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

O N O
O
34 (R = CN, Ph, p-Tol, CH2Br, Ac)
REACTIONS OF NITRILE OXIDES 23
Reaction of methoxycarbonylformonitrile oxide (generated from
MeO2CCCl=NOH in the presence of Et3NinEt2O) with methyl undec-10-enoate
gave 90% of isoxazoline 35 [R = (CH2)8CO2Me, R 1 = H] whereas a similar reaction
with methyl oleate gave a 40% isomeric mixture of 35 [R = 1-octyl, R 1 =
(CH2)7CO2Me and R = (CH2)7CO2Me, R 1 = 1-octyl] (176).
R 1
O
N
R
35
CO2Me
Formation of mixtures of the above type, which is common with internal
oleÞns, do not occur with many functionalized alkenes. Thus, tertiary cinnamates
and cinnamides undergo cycloadditions with benzonitrile oxides to give the 5-Ph
and 4-Ph regioisomers in a 25–30:75–70 ratio. This result is in contrast to that
obtained when methyl cinnamate was used as the dipolarophile (177). 1,3-Dipolar
cycloaddition of nitrile oxides to ethyl o-hydroxycinnamate proceeds regioselectively
to afford the corresponding ethyl trans-3-aryl-4,5-dihydro-5-(2-hydroxyphenyl)-4-isoxazolecarboxylates
36 (178). Reaction of 4-[(E)-(2-ethoxycarbonylvinyl)]
coumarin with acetonitrile oxide gives 37 (R = Me) and 38 in 73% and
3% yields, respectively, while reaction of the same dipolarophile with 4-methoxybenzonitrile
oxide affords only 37 (R = 4-MeOC6H4) (85%) (179).
O
H
N
CO2Et
R
N
O
H
R
CO2Et R
N O
H
CO2Et
H
OH
O O
O O
36 (R = H, 4-Me, 2,4,6-Me3)
37
38
1,3-Dioxolanes 39 derived from α,β-unsaturated aldehydes react with nitrile
oxides R 2 CNO to give the corresponding isoxazolines 40 with the 1,3-dioxolan-
2-yl substituent in position 4 as main products, and their 5-isomers as minor products
with good regioselectivity and synthetically useful yields. The corresponding
R