Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

RO
R 4
R 3
R 2
O N
R 1
O
P(OAlk) 3
R 4
R 3
REACTIVITY OF NITRONATES 535
R 2
O N
Alk – Me, Et; R1 – CO2Alk; R2 – OSiAlk3, aryl, But ;
R3 – H or alkyl; R4 – AlkylCO or AlkylO2C.
( ) n
H
H
Ar
O N
O N
CO2Me
O
SO 2Ph
O
P(OEt) 3
SnCl 2 • H 2O
RO
Scheme 3.114
( ) H
n
R 1
Ar
O N
O N
140a,b 141a,b
H
Scheme 3.115
65%–90%
SO2Ph
CO2Me
60%–75%
(1)
(2)
a - n = 1 (94%)
b - n = 2 (92%)
3.4.2.5.3. Hydrogenolysis of Cyclic Nitronates Since hydrogenolysis of substituted
cyclic nitronates afford polyfunctional derivatives, this reaction is of interest
for the development of a methodology of organic synthesis. However, this process
has been poorly studied.
Generally, hydrogenolysis involves cleavage of the endocyclic N–O bond
as the Þrst step. For example, four-membered cyclic nitronates can be reduced
successively to α-hydroxy-oximes (167) (Scheme 3.116, Eq. 1). Unfortunately,
the yield of the target product was not reported.
An analogous result was obtained by the hydrogenation of Þve-membered
cyclic nitronate. The latter reaction afforded α-oximino-γ-hydroxypentane-1,5dicarboxylic
acid derivative (Scheme 3.116, Eq. 2) in high yield (83%) (319).
Professor Denmark studied in detail hydrogenolysis of six-membered cyclic
nitronates containing the alkoxy group at the C-6 atom (137, 139). The mechanistic
interpretation of the reaction is given in Scheme 3.117.
Here hydrogenolysis also starts with the cleavage of the endocyclic N–O bond;
however, the resulting oximes A undergo the following transformations: hemiacetals
at the C-6 atom are transformed into the carbonyl group, and the oximino
fragment is successively hydrogenated to imine and then to amine followed by
condensation of the amino group with the carbonyl group. The resulting imine is
reduced to pyrrolidine B. Unstable pyrrolidines B are immediately tosylated in