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

546 NITRONATES
N
R
H
OSi
K 1
O
O
N
OSi
R
X
X
K2 >> K1
N
H
R
OSi
K 2
O
O
N
OSi
X
R
Scheme 3.128
X
R = Me, CO2Me; X = CO2Me
six-membered cyclic nitronates are involved in this reaction much faster than
analogous Þve-membered nitronates.
For acyclic nitronates, the reaction rate can depend on the conÞguration of the
dipole. On the basis of 15 N NMR spectroscopic data, it was demonstrated (338a)
that thermodynamically more favorable trans isomers of nitronates (1) are much
less reactive in [3 + 2]-addition to methyl acrylate (Scheme 3.128).
Analogous data on O-methyl derivatives of methyl nitroacetate were reported
in the study (338b).
Evidently, this situation determined by the inßuence of the steric factors should
also be observed in reactions of analogous nitronates with other dipolarophiles.
3.4.3.1.1. Alkyl Nitronates In spite of the low stability of acyclic alkyl nitronates,
these compounds were rather extensively studied in [3 + 2]-addition reactions with
various alkenes (9, 18, 28, 49, 300, 301, 306, 307, 338b–354) (Chart 3.11).
Regardless of the nature of substituents in monosubstituted and α,α-disubstituted
alkenes, these compounds are involved in regioselective addition in a
head-to-head fashion to give the corresponding isoxazolidines substituted at the
C-5 atom (for transformations of cycloadducts, see the special Section 3.4.3.4.).
In the case of α,β-di- or trisubstituted oleÞns, the regioselectivity of the approach
of oleÞn and the regioselectivity of cycloaddition depend on the nature of substituents
in the oleÞn. As a rule, the EWG groups of oleÞn appear at the C-5 atom
in the resulting cycloadduct. To estimate the preferable approach of the oleÞn
responsible for the attachment of the discussed substituent at the C-4 atom, one
can use the series H > Si > C > O suggested by Prof. Denmark (162) for cycloadditions
of disubstituted oleÞns to six-membered cyclic nitronates.
It is of no beneÞt to analyze, stereoselectivity, the reaction under consideration
in the general case. However, it should be noted that the exo approach of oleÞn
is preferable in more cases. As can be seen from Chart 3.11, if oleÞn molecules
contain two different double bonds or a double bond and a triple bond, only one
double bond can be selectively involved in the reaction with acyclic nitronates.