An unexpected thermal-ring-rearrangement of benzochromenes to inden-3-yl-naphthols with: P TsOH

Article abstract of DOI:10.1039/c8ob02264j

Described here is the first report of an unexpected thermal-ring rearrangement (TRR) of benzochromenes to indene derivatives promoted by pTsOH. This cascade ring-rearrangement proceeds through the protonation of benzochromenes by an acid catalyst followed by ring-opening and ring-closure by an intramolecular Friedel-Crafts cyclization to provide a new bicyclic framework, inden-3-yl-naphthols bearing a quaternary center, which also exhibited atropisomerism. Regioselectivity, broad substrate scope, high yields, solvent-free conditions and atom economy are the additional high points of this ring-rearrangement.

Full text of DOI:10.1039/c8ob02264j

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An unexpected thermal-ring-rearrangement of
benzochromenes to inden-3-yl-naphthols with
pTsOH†
Cite this: Org. Biomol. Chem., 2018,
16, 7920
Srinivasarao Yaragorla * and Tabassum Khan
Described here is the first report of an unexpected thermal-ring rearrangement (TRR) of benzochromenes
to indene derivatives promoted by pTsOH. This cascade ring-rearrangement proceeds through the proto-
nation of benzochromenes by an acid catalyst followed by ring-opening and ring-closure by an intra-
molecular Friedel–Crafts cyclization to provide a new bicyclic framework, inden-3-yl-naphthols bearing a
quaternary center, which also exhibited atropisomerism. Regioselectivity, broad substrate scope, high
yields, solvent-free conditions and atom economy are the additional high points of this ring-
rearrangement.
Received 13th September 2018,
Accepted 3rd October 2018
DOI: 10.1039/c8ob02264j
rsc.li/obc
tions,6 whereas in ring-rearrangement reactions a single sub-
strate is involved and rearranged to a new cyclic compound.
Introduction
Benzochromenes are versatile building blocks of natural pro- Intramolecular ring-opening-cyclization reactions are also
ducts and part of many biologically significant molecules.1 They known as depicted in Fig. 1, but it is a reversible reaction (no
are known to possess a spectrum of biological activities such as rearrangement). To the best of our knowledge, these kinds of
antifungal, anti-inflammatory, anti-hyperglycemic, cytotoxic ring-rearrangements (RR) are not extensively investigated.
and molluscicidal activities.2 In addition to these features, However, there are some indirect examples, such as a cyclo-
benzochromenes are known to undergo a reversible ring- isomerization reaction, where the transformation of a polyun-
opening-cyclization process (Fig. 1) in a thermal/photochemical saturated acyclic substrate initially cyclizes it to a less stable
environment and hence exhibit photochromic properties,3 and cyclic intermediate which further rearranges to a stable
if this ring-opening-cyclization occurs in a specific acid–base product.7 Therefore in cycloisomerization reactions, the
medium, it leads to acidochromism.3,4 As a result of photochro- rearrangement of the less stable cyclic intermediate to a final
mic and acidochromic properties, benzochromenes are known (cyclic) product can be considered as a ring-rearrangement
to possess significant material applications.3–5
(RR) reaction. A couple of examples of RR are shown in
Ring-rearrangement is conceptually different from the Scheme 1, including our recent report on the cycloisomeriz-
known ring-opening-cyclization reactions because at least two ation of 1,5-enyne, which proceeded through the formation of
substrates are involved in the ring-opening-cyclization reac- a less stable spirane, which then underwent a thermal-ring-
rearrangement (TRR) with chloranil to yield the stable phenan-
thridinone (eqn (i), Scheme 1).8 Another example is the cyclo-
isomerization of bis(indolyl)methane tethered propargyl
alcohol to yield a less stable spirane intermediate, which then
underwent ring-rearrangement (RR) through 1,2-migration
and yielded a stable carbazole (eqn (ii), Scheme 1).9 With this
background, herein we would like to report our first obser-
vation on the pTSA catalyzed thermal ring-rearrangement
Fig. 1 Photochromism of benzochromenes (intramolecular-ring-
opening-cyclization).
