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

REACTIVITY OF NITRONATES 581
orientation of the lone pair for model six-membered nitrosoacetals is thermodynamically
much less favorable than the other conformer D ′ (408).
The conÞgurations of Þve-membered cyclic nitronates A in solution were
established and conformational analyses were performed by NMR spectroscopy
(178, 184, 273, 338a, 338b, 410–414). In most of these studies, the mutual
arrangements and positions of the protons were determined with the use of the
Karplus dependence 3 Jvic,H,H. However, other evidence, such as NOEdif (410) and
the appearance of the small constant 3 JH,H allows one to determine the proton at
C-3 in the pseudoequatorial position (414). The determination of long-range constants
JH,H, based on the application of known rules (415), the use of shift reagents
(335a) and the long-range constants JH,15N (273, 338a), were also considered. The
absolute values of the differences in the vicinal constants appeared to be useful
in determining the positions of the protons of ABX systems in fused bicyclic systems
(337). In more recent studies, different 2D NMR methods were employed
(162, 416). The same indications were used for the determination of the con-
Þgurations and in conformational analyses of N-siloxy-tetrahydro-4H -oxazines
(274).
Investigations of the stereodynamics of cyclic nitroso acetals and, particularly,
different types of isoxazolidines are of great importance.
Only one process was observed in these derivatives by low-temperature NMR
(see, e.g., Ref. 417). As a rule, the barrier of this process was very high (133,203,
346); although, for unknown reasons, the introduction of two nitro groups at the
C-3 atom leads to a noticeable decrease in the barrier height (see Scheme 3.166).
On the basis of all the above mentioned results, Russian researchers provided
an explanation for the stereodynamics of isoxazolidines shown in Scheme 3.166.
There is a so-called combined inversion-buckling process (417), which occurs
through a planar transition state A with an unhybridized p electron pair at the
nitrogen atom (see Scheme 3.166).
Subsequently, this interpretation has been commonly accepted; however, it
does not account for the factors responsible for a decrease in the rate of the fast
classical ring inversion. Hence, in our opinion, it is more reasonable to consider
the stereodynamics of isoxazolidines as a result of superposition of two processes,
a fast ring inversion (IR) and slow nitrogen inversion (shown in the lower portion
of Scheme 3.166) (418).
The observed degeneration of the stereodynamic pattern of these processes is
due to the fact that all conformers containing the axial lone pair at the nitrogen
atom are thermodynamically highly unfavorable. Calculations also conÞrmed that
the axial position of the nitrogen lone electron pair is highly unfavorable (see
Chart 3.15).
It should be noted that the equatorial position of the nitrogen lone pair in
six-membered cyclic nitroso acetals is not so favorable. Hence, both stereodynamic
processes can be observed for these compounds (Scheme 3.167, see also
Ref. (274)).
In conclusion, the barrier of nitrogen inversion in cyclic nitrosoacetals is very
high. This circumstance allows one to distinguish kinetic and thermodynamic