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

270 NITRONES: NOVEL STRATEGIES IN SYNTHESIS
O
O
PhSO2
R
386
+
N
O
−
Li
+
Ph
−
N O +
PhSO 2
THF
−78 °C, 0.25 h
Scheme 2.168
PhSO 2
O
Ph
PhSO 2CH 2Li 20°C
387 388
O
R
Scheme 2.169
N
OH
+
N
O
−
PhSO2
CHCl 3
20 °C, 96 h
Ph
O
O
N
O
PhSO2
389
H
R
+
N O
Another approach is based on the condensation of lithiated sulfones to unsaturated
nitrones (387). Good yields of single stereoisomers of unsaturated hydroxylamines
(388) are obtained. They undergo a reverse-Cope elimination leading
to a single enantiomer of pyrrolidine-N -oxide (389) (Scheme 2.169) (626).
Nucleophilic addition of lithiated allylphenylsulfone (390) to nitrones at
−80 ◦ C proceeds exclusively α to the phenylsulfonyl group (391, α-addition)
affording anti-adducts (392a–h) in high yields, which at 0 ◦ C give isoxazolidines
(393a–h). The formation of these compounds involves isomerization of
the allylsulfonyl moiety (A-B-C) to give a transient vinylsulfone (392 C), which
then undergoes an intramolecular Michael addition (Scheme 2.170). The addition
to several nitrones has been studied; the formation of γ-addition products (391)
has never been observed. Theoretical calculations have been reÞned to explain
accurately the selectivity of the allylation reaction (627).
2.6.6.1.6. Addition of CN − Anion Stereoselective addition of the CN-group to
nitrones has received considerable attention for the synthesis of optically active
α-hydroxyamino nitriles which can be further transformed into α-hydroxyamino
acids and α-amino acids. Me3SiCN (TMSCN), Et2AlCN, Bu4NCN, and LiCN
−