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

708 NITRONATES
compounds containing EWG groups in the α-position. In particular, these compounds
are active N -electrophiles. At the same time, the electrophilic center
can be transferred to the β-C atom. It is in these in which the transformations
of EWG-substituted α-nitrosoalkenes, generated by silylation of β-functionalized
AN, are interpreted (491) (Schemes 3.270 and 3.271).
Silylation of primary AN of the general formula R 1 CHXCH2NO2 (528a–e)
with BSA affords trimethylsilyl derivatives of divinylhydroxylamines (531a–e)
in moderate to good yields. This reaction is regio- and stereospeciÞc. Both vinyl
fragments in products (531a,b,e) adopt a trans conÞguration. The corresponding
divinylhydroxylamine (532a) was obtained after removal of the silyl protecting
group from derivative (531a).
Silylation of AN (528b,c,e) with another silylating agent (Me3SiCl/Et3N) gives
poorly separable mixtures of unidentiÞed products. However, the reaction of AN
(528a) under these conditions produces the silyl derivative of bis-oxime (533),
which can be subjected to desilylation to prepare free bis-oxime (534) (491, 497).
The stereoselectivity of the reaction with respect to the new C,C double bond
is low (E/Z ≈ 1.3:1). Silylation of sterically more hindered nitroalkane (528 d)
with Me3SiCl/Et3N affords derivative (535), which can be desilylated to form
en-oxime (536) (216, 491). As mentioned above, secondary nitroalkane (528f)
gives chloro oxime (537). The latter is the only product of this type, which was
isolated upon silylation of acyclic AN (216, 491, 498).
Silylation of AN with BSA involves N,C-coupling of two intermediates,
nitronate (529) and nitrosoalkene (530), as the key step, which apparently proceeds
through the cyclic transition state A (Scheme 3.271). (Here, it should
be noted that intermediates (529a) and (530a) were detected with by trapping
agents.(491) Nitronate (529d) was found in the reaction mixture by NMR monitoring
(491). Evidently, elimination of nitrous acid is only one of possible
pathways for the transformation B→(531).
From this it follows that it is necessary to maintain a high concentration of
nitronate (529) but a low current concentration of nitrosoalkene (530) for the
successful N,C-coupling reaction (528→531). The highest yield of the target
derivative (531 d) was achieved in the reaction with (529d), because nitronate is
relatively slowly transformed into the corresponding nitrosoalkene (530 d). Steric
hindrance in nitrosoalkene (530f) prevents its coupling with nitronate (529f).
An increase in nucleophilicity of the silylating agent apparently leads to
reversible ionization of nitronate (529) (see529a⇄529a ′ in Scheme 3.271). Evidently,
anion (529a ′ ) reacts with nitrosoalkene (530a) in a Michael-type fashion
to give, after a series of transformations, derivative (533). (These transformations
are described in more detail in Section 3.5.6.)
3.5.5.2. Reactions of β-Functionalized α-Nitrosoalkenes with the C,C Double
Bond The reactivity of α-nitrosoalkenes toward double bonds depends drastically
on the structure and conÞguration of the reagents. The characteristic features
of the behavior of β-functionalized α-nitrosoalkenes were studied using readily
available methyl β-nitrosoacrylate (530a). It appeared that this derivative is readily
involved in the ene reactions with electron-donating and neutral oleÞns either