Reinvestigation of the synthetic and mechanistic aspects of Mn(III) acetate mediated oxidation of enones

Article abstract of DOI:10.1016/j.tet.2004.02.039

Mn(OAc)₃ mediated α′-acetoxylation of α,β-unsaturated enones is reinvestigated from a synthetic and mechanistic point of view and an improved procedure based on the use of acetic acid as a co-solvent is presented. Excellent results were obtained for a variety of structurally diverse and synthetically important enones under the optimized conditions.

Full text of DOI:10.1016/j.tet.2004.02.039

Tetrahedron 60 (2004) 3427–3432
Reinvestigation of the synthetic and mechanistic aspects of
Mn(III) acetate mediated oxidation of enones
¨
Ayhan S. Demir, Omer Reis and A. Cigdem Igdir
*
Department of Chemistry, Middle East Technical University, Inonu Bulvari, 06531 Ankara, Turkey
Received 1 December 2003; revised 30 January 2004; accepted 19 February 2004
Abstract—Mn(OAc)3 mediated a0-acetoxylation of a,b-unsaturated enones is reinvestigated from a synthetic and mechanistic point of view
and an improved procedure based on the use of acetic acid as a co-solvent is presented. Excellent results were obtained for a variety of
structurally diverse and synthetically important enones under the optimized conditions.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
carry out the desired conversion, and irreproducible yields/
reaction times were observed under the same set of
conditions. A representative example of this is the a0-
acetoxylation of 1-indanone (1a) to 2-acetoxy-1-indanone
(2a) that is a key precursor of the HIV protease inhibitor
indinavir. Hiyama4 reported a 53% yield as compared to the
82% previously reported by Demir.5a Possibly, the most
striking contradictory observation is the different chemo-
selectivity observed for the same substrate under seemingly
identical reaction conditions. For example, Mn(OAc)3
mediated oxidation of b-ethoxy cyclohexenone 1c was
recently reported to be giving tandem a0-acetoxylation, and
a0-phenylation product 3 in a 56% yield together with a
35% acetoxylation product 2c. This observation was
shown to be general for a variety of b-alkoxy cyclo-
hexenone and cyclopentenone derivatives.6 The presence of
the arylation product was explained on the basis of fast
trapping of the a0-keto radical by the solvent benzene.
We reported the acetoxylation of b-methoxy cyclo-
hexenone in a 79–83% yield and only a trace amount
of unidentified arylated products were detected in crude
1H NMR spectra.7
Selective a0-acetoxylation of a,b-unsaturated ketones 1
provides key precursors for pharmaceutically important
compounds and useful chiral ligands.1 As Williams and
Hunter reported that the Mn(OAc)3 oxidation of enones in
AcOH produces a0-acetoxy enones, with low yields when
compared to more commonly used lead(IV) acetate, the
utility of this reagent was diminished.2 Watt and co-workers
greatly improved the yields by using excess dried
Mn(OAc)3 in refluxing benzene.3 Today, Mn(OAc)3
mediated acetoxylation is one of the most useful methods
for the synthesis of a0-acetoxy a,b-unsaturated ketones 2
(Scheme 1).
Scheme 1.
Although successful a0-acetoxylation of a great variety of
substrates have been reported so far by us and others
(Table 1), there are some problems associated with the use
of Mn(OAc)3. A brief list of them is as follows: (1) excess
Mn(OAc)3 (4–6 equiv.) is generally used for acceptable
yields and reaction times; (2) many contradictory results can
be seen when literature reports are closely inspected. These
include, the amount of Mn(OAc)3 which was employed to
These inconsistencies and the use of an undesirable amount
of Mn(OAc)3 reduced the value of the method. This
indirectly indicates that the mechanism and the factors
governing the outcome of this reaction are not so clear.
Considering that there are not many simple methods for the
direct acetoxylation of enones, optimization of Mn(OAc)3
Keywords: Enones; Oxidation; Mn(OAc)3; Alkenes.
