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

62 NITRILE OXIDES
and modern preparative procedures. In particular, reagents and approaches in
nitrile oxide chemistry become closer to those of organometallic and coordination
chemistry.
Chromone-3-carbonitrile oxide undergoes cycloaddition reactions with phenyland
diphenylacetylenes to give isoxazoles 205 (R = H, Ph). The nitrile oxide is
obtained from 3-formylchromone oxime by bromination and subsequent dehydrobromination
(175).
O
O
205
The reaction of ethoxycarbonylformohydroximinoyl chloride EtO2CCl:NOH
with acetylene derivatives gave isoxazoles 206 (R 2 = H, CO2Et, CO2Me,
COC6H4NO2-4; R 3 = CO2Et, CO2Me, Ph, COPh) (366).
EtO 2C
N
N
R
O
O
R
206
2 R3 The cycloaddition of Weinreb amide functionalized nitrile oxide with a range
of dipolarophiles has been studied. N-Methoxy-N-methylcarbonocyanidic amide,
nitrile oxide 207 (i.e., a nitrile oxide of Weinreb amide type derivative) was generated
from 2-chloro-2-(hydroxyimino)-N-methoxy-N-methylacetamide as intermediate
and used in situ. Thus, addition of 3-bromo-1-propyne as dipolarophile to
this precursor of 207, followed by quenching with triethylamine, gave 5-(bromomethyl)-N-(methoxy)-N-methyl-3-isoxazolecarboxamide
208 in 55% to 60%
yield (367).
Me N
OMe
N O
Me N
OMe N O
C
Br
O
207
Iodoacetylene (prepared in situ from ethynylmagnesium bromide or tributyl
(ethynyl)tin with iodine) was used as a dipolarophile in the 1,3-dipolar cycloaddition
reactions with nitrile oxides to produce 2-(5-iodoisoxazol-3-yl)pyridine
and 3-(4-ßuorophenyl)-5-iodoisoxazole in good yield (70%–90%). Subsequently,
O
Ph
208