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

98 NITRILE OXIDES
t-Bu
t-Bu
O
O
O
O
Me
Me
Me Me O
O
Me Me O
O
A
O N
Me
443
O H
O
H 2C Me
444
C
O H O
O
O
OSiMe2Bu-t
TMS
N OH
OSiMe2Bu-t
TMS
Starting from the Ni meso-formyloctaethylporphyrin oxime complex, the
meso-cyanooctaethylporphyrin N-oxide complex has been synthesized for the
Þrst time. The double addition of the nitrile oxide to 2,5-norbornadiene afford a
porphyrin dimer, whose structure has been established by X-ray diffraction analysis
(485). The 1,3-dipolar cycloaddition reaction of meso-tetraarylporphyrins with
2,6-dichlorobenzonitrile oxide yields isoxazoline-fused chlorins and stereoisometric
bacteriochlorins. The crystal structure of one of bacteriochlorins has been
characterized by X-ray diffraction (486, 487).
An efÞcient synthetic route to (10Z )- and (10E)-19-ßuoro-1α,25-dihydroxy
vitamin D3 has been developed (488). The key feature of this pathway is the introduction
of a 19-ßuoromethylene group to a (5E)-19-nor-10-oxo-vitamin D derivative.
The 10-oxo compound 445 has been obtained via a 1,3-dipolar cycloaddition
reaction of (5E)-1α,25-dihydroxyvitamin D with in situ generated nitrile oxide,
followed by ring cleavage of the formed isoxazoline moiety with molybdenum
hexacarbonyl. Conversion of the keto group of (5E)-19-nor-10-oxo-vitamin D to
the E and Z ßuoromethylene group has been achieved via a two-step sequence,
involving a reaction of lithioßuoromethyl phenyl sulfone, followed by the reductive
de-sulfonylation of the α-ßuoro-β-hydroxysulfone. The dye-sensitized
photoisomerization of the (5E)-19-ßuorovitamin D affords the desired (5Z )-19ßuorovitamin
D derivatives, (10Z )- and (10E)-19-ßuoro-1α,25-dihydroxyvitamin
D3.