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

R
N O
H
H
Fe(CO) 3
R
N O
R
R
N
REACTIONS OF NITRILE OXIDES 35
N O
O
H
H
H
H
84 85
86
Fe(CO) 3
Fe(CO)3
1.3.4.1.4. Aromatic and Related Compounds The main part of aromatic compounds,
able to undergo 1,3-dipolar cycloaddition reactions with nitrile oxides,
are polycyclic hydrocarbons and their derivatives. Dipolarophilic reactivity
toward nitrile oxides is known for phenanthrene and pyrene (247). Microwave
irradiation in the absence of a solvent improves product yields and reduces reaction
times compared with classical heating with and without reßuxing solvents
(248). Quantum chemical DFT calculations at the B3LYP/6–31G(d) level for
reactions of mesitonitrile oxide with anthracene and its aza-analog, acridine, are
in agreement with the observed regioselectivity and do not agree with the predictions
of frontier molecular orbital (FMO) theory (249).
p-Quinones are active dipolarophiles, used in particular in natural product syntheses
(250). 2,5-Di(tert-butyl)-p-benzoquinone is well known as a dipolarophile
in reactions with a series of substituted benzonitrile oxides (251, 252). This
quinone gives not only 1:1 but also 1:2 cycloadducts, the latter, for example, 87,
with p-substituted benzonitrile oxides, probably, because of less steric hindrances
(251). Normal 1:1 and 1:2 adducts of the 1,3-cycloaddition at C=C bond(s) are,
however, rather unstable and, in particular, undergo base-induced transformations.
The structure of one of the Þnal products 88, obtained from 1,3-dipolar
1:1 cycloadduct of 2,5-di(tert-butyl)-p-benzoquinone with 2,6-dichlorobenzonitrile
oxide was determined by X-ray diffraction analysis. The t-Bu group at
R
O
CMe 3
N
O
O
N
Me3C O
R
87 (R = 4−BrC6H4, 4−CIC6H4) t−Bu
HO Bu−t
O
N
O
88
CI
CI
I