Reductive cleavage molybdenum hexacarbonyl

Pyrrolo[3,4-c]isoxazoles 21 were designed to act as non-polar scaffolds for elaboration to acyltetramic acids. In particular, hydrogenolytic N-0 cleavage followed by hydrolysis of the resultant enaminone afforded the 3-(3-arylpropanoyl)tetramic acids 23, while N-0 reduction by molybdenum hexacarbonyl gave the corresponding 3-arylpropenoyl compounds 22 <04SL2815>. 

An efficient synthetic route to (10Z)- and (10 )-19-lluoro-la,25-dihydroxy vitamin D3 has been developed (488). The key feature of this pathway is the introduction of a 19-fluoromethylene group to a (5 )-19-nor-10-oxo-vitamin D derivative. The 10-oxo compound 445 has been obtained via a 1,3-dipolar cycloaddition reaction of (5 )-la,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 (5 )-19-nor-10-oxo-vitamin D to the E and Z fluoromethylene group has been achieved via a two-step sequence, involving a reaction of lithiofluoromethyl phenyl sulfone, followed by the reductive de-sulfonylation of the u-lluoro-j3-hydroxysulfone. The dye-sensitized photoisomerization of the (5 )-19-fluorovitamin D affords the desired (5Z)-19-fluorovitamin D derivatives, (10Z)- and (10 )-19-fluoro-la,25-dihydroxy-vitamin D3. 

The cycloadducts are readily converted to the corresponding enantiomerically pure amino acids by reductive cleavage of the N —O bond with molybdenum hexacarbonyl in acetonitrile/water 93 7, followed by hydrolytic removal of the chiral auxiliary, performed with 6 M hydrochloric acid 11. 

A number of attempts have been made over the years to develop reproducible synthetic routes to six- and seven-coordinate isocyanide complexes of molybdenum and tungsten. Two of the older methods, namely, the reaction of the hexacarbonyls with halogens in the presence of an isocyanide (775,116) or reactions of the salt Ag4Mo(CN)8 with isocyanides (74), have given six- and seven-coordinate products. Recently, however, the discovery of the reductive or nonreductive cleavage of multiple metal - metal bonds in dinuclear group VIA compounds by isocyanides has provided a facile route to the synthesis of a variety of homoleptic and related isocyanide complexes of these metals in reasonable yields. 

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