8202
B. Batanero, F. Barba / Tetrahedron Letters 47 (2006) 8201–8203
O
O
O
O
O
N
H
N
+ e-
+ 1
O
O
- H.
N
N
O
_
-
H
a
1
O
H
O
O
O
N
N
H
N
O
O
O
O
.
O
-
work-up
+ O2
N
a
+
O2
O
.
b
COOH
O
O
H
N
HN
O
O
N
O
O
O
O
O
- CO2
- H2O
H2O
O
N
N
N
O
2
Scheme 1.
isatin recovered in the process. When platinum plate is
used as a cathode, the charge consumption of the pro-
cess is slightly higher (0.5–1 eÀ/substrate–molecule) than
when mercury is used as a cathode. Nevertheless, as isa-
tin is the side product, it can be recovered and electro-
lyzed again.
References and notes
1. Witt, A.; Bergman, J. Curr. Org. Chem. 2003, 7, 1–19;
Mitscher, L. A.; Wong, W. C.; De Meulenere, T.; Sulko,
J.; Drake, S. Heterocycles 1981, 15, 1017.
2. Baker, W. R.; Mitscher, L. A. US Patent 5, 441,955, 1995.
3. Bhattacharjee, A. K.; Skanchy, D. J.; Jennings, B.;
Hudson, T. H.; Brendle, J. J.; Werbovetz, K. A. Bioorg.
Med. Chem. 2002, 10, 1979.
4. Scovill, J.; Blank, E.; Konnick, M.; Nenortas, E.; Shapiro,
Th. Antimicrob. Agents Chemother. 2002, 46, 882.
5. Honda, G.; Tabata, M. Planta Med. 1979, 36, 85.
6. Fiedler, E.; Fiedler, H. P.; Gerhard, A.; Keller-Schierlein,
W. Arch. Microbiol. 1976, 107, 249.
7. Staskun, B.; Wolfe, J. F. S. Afr. J. Chem. 1992, 45, 5.
8. Eguchi, S. Top Heterocycl. Chem. 2006, 6, 113–156;
Eguchi, S.; Takeuchi, H.; Matsushita, Y. Heterocycles
1992, 33, 153.
However, preparative electrolysis using a platinum
cathode with a circulation of 50% of the theoretical
charge for a 0.5 eÀ/substrate–molecule process has
been performed. Once the potentiostat was switched
off, the crude mixture was divided into two identical
portions. The first one was immediately treated as indi-
cated in the experimental section,10 and the second one
was allowed to stay for 3 h under an argon atmosphere
and with continual stirring. Afterwards it was treated
in exactly the same way. It was observed that the yield
of 2 in the second portion was two times higher than
that of the first one. From this result it can be postu-
lated that with a platinum electrode the formation of
the anion a is slow, compared to a Hg cathode. A
plausible explanation is that when mercury is used,
the fresh electrogenerated anion immediately attacks
the carbonyl group of adsorbed 1. However with plat-
inum this attack takes place in solution and conse-
quently the formation of 2 is slow and needs time to
be completed.
9. Swern, D. In Organic Peroxides; Wiley Interscience. Wiley
and Sons, 1971; Vol. II, Chapter VII, pp 708–712.
10. The electrochemical reductions were performed under
constant-potential conditions in a concentric cell with two
compartments separated by a porous (D4) fritted-glass
diaphragm and equipped with a magnetic stirrer. A
mercury pool (20 cm2), a platinum plate (12 cm2) or a
lead plate (12 cm2) were used as the cathode, another
platinum plate as the anode, and a saturated calomel
electrode (SCE) as the reference. The solvent-supporting
electrolyte system (SSE) was nominally anhydrous dichlo-
romethane containing 0.05 M Et4NCl.
A solution of the electroactive isatin (3.0 mmol in 50 ml of
SSE) was electrolyzed, under an argon atmosphere, at a
constant potential of À0.9 V (vs SCE). The initial current,
200 mA, decreased as the substrate was consumed. Once the
reduction was finished, the solvent in the cathodic solution
was removed under reduced pressure. The residue was
extracted with diethyl ether (3 · 100 cm3)/H2O and the
organic phase dried over Na2SO4 and concentrated by
evaporation. The resulting solid was chromatographed on a
Acknowledgments
This study was financed by the Spanish Ministry of
Science and Education CTQ2004-05394/BQU. B.B.
thanks the Spanish Ministry of Science and Technology
for the ‘Ramon y Cajal’ financial support.
Batanero, Belen
