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

REACTIVITY OF NITRONATES 587
Experimental (for 250a) Calculated
For ΔH ≠ : 249c≥249e>249a≈249d>249b>249f 249a>249f>249c>249e>249d>249b
For Δ G ≠ 293: 249e>249c≥249a>249d≥249f>249b
Experimental (for 250b) Calculated
For Δ H ≠ : 249e>249c>249d 249a>249f>249c>249d>249e>249b
For Δ G ≠ 293: 249e>249c>249d
Chart 3.17 The experimental and calculated rates of [3 + 2] cycloaddition of some
six-membered cyclic nitronates with model oleÞns.
difference in the lengths of the bonds formed in the transition state of nitronates
discussed.
This approach did not provide a complete explanation for the observed degree
of stereoselectivity. On the whole, the endo approach of oleÞn to nitronate is
stabilized by secondary orbital interactions but, at the same time, is destabilized
due to steric hindrance.
In terms of the approach used, the authors drew a general conclusion that
the [3 + 2]-cycloaddition reactions of nitronates with dipolarophiles under study
should be considered as either nonconcerted or as those, which substantially differ
in the positions of transition states on the reaction coordinate (337).
Another attempt to perform a general mechanistic consideration of [3 + 2]cycloaddition
reactions of nitronates with oleÞns has been made relatively
recently by Prof. Denmark and coworkers (162) using modern quantum-chemical
methods, which allow one to correctly calculate the potential energy surfaces
for model substrates. Since these data have been summarized in the recent
review (335), it is not necessary to consider them as comprehensively as the
study in (337).
Cycloadditions of six-membered cyclic nitronates (249i,j) with various oleÞns,
both monosubstituted (250a,i,k–n) (Scheme 3.169) and disubstituted (253a–g)
(Scheme 3.170), were also investigated.
Kinetic studies were not performed and, consequently, the activation parameters
were not determined.
Data on related reactivities of oleÞns were extracted from competitive reactions.
Hence the main emphasis has been given to investigations of regio-, stereo-,
and facial preferences.
The structures of the resulting nitroso acetals (252) (Scheme 3.169) or
(254 + 255) (Scheme 3.170) and their stereoisomeric compositions were studied
in detail by NMR and X-ray analysis.
As in the above described study (337), the addition of monosubstituted oleÞns
(250a,i–m) to nitronates (249) occurs strictly regioselectively in a head-to-head