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

SILYLATION OF NITRO COMPOUNDS AS A PROCESS 645
The fragmentation of the latter, after elimination of silyl carboxylate, affords silyl
derivatives of conjugated enoximes (379) in moderate yields.
Retro-[4 + 2]-cycloaddition of dihydrooxazines of type C was considered in
Section 3.5.1.3. An analogous process was described for N-alkyl- (490) or Ntrimethylsilyl-2H
- dihydrooxazines (491). However, 2H - dihydrooxazines dealt
with in studies (490, 491) could be isolated or, at least, characterized. At the same
time, accumulation of heteroatoms bound to each other dramatically decreases
the stability of intermediates C.
The process under consideration could be discussed in terms of ring-chain
tautomerism of nitronates (378⇄381) (see the lower part of Scheme 3.215),
the more so that such examples were documented for proton analogs of similar
SENA (492). However, both nitronates ((378) and (381)), which were prepared
by independent syntheses, are quite stable, and therefore their isomerization in the
presence of a silyl Lewis acid should involve ring-chain tautomerism of cations.
Evidently, cyclization of nitronate (378) is attributed to high electrophilicity of
the carbon atom of the carbonyl group, provided that this group is not involved
in the conjugation chain.
3.5.2.4.2. Intramolecular C,C-Coupling Reactions of Bis-N,N-(trimethylsiloxy)
Iminium Cations Here we consider one of the mechanistic schemes of intramolecular
C,C-coupling reactions of bis-N,N-(siloxy)iminium cations generated
by silylation of β-nitroalkylated derivatives of malonic ester (382) (Scheme
3.216).
The initially formed SENA (383a) (R’ = H) undergoes rapid deprotonation
at the C-2 atom to give unstable enamine C which is rapidly transformed into
enoxime (384). In the presence of the substituent R ′ , which shields the C-2 atom,
thus hindering its deprotonation, SENAs (383) undergo further silylation of both
functionalized fragments to give cationic intermediates B. The latter undergo
intramolecular cyclization accompanied by eliminating trimethylbromosilane to
give cyclopropanes (385) in good yields (470, 489, 493). The formation of cyclopropanes
(385) is characterized by strict stereoselectivity (the substituent R ′ and
the nitroso acetal fragment have a trans conÞguration).
3.5.2.4.3. Selected Features of the Chemistry of N,N-Bis-Trimethylsiloxy-Substituted
Cyclopropanes and Dihydrofurans Scheme 3.217 shows selected features
of the chemistry of compounds prepared by intramolecular trapping of bis-N,N-
(trimethylsiloxy)iminium cations.
The strained C,C bond in cyclopropanes is prone to heterolysis, particularly,
in the presence of substituents stabilizing the resulting charges. These facts can
be used to interpret easy methanolysis of cyclopropanes (385), resulting in regeneration
of the starting functionalized AN (382) in high yields (Eq. 1) (493).
The –N(OSiMe3)2 fragment is an equivalent of the nitroso group. It was
demonstrated (493) that cyclopropanes (385) can serve as equivalents of the
corresponding nitrosocyclopropanes in coupling reactions of the nitroso group
with anions of nitro compounds (Eq. 2).