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

358 NITRONES: NOVEL STRATEGIES IN SYNTHESIS
O
O
HN O
Bn
N
O
Rf
Ph Ph Bn
O
N
645 b
O
Ph Ph Bn
645 d
O
N O
Rf
O
N O
Rf
O O
O
LiCl, DIPEA, THF
Rf = (CH 2) 2C 6F 13
endo
exo
+
+
O
N
O
Ph Ph Bn
O
N
O
Ph Ph Bn
Scheme 2.281
645 a
645 c
O
N O
O
N O
Bn Rf
O
O
Rf
N O
Rf
CH 2Cl 2
Ph
+ N
−
O Ph
637 b
The 1,3-dipolar cycloaddition of nitrones to vinyl ethers is accelerated by
Ti(IV) species. The efÞciency of the catalyst depends on its complexation capacity.
The use of Ti( i PrO)2Cl2 favors the formation of trans cycloadducts, presumably,
via an endo bidentate complex, in which the metal atom is simultaneously
coordinated to the vinyl ether and to the cyclic nitrone or to the Z-isomer of
the acyclic nitrones (800a). Highly diastereo- and enantioselective 1,3-dipolar
cycloaddition reactions of nitrones with alkenes, catalyzed by chiral polybinaphtyl
Lewis acids, have been developed. Isoxazolidines with up to 99% ee
were obtained. The chiral polymer ligand inßuences the stereoselectivity to the
same extent as its monomeric version, but has the advantage of easy recovery
and reuse (800b).
It was shown (801) that the diastereoselectivity of α-ßuoroalkyl nitrones is
reversed to that of the corresponding α-alkyl nitrones. This fact supports the
conclusion that the conformation, due to the relief of the dipole repulsion between
the ßuorine atom and the oxygen atom of the nitrone is preferred in α-ßuoroalkyl
nitrones.
The addition of nitrones (647) and (312) to 4-pentenofuranoside (648) derived
from D-ribose, followed by reductive opening of the isoxazolidine ring in (649)