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

REACTIONS OF NITRILE OXIDES 33
It was also shown that for some other related 4-substituted cyclopent-2-enone
derivatives the regiofacial selectivity is lower, and completely reversed in the
case of 4-acetoxycyclopent-2-enone, giving 100% of the adduct 77.
H
AcO
O
N R
The study of benzonitrile oxide additions to 4-benzoylaminocyclopent-2-en-
1-ol and its derivatives (240) demonstrated that mainly anti-adducts are formed.
This was interpreted as the result of the syn-directing ability of the cyclopentene
substituents by strong intramolecular hydrogen bonding. Indeed, removal of
the intramolecular hydrogen bond, by OH protection or oxidation, activates the
syn-directing ability of the amido substituent and provides a route for obtaining
syn-adducts (240).
The cycloaddition of nitrile oxides RCNO (R = alkyl, alkenyl, aryl), generated
in situ from either RCH2NO2/PhNCO or RCH=NOH/NaOCl to (R)-( + )limonene,
proceeds regioselectively at the extracyclic double bond, but not
stereospeciÞcally, to form (5R/S )-isoxazoles 78 in 64% to 81% isolated yield
(241).
Me
O
N
77
Me
78
Isoxazolines 79, obtained from aromatic nitrile oxide cycloadditions to
cyclohex-2-enone, reacted with nickel peroxide to give 3-aryl-6,7-dihydro[1]
benzoisoxazol-4(5H )-ones 80. In contrast, the corresponding 2-bromocyclohex-
2-enone underwent nitrile oxide cycloaddition, followed by dehydrobromination,
to afford the regioisomeric 3-aryl-4,5-dihydro[1]benzoisoxazol-7(6H )-ones 81
(Scheme 1.23) (242).
The 1,3-dipolar cycloaddition reactions of nitrile oxides to unsymmetrically
substituted norbornenes (243) and to dicyclopentadiene and its derivatives (244)
proceed with complete stereoselectivity. The approach of the dipole takes place
exclusively from the exo-face of the bicycloheptane moiety, generally
H
R
O