Synthesis and enzyme-catalysed reductions of 2-oxo acids with oxygen containing side-chains

A series of novel 2-oxo esters with protected alcohols at the 3-, 4-, 5-, 6- and 7-positions has been prepared either via coupling of an aldehyde with an organometallic reagent (Zn, In or Cr) or via a one-carbon homologation of the precursor acid. A one-pot dual enzyme system was used to convert the simpler 2-oxo acids (with a single MOM ether at either C-3 or C-4) into enantiopure protected 2,3- and 2,4-dihydroxy acids in good yields, but in the cases of the more complex trisubstituted substrates, significant decomposition occurred. Biotransformations have proved valuable for the enantioselective synthesis of protected 2,6,7-trihydroxyhept-3-enoic acids.