(TRR) of benzochromenes to inden-3yl-naphthols.
School of Chemistry, University of Hyderabad, 500046 Telangana, India.
Results and discussion
E-mail: Srinivas.yaragorla@uohyd.ac.in, ysruoh@gmail.com
†Electronic supplementary information (ESI) available. CCDC 1858026 and
We commenced our investigations with 3,3-dimethyl-1-phenyl-
3H-benzo[f]chromene (1a), which was synthesized using a
1851351. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c8ob02264j
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of catalyst loading and temperature did not have much influ-
ence on the reaction yield and time (entry 10). Among the
various solvents screened (1,2-DCE, EtOH, CH3CN, toluene,
and water), it was found that the reaction yield of 2a was best
under solvent-free conditions (entries 11–15, Table 1).
Having discovered the optimum conditions for the ring-
rearrangement of benzochromene (1a) to 1-(1,1-dimethyl-1H-
inden-3-yl)naphthalen-2-ol (2a), we were keen to test the scope
of different benzochromenes to rearrange into indene deriva-
tives under the optimum conditions (Scheme 2). Therefore, we
prepared several benzochromenes with different substitutions
and subjected them to the standard reaction conditions (s.r.c).
The ring rearrangement was compatible with alkyl (methyl,
n-butyl), phenyl and ethoxy substitutions on the aryl ring to
produce the corresponding-indene derivatives, 2b, 2c, 2d and
2e in excellent yields (Scheme 2). Substitutions at the quatern-
ary center (phenyl–phenyl; phenyl–methyl; methyl–hexyl) of
benzochromene (1) were retained during the TRR reaction to
furnish indenes 2f–2h in good yields. Interestingly, we
observed that products 2g and 2h appeared as diastereomers
Scheme 1 Representative examples of ring-rearrangement (RR) reac-
tions in cycloisomerizations.
known procedure.10 Initially, 1a was treated with the Lewis
acid catalyst, Ca(OTf)2 at 120 °C under solvent-free conditions
for 48 h, unfortunately, we found that there was no change in
the reaction; considering the solvent effect, the reaction was
refluxed in toluene for 48 h, but no initiation of reaction was
observed (entries 1 and 2, Table 1). Among the variety of
Brønsted acids studied, TFA, CSA and benzoic acids (entries
4–6) were unable to catalyze the reaction, however to our
delight PTSA gave 60% of product 2a at 100 °C under solvent-
free conditions after 16 h (entry 3).11 The increase of tempera-
ture from 100 °C to 120 °C with 10 mol% PTSA was beneficial
for elevating the yield of 2a and also lessening the reaction
time (entries 7 and 8). Interestingly, the maximum reaction
yield (95%) was obtained with 20 mol% PTSA at 110 °C within
1.5 h (entry 9). It is worth mentioning that a further increase
1
in the H and 13C NMR spectra, which is due to the presence
of a chiral carbon on the indene.12 The crystal structure of the
product (Fig. 2) shows that naphthyl and indenyl moieties are
perpendicular to each other, and this may be analogous to a
biphenyl system, so we presume that there could be a
restricted rotation existing between the C–C single bond con-
necting naphthyl and indenyl moieties, which leads them to
show atropisomerism. We may need to study this property
more deeply to understand this phenomenon; however, our
main objective here is to show the RR of benzochromenes.
