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

NO2CH2CH2CH2CO2ME
Me CO2Me
NO2CH CH
61b
61a
CHCO2Me
Me
Me 3SiCl/Et 3N
SYNTHESIS OF NITRONATES 483
MeO 2CCH 2CH 2CH N
51m
O
OSiMe 3
BSTFA/Et3N, C6H6, 80°C, 2h – 91% Ref.186
Ref. 216
Me3SiCl/Et3N, CCl4, 20°C – 89%
BSA, 100°C, 2h or 80°C, 30h
Scheme 3.61
Me3SiO N
O
51n
Me
CHCHMeCO 2Me
CO2Me
98% two diastereomeres ~ 1:1
Ref. 216
AN (61a), which is easily generated from nitromethane and methyl acrylate
by the Michael reaction, is smoothly silylated by both Me3SiCl/Et3N (211)
and amide BSTFA (182) to give the corresponding SENA (51 m) in high yield.
However, its sterically hindered analog, viz., product (61b), does not react with
Me3SiCl/Et3N and gives the corresponding SENA (51n) in the reaction with
BSA only under very drastic conditions. The removal of the Me substituent from
the α or γ position of (61b) leads to a sharp increase in the silylation rate and a
change in the character of the process.
3.2.3.6. Synthesis of Silyl Nitronates by the Replacement of an Heteroorganic
Fragment in Organoboron Nitronates An alternative procedure for the synthesis
of SENAs is based on the replacement of a elementorganic fragment in
elementorganic derivatives of nitronic acids with specially selected silylating
agents. Evidently, these reactions are reversible and can proceed only if the formation
of new bonds is energetically favorable. This approach was studied only
as applied to boron derivatives of alkanenitronic acids (217) (Scheme 3.62).
For example, the reactions of silylamines with dimeric dialkylboron nitronates
(62a–c) rapidly afford the corresponding SENA (51o–r) in satisfactory yields,
and these products can be isolated and puriÞed by distillation. Evidently, the gain
in energy due to the replacement of the weak Si–N bond by the stronger Si–O
bond is the driving force of the process. Besides, there is apparently an additional
gain in energy in the case of trimeric borylated amines compared to dimeric
nitronates (62) because of the stronger coordination bond, although the energy
of the > B–O single bond (544 kJ/ mol) is substantially higher than that of the
B–N single bond (443 kJ/ mol) (218). Interestingly, as opposed to the reaction
of silylamine presented in Scheme 3.62, the reaction with the corresponding
silyl halide does not occur. On the contrary, the backward reaction with the