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

R′
R′′
HO
R
O N
213
O
ClSi
R′
R′′
Si
O
O N
R
Scheme 3.150
REACTIVITY OF NITRONATES 569
O
R′
R′′
O Si
R
O
N O
Intramolecular [3 + 2]-cycloaddition of six-membered cyclic nitronates was
extensively studied by Prof. Denmark and coworkers for the tandem [4 + 2]
[3 + 2]-cycloaddition reactions of nitroalkenes. Detailed considerations of this
problem were summarized in two reviews (394a, b). Most data were comprehensively
discussed in Reference 394b. It is unnecessary to repeat this information;
however, it is worthwhile to brießy review the available data.
At the outset, it should be noted that, as in the above-considered intramolecular
[3 + 2]-cycloaddition reactions, the regioselectivity of the process is retained, that
is, the terminal (or more removed) atom of the C,C double bond is always bonded
to the oxygen atom of the nitronate.
The character of the resulting fused system completely depends on the site of
attachment of the tether to the six-membered ring (Scheme 3.151).
If a tether is attached to the C(3) atom of nitronates (214), the reaction gives
rise to spiro systems (215). For these systems, generating a linkage containing
three atoms between the six-membered ring and the C,C double bond is a case
of choice. It necessarily leads to cis-fusion of two Þve-membered rings.
If a tether is bound to the C(4) atom, annelated (fused) systems (216) are
formed. In this case, two atoms can be present in the above mentioned linkage,
the protons in product (216) beinginthecis position.
RO
218
O
N
O
OR
217
O
N
C(6)
tether
C(5)
tether
O
RO
Bridged
O
6 N
5
4
3
214
O
RO
Scheme 3.151
C(4)
tether
H
C(3)
tether
O
RO
N
216
O
215
O
N
H
Fused
O
Spiro