited to facilitate o-C(sp²)-H functionalization to afford diaryl ethers. The presence of an o-

Full text of DOI:10.1021/acs.orglett.7b03328

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Metal-Free Diaryl Etherification of Tertiary Amines by Ortho-C(sp2)−
H Functionalization for Synthesis of Dibenzoxazepines and -ones
Vellekkatt Jamsheena, Chikkagundagal K. Mahesha, M. Nibin Joy, and Ravi S. Lankalapalli*
Chemical Sciences and Technology Division and Academy of Scientific and Innovative Research (AcSIR), CSIR-National Institute for
Interdisciplinary Science and Technology (CSIR-NIIST), Thiruvananthapuram 695019, India
S
* Supporting Information
ABSTRACT: A phenyliodine(III) diacetate mediated umpo-
lung reactivity of the tertiary amines with suitably substituted
o-hydroxybenzyl and phenyl groups is exploited to facilitate o-
C(sp2)−H functionalization to afford diaryl ethers. The
presence of an o-CHO and secondary amine functionalities
in the resulting diaryl ether, generated in situ, were utilized for
synthesis of dibenzoxazepines and dibenzoxazepinones. Mild conditions and relative broad substrate scope, and potential for
further diversification of the diaryl ethers are highlights of this methodology.
ibenzoxazepines and dibenzoxazepinones are pharma-
ceutically relevant molecules present in antidepressants
various intramolecular seven-membered ring formation strat-
egies such as reductive lactamization, Pd-catalyzed condensation,
cyclocarbonylation, Smiles rearrangement, and Cu-catalyzed
Goldberg reaction to afford dibenzoxazepinones.5 Alternatively,
intramolecular diaryl etherification as the later annulation event
under metal-catalyzed and base-promoted conditions from
substrates with suitably tethered phenol- and halo-substituted
phenyl units was also developed.6 Dibenzoxazepines, as valuable
synthetic intermediates, afforded more complex heterocyclic
structures.7
Recently, dibenzoxazepinone synthesis was reported by
hypervalent iodine(III) reagent (HIR) mediated intramolecular
C−N bond formation from 2-(aryloxy)benzamides synthesized
by Cu-mediated diaryl etherification (Figure 1b)8 but failed with
substrates containing strong electron-withdrawing and -donating
groups. Another example involving use of HIR reagent involved
intramolecular cyclization of two aryl groups from 2-hydroxy-N-
phenylbenzamides affording dibenz[d,f] [1,3]oxazepin-6(7H)-
ones.9 HIR is considered as a mild and metal-free alternative for
the construction of C−C and C−heteroatom bonds.10 HIR-
mediated oxidative dearomatizing transformations for ortho-
substituted phenols were utilized in the synthesis of complex
natural products.11 Similar to phenolic substrates, tertiary amines
were also efficient substrates in HIR mediated transformations
involving functionalization of C(sp3)−H bond adjacent to
nitrogen.12 Herein, we report ortho-C(sp2)−H bond function-
alization of tertiary amines in the presence of PIDA to afford
diaryl ether (Figure 1). Diaryl etherification is an important
synthetic strategy achieved mostly by Cu- and Pd-catalyzed
cross-coupling reactions (Figure 1c).13 The metal-free alter-
native for diaryl etherification with HIR involving diary-
liodonium salts is an attractive strategy,14 applied toward the
synthesis of o-CHO diaryl ethers (Figure 1d).15 In the present
D
and antipsychotics. These privileged structural motifs possess a
broad range of biological activities such as anti-HIV, antitumor,
antioxidant, oral contraceptive, TRPA1 agonist, sodium channel
blocker, CNS depressant, antiinflammatory, and antinociceptive
properties.1 As natural products, dibenzoxazepinones were
isolated from the leaves of Carex distachya and from the
ethanolic extracts of streptomycetes.1c,d Many approaches have
been developed for the synthesis of the dibenzoxazepine core
skeleton. Traditionally, a base-promoted nucleophilic aromatic
substitution (SNAr) reaction was employed to construct the
seven-membered ring of dibenzoxazepinones via Smiles
rearrangement of suitable electrophilic substrates by a domino
C−O and C−N coupling reactions (Figure 1a).2 A base-
Figure 1. Strategies for dibenzoxazepinone and diaryl ether synthesis
and present work.
promoted green protocol has been developed for the synthesis of
dibenz[b,f ][1,4]oxazepin-11-amines by SNAr with concomitant
addition reaction.3 Post-Ugi reaction, an intramolecular micro-
wave-assisted diaryl etherification was used as an attractive
strategy to synthesize the highly substituted dibenz[b,f ][1,4]-
oxazepine scaffold.4 Key precursors generated from diaryl
etherification methodology served as suitable substrates for
Received: October 25, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03328
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
study, PIDA induced umpolung reactivity of tertiary amines
affords diaryl ether with an o-CHO and secondary amine
substituents that upon subsequent treatment with NaBH(OAc)3
and PCC provided dibenzoxazepines and dibenzoxazepinones,
respectively (Figure 1). The present method serves as a metal-
free alternative to the existing methods vide supra with a broad
substrate scope. Further synthetic applications of this method-
ology from diaryl ether were demonstrated with an array of
transformations to access other bioactive skeletons.
as shown in Scheme 1. Under the optimized conditions (entry 5,
Table 1), tertiary amines bearing activating as well as deactivating
Scheme 1. Substrate Scope of Dibenzoxazepines
In an initial attempt (Table 1), tertiary amine 1a with suitably
substituted o-hydroxybenzyl and phenyl groups was treated with
a
Table 1. Optimization of Reaction Conditions
g
g
g
entry
1
[H]/[O]
NaBH4
2a
34
3b (two steps)
4c (two steps)
b
2
67
71
65
77
b
3
NaBH(OAc)3
NaCNBH3
NaBH(OAc)3
PCC
b
4
c
5
d
6
70
73
57
e
7
PCC
f
8
NaClO
a
Reaction conditions: All reactions were conducted at room
temperature using undistilled solvents. Compound 1a/1b/1c (0.18
mmol) in HFIP (1 mL) was the scale of the reactions for the first step.
b
Reductant (3 equiv) in MeOH (1 mL) was added to the crude
c
mixture of compound 2 after quenching. NaBH(OAc)3 (3 equiv) was
added to the same pot. To the isolated compound 2c was added
oxidant (1 equiv) in DCM (1 mL). PCC (2 equiv) was used. The
solvent in the second step was AcOH. Isolated yields (%).
d
groups in either the mono- or disubstituted A ring and para-
substituted B ring conveniently converted to dibenzoxazepines
3b−ab in 24−92% yields. In general, tertiary amines with
deactivating groups (halo, nitro) in the A ring and ones with
activating groups (alkyl, methoxy) in the B ring afforded
dibenzoxazepines in good yields. For instance, substrate 1c with
nitro substitution in the A ring and 1i with tert-butyl substitution
in the B ring afforded dibenzoxazepines 3c and 3i in 83 and 92%
yields, respectively. Accordingly, substrate 1v with both nitro and
tert-butyl substitutions in the A and B rings, respectively, afforded
dibenzoxazepine 3v in 87% yield. Meta-substituted B rings
afforded a mixture of separable regioisomers 3ac, 3ac′−3ae, and
3ae′ since the nucleophilic attack by OH group is feasible in
either of the ortho-positions of the B ring. Dimethoxy-substituted
B ring offered a single regioisomer 3af−ah, arising due to
electronic reasons. Substrates with N-benzyl substitution
afforded dibenzoxazepines 3ai and 3aj which serves as a third
variable group in the tertiary amine for diversification. However,
when the substrate bears an ortho-substitution in the B ring the
cyclization did not occur. Presence of a 3,4-(methylenedioxy)
group in the B ring produced the desired dibenzoxazepine 3ak in
40% yield similar to substrates, vide supra, with a dimethoxy-
substituted B ring. Compound 3ak with a tetracyclic framework
could be a novel entry in the class of tetracyclic antidepressants.
Tertiary amine with two benzylic alcohols, where one of them,
containing nitro substitution, did not undergo ring formation,
and the other, with no substitution, participated in dibenzox-
azepine 3al formation in 52% yield, confirmed by HMBC
analysis of its methylated analogue (see the Supporting
Information). An attempt involving further cyclization using
dibenzoxazepine 3al under the optimized conditions led to
e
f
g
2 equiv of PIDA at room temperature in the presence of HFIP,
and within 10 min compound 2a formed (entry 1),
unambiguously confirmed by single-crystal X-ray analysis (see
the Supporting Information). Further variations in the mode of
additions, solvents, and oxidants were not effective, which led us
to choose PIDA and HFIP as the optimal combination.
Compound 2 with appropriately substituted aldehyde and
secondary amine is prone to undergo reductive amination to
afford the desired dibenzoxazepine. Accordingly, a simple tertiary
amine 1b was subjected to PIDA-mediated oxidation, and the
crude mixture was treated with excess NaBH4 in methanol to
afford dibenzoxazepine 3b in 67% yield (entry 2). Variation of
reductants (entries 3 and 4) did not improve the yield further.
Interestingly, addition of 3 equiv of NaBH(OAc)3 in the same
pot after complete consumption of tertiary amine afforded 3b
with an enhanced yield of 77% (entry 5). To access
dibenzoxazepinones, treatment of 1 equiv of PCC on the
isolated intermediate 2c in an overnight reaction afforded
dibenzoxazepinone 4c in 70% yield (entry 6) and an increase of
PCC to 2 equiv led to reaction completion within 1 h with 73%
yield (entry 7). Even though other oxidants (see the Supporting
Information) failed to afford the desired product, sodium
hypochlorite in acetic acid produced compound 4c in 57%
yield (entry 8).
The optimization studies revealed the broad scope of this
methodology involving various substitutions in the benzyl (A
ring), phenyl (B ring), and N-alkyl groups of the tertiary amines
B
DOI: 10.1021/acs.orglett.7b03328
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
seven-membered ring opening to afford the corresponding
aldehyde 2al, which upon subsequent reduction reformed
dibenzoxazepine 3al. A successful dibenzoxazepinone 4c
formation by successive PIDA and PCC oxidations from the
optimization studies prompted us to further demonstrate the
substrate scope of this reaction (Scheme 2). Accordingly, tertiary
amines with different substitutions in the A and B rings afforded
dibenzoxazepinones 4 in 28−73% yields.
Information), as demonstrated with methyl-protected tertiary
amine 1c.
To further demonstrate the broad applicability of this
methodology, an array of synthetic transformations were carried
out on diaryl ether 2c (Scheme 4). Treatment of compound 2c
a
Scheme 4. Demonstration of Applicability of Methodology
Scheme 2. Substrate Scope of Dibenzoxazepinones
A plausible mechanism has been proposed similar to activation
of tertiary amines in the presence of PIDA.12 Oxidative
dearomatization of phenols in the presence of PIDA are well
documented;16 however, tertiary amine will be more reactive for
the initial ligand exchange with PIDA. Accordingly, reaction of
tertiary amine 1 with PIDA affords intermediate I (Scheme 3).
a
Reaction conditions: (a) 2c (1 equiv), t-BuNC (1 equiv), InCl3
(cat.), MeOH, 60 °C; (b) 2c (1 equiv), NaClO (2 equiv), 1 M NaOH
(cat), Bu4NI (1.5 equiv), DCM, rt; (c) 2c (1 equiv), AcOH (cat.),
DDQ (3 equiv), DCM, rt; (d) 7 (1 equiv), DMP (1.5 equiv), DCM,
rt; (e) 7 (1 equiv), p-TsOH (cat.), EtOH, 60 °C.
Scheme 3. Plausible Mechanistic Pathway
with tert-butyl isocyanide afforded a pharmaceutically relevant
dibenzoxazepine-11-carboxamide 5. A facile DDQ-mediated
oxidation of benzylic carbon afforded diaryl ether 6 flanked by
three reactive functionalities. The presence of p-nitro sub-
stitution in ether facilitates an intramolecular ipso-substiution
with secondary amine by Smiles rearrangement, interestingly,
undertaken in the presence of bleach to afford tertiary amine 7 in
85% yield. Furthermore, tertiary amine 7 in the presence of
Dess−Martin periodinane and p-TsOH afforded p-benzoqui-
none 8 and dihydroacridine 9 in 40 and 55% yields, respectively.
Structural confirmation of all the products was carried out by
extensive 2D NMR analysis (see the Supporting Information). A
further extension of this methodology to afford dibenzodiazepine
11, categorized as privileged structure by Evans et al.,17 was
obtained from tertiary amine 10 with an appropriate NHTs
substitution.
In conclusion, tertiary amines with suitably substituted o-
hydroxybenzyl, phenyl groups with varied substituents under-
went diaryl ether formation endowed with o-CHO and
secondary amine functionalities in the presence of PIDA by o-
C(sp2)−H functionalization under mild conditions. An intra-
molecular seven-membered ring formation facilitated by NaBH-
(OAc)3 and PCC provided dibenzoxazepines and dibenzox-
azepinones, respectively. A broad substrate scope for dibenzox-
azepine has been demonstrated; in particular, substrates with
deactivating groups in A ring and activating groups in B ring
offered good yields in a one-pot reaction. Furthermore, an array
of synthetic transformations to further demonstrate the broad
applicability of this methodology was carried out, which afforded
diverse molecular motifs.
The key nucleophilic attack of the phenoxide on the electron-
deficient ortho-carbon of the aniline ring of I is proposed
analogous to a reported PIDA activation.9 This key seven-
membered ring formation accompanied by elimination of phenyl
iodide affords intermediate II that rearomatizes to provide
dibenzoxazepine 3. It is worth mentioning that stabilization of
intermediate I by the presence of deactivating groups in A ring
and activating groups in B ring promotes formation of
dibenzoxazepines in good yields, observed in substrate scope.
Use of 1 equiv of PIDA produced a mixture of dibenzoxazepine 3
and diaryl ether 2 along with unreacted tertiary amine 1 as
observed over TLC (see the Supporting Information). A reactive
compound 3 in the presence of a second equivalent of PIDA
afforded intermediate iminium ion IV, formed by abstraction of
benzylic proton from the activated tertiary amine III. The
reaction mixture was quenched upon complete consumption of
compound 3, producing ring opening of iminium intermediate
IV to afford diaryl ether 2. This was again confirmed by the
reaction of compound 3g with 1 equiv of PIDA, which led to the
formation of diaryl ether 2g in 87% yield (see the Supporting
Information). In the absence of nucleophilic hydroxyl group, the
competing benzylic proton abstraction takes place which upon
hydrolysis produces benzaldehyde (12, see the Supporting
C
DOI: 10.1021/acs.orglett.7b03328
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03328.
■
S
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Reaction optimization, crystal data, experimental proce-
dures, and NMR (PDF)
Accession Codes
CCDC 1503980 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing 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.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ravishankar@niist.res.in.
ORCID
(9) Shang, S.; Zhang-Negrerie, D.; Du, Y.; Zhao, K. Angew. Chem., Int.
Ed. 2014, 53, 6216−6219.
(10) Yoshimura, A.; Zhdankin, V. V. Chem. Rev. 2016, 116, 3328−
3435.
Ravi S. Lankalapalli: 0000-0001-8223-7674
Notes
́
(11) (a) Companys, S.; Pouysegu, L.; Peixoto, P. A.; Chassaing, S.;
Quideau, S. J. Org. Chem. 2017, 82, 3990−3995. (b) Roche, S. P.; Porco,
J. A., Jr. Angew. Chem., Int. Ed. 2011, 50, 4068−4093.
The authors declare no competing financial interest.
(12) (a) Zhang, N.; Cheng, R.; Zhang-Negrerie, D.; Du, Y.; Zhao, K. J.
Org. Chem. 2014, 79, 10581−10587. (b) Shen, H.; Zhang, X.; Liu, Q.;
Pan, J.; Hu, W.; Xiong, Y.; Zhu, X. Tetrahedron Lett. 2015, 56, 5628−
5631. (c) Yang, L.; Zhang-Negrerie, D.; Zhao, K.; Du, Y. J. Org. Chem.
2016, 81, 3372−3379. (d) Rong, H. J.; Yao, J. J.; Li, J. K.; Qu, J. J. Org.
Chem. 2017, 82, 5557−5565. (e) Deb, M. L.; Pegu, C. D.; Borpatra, P. J.;
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A.; Kalbandhe, A. H.; Thorat, P. B.; Karade, N. N. Tetrahedron Lett.
2016, 57, 680−683. (g) Shu, X.-Z.; Xia, X.-F.; Yang, Y.-F.; Ji, K.-F.; Liu,
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ACKNOWLEDGMENTS
■
Financial support from The Kerala State Council for Science,
Technology & Environment (KSCSTE), India, received by
R.S.L. in the form of a Kerala State Young Scientist Award is
gratefully acknowledged. R.S.L. is thankful to Dr. Sunil
Varughese, CSIR-NIIST, for single-crystal XRD analysis.
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