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

192 NITRONES: NOVEL STRATEGIES IN SYNTHESIS
Ph
O N O
E-217
8.80 d
H
O
+ − +
Fig. 2.15
O N Ph
Z-217
−
6.02 d
with the data of other authors (398, 399).Determination of the nitrone group
conÞguration is based on the signiÞcant difference between the vicinal constants
of cis- and trans- 3 Jc,H; in cyclic E-isomers this is noticeably two to three times
larger than in Z -isomers. In methylene-nitrone (215) Jtrans equals 6.5 Hz, while
Jcis equals 1.5 Hz.
Using a variety of NMR 2-D techniques, such as H–H COSY, C–H COSY,
DEPT, HMBC, and NOESY, an accurate assignment of the signals of 18 α-(5substituted-2-hydroxyaryl)-N
-aryl nitrones has been made (159).
Convincing evidence was found that the majority of acyclic aldo-nitrones exist
in the Z-form, by investigating the ASIS-effect (aromatic solvent induced shift
effect) (399). However, in some cases, speciÞed by structural factors and solvent,
the presence of both isomers has been revealed. Thus, in C -acyl-nitrones the existence
of Z -and E-isomers was detected. Their ratio appears to be heavily dependant
on the solvent; polar solvents stabilize Z -isomers and nonpolar, E-isomers
(399). A similar situation was observed in α- methoxy-N-tert-butylnitrones. In
acetone, the more polar Z -isomer was observed, whereas in chloroform, the less
polar E-isomer prevailed. The isomer assignments were made on the basis of the
Nuclear Overhauser Effect (NOE) (398). E/Z-Isomerization of acylnitrones can
occur upon treatment with Lewis acids, such as, MgBr2 (397). Another reason
for isomerization is free rotation with respect to the C–N bond in adduct (218)
resulting from the reversible addition of MeOH to the C=N bond (Scheme 2.74).
The increase of the electron acceptor character of the substituent contributes to
the process (135).
R
C
H
−
N
But O R But R OH
MeO
C N C N
Bu H OH
t
H
+ +MeOH
−MeOH
MeO C
−MeOH
+MeOH
H
R
218
Scheme 2.74
H
−
N
But O
+