*
Corresponding author. Tel.: þ90-312-2103242; fax: þ90-312-2101280;
e-mail address: asdemir@metu.edu.tr
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.02.039

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A. S. Demir et al. / Tetrahedron 60 (2004) 3427–3432
Table 1. Yields and structures of a0-acetoxy a,b-unsaturated ketonesa
Entry
1
Enone 1
Acetoxy enone 2
Yield (%)b
98
Time (h)
1
Yield (%)c
1a
1b
53, A4
82, A5a
2
97
7
81,d A7
35, A6
3
4
1c
99
98
7
5
78, B10
1d
59, B11
86, A5a
5
6
1e
1f
97
96
5
5
82, A5b
8212
7
8
1g
1h
96
75
5
2
8912
65, A13
9
1i
1j
83
98
2
81, A13
10e
11
20, B14
a
1 mmol enone and 1.25 mmol Mn(OAc)3 in 11 mL benzene–AcOH (10:1) was stirred under reflux; although 1.25 equiv. of Mn(OAc)3 was used for full
conversion, the addition of 0.25 equiv. of Mn(OAc)3 towards the end of the reaction greatly shortens the reaction times.
Isolated yields.
Yields of previously reported acetoxylations mediated by Mn(III) acetate (Method A) or lead(IV) acetate (Method B).
For methoxy derivative.
1:6 mixture of easily separable isomers was obtained.
b
c
d
e
mediated a0-acetoxylation of enones and reaching its
2. Results and discussion
maximum potential has a great importance from a synthetic
and economical point of view. Herein we report our
investigation towards understanding the nature of this
reaction together with increasing its efficiency and
reproducibility.
We started our investigations with the synthesis of
Mn(OAc)3·2H2O from Mn(OAc)2·4H2O and KMnO4
according to well established methods8 in order to provide
consistency for the source of Mn(OAc)3 because Bush and

A. S. Demir et al. / Tetrahedron 60 (2004) 3427–3432
3429
Scheme 2.
Finkbeiner9 showed that the reactivity of Mn(OAc)3 may
change with the way it is synthesized. Next, we chose
1-indanone 1a as the model substrate for our experiments
and investigated its reaction with 4 equiv. Mn(OAc)3. We
first directed our attention to the reactivity of Mn(OAc)3 and
conversion times. According to the optimized procedure by
Watt,3 Mn(OAc)3 was dried over P2O5 under high vacuum
to remove water and was then reacted with 1a in refluxing
benzene monitored by TLC and GC–MS. Although a
smooth reaction was observed, conversion was slow,
therefore Mn(OAc)3 was further dried in a heating gun
(refluxing xylene) under high vacuum to obtain a dark
brown colored Mn(OAc)3. Reaction with this extensively
dried Mn(OAc)3 provided only a trace amount of product.
An interesting property of this dry Mn(OAc)3 was the lack
of AcOH odor that is typical for any Mn(OAc)3 either from
a commercial source or synthesized from Mn(OAc)2·4H2O
and KMnO4. Considering that the absence of AcOH might
be responsible for the slow conversions, we carried out the
reaction in a AcOH–benzene (1:100) mixture. Reaction
under this condition was quite successful with increased
conversion rates. A careful monitoring revealed that
increasing the AcOH content shortened the reaction time
and full conversion was observed after 15 min at 1:10
AcOH–benzene affording 2a in .99% yield (GC–MS).
