Cobalt complexes Reformatsky reactions

The unusual chiral (3-methoxy-y-amino acid dolaproine (Dap) is the most complex unit of dolastatin 52, which has a remarkable antineoplastic activity and is now in Phase II human cancer clinical trials. Many synthetic strategies such as aldol condensation and a cobalt-catalyzed Reformatsky reaction have been employed in its synthesis. Almeida and Coelho have demonstrated a stereoselective synthetic method for A-Boc-dolaproine (53) through a sequence of MBH reaction, a diastereoselective double bond hydrogenation and hydrolysis of the ester functional group (Scheme 5.8). ... 

Cyclic cobalt-acyl complexes can be deprotonated, and subsequent reaction of these enolates with aldehydes gives predominantly the anti/threo product (Scheme 63). Rhenium-acyl complexes can be deprotonated in the same manner. These lithium enolates can be alkylated or can react with [M(CO)5(OTf)] (M = Re, Mn) to give the corresponding enolates (Scheme Many transition metal enolates of type (21) or (22) are known, - but only a few have shown normal enolate behavior , e.g. aldol reaction, reaction with alkyl halides, etc. Particularly useful examples have been developed by Molander. In a process analogous to the Reformatsky reaction, an a-bromo ester may be reduced with Smia to provide excellent yields of condensation products (Scheme 65) which are generated through intermediacy of a samarium(III) enolate. ... 

Catalysed aerobic epoxidation of the styrene-cobalt complex, followed by the addition of trimethylsilyl isothiocyanate, yields 4-phenyloxazolidine-2-thione 123 <96TL7315>. The 4-substituted 2-oxazolidinone 124 undergoes a "chromium Reformatsky" reaction with aldehydes RCHO (R = i-Pr or Ph) in the presence of chromium(ll) chloride and lithium iodide to give mainly the unusual anti-aldol products 125 in excellent dia,stereomeric excess and yield <97TL4,387>. 

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