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

REACTIONS OF NITRILE OXIDES 51
gave 1,2,4-oxadiazoles 153. When mesitonitrile oxide was used, bis-adducts
154 (at C=N and C=O bonds) were also formed. The cycloadditions showed
a remarkable site selectivity toward one of two carbon–nitrogen double bonds.
The structures of both adducts were conÞrmed by X-ray analysis (322).
Ph
N
N O
R
O
N
O
CO2H
Ph
NNHCO
R 1
R
R 1
N OPh
O N
N
N O
O
152 153 154
Cycloaddition of benzonitrile oxide to di-exo- and di-endo-norbornane and
norbornene-fused dihydro-1,3-oxazine structural isomers gave tetracyclic 1,3oxazino-1,2,4-oxadiazolines.
With norbornene dipolarophiles, which contain
C=N and C=C bonds, the cycloaddition with PhCNO takes place at the oleÞnic
bond. The di-exo compound yields one tetracyclic isoxazoline, regioselectively,
whereas the di-endo-isomer gives an isomeric mixture of isoxazolines. The
di-exo-norbornene derivative 155 and PhCNO, however, gave a bis-adduct (323).
cis-5,6-Tetramethylene-1H -1,3-dihydrooxazines 156 (Z = CH2CH2, R= 4-
ClC6H4, 4-MeC6H4), and analogs unsaturated in the carbocyclic ring 156 (Z =
CH:CH) gave adducts at the hetero-double bond with benzonitrile oxide, furnishing
1,3-oxazino-1,2,4-oxadiazolines 157 (Z = CH2CH2, CH:CH; X= O,
R = 4-MeC6H4). The site selectivity of the cycloaddition differs from that of
the above-mentioned norbornene-fused dihydrooxazines, where the nitrile oxide
dipole attacks Þrst the C:C rather than the C:N bond (324).
O
N
Z
H
H
N
O
R
Z
H
H
Ph
N
O
R
X
N
Cl
155 156 157
Methoxymethyldiazepines 158 (R = Me, R 1 = H; R = H, R 1 = Me) undergo
regioselective 1,3-cycloaddition with benzonitrile oxide and its 4-substituted
R
R 1