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

SILYLATION OF NITRO COMPOUNDS AS A PROCESS 649
In the presence of ßuoride anion as the catalyst, dihydrofurans (376) can
also apparently generate the respective nitroso derivatives. The latter eliminate
HNO and give the corresponding furans (387a–h) in moderate yields (292)
(Eq. 3).
Finally, treatment of dihydrofuran (388) with ßuoride anion in CH2Cl2 gives
rise to the Z- isomer of conjugated ene nitrile (390-Z ) rather than the corresponding
furan (Eq. 4) (292). Unfortunately, attempts to extend the formation of ene
nitriles to other dihydrofurans (376) failed.
3.5.2.5. Ring-Chain Tautomerism of Bis-N,N-Oxyiminium Cations Scheme
3.215 shows the ring-chain tautomerism of selected bis-N,N -oxyiminium cations,
which was suggested as an explanation of the reactivity of γ-functionalized AN.
This problem is not only of theoretical interest but also of importance for the
development of a simple and efÞcient procedure for the synthesis of conjugated
enoximes (395) containing a remote functional group (153, 293) (Scheme 3.218).
These enoximes can be prepared through available annelated six-membered
cyclic nitronates (391) by their silylation giving rise to the iminium cations A,
whose deprotonation affords annelated 2H-5,6-dihydrooxazines (392). After the
known retro-fragmentation (490), the latter compounds are transformed into the
target conjugated en oximes (395).
For compounds, in which n = 1 and X = SiMe3 or OMe, silylation was studied
in more detail (293). For this purpose, the specially prepared individual stereoisomers
of nitronates (391) were subjected to silylation. The conÞguration of the
C,C double bond in the resulting oximes (393) and (395) was determined in
each case. It appeared that if the fragmentation (392→393) is considered as a
concerted process (according to Reference 490a), the conÞguration of the C=C
bond in derivative (393) does not correspond to the conÞguration of its direct
precursor, oxazine (392). This contradiction can be resolved by considering the
ring-chain tautomerism of cationic intermediates A⇄A ′ , which can lead to a
change in the initial conÞguration of the C-6 stereocenter of (392).
Variations in the size of the carbocycle (n = 1–3), the nature of the group X
stabilizing the positive charge in the cation A ′ , and the steric hindrance of the
base in the silylating agent demonstrated that both the character of silylation
products and the conÞguration of the C,C double bond in oxime (395) depend
dramatically on these factors. An increase in ring size, the replacement of the
alkoxy group at the C-6 atom by the amino fragment, and the use of Hunig’s base,
sterically more hindered than Et3N, lead to an increase in the percentage of nitro
compound (394) in silylation products. The reactions of compounds, in which
n = 1 and X is –N-morpholyl or n = 3 and X=OMe, afford nitro compound (394)
as the only silylation product. The transformation (391→394 ′ ) does not lead to
changes in the relative conÞguration of the stereocenters at the C-4 and C-5
atoms.
Attempts to directly observe the cations A and A ′ by low-temperature NMR,
developed in the studies 274 and 478, made it possible to detect both types of
cations (293) (Scheme 3.219).