2
L. Munive et al. / Tetrahedron Letters xxx (2017) xxx–xxx
Table 1
Reaction conditions for Manganese-mediated oxidation of methyl 1-hydroxy-2-naphthalene carboxylate.
Entry
Mn(III) (equiv)
Time (h)
Solvent
Temp (°C)
Ratioa 8a/9
Yield (%)
1
2
3
4
5
6
7
8
2
2
2
3
2
2
2
1.25
24
3
6
12
3
12
4
AcOH
35
35
35
35
80
80
80
80
25:75
24:76
15:85
–:100
50:50
–:100
100:–
100:–
92
92
90
89
88
89
97
98
AcOH:CH
AcOH:CH
AcOH:CH
AcOH:CH
AcOH:CH
Benzene
Benzene
3
CN, 1:1
3
CN, 1:1
3
CN, 1:1
3
CN, 1:1
3
CN, 1:1
4
a
Ratio determined by 1H NMR spectra of crude product.
Scheme 2. Dimerization of methyl 1-hydroxy-2-naphthalene-carboxylate.
Results and discussion
This is the first observation of a Wessely acetoxylation of aro-
matics with manganese triacetate. Adding an equal amount of
acetic acid and acetonitrile gave similar results in less time (entry
Scheme 3. Oxidation of methyl 1-hydroxy-2-naphthalene-carboxylates in AcOH:
CH CN.
3
2
). Increasing reaction time and the amount of manganese complex
resulted in complete acetoxylation of the naphthol exclusively on
the para-position to the hydroxyl group (entries 3 and 4). Increas-
ing the reaction temperature and three hours of stirring delivered
an equal mixture of dimer and acetoxylation products (entry 5).
Again, longer reaction times at high temperature gave only ace-
toxylation product when the reaction was run for twelve hours
We then explored the manganese triacetate oxidation in acetic
acid/acetonitrile with the same substrates 7a–c and 10a–c,
Scheme 3. Interestingly, employing this solvent system, the pro-
duct obtained depended on the substituent on C-3. Acetoxylation
occurred on the para position when naphthalenecarboxylate
lacked a substituents on C-3 and 8 (acetoxynaphthalene 9). Oxida-
tion to p-quinones occurred when the naphthalenecarboxylate
possessed an 8-methoxy group (p-quinone 12) or a 3-methyl group
(
entry 6). Interestingly, when the reaction was carried out in ben-
zene under anhydrous conditions at high temperature, only dimer-
ization occurred even with only a slight excess of metal complex
(p-quinone 13). An inseparable mixture of products was obtained
(
entries 7 and 8).
for carboxylate 10b. Wessely acetoxylation, solely on carbon-2,
occurred for the sterically hindered 3-phenylnaphthalenecarboxy-
lates 7c and 10c (products 14 and 15).
Having found reaction conditions that favored dimerization
over acetoxylation of the hydroxynaphthalene carboxylate, we
applied these to the dimerization of 3-substituted 1-hydroxy-
napthalenecarboxylates
7a–c
and
8-methoxy-1-hydroxy-
napthalenecarboxylates 10a–c, Scheme 2. Dimerization occurred
in excellent to good yield when substrates lacked a substituent
on C-3 (dimers 7a and 10a). Dimerization also occurred in excel-
lent and good yields when the 3-substituent was a methyl group
Conclusions
In summary, we investigated the Mn-triacetate oxidation of 1-
hydroxy-2-napthalene carboxylates. Dimerization occurred selec-
tively employing anhydrous conditions in benzene, except for the
(
dimers 8b and 11b). However, no reaction was observed when
the substituent was a phenyl group (7c and 10c). Lack of reactivity
towards dimerization of the 3-phenyl substituted hydroxynaph-
thalenecarboxylates is probably because of steric effects.
3
-phenyl derivatives. Oxidation employing acetic acid-acetonitrile
gave Wessely acetoxylated products or quinones depending on the
substituents of the napthalenecarboxylate. This oxidative homo-
Munive, Laura
