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M. Rueping, E. Sugiono
the BINOL-phosphate catalyzed reduction of the tri-substituted pyridines 18, whereby the reaction parameters including the solvent, temperature, catalyst loading, substrate concentration and the hydride
source were varied. The results showed that our new enantioselective,
Brønsted-acid catalyzed cascade transfer hydrogenation of pyridines
can indeed be performed and that the partially reduced enantiomerically
enriched pyridines 19 can be isolated (Table 7).
In general this organocatalytic pyridine reduction procedure provides
azadecalinones 19a–f as well as the tetrahydropyridines 19g–j in good
isolated yields and with excellent enantioselectivities (up to 92% ee)
(Rueping and Antonchick 2007).
The newly developed enantioselective organocatalytic reduction of
pyridines can, for example, be used as a key step in the synthesis of decahydroquinolines from the pumiliotoxin family (Scheme 3). Hence, the
BINOL-phosphate catalyzed reduction of the corresponding pyridine,
which can readily be prepared according to a procedure described by
Bohlmann and Rahtz (Bohlmann and Rahtz 1957; Bagley et al. 2002),
gives the corresponding 2-propyl-hexahydroquinolinone. This can subsequently be transformed, according to a reported sequence into diepipumiliotoxin C (Sklenicka et al. 2002).
The first enantioselective Brønsted acid catalyzed cascade reduction
of pyridines provides the corresponding products in good yields and
with excellent enantioselectivities (up to 92% ee). The hexahydroquinolinones and tetrahydropyridines isolated are not only known starting
products for various natural products such as pumiliotoxin C but also
for numerous interesting biologically active compounds. As previously
only metal catalyzed hydrogenations of pyridines have been described,
and these did not result in the valuable products described herein and
gave lower enantioselectivities, our newly developed metal-free Brønsted acid catalyzed procedure represents an important contribution to
the enantioselective reduction of pyridines.
M. Rueping, E. Sugiono
the BINOL-phosphate catalyzed reduction of the tri-substituted pyridines 18, whereby the reaction parameters including the solvent, temperature, catalyst loading, substrate concentration and the hydride
source were varied. The results showed that our new enantioselective,
Brønsted-acid catalyzed cascade transfer hydrogenation of pyridines
can indeed be performed and that the partially reduced enantiomerically
enriched pyridines 19 can be isolated (Table 7).
In general this organocatalytic pyridine reduction procedure provides
azadecalinones 19a–f as well as the tetrahydropyridines 19g–j in good
isolated yields and with excellent enantioselectivities (up to 92% ee)
(Rueping and Antonchick 2007).
The newly developed enantioselective organocatalytic reduction of
pyridines can, for example, be used as a key step in the synthesis of decahydroquinolines from the pumiliotoxin family (Scheme 3). Hence, the
BINOL-phosphate catalyzed reduction of the corresponding pyridine,
which can readily be prepared according to a procedure described by
Bohlmann and Rahtz (Bohlmann and Rahtz 1957; Bagley et al. 2002),
gives the corresponding 2-propyl-hexahydroquinolinone. This can subsequently be transformed, according to a reported sequence into diepipumiliotoxin C (Sklenicka et al. 2002).
The first enantioselective Brønsted acid catalyzed cascade reduction
of pyridines provides the corresponding products in good yields and
with excellent enantioselectivities (up to 92% ee). The hexahydroquinolinones and tetrahydropyridines isolated are not only known starting
products for various natural products such as pumiliotoxin C but also
for numerous interesting biologically active compounds. As previously
only metal catalyzed hydrogenations of pyridines have been described,
and these did not result in the valuable products described herein and
gave lower enantioselectivities, our newly developed metal-free Brønsted acid catalyzed procedure represents an important contribution to
the enantioselective reduction of pyridines.