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

544 NITRONATES
R 1
R 2
O
N
OR 3
+
R 4
R 5
R 1
R 5
R 4
R 3 O
R 2
R 4
O
N
N
O
R 5
R1 R2 OR 3
and/
or
R 4
R 5
O
N
R1 R2 R3 – alkyl, Si R3 + R2 – alkylene C2 or C3, R1 , R2 , R4 , R5 – H, alkyl, aryl, EWG or
electron donating groups.
Scheme 3.126
Nowadays, it is commonly accepted that [3 + 2]-cycloaddition is a concerted
process, and that the C,C double bond serves as the main type of dipolarophiles
(Scheme 3.126).
Regio- and stereoselectivity of the process depend on the nature of its participants
and are determined by the character of the approach of the dipolarophile
to the dipole. (In Scheme 3.127, this is demonstrated for the reaction of monosubstituted
nitronates with monosubstituted oleÞns.)
For the overwhelming majority of nitronates, the reactions with monosubstituted
oleÞns are characterized by the head-to-head approach of the oleÞn, that is,
the substituent R is present at the C-5 atom. A general conclusion about stereoselectivity
of this reaction (endo or exo approach of oleÞn to the dipole (Scheme
3.127)) cannot be drawn. However, the exo approach prevails for nitronates.
(Possible factors responsible for discrimination of the facial approach will be
discussed below in Section 3.4.3.5).
3.4.3.1. Intermolecular [3 + 2]-Addition of Nitronates to OleÞns Of all known
types of nitronates (see Section 3.2), alkyl- and silyl nitronates as well as cyclic
C5-C6 nitronates are involved in [3 + 2]-cycloaddition reactions. Detailed comparative
kinetic studies for different types of nitronates have not been reported.
However, a few data (162, 336, 337) allow one to deduce some sequences
(Chart 3.10).
In spite of the fact that these series are conventional, the choice of the optimal
conditions for concrete [3 + 2]-addition reactions can be guided by these series
to a Þrst approximation.
A
OR 3