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

564 NITRONATES
In addition to the general procedure for the construction of the starting substrates
for intramolecular [3 + 2]-cycloaddition (see Schemes 3.140 and 3.143),
several other routes to these products are known.
A procedure developed by Moiseenkov, Veselovskii, and coworkers (207–211,
391) (Scheme 3.145) is of most interest.
Here substituted acyclic dienes (199) serve as the starting reagents. The method
involves:
a. The introduction of the nitro group with retention of both C,C double bonds
in the molecule;
b. Silylation of the nitro fragment in substrate (200) with BSA;
c. Intramolecular cycloaddition giving rise to fused isoxazolidine (201);
d. Cleavage of the isoxazolidine ring to form conjugated oxime (202).
This method is very original. Intramolecular cycloaddition to the double bond
remote from the nitro fragment can be considered as the key step (step c). This
step is regio- and stereoselective. Cleavage of the isoxazolidine ring with ßuoride
ions was accompanied by an interesting allyl migration of the nitroso group
(B→C), which allows one to synthesize conjugated ene oximes (202). The latter
are very convenient reagents for the formation of the iridane skeleton involved
in a series of biologically active substrates.
Russian researchers applied this scheme to readily available chiral dienes or
their precursors and prepared several enantiomerically pure target substrates, the
enantioselectivity of intramolecular [3 + 2]-cycloaddition being virtually
complete.
Professor Seebach and coworkers (96) used silyl enolate (203) containing an
additional C,C double bond in the Michael reaction with α-nitroalkene (204) in
the presence of chiral LA (Scheme 3.146).
As a result, diastereo- and enantiomerically pure functionalized nitro derivative
(205) was synthesized in satisfactory yield. This compound was used in the ISOC
procedure, which gave diastereo- and enantiomerically pure isoxazolidine (206)
in good yield. The latter compound can be considered as a possible reagent
for asymmetric synthesis. It should be noted that silylation in this procedure,
like that described above (Scheme 3.145), was performed with the use of BSA
as the silylating agent in the presence of a small amount of Hunig’s base, the
latter being evidently added for acceleration of silylation and stabilization of
intermediate silyl nitronate.
Taiwan researchers (392) described the transformation of fused dihydrofurans
(207), in the presence of LA, giving rise to 3-benzofuryl-substituted formaldoximes
(208) in good yields (Scheme 3.147).
A long reaction sequence was suggested for this very unusual transformation.
In this process, the key step (A→B) involves intramolecular [3 + 2]-cycloaddition
of SENA A, which was generated through 1,5–C,O-migration of the trimethylsilyl
group in compound (207). Then the tricyclic system B is rearranged to give