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

96 NITRILE OXIDES
the unambiguous assignment of stereogenic centers, not previously assigned for
these compounds (475, 476).
OH
O
O OH
OH
OH
OH
O
OH
O
OH
O
OH
O
437 438
O
436
OH O
OH
OH O
Carbohydrate derivatives with a spiroisoxazoline moiety, present in psammaplysins
and ceratinamides (metabolites isolated from marine sponges) have been
prepared in good yields and excellent regio- and diastereoselectivity by a route
involving Wittig oleÞnation and 1,3-dipolar cycloaddition as key steps (477).
3,4-Di(2,3,4,6-tetra-O-acetyl-b-D-mannopyranosyl)-1,2,5-oxadiazole 2-oxide
has been synthesized from D-mannose by a route, involving as the key step,
dimerization of mannopyranosyl nitrile oxide. Three methods have been used
for the generation of the nitrile oxide: isocyanate-mediated dehydration of nitromethylmannose
derivative, treatment of aldoxime with aqueous hypochlorite and
base-induced dehydrochlorination of hydroximoyl chloride. D-gluco, D-galacto,
D-xylo and L-fucopyranosyl analogs has been prepared similarly. The structure
of D-mannose-derived 1,2,5-oxadiazole 2-oxide has been established by X-ray
crystallography (478).
A strategy based on the diastereoselective dipolar cycloaddition reaction of
nitrile oxides and allylic alcoholates, has been applied to the synthesis of bis-
(isoxazolines) that are precursors to polyketide fragments. These intermediates
can be elaborated into protected polyols, for example, 439, by sequential chemoselective
reductive opening of each isoxazoline or, alternatively, by simultaneously,
providing access to all stereoisomers of this carbon skeleton (479).
Ph
Me
O
O
Me
O O
Me
Me
Me
439
Me
Me
O O
Me O
Me
SiPh 2Bu-t