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

580 NITRONATES
3.4.3.4.4. Determination of the ConÞgurations and Study of Stereodynamics of
Cyclic Nitroso Acetals This determination is of obvious fundamental importance
by itself and, in addition, it is of importance in considering the mechanism of
[3 + 2]-cycloaddition and in predicting the conÞgurations of the resulting stereocenters.
The most reliable data on the relative conÞgurations of different types of cyclic
nitroso acetals can be obtained by X-ray diffraction (Chart 3.15).
These studies were carried out for Þve-membered monocyclic N-alkoxy-
(406–408) and N-siloxyisoxazolidines (178) (A), for N-siloxytetrahydro-4H -
oxazines (274), and bicyclic fused derivatives containing the isoxazolidine ring
(B) (99, 157, 337).
These data demonstrate that the nitrogen atom in practically all cyclic nitroso
acetals A or B, containing the isoxazolidine ring, deviates from the plane through
the other four atoms (envelope). The nitrogen lone pair always pseudoequatorial.
(The exception is one stereoisomer of bicyclic compound B(R=CN, R ′ =
p–Br–C6H4 (trans)), in which the C-6 atom rather than the nitrogen atom
deviates from the plane through four atoms, and the nitrogen lone pair is pseudoequatorial
in this compound as well.) All known N-siloxy-tetrahydro-4H -oxazines
(274) adopt a chair conformation, whereas the nitrogen lone pair can be in
either equatorial or axial orientation. In the majority of fused compounds B, the
six-membered ring also adopts a chair conformation, although the six-membered
ring having a skewed boat conformation was found in one compound (157).
Quantum-chemical calculations for model isoxazolidines C and C ′ demonstrated
that the conformer C containing the pseudoequatorial nitrogen lone pair is
thermodynamically highly favorable, whereas the conformer D with the equatorial
Me
H
Ha O
N
OR
2
H2 H1 3JH A
a H 1 is smool
4JCH3, 3
H2 2 Hz
3J1H,15N are observed
3J2H,15N not observed
R – alkyl, trialkylsilyl
N
O
OMe
C
H 1
ΔHc-c′ =−12,4 ccal/mol
MeO N
O N
O
C′
OMe
D
R Hb 6 7 Ha 5 9 Ha 8
1 O Hxr
N
R′
4
2
O
B
3
D = 3 /J AX-J BX/ = 4 – 7Hz (for trans)
1.7–2.2 Hz (for cis)
Dcis > Dtrans
O
MeO N ΔHD-D′ = −2 ccal/mol
~
Chart 3.15 Some criteria for determination of conÞguration of cyclic nitrosoacetals.
D′