In the case of TRR with 2-methyl-2,4-diphenyl-2H-benzo[h]
chromene (1j) the product 2-(1-methyl-1-phenyl-1H-inden-3-yl)
naphthalen-1-ol (2j) was yielded in 79% as a 1 : 1 mixture of
diastereomers. In a similar way, other benzo[h]chromenes
were also subjected to the standard TRR conditions and the
1-naphthol derivatives of indenes (2i, 2k) were obtained in
good yields. Bromonaphthol derivatives 2l–2o were also
formed from the corresponding benzochromenes through the
TRR reaction in good to excellent yields. As expected, com-
pound 2o also exhibited atropisomerism due to the presence
of a chiral carbon on the indene. The product 6-bromo-1-(1,1-
dimethyl-1H-inden-3-yl)naphthalen-2-ol (2m) was found as a
good crystalline solid, and hence the single crystal X-ray data
of 2m were obtained to confirm its structure (Fig. 2).13
Furthermore, we examined the TRR of benzochromenes
bearing the spiro-centre such as 1-phenylspiro[benzo[f]chro-
mene-3,1′-cyclohexane] 1p under standard reaction conditions
and obtained the corresponding 1-(spiro[cyclohexane-1,1′-
Table 1 Optimization of the reaction conditionsa
Entry Catalyst (mol%)
Reaction conditionsa Yieldb (%)
1
2
Ca(OTf)2/Bu4NPF6, (10/10) Neat, 120 °C, 48 h
Ca(OTf)2/Bu4NPF6, (10/10) Toluene, 120 °C,48 h nr
nr
3
4
5
6
7
8
9
10
11
12
13
14
15
PTSA (10)
TFA (100)
CSA (20)
Neat, 100 °C, 16 h
Neat, rt, 48 h
60
nr
nr
nr
91%
92%
95%
78%
Neat, 110 °C, 48 h
Neat, 110 °C, 48 h
Neat, 110 °C, 6 h
Neat, 120 °C, 4 h
Neat, 110 °C, 1.5 h
Neat, 120 °C, 1.5 h
1,2-DCE, 90 °C, 14 h 20%
EtOH, 90 °C, 48 h
CH3CN, 90 °C, 48 h
PhCO2H (30)
PTSA (10)
PTSA (10)
PTSA (20)
PTSA (40)
PTSA (20)
PTSA (20)
PTSA (20)
PTSA (20)
PTSA (20)
inden]-3′-yl)naphthalen-2-ol 2p in 93% yield after
3 h
(Scheme 2). Encouraged by this result, we prepared indene
derivatives bearing cyclohexane (2p, 2v, 2w, 2y), cycloheptane
(2q, 2r, 2x), cyclooctane (2s) and fluorenyl (2t, 2u) substitutions
at the C1-spiro center in excellent yields from the corres-
ponding spirocyclic-benzochromenes.
10%
nr
Toluene, 120 °C, 6 h 10%
H2O, 100 °C, 48 h nr
a Oil bath temperature. b Isolated yield; nr = no reaction, PTSA:
p-toluene sulfonic acid, TFA: trifluoroacetic acid, CSA = 10-camphor
sulfonic acid, OTf – trifluoromethane sulfonate.
As depicted in Scheme 3, the synthesis of benzochromene
(1) is known to begin from propargyl alcohol 3 and naphthol 4
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Fig. 2 ORTEP diagram of 2m (top) and 2ab (bottom) with 50%
probability.
Scheme 3 The rationale for the one-pot cascade protocol.
using acid catalysts (step-1),10 and here we discovered the TRR
of benzochromene (1) to indene (2) in the presence of an acid
catalyst (step-2, Scheme 3). Therefore, presuming that it is
possible to develop a one-pot cascade reaction,14 in such a way
that 3 and 4 will form benzochromene 1 with PTSA, and then
ring-rearrangement of 1 should give indene derivative 2
(Scheme 3).
To implement this one-pot proposal, a mixture of 2-methyl-
4-phenylbut-3-yn-2-ol (3a), 2-naphthol and PTSA was heated at
110 °C under solvent-free conditions and the formation of 3,3-
dimethyl-1-phenyl-3H-benzo[f]chromene (1a) was observed in
20 min, and delightfully, further continuation of the reaction
yielded the indene 2a in 80% through the ring-rearrangement
reaction (Scheme 4). Knowing the importance of a one-pot,
cascade protocol towards a sustainable synthetic process,14 we
were encouraged to elaborate this method to synthesize some
of the inden-3-yl-naphthols with diversity at the quaternary
center (2f, 2z and 2aa) and with the spirocyclic system (2p, 2ab
and 2ac) in a good overall yield. The structure of 2ab was
further confirmed by using single crystal X-ray data (Fig. 2).13
Scheme 2 Substrate scope of TRR reaction.
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observed. This result emphasizes the importance of having an
aryl group at C1 to accomplish the TRR reaction (eqn (i),
Scheme 5). The second experiment was performed with benzo-
chromene 1a bearing a phenyl group at C1 with chemically
equivalent ortho-carbons and accomplished the TRR (eqn (ii),
Scheme 5). The third reaction was performed with a benzo-
chromene (6) bearing a meta-tolyl group (with chemically
inequivalent ortho-carbons) and a regioisomeric mixture of
two indene derivatives (6a/6b) was obtained in a 1 : 1.24 ratio
with 96% combined yield (eqn (iii), Scheme 5).15 The fourth
experiment commenced with the benzochromene 7 bearing a
phenanthrenyl group at C1, with a single reactive ortho-carbon
and accomplished the TRR to yield indene derivative 7a in
78% yield (eqn (iv), Scheme 5). All these experiments (i–iv) con-
cluded that it is must to have an aryl (phenyl) group at C1 with
at least one reactive ortho-carbon to accomplish the TRR reac-
tion. Later, we performed another reaction to check the impor-
tance of a quaternary center at C3 in TRR. Therefore we chose
1,3-diphenyl-3H-benzo[f]chromene (8), having a phenyl group
with a reactive ortho carbon at C1 but not a quaternary center
at C3 and subjected it to s.r.c., surprisingly, we found that the
reaction did not yield the desired product (eqn (v),
Scheme 5).16 This experimental observation as a whole
suggests that the TRR works with benzochromenes bearing a
C3-quaternary center and aryl group at C1 with at least one
reactive ortho-carbon.
Scheme 4 One-pot synthesis. A mixture of 3 (1 equiv.), 4 (1.2 equiv.)
and PTSA (30 mol%) was heated at 110 °C (oil bath temperature) for a
specified time.
After developing a new TRR reaction of benzochromenes
(Scheme 2) to indene derivatives and the one-pot reaction
from propargyl alcohols (Scheme 4), we were interested in
checking the scope and limitations of this method by perform-
ing control experiments as depicted in Scheme 5. The first
reaction was performed with benzochromene 5 bearing a
cyclohexyl moiety at C1 under standard reaction conditions
(s.r.c.) and found that no change in the starting material was
Based on the observations made through the control experi-
ments the plausible reaction mechanism for the PTSA cata-
lyzed, thermal-ring-rearrangement (TRR) of benzochromene
(1) to inden-3-yl-naphthol (2) is proposed in Scheme 6. Here,
the Brønsted acid protonates the benzochromene 1 to form
the oxonium 1A, which then undergoes the ring-opening to
yield carbocation intermediate 1B. Isomerization of 1B to 1C
followed by an intramolecular Friedel–Crafts cyclization and
aromatization cascade provides the indene 2 with a ring
rearrangement.
Scheme 5 Control experiments.
Scheme 6 The reaction mechanism for the TRR of benzochromenes.
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Conclusions
In conclusion, we disclose here the first report on PTSA cata-
lyzed, intramolecular thermal-ring-rearrangement (TRR) of
benzochromenes to indene derivatives. Based on control
experiments we confirmed that the presence of an aryl group
with at least one reactive ortho-carbon at C1 and a quaternary
center at C3 is must for the TRR of benzochromenes. We
observed that these compounds exhibit atropisomerism, and
in the case of 2g, 2h, 2j, 2o, and 2aa we have noticed the for-
mation of diastereomers in the 1H and 13C NMR spectra.
Taking advantage of the benzochromene synthesis, from pro-
pargyl alcohols and naphthols, we developed further a green
synthetic protocol by combining the benzochromene synthesis
and their TRR in a one-pot, cascade strategy under solvent-free
conditions. Further investigations towards the TRR concepts
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