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

452 NITRONATES
RCOO OCOR + [PhSO2CH(NO2)]Li
Ph
OCO2Me
19
18
OCO 2Me
+ [PhSO 2CH(NO 2)]Li
Scheme 3.21
(dba)2Pd
Ph 3P, THF
Pdcl/ 2
Ph
5e
SO 2Ph
N O
O 94%(ee 95%)
O N
O
5f
SO 2Ph
of the anionic intermediate with the C,C double bond accompanied by the
displacement of the second acyloxy group. Special experiments with the use
of oleÞn (19) demonstrated that the reaction is regioselective, that is, the sterically
least hindered acyloxy group is initially displaced. The reaction of oleÞn
(19) with the involvement of Pd coordinated to enantiomerically pure ligands
is highly enantioselective (ee > 97%). Therefore, this approach holds promise in
the synthesis of enantiomerically pure nitronates similar to (5f).
Lastly, the radical inter- and intramolecular cyclizations in the presence of
one-electron oxidizing agents as a procedure for the synthesis of Þve-membered
cyclic nitronates can be considered. Radical oxidation of α-nitro ketones (19) in
the presence of disubstituted oleÞns under the action of Mn(OAc)3 was documented
(72a) (Scheme 3.22, Eq. 1).
The initially formed nitronate radical reacts with oleÞn R ′ C=CR ′′ to give an
“elongated” radical A whose successive cyclization and oxidation affords target
nitronates (5g) in 18% to 81% yields. Manganese triacetate is regenerated by
electrochemical methods.
A similar process can be performed with the use of cerium ammonium nitrate
(CAN) for generation of radicals from another CH acid, viz., methyl nitroacetate
(72b) (Scheme 3.22, Eq. 2).
Analogous intramolecular cyclization can be carried out by performing the
reaction of CAN with nitro oleÞn(20) (73) (Scheme 3.23). However, this reaction
is unlikely to be useful in the synthesis of a broad range of cyclic nitronates
because the starting nitro compounds (similar to (20)) are difÞcult to prepare.
3.2.2.1.2. Synthesis of Five-membered Cyclic Nitronates by Cycloaddition
Reactions
[3 + 2]-Cycloaddition of Nitrocarbenes. Scheme 3.24 presents possible approaches
to the synthesis of Þve-membered cyclic nitronates (24), where [3 + 2]-cycloaddition
of intermediate nitrocarbenes (as dipoles B) tooleÞns as trapping agents
is the key step.