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

540 NITRONATES
of the observed transformations. The main characteristic feature of this process
is generation of the nitrosonium cation A after cleavage of the endocyclic N–O
bond. These cations are ambident and can be involved in intramolecular reactions
with nucleophilic aryl fragments either through the nitrogen atom or through the
oxygen atom to give nitrones (148) or heterocycles (149), respectively. In the
authors’ opinion, the presence of substituents X 1 in the para position of nitronate
(147) facilitates the reaction pathway to form products (149).
Japanese researchers varied aryl substituent at C-4 in nitronate (147), LA, and
the reaction conditions, they used successfully diastereomerically pure nitronates
(147) (Scheme 3.121) for the synthesis of various fused-ring systems, such as
benzofuro-[3,2-d]-1,2-oxazines (322), furo-[3,4-d]-isoxazoles (323–326), indolo-
[2,3-b]-1-pyrroline 1-oxides (327), 4H -1,2-benzoxazines (328), benzofuro-
[2,3-c]-tetrahydropyrans (329), and monocyclic 1,2-oxazines (330).
All of these processes are interpreted through intermediate nitrosonium cations,
which are generated upon cleavage of the endocyclic N–O bond with LA. A
detailed consideration of all these transformations is beyond the scope of the
present monograph.
The transformation of N -oxides (150a,b) into butyrolactone derivatives
(151a,b), accompanied by the formation of oximes (152) as by-products, is yet
another example (331) (Scheme 3.122).
In the authors’ opinion, the nitrosonium cations A are chlorinated by TiCl4(LA)
and undergo cyclization with one of the methoxycarbonyl groups to give butyrolactones
(151). This process can be accompanied by ortho-cyclization giving
rise to oximes (152) as by-products.
Six-membered cyclic nitronates can undergo rearrangements accompanied by
cleavage of the endocyclic N–O bond or by cleavage of the endocyclic C–O
bond. The latter fact is evidently associated with the presence of substituents,
which efÞciently stabilize the positive charge, at the C-6 atom. This occurs upon
treatment of nitronates (154a,b) with silicon dioxide (Scheme 3.123)
(117).
The reactions of the resulting stabilized ions 153a,b with silicon dioxide produces
isolable polyfunctional compounds 155a,b. It should be emphasized that
the conÞguration of the stereocenters in nitronate (153) remains unchanged in
the course of the transformation and the reaction is stereoselective with respect
to the new stereocenter at the atom bearing the nitro group.
Interestingly, the character of the isolable products strongly depends on the
nature of the substituent R 3 . In the presence of the piperidine radical instead of
the morpholine group, the resulting acyclic ions (157a,b) are again involved in
cyclization giving rise to a new C,C bond and annelated bicyclic compounds
(158a,b). This reaction is also rather diastereoselective.
Similar tautomeric rearrangements of intermediate six-membered cyclic nitronates
were described by Huffman and coworkers (116) (Scheme 3.124).
By contrast, six-membered cyclic nitronates A containing the trimethylsilyl
group at the C-6 atom are apparently stabilized through cleavage of the N–O