Chiral Methyl Group - Stereochemistry of Enzymatic Reactions

Chiral methanol is the smallest chiral organic molecule and represents a synthetic challenge. The submitted project presents a method for its direct chemical synthesis, which so far could not be achieved. The crucial steps are the chemical resolution of a boronic acid ester containing a silyl group, and the reductive removal of a carbamoyloxy group by lithium aluminumdeuteride to generate a silylmethyl boronate. Oxidative cleavage of the boron carbon bond furnishes a chirally labelled silylmethanol which is rearranged (Brook rearrangement) under basic conditions. When the sequence was repeated using the three isotopes of hydrogen, chiral methanol was obtained having an enantiomeric excess of 96% as determined by tritium NMR. The diastereoselectivity of the addition of the metallated carbamate to the boric acid ester has to be improved. This can possibly be achieved by testing boric acid esters of different diols and carbamates and thiocarbamates of various secondary amines. The chiral methanol will be transformed into its tosylate and used to prepare methionine with a chiral methyl group. This will be used to elucidate the stereochemistry of two very interesting methyl transfers in the biosynthesis of fosfomycin, a clinically used antibiotic, and phosphinothricin, a builing block of the herbicide bialaphos used in agriculture. The two methyl transfers are supposed to occur, contrary to the majority of methylations in biological systems, with retention of configuration. The methyl groups transferred to intermediates of the biosyntheses will eventually end up as chiral acetic acids. Determination of their configurations and enantiomeric excesses by a known procedure based on malate synthase and fumare will prove whether the methyl groups have been transferred with net inversion or retention of configuration. This methodology will also be used to determine the stereochemistry of the phosphonatase-catalysed reaction, the cleavage of the P-C bond in phosphonoacetaldehyde, yielding inorganic phosphate and acetaldehyde which as to be oxidised to acetic acid. This enzyme is crucial in the biodegradation of 2-aminoethylphosphonic acid the most widely distributed natural phosphonic acid. To achieve that, 2-aