Stereoselective synthesis of 9-vinyl substituted unsymmetrical xanthenes and thioxanthenes

Article abstract of DOI:10.1016/j.tetlet.2020.152347

Activated 2°-allylic alcohols derived from 2-aryloxybenzaldehydes and 2-(arythio)benzaldehydes undergo intramolecular Friedel-Crafts alkylation reaction when heated with catalytic amount of a Lewis acid in 1,2-dichloroethane to provide highly E-selective 9-vinyl substituted unsymmetrical novel xanthenes and thioxanthenes in good yields.

Full text of DOI:10.1016/j.tetlet.2020.152347

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Stereoselective Synthesis of 9-Vinyl Substituted Unsymmetrical Xanthenes
and Thioxanthenes
Anamika Prajapati, Mahendra Kumar, Ranjit Thakuria, Ashok K. Basak
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DOI:
Reference:
S0040-4039(20)30825-X
https://doi.org/10.1016/j.tetlet.2020.152347
TETL 152347
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Tetrahedron Letters
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5 August 2020
6 August 2020
Please cite this article as: Prajapati, A., Kumar, M., Thakuria, R., Basak, A.K., Stereoselective Synthesis of 9-
Vinyl Substituted Unsymmetrical Xanthenes and Thioxanthenes, Tetrahedron Letters (2020), doi: https://doi.org/
10.1016/j.tetlet.2020.152347
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Stereoselective Synthesis of 9-Vinyl Substituted Unsymmetrical Xanthenes
and Thioxanthenes
Anamika Prajapati,a Mahendra Kumar,a Ranjit Thakuria,b Ashok K. Basak*c
aDepartment of Chemistry, University of Rajasthan, JLN Marg, Jaipur 302004, India
bDepartment of Chemistry, Gauhati University, Guwahati 781014, India
cDepartment of Chemistry, Institute of Science, Banaras Hindu University, Varanasi 221005, India
Corresponding author e-mail ID: akbasak.chem@bhu.ac.in
Abstract: Activated 2o-allylic alcohols derived from 2-aryloxybenzaldehydes and 2-
(arythio)benzaldehydes undergo intramolecular Friedel-Crafts alkylation reaction when heated
with catalytic amount of a Lewis acid in 1,2-dichloroethane to provide highly E-selective 9-vinyl
substituted unsymmetrical novel xanthenes and thioxanthenes in good yields.
Introduction: Xanthenes and thioxanthenes are important polycyclic motifs found in many
biologically important compounds including natural products.1 Xanthene and thioxanthene
derivatives are known to exhibit antimicrobial,2 antiparasitic,3 anti-cancer,4 antitumor,5 anti-
proliferic,6 anti-inflamatory and antioxidant activities.7 In addition, thioxanthenes are utilized for
the development of neuroleptic and antidepressive drugs. Some thioxanthenes are used as
antipsychotic drugs used in the treatment of schizophrenia and other psychotic disorders.8
Besides these valuable biological properties, xanthene derivatives are also used in photodynamic
therapy,9 as chirooptical molecular switches10 and as commercially available dyes11 such as
fluorescence, rhodamines, etc. Unsubstituted 9H-xanthenes and thioxanthenes are extremely
useful advanced starting material for the development of novel C-H functionalization reactions,12
organocatalytic asymmetric cross-dehydrogenative coupling reactions,13 anodic oxidative cross-
dehydrogenative reactions14 and kinetic as well as mechanistic studies.15 Due to these
widespread importance, that range from medicinal chemistry to material science, synthesis of
xanthene and thioxanthene derivatives have continued attracting research interest of synthetic
chemists worldwide.16
2-Aryloxybenzaldehydes and 2-(arylthio)benzaldehydes are easily accessible from commercially
available 2-flurobenzaldehydes and phenols/thiophenols via direct aromatic nucleophilic
substitution reaction,17 and thus have been extensively utilized for the synthesis of xanthene and
thioxanthene derivatives, particularly 9-aryl/heteroaryl substituted xanthenes and
thioxanthenes.18 For instance, Li et al. reported a FeCl3-catalyzed domino reaction wherein two
equivalent of electron-rich (hetero)arenes undergo condensation with 2-aryloxybenzaldehydes/2-
(arylthio)benzaldehydes to provide 9-(hetero)aryl xanthenes/thioxanthenes (Scheme 1a).18a
Similar domino reaction catalyzed by Sc(OTf)3 was disclosed by Panda et al.18b Addition of
(hetero)aryl Grignard reagents to 2-aryloxybenzaldehydes and 2-(arylthio)benzaldehydes leads to
π-activated 2o-alcohols which were converted into 9-arylxanthenes/thioxanthenes via
intramolecular Friedel-Crafts alkylation (FCA) reaction using catalytic amount of FeCl3
(Scheme 1b)17 as
1

Scheme 1: Synthesis of 9-substituted xanthene and thioxanthenes from 2-aryloxybenzaldehydes
and 2-(arylthio)benzaldehydes.
well as organic Brønsted super acid18c,18d (Scheme 1c). Notably, allylic alcohols, being another
important class of π-activated alcohols,19 have also been widely used in Brønsted/Lewis acid
catalyzed inter- and intramolecular FCA reaction. For 2o-allylic alcohols, these reactions often
lead to stereoselective formation of alkenes.20 For example, Morita-Baylis-Hillman (MBH)
adducts undergo intermolecular FCA reaction with arenes providing E- or Z-selective
trisubstituted olefins depending on the electron-withdrawing group present in the MBH
adducts.21 Drawing inspiration from these literature reports, we devised a strategy to synthesize
9-vinyl xanthenes/thioxanthens via intramolecular FCA reaction on 2o-allylic alcohols derived
from 2-aryloxybenzaldehydes and 2-(arylthio)benzaldehydes (Scheme 1d). In continuation with
our ongoing effort to synthesize novel xanthene and thioxanthenes,22 herein we report a
In(OTf)3-catalyzed intramolecular FCA reaction for the synthesis of E-selective 9-vinyl
unsymmetrical novel xanthenes and thioxanthenes.
Our investigation on the intramolecular FCA reaction commenced with (E)-1-(4-nitrophenyl)-3-
(2-phenoxyphenyl)prop-2-en-1-ol 3a which was obtained in two steps from 2-
2

phenoxybenzaldehyde 1a following reported procedures (Scheme 2).23 Knoevenagel type
condensation of 2-phenoxybenzaldehyde 1a with 4´-nitroacetophenone in the presence of 20
mol% piperidine and 40 mol% AcOH in refluxing benzene provided (E)-1-(4-nitrophenyl)-3-(2-
phenoxyphenyl)prop-2-en-1-one 2a in 78% yield. Chemoselective reduction of enone 2a under
Luche conditions24 provided allylic alcohol 3a in nearly quantitative yield (98%). With the
allylic alcohol 3a in hand, we sought to find conditions for the envisaged intramolecular FCA
reaction.
Scheme 2: (i) 4´-nitroacetophenone, piperidine, AcOH, benzene, reflux, 4 h, 78%; (ii)
CeCl3.7H2O, NaBH4, MeOH-DCM, 0 oC, 15 min, 98%.
o
Reaction of 3a in the presence of 10 mol% In(OTf)3 in DCM at rt (35 C) was slow and
underwent approximately 50% conversion (40% yield) to the xanthene 4a after 24 h (entry a,
1
13
Table 1). Compound 4a was characterized unambiguously by H & C NMR and HRMS data.
Notably, formation of only the E-isomer was observed to the limits of detection by 1H NMR of
o
the crude reaction mixture. Carrying out the reaction at 50 C in 1,2-dichloroethane (DCE),
better conversion (80%) and yield (65%) was observed after 4 h (entry b, Table 1). However, to
our delight, complete conversion and satisfactory yield (78%) was obtained within 15 min when
the reaction was carried out at 70 oC. The In(OTf)3-catalyzed cyclization reaction in CH3CN and
THF as solvent provided slightly lower yield of xanthene 4a (entries d-e, Table 1). When 10
mol% Sc(OTf)3 was used as a catalyst, the reaction provided xanthene 4a in 72% yield after 30
min (entry f, Table 1). Yb(OTf)3 was found to be less effective when compared to In(OTf)3 for
the intramolecular FCA reaction. InCl3 provided 75% yield of xanthene 4a whereas FeCl3
provided 65% yield of xanthene 4a (entries h-i, Table 1). It is worth mentioning that the
intramolecular FCA reaction of 3a gave only a small amount of xanthene 4a and a mixture of
unidentified products (entry j, Table 1) under solvent free conditions at 50 oC.
With the optimized conditions in hand, we sought to evaluate the scope of the intramolecular
FCA reaction. As shown in Scheme 3, the In(OTf)3-catalyzed intramolecular FCA reactions were
successful on a number of 2o-allylic alcohols synthesized in two steps (as shown in Scheme 2)
from a variety of 2-aryloxybenzaldehydes and 2-(arythio)benzaldehydes. In all cases,
acetopheneones bearing an electron-withdrawing group (NO2, CN, Cl, Br) at p- or m-position
were utilized for the Knoevenagel type condensation reaction. Interestingly, 2-aryloxyquinoline
3-carboxyaldehyde could also be employed for the preparation of 2o-allylic alcohols. As evident
from the experimental results presented in Scheme 3, the electron density of the aryl ring B as
3

well as the nature of substituent on the aryl rings A & C have significant effect on the reactivity
of the allylic alcohols towards the intramolecular FCA reaction. For example, the cyclization
reaction occurred at lower temperature when a strong electron-withdrawing NO2-group is
Table 1: Optimization of reaction conditionsa
entry catalyst
conditions
DCM, rt, 24 h
DCE, 50 oC, 4 h
yield (%)
40b
65
78
72
70
72
68
75
a
b
c
d
e
f
g
h
i
In(OTf)3
In(OTf)3
In(OTf)3 DCE, 70 oC, 15 min
In(OTf)3
In(OTf)3
Sc(OTf)3
CH3CN, 70 oC, 15 min
THF, 70 oC, 30 min
DCE, 70 oC, 30 min
Yb(OTf)3 DCE, 70 oC, 30 min
InCl3
FeCl3
DCE, 70 oC, 15 min
DCE, 70 oC, 20 min
neat, 50 oC, 2 h
64
ND
j
In(OTf)3
aAll reactions were carried out in 0.20 mmol scale using
10 mol% catalyst in appropriate solvent (0.2M
concentration); b50% conversion; ND = yield not
determined.
replaced by a Cl-atom in the aryl ring C (e.g., 4a vs 4b). Due to the positive inductive effect,
alkyl groups present in the aryl ring B increased the reactivity significantly (for comparison, 4a
vs 4h vs 4l). Presence of the electronegative Cl-atom in the aryl ring A had a negative impact on
the reactivity (4e vs 4f). Allyl alcohols derived from 2-(arylthio)benzaldehydes were, in general,
more reactive towards the intramolecular FCA reaction due to the lower electronegativity of S-
atom. Allyl alcohols having unsymmetrically substituted aryl ring
B
provided
xanthenes/thioxanthenes as a variable mixture of regioisomers which were separated by column
chromatoghaphy. Isolated yield for the major regioisomers (4e-4h, 4l, 4n) of the cyclized
products are shown in Scheme 3. However, unfortunately, regioisomeric mixture of xanthenes
(4l, 4l´) could not be separated by column chromatography and were obtained as a mixture in
3.5:1 ratio in 82% yield. In general, the p-position to an alkyl (methyl) group is preferred over
the o-position in the FCA process to minimize the steric effect. It is worth noting that the
cyclization also occurs at the o-position to an alkyl (methyl) group at moderate temperature to
provide xanthenes in good yields (4i-4j, Scheme 3). Due to the presence of an additional
4

electronegative atom (N-atom), the 2o-allylic alcohols bearing quinoline moiety as the aryl ring
A, exhibited low reactivity at lower temperature. However, gratifyingly, complete conversion
occurred at higher temperature using 20 mol% of In(OTf)3 to furnish the desired novel quinoline
Scheme 3: Synthesis of 9-vinyl xanthene and thioxathenea
5

b
c
aReactions were carried out in 0.20 mmol scale using 10 mol% In(OTf)3 in DCE (0.2M); major isomer; yield of
major isomer; dreaction was carried out using 20 mol% In(OTf)3.
fused xanthenes 4m and 4n in good yields. In all cases, the stereochemistry of the olefin in the
synthesized 9-vinyl xanthenes and thioxanthenes was invariably found to be the
thermodynamically stable E-configuration possessing a high coupling constant, J = 15.6 Hz.
Interestingly, adding to our advantage, a solution of compound 4l in hexanes upon storage in a
freezer yielded single crystals suitable for X-ray diffraction studies. As shown in Figure 1, the
ORTEP diagram further confirms the E-geometry of the disubstituted olefin as well as the
orientation of alkyl groups in the aryl ring B. The vinyl side chain in xanthene 4l orients in a
linear fashion perpendicular to the plan containing the xanthene nucleus.
Figure 1: ORTEP diagram of compound 4l (CCDC No. 2010351)
The tentative mechanism of the xanthene synthesis is outlined in Scheme 4. In(OTf)3 promotes
the formation of resonance stabilized allylic carbocation 7 at the benzylic position (C15).
Intramulecular FCA reaction forming C12-C13 bond provides xanthene 10 with the loss of a
neutral water molecule from intermediate 9. Intramolecular FCA forming C12-C15 bond would
lead to a strained 8-membered ring that incorporates an alkene making this pathway for
6

cyclization thermodynamically unfavorable. Although generation of a carbocation at the benzylic
position of the aryl ring C that carries a strong electron-withdrawing group at the para position is
of formidable challenge, the overall success of the reaction is attributed to the resonance
stabilization of the resulting benzylic/allylic carbocation. Notably, the water molecule released in
the intramolecular FCA reaction did not participate in hydration of the olefin present in the
xanthenes/thioxanthenes under the reaction conditions. This could be attributed to the partial
electron-deficient nature of the resulting olefin due to the electron-withdrawing group present in
the aryl ring C and the use of a Lewis acid for the intramolecular FCA reaction.
Scheme 4: Plausible mechanism of the intramolecular FCA reaction
In summary, a novel method for highly stereoselective synthesis of 9-vinyl xanthenes and
thioxanthenes has been developed. The syntheses rely upon a In(OTf)3-catalyzed regio- and
stereoselective intramolecular Friedel-Crafts alkylation reaction of easily accessible π-activated
2o-allylic alcohols derived from a variety of 2-aryloxybenzaldehydes/2-(arylthio)benzaldehydes
and commercially available acetophenones. Remarkably, the mild reaction conditions are also
suitable for efficient synthesis of quinoline fused xanthenes. As the electronegativity of N-atom
is less than O-atom, we anticipate that the aza-analogues of xanathenes and thioxanthenes could
also be synthesized by this methodology. The presence of a versatile vinyl functionality in the
products suggests that further annulation with appropriately substituted aryl ring B might be
possible to generate novel polycyclic compounds. We will explore this in our ongoing study of
the chemistry of xanthenes and thioxanthenes.
Notes: Authors declare no competing financial interest
Acknowledgement: AP thanks CSIR, New Delhi for a research fellowship. We are thankful to
MRC, MNIT Jaipur for NMR spectra. We thank USIC, University of Rajasthan, Jaipur for
HRMS data. We also thank SAIF, Gauhati University for providing the single crystal X-ray
diffraction data. Research grants from Council of Scientific and Industrial Research, New Delhi
(Grant No. 02(0346)/19/EMR-II) is gratefully acknowledged.
Associated content:
7

The Supplementary Information is available (Experimental procedures, analytical data and
copies of 1H & 13C NMR spectra).
*Corresponding Author:
Ashok K. Basak-Department of Chemistry, Institute of Science, Banaras Hindu University,
Varanasi 221005, India. E-mail ID: akbasak.chem@bhu.ac.in
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9

 E-selective 9-vinyl novel unsymmetrical xanthene/thioxanthene synthesis
 Novel quinoline fused xanthene synthesis
 Lewis acid catalyzed cyclization of π-activated allylic alcohols
 High yielding reactions
Graphical Abstract
Stereoselective Synthesis of 9-Vinyl Substituted
Unsymmetrical Xanthenes and Thioxanthenes
Anamika Prajapati, Mahendra Kumar, Ranjit Thakuria and Ashok K. Basak
10