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

RCNO
NMO
− NMM
R = Ph, 4-ClC6H4, Mes
R
N O
NMO is N-methylmorpholine N-oxide
NMM is N-methylmorpholine
REACTIONS OF NITRILE OXIDES 79
(20 equiv.)
OH
N R
O
O
373 374
Scheme 1.43
of stable supported nitrile oxides to amidoximes. The photochemical cycloreversion
of these heterocycles affords free nitrosocarbonyl intermediates, which are
trapped by suitable dienes or enes. The method is considered a clean and environmental
friendly approach to the ßeeting nitrosocarbonyl intermediates, which
afford valuable adducts in various synthetic applications. The isomeric heterocycles,
adhered at position 5 of the ring, are also obtained by cycloaddition of nitrile
oxides to supported amidoximes. Their photolysis affords resin-bound nitroso carbonyls
that are trapped with dienes, affording valuable supported adducts suitable
for further elaboration in solid-phase chemistry (422).
Rather than the expected [3 + 2] cycloaddition, a novel ene-like cycloisomerization
occurs on deprotonation of allyltrimethylsilyl-oxime compounds, when
the β-sp 2 carbon atom of the allyltrimethylsilyl moiety is tethered to the oxime
unit. The resulting nitrile oxide group serves as an enophile, and the Þnal cyclized
product still has two functional groups suitable for further manipulations. Thus,
ene-like cycloisomerization of allyltrimethylsilyl-oxime 375 with NaOCl in
CH2Cl2 gives 82% of cyclized product 376 (423). See also Reference 424.
CH 2
N
OH
SiMe3
N
OH
375 376
SiMe 3
DFT studies of the intramolecular ene-like (or the so-called 1,3-dipolar ene)
reaction between nitrile oxides and alkenes show that this reaction is a three-step
process involving a stepwise carbenoid addition of nitrile oxide to form a bicyclic
nitroso compound, followed by a retro-ene reaction of the nitrosocyclopropane
intermediate. The competitive reactions, either the intramolecular [3 + 2] cycloaddition
between nitrile oxides and alkenes or the intermolecular dimerization of
nitrile oxides to form furoxans, can overwhelm the intramolecular 1,3-dipolar
ene reaction if the tether joining the nitrile oxide and alkene is elongated, or if
substituents such as trimethylsilyl are absent (425).