This is quite an improvement when compared to previous
reports in terms of its yield, reaction time and reproduci-
bility. Mn(OAc)3 from a commercial source provided
similar results as long as it was extensively dried. Although
benzene is the most frequently used solvent after the report
by Watt, we found that cyclohexane and MeCN can also be
used instead of benzene and a0-acetoxylation was the only
detectable product in a GC–MS analysis despite slower
conversion rates. Reactions in THF and DMF were too slow
to be useful for a0-acetoxylation. It is worth mentioning that
acetic anhydride as a co-solvent instead of acetic acid shows
a similar rate enhancement whereas the latter is more
effective. Another optimization was made for the determi-
nation of the amount of Mn(OAc)3 required for full
conversion. Based on the largely accepted radical mechan-
ism, a minimum 2 equiv. of Mn(OAc)3 was necessary. After
a series of reactions, we have found that 1 equiv. of
Mn(OAc)3 is sufficient for this conversion. A slight excess
(1.25 equiv.) of the Mn(OAc)3 was used to ensure complete
conversion. Several structurally diverse enones were tested
under optimized conditions for 1a and where possible a
comparison was made with previously reported results as
shown in Table 1.
Although conditions were not optimized for any particular
enone type, excellent results were obtained for a variety of
enones. As can be seen in Table 1, these are the best
available yields for the specified conversion except the fact
that highly toxic reagent thallium(III) triflate (not shown in
table) provides 2a in a 99% yield.15 Considering many high
yield (.90%) hydrolysis methods for a-acetoxy groups that
were developed thus far, this optimized protocol can also be
considered as one of the most useful ways to obtain a0-
hydroxy a,b-unsaturated ketones.16 Generally, no product
other than acetoxylation was detected in the 1H NMR
spectrum of crude products and they were pure enough for
further synthetic manipulations. An apparent exception was
observed for the cyclopentenones 1h and 1i in which a small
amount of side products were detected in crude mixtures by
NMR and GC–MS. These products were identified for 1i
and found to be an a0-phenylation product 4i, a0-phenyl-
ation and a0-acetoxylation product 5i and a dimerization
product 6i. Products 4i, 5i and 6i (two isomers were
separated providing a 1:2.5 ratio) were isolated in an
approximate ratio of 1:2.5:2 accounting for ,15% of the
starting material. These seem to be pointing out the
intermediacy of 4i, which is acetoxylated at the benzylic
position or dimerized in an alternative pathway (Scheme 2).
Thus, phenylation instead of acetoxylation is a minor
alternative route, resulting in aromatic signals in NMR
spectra of crude products of Mn(OAc)3 mediated
Scheme 3.

3430
A. S. Demir et al. / Tetrahedron 60 (2004) 3427–3432
acetoxylation reactions. In a previous report, the formation
of products 6i and 7i was reported, but not 4i and 5i.13
Although these products do not have any importance from a
synthetic point of view, they would be useful towards
understanding the nature of the reaction as discussed below.
presented conditions seem to be the best choice. However,
combinations of these possibilities may prove to be more
useful for different substrates and applications. We have
also shown in unoptimized reactions that saturated ketones
can be substrates of the Mn(OAc)3 mediated acetoxyl-
ation.18 Besides, these optimized conditions can be useful
for the intermolecular addition of ketones to alkenes
previously reported to be a low yield process that generally
requires the use of an excess amount of carbonyl compound.
Applicability and generality of these reactions together with
the kinetic and mechanistic investigations in this context
will be reported in due course. As a result, this report will be
helpful by providing not only reliable and reproducible
results for the Mn(OAc)3 mediated acetoxylation of enones,
but also a better understanding of the reaction for other
applications utilizing Mn(OAc)3.
It is hard to justify the role of acetic acid, but it could be
related to an increased solubility of Mn(OAc)3 in the
reaction mixture. Although the structure of Mn(OAc)3 has
been shown to be an oxo-centered triangle of Mn(III) with
bridging acetates,17 the prevalent form in benzene and a
benzene–acetic acid mixture can be quite different resulting
in different conversion rates and isolated yields.
For the mechanism of the reaction, both radical mechanism
(route a) and ligand transfer via metal-enolate intermediate
(route b, proposed based on Pb(OAc)4 mediated acetoxyl-
ation of carbonyl compounds) have been proposed
(Scheme 3).2 Since Mn(OAc)3 is a single electron oxidant
and a vast majority of the reactions mediated by it have been
shown to be taking place via a radical mechanism, route a is
widely accepted.17
3. Experimental
NMR spectra were recorded on a Bruker DPX 400.
Chemical shifts d are reported in ppm relative to CHCl3
(1H: d¼7.26) and CDCl3 (13C: d¼77.0) as an internal
standard. Column chromatography was conducted on silica
gel 60 (mesh size 40–63 mm). IR spectra were obtained
from a Perkin–Elmer Model 1600 series FT-IR spec-
trometer and are reported in cm21. TLC was carried out on
aluminum sheets precoated with silica gel 60F254 (Merck),
and the spots were visualized with UV light (l¼254 nm).
GC–MS spectra were determined using a ThermoQuest
(TSP) TraceGC-2000 Series equipped with Phenomenex
Zebron ZB-5 capillary column. All known compounds have
data in agreement with the previously published values; 2a,
d,5a 2b,6 2c,10 2e,5b 2f, g,12 2h, i,13 4i,22 6i, 7i,13 10,18
Mn(OAc)3.2H2O was prepared as reported previously (Ref.
8, p 308). As reported by Bush and Finkbeiner,9 increasing
the amount of water at the end of the reaction yields
Mn(OAc)3 that results in much slower conversions and
slightly reduced yields, possibly related to longer reaction
times.
Although discrimination between the mechanisms is not a
trivial question at this stage, we wish to underline a few
points and address the questions to be clarified to improve
our understanding of these types of processes. First, we
carried out preliminary reactions to compare the behavior of
enones with saturated ketones in the presence or absence of
vinyl acetate. Oxidation of cyclohexanone afforded
acetoxylation product 1018 together with side products in
an unoptimized reaction (GC–MS). Same reaction in the
presence of vinyl acetate mainly afforded product 11 along
with little acetoxylation. Oxidation of 1a in the presence of
vinyl acetate afforded acetoxylation together with small
amount (,15%) of alkene addition product 12 in benzene
(Scheme 4). Different behavior of enones and saturated
ketones might be the indication of different mechanisms
(routes a and b in Scheme 3) depending on the types of
substrates and solvents.19 The structure and the amount
of Mn(OAc)3 employed in this study, possible presence of
homolysis labile organomanganese20 or manganese bound
radical21 intermediates should be clarified to elucidate the
preferred mechanistic pathways and widen the scope of the
reaction, a subject already under investigation.
3.1. General procedure for the a0-acetoxylation of a,b-
unsaturated ketones
A solution of 1 mmol a,b-unsaturated ketone and
1.25 mmol Mn(OAc)3 in 11 mL benzene–AcOH (10:1)
was stirred under reflux (Dean–Stark apparatus) during
which the dark brown color of Mn(OAc)3 disappeared by
time which was also monitored by GC–MS and TLC. After
all starting material was consumed, the reaction mixture was
diluted with ether and washed with brine. Resulting organic
phase was dried over MgSO4 and concentrated under
vacuum. If necessary, crude products were purified by
column chromatography using EtOAc–hexane as eluent. In
some cases, direct filtering of the reaction mixture through a
pad of silica provided pure acetoxy enones. Reactions with
5–10 mmol substrates worked equally well.
Scheme 4.
In conclusion, we have presented an improved procedure
based on the use of acetic acid as a co-solvent. From a
synthetic point of view, excellent results were obtained for a
variety of structurally diverse and synthetically important
enones under optimized conditions. From an economical
point of view, as low as 1.25 equiv. Mn(OAc)3 can be used
as compared to the previously used 4–6 equiv. Moreover,
MeCN and cyclohexane can also be used instead of benzene
and acetic anhydride instead of acetic acid even though the
For the oxidation of cyclohexanone and alkene addition
reactions, the same procedure was applied according to the
following points; 2 equiv. of alkene with respect to carbonyl
compound was used for addition reactions. Due to the high
volatility of vinyl acetate, Dean–Stark apparatus was

A. S. Demir et al. / Tetrahedron 60 (2004) 3427–3432
3431
removed after refluxing the Mn(OAc)3 solution for 15 min
and substrates were added on to the resulting mixture.
3.1.5. 2-(2,2-Diacetoxy-1-ethyl) indanone (12). Colorless
viscous oil, IR (CHCl3, cm21) 1712, 1759; 1H NMR
(CDCl3) d 1.78 (1H, m), 1.97 (3H, s), 2.03 (3H, s), 2.48 (1H,
m), 2.66 (1H, m), 2.89 (1H, dd, J¼17.1, 4.5 Hz), 3.35 (1H,
dd, J¼8.1, 17.1 Hz), 6.88 (1H, dd, J¼4.6, 6.6 Hz), 7.3 (1H,
m), 7.6 (1H, d, J¼7.3 Hz), 7.52 (1H, m), 7.6 (1H, d,
J¼7.7 Hz); 13C NMR (CDCl3) d 21.1, 33.4, 35.0, 43.2, 89.7,
124.4, 126.7, 127.8, 135.1, 136.6, 153.4, 168.6, 168.7,
206.1. Anal. Calcd for C15H16O5: C, 65.21; H 5.84 found C,
65.31; H, 5.97.
3.1.1. 2-Acetoxy-4-cholesten-3-one (2j).14 Major isomer.
White solid, mp 102–104 8C (EtOAc–hexane); IR (CHCl3,
1
cm21) 1683, 1739, 2867, 2948; H NMR (CDCl3) d 0.63
(3H, s), 0.79 (6H, d, J¼5.5 Hz), 0.83 (3H, d, J¼6.5 Hz),
0.9–1.1 (9H, m), 1.12 (3H, s), 1.15–1.6 (10H, m), 1.7–2.0
(4H, m), 2.07 (3H, s), 2.14–2.23 (2H, m), 2.37–2.47 (1H,
m), 5.25 (1H, dd, J¼12.5, 4.9 Hz), 5.7 (1H, s); 13C NMR
(CDCl3) d 12.4, 18.9, 21.3, 22.6, 22.9, 23.2, 24.2, 24.5, 28.3,
28.6, 33.3, 35.0, 36.1, 36.4, 37.8, 39.8, 39.9, 41.4, 43.2,
51.0, 56.1, 56.4, 70.9, 120.8, 129.4, 170.6, 173.8, 194.0.
Anal. Calcd for C29H46O3: C, 78.68; H 10.47 found C,
78.85; H, 10.66
Acknowledgements
Financial support from the Middle East Technical
University (BAP-2003), Scientific and Technical Research
Council of Turkey (TUBITAK), Turkish Academy of
Sciences (TUBA) and Turkish State Planning Organization
(DPT) is gratefully acknowledged.
Minor isomer. White solid, mp 121–123 8C (EtOAc–
1
hexane); IR (CHCl3, cm21) 1686, 1742, 2868, 2944; H
¨
NMR (CDCl3) 0.63 (3H, s), 0.79 (6H, d, J¼6.5 Hz), 0.84
(3H, d, J¼6.4 Hz), 0.85–2.35 [22H(1.24, CH3 and 2.10,
CH3), m], 5.38 (1H, dd, J¼15, 5.3 Hz); 5.67 (1H, s); 13C
NMR (CDCl3) d 12.3, 18.5, 19.0, 21.2, 21.3, 22.9, 23.2,
24.1, 24.4, 28.4, 28.5, 32.3, 32.9, 35.4, 36.1, 36.5, 39.84,
39.88, 41.0, 41.8, 42.7, 54.7, 56.1, 56.4, 71.7, 122.0, 170.7,
171.6, 194.1. Anal. Calcd for C29H46O3: C, 78.68; H 10.47
found C, 78.75; H, 10.57.
References and notes
1. Demir, A. S.; Jeganathan, A. Synthesis 1991, 235.
2. Williams, G. J.; Hunter, N. R. Can. J. Chem. 1976, 54, 3830.
3. Dunlap, N. K.; Sabol, M. R.; Watt, D. S. Tetrahedron Lett.
1984, 25, 5839.
3.1.2. 5-Acetoxy-3-methyl-5-phenyl-2-cyclopentenone
(5i). Colorless oil, IR (CHCl3, cm21) 1624, 1719, 3020;
1H NMR (CDCl3) d 2.09 (3H, s), 2.16 (3H, d, J¼1 Hz),
3.05 (1H, d, J¼18.3 Hz), 3.24 (1H, d, J¼18.3 Hz), 5.99
(1H, d, J¼1 Hz), 7.2–7.3 (5H, m); 13C NMR (CDCl3) d
19.9, 21.5, 47.2, 84.9, 124.9, 128.51, 128.54, 128.7, 138.2,
170.1, 174.2, 202.4; MS (EI), m/z 230 (Mþ., 7), 187 (33),
170 (15), 141 (10), 128 (9), 105 (100), 76 (29). Anal.
Calcd for C14H14O3: C, 73.03; H 6.13 found C, 73.15; H,
6.24.
4. Kajiro, H.; Mitamura, S.; Mori, A.; Hiyama, T. Tetrahedron:
Asymmetry 1998, 9, 907.
5. (a) Demir, A. S.; Hamamci, H.; Tanyeli, C.; Akhmedov, I. M.;
Doganel, F. Tetrahedron: Asymmetry 1998, 9, 1673. (b) Demir,
A. S.; Camkerten, N.; Akgun, H.; Tanyeli, C.; Mahasneh, A. S.;
Watt, D. S. Synth. Commun. 1990, 20, 2279.
¨
6. Tanyeli, C.; Ozdemirhan, D.; Sezen, B. Tetrahedron 2002, 58,
9983.
7. Demir, A. S.; Sesenoglu, O. Org. Lett. 2002, 4, 2021.
8. de Klein, W. J. In Organic synthesis by oxidation with metal
compounds; Mijs, W. J., de Jonge, C. R. H., Eds.; Plenum:
New York, 1986; pp 261–314.
3.1.3. 4,040-Dimethyl-1,10-diphenyl-bicyclopentyl-3,30-
diene-2,2 -dione (6i).13 Separation of isomers provided a
2.5:1 ratio.
9. It has been shown that the amount of water used for the
synthesis change the yield and the nature of the Mn(OAc)3
together with the rate of the reaction. However, yields were
relatively unaffected: Bush, J. B., Jr.; Finkbeiner, H. J. Am.
Chem. Soc. 1968, 90, 5903.
Major isomer. White solid, mp .165 8C (decompose); IR
1
(CHCl3, cm21) 1629, 1683; H NMR (CDCl3) d 1.71 (6H,
s), 2.94 (2H, d, J¼19.6 Hz), 3.95 (2H, d, J¼19.6 Hz), 5.47
(2H, s), 7.05–7.2 (6H, m), 7.57 (4H, d, J¼7.4 Hz); 13C
NMR (CDCl3) d 19.1, 45.9, 60.7, 127.4, 128.1, 129.9, 130.6,
138.9, 177.8, 209.7
10. Ellis, J. W. J. Org. Chem. 1969, 34, 1154.
11. Naemura, K.; Wakebe, T.; Hirose, K.; Tobe, Y. Tetrahedron:
Asymmetry 1997, 8, 2585.
Minor isomer. White semi-solid; IR (CHCl3, cm21) 1630,
1689; 1H NMR (CDCl3) d 2.0 (6H, s), 2.51 (2H, d,
J¼18 Hz), 3.9 (2H, d, J¼18 Hz), 5.74 (2H, s), 7.05 (4H, m),
7.1–7.2 (6H, m); 13C NMR (CDCl3) d 19.7, 46.0, 60.1,
127.6, 127.7, 129.1, 129.7, 139.7, 177.0, 209.3.
12. Demir, A. S.; Aybey, A.; Sesenoglu, O.; Polat, F. Tetrahedron:
Asymmetry 2003, 14, 1489.
13. Tanyeli, C.; Tosun, A.; Turkut, E.; Sezen, B. Tetrahedron
2003, 59, 1055.
14. (a) Mihailovic, M. L.; Forsek, J.; Lorenc, L.; Maksiomovic, Z.;
Fuhrer, H.; Kalvoda, J. Helv. Chim. Acta 1969, 52, 459.
Improved Pb(OAc)4 mediated acetoxylation of steroidal
substrates has been reported; for recent examples see:
(b) Ferreira, M. R. C.; Hernando, J. I. M.; Lena, J. I. C.;
Moral, J. Q.; Arseniyadis, S. Tetrahedron: Asymmetry 1999,
10, 1527. (c) Hernando, J. I. M.; Ferreira, M. R. C.; Lena, J. I.
C.; Toupet, L.; Birlirakis, N.; Arseniyadis, S. Tetrahedron:
Asymmetry 1999, 10, 3977.
3.1.4. 2-(2,2-Diacetoxy-1-ethyl) cyclohexanone (11).
Colorless liquid, IR (CHCl3, cm21) 1709, 1757, 2863,
2938; 1H NMR (CDCl3) d; 1.34 (1H, m), 1.47 (1H, m), 1.60
(1H, m), 1.81 (1H, m), 1.98 (3H, s), 2.0 (3H, s), 2.16–2.38
(6H, m); 6.70 (1H, dd, J¼5.7, 5.3 Hz); 13C NMR (CDCl3) d
21.0, 25.5, 28.3, 33.3, 34.8, 42.1, 46.0, 89.7, 168.7, 168.8,
210.3. Anal. Calcd for C12H18O5: C, 59.49; H 7.49 found C,
59.65; H, 7.54.
15. Lee, J. C.; Jin, Y. S.; Choi, J. Chem. Commun. 2001, 956.

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A. S. Demir et al. / Tetrahedron 60 (2004) 3427–3432
16. Kajiro, H.; Mitamura, S.; Mori, A.; Hiyama, T. Bull. Chem.
Soc. Jpn. 1999, 72, 1553.
Tetrahedron 1996, 52, 6073. Successful intermolecular
addition of enones to alkenes providing dihydrobenzofurans
was reported: (b) Snider, B. B.; Han, L.; Xie, C. J. Org. Chem.
1997, 62, 6978.
17. (a) Snider, B. B. Chem. Rev. 1996, 96, 339. (b) Iqbal, J.;
Bhatia, B.; Nayyar, N. K. Chem. Rev. 1994, 94, 519.
(c) Melikyan, G. G. Synthesis 1993, 833.
20. We recently showed the potential of Mn(OAc)3 to mediate
18. It has been shown that Mn(OAc)3 oxidation of saturated
ketones provide complex mixtures: Okano, M.; Aratani, T.
Bull. Chem. Soc. Jpn. 1976, 49, 2811.
19. Snider stated that oxidation of a0-keto radical derived from
enones is a very fast process relative to oxidation of radicals
derived from saturated ketones. Attempted alkene addition to
an enone in the presence of 1-octene provided only
acetoxylation and oligomer: (a) Snider, B. B.; Kiselgof, E. Y.
reactions that is possibly based on reductive elimination of
¨
arylmanganese intermediates: Demir, A. S.; Reis, O.;
Emrullahoglu, M. J. Org. Chem. 2003, 68, 10130.
21. Curran, D. P.; Morgan, T. M.; Schwartz, C. E.; Snider, B. B.;
Dombroski, M. A. J. Am. Chem. Soc. 1991, 113, 6607.
22. Murakami, M.; Takahashi, K.; Amii, H.; Ito, Y. J. Am. Chem.
Soc. 1997, 119, 9307.