Thioesters reductive coupling

The oxidation and reduction steps are coupled through the binding of G3P to a sulfhydryl group on an enzyme that already has bound a NAD+ molecule. The enzyme facilitates a H (hydride ion) shift from G3P to reduce the NAD+ to NADH. The oxidation of G3P results in the formation of a high-energy thioester bond that is subsequently replaced by an orthophosphate group to yield the 1,3-BGP. 

Oxidative addition of the thioester is followed by transmetalation from the zinc compound. Reductive elimination leads to the coupled product. 

This coupling procedure with the thioesters proved sensitive to the substitution pattern of both the amino acid and alkene. In contrast, coupling reactions with the M-acyl oxazolidinone derivatives such as 22 proved to be much more effective (Scheme 14) [20]. Mechanistic studies suggested that an alternative pathway was operating in these cases, where reduction of the al-... 

In the original formulation [81a], DBN and a slight molar excess of AgN03 in MeCN solution were used. Depletion of the starting penicillanic ester was followed by tic and, when complete, obtained silver azetidinyl mercaptides 126 were in situ treated with acyl chlorides to afford thioesters 127 these in turn were oxidized at the butenoate double bond to give lH-4-thioesters 128, key intermediates of Woodward s original phosphorane-thioester cyclization route to penems [1, 48], or ozonized to the iV-oxalo compounds 129, immediate penem precursors by a newly discovered reductive dicarbonyl coupling [82]. 

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