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

H
O
N
H
N CN
Bz
R 2
R 1
R
REACTIONS OF NITRILE OXIDES 41
104 105
O R2 H
O
N
O
R
H
106 107 108
R = Ph, 2,4,6-Me3C6H2; R 1 = OAc, R 2 = CN; R 1 R 2 = O, OCH2CH2O
Benzonitrile oxide, generated by dehydrochlorination of benzohydroximoyl
chloride, undergoes regio- and face-selective cycloadditions to 6,8-dioxabicyclo
[3.2.1]oct-3-ene 108a yielding a 4:1 mixture of 4,5-dihydroisoxazoles 109 and
110. Both products have exo-stereochemistry, resulting from the approach of the
nitrile oxide from the face opposite to the the methyleneoxy bridge. Structures
of the adducts were determined by 1 H NMR spectroscopy and, in the case of
compound 109, by X-ray diffraction analysis (275).
O
O
O
O
R 1
R
N
O
H
R
N
R 1
H
H
O
O
Ph Ph
O N
N O
108a 109
110
Pentanenitrile oxide, BuCNO, formed in situ from 1-nitropentane, PhNCO and
Et3N in benzene, added stereo- and regioselectively to 8-syn-(dimethoxymethyl)-
3-oxo-2-oxabicyclo[3.2.1]oct-6-ene to give 75% of the tricyclic lactone 111 (276).
Introduction of a methoxycarbonyl group into the plane asymmetrical double
bond of 2,3-dioxa- and 2,3-oxazabicyclo[2.2.2]oct-5-enes, brought about a
clear-cut increase in syn selectivity of their reactions with 1,3-dipoles (277).
N
O
H
O
R 2
R 2
R 1