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R. S. Phatake, C. V. Ramana / Tetrahedron Letters 56 (2015) 3868–3871
Oxone (2eq.)
Table 1 (continued)
NaHCO3 (12eq.)
O
O
Entry
14
Substrate
MeO
Product
Yieldb (%)
acetone:H2O:EtOAc (5:1:1)
rt, 20h
MeO
1p
1q
3p (69%)
3q (68%)
O
p-tolyl
69
p-tolyl
''
1o
3o
Reaction conditions.
a
Substrate 1 (1 equiv), oxone (2 equiv), NaHCO3 (12 equiv), acetone/H2O/EtOAc
(5:1:1), rt, 15–20 h.
PdCl2 (10 mol %)
Cu(OAc)2 (1.2 eq.)
b
Isolated yields.
O
O
2 balloon pressure
CH3CONMe2:H2O (4:1)
rt, 10 h
employed was found to be tolerable for a range of functional
groups like chloro and azide without affecting the yields of the
final products much (Table 1).
1q
5
(62%)
Scheme 3. Regioselectivity of Wacker-type oxidation of diene 1p and 1q.
To explore the scope of this reaction, we next examined the
oxidation of 1,2-dihydronaphthalene leading to 2-tetralones. 2-
Tetralones have been found to have a wide range of applications
as valuable intermediates in natural products synthesis and also
in the synthesis of pharmaceuticals.11 The 1,2-transposition of
scavenger for metals such as Pd, Cu(I), Cu(II), Ni, Pt, etc.) as an addi-
tive in the reaction medium.15 As shown in Scheme 2, there was no
interference from this added metal scavenger and there was no
change either in the time or the yield of the product. This control
experiment has precisely ruled out the possible involvement of
traces of metal impurities present in the reagents in the present
oxidation.
Next, examined was the olefin selectivity of the present process.
3-(but-3-en-1-yl)-1H-indene (1q) and 3-allyl-1H-indene (1p) have
been subjected for the present oxidation under established condi-
tions. Interestingly, the oxidation of both dienes 1p and 1q
occurred in a way such that the internal olefins only got oxidized
resulting in the corresponding indan-2-ones exclusively
(Scheme 3). Interestingly, when 1q was exposed to Pd-catalyzed
Wacker oxidation, the exclusive oxidation of the external olefin
without disturbing the internal olefin has been noticed. This com-
plementary selectivity clearly explains that the oxone–acetone
mediated oxidation is very selective for the benzo-fused olefins
despite the fact that this olefin is sterically hindered.
To conclude, we have documented the direct oxidation of inde-
nes and 1,2-dihydronapthalenes, respectively, to the correspond-
ing 2-indanones and 2-tetralones employing oxone–acetone and
sodium bicarbonate under relatively simple conditions. The control
experiments primarily reveal that the current reaction is similar to
the acetonide formation and did not occur due to the presence of
trace-metal impurities, nor is the epoxide an intermediate in the
path of the reaction. However, further investigation is underway
in our laboratory to rule out the other possibilities. In a nutshell,
it must be noted that a subtle variation in the reaction conditions
either in terms of the solvents employed and/or the amount
of base used seems to causes a complete change in the course of
the reaction. The current base-driven switching in reaction path
is very unusual and creates substantial opportunities for further
exploration.
the carbonyl group of a-tetralones is one of the generally adopted
direct methods for the preparation of b-tetralones.12 Also, the 1,2-
dihydronaphthalene derivatives have been converted into 2-te-
tralones by employing a sequence of hydroboration followed by
oxidation of the resulting alcohols.13
The 1,2-dihydronaphthalenes 1i–1o have been synthesized
following the established procedures and then subjected for the
oxone–acetone mediated olefin oxidation under the established
conditions. As shown in Table 1 (entries 8–14), in all the cases,
the reaction proceeded efficiently and provided the corresponding
2-tetralones with complete regioselectivity. It is worth mentioning
here that tetralones 3k and 3n have been prepared earlier in the
context of synthesis of an ABCD tetracyclic Bruceantin precursor
and a 2 step protocol has been employed.14
In order to have preliminary information on the course of this
oxidation reaction, some control experiments have been carried
out. At first, both epoxide 2a and acetonide 4a have been exposed
to the present conditions for a longer period. It has been found that
both 2a and 4a are intact which indicated that either epoxide or
the acetonide are not involved as intermediates in the current
transformation. Indeed, the GC–MS monitoring of this reaction
during the optimization studies has clearly indicated that none of
these intermediates are present. Next, the oxidation of 1a has been
conducted employing excess QuadraPure™ DMA (a known
Oxone (2 eq.)
NaHCO3 (12 eq.)
O
No Reaction
acetone:EtOAC:H2O
(5:1:1), rt, 24 h
2a
O
Oxone (2 eq.)
NaHCO3 (12 eq.)
Acknowledgments
O
No Reaction
acetone:EtOAC:H2O
(5:1:1), rt, 24 h
The authors would like to acknowledge the CSIR (India) for pro-
viding the financial support for this project under the NICE
(CSC0109). Financial support from CSIR (New Delhi) in the form
of a research fellowship to R.S.P. is gratefully acknowledged.
4a
1a
Oxone (2 eq.)
NaHCO3 (12 eq.)
O
acetone:EtOAC:H2O
(5:1:1), rt, 20 h
QuadraPure DMA
Supplementary data
3a
(72%)
Supplementary data (the NMR spectra of all new compounds)
associated with this article can be found, in the online version, at
Scheme 2. Control experiments conducted to examine the involvement of epoxide
and trace metal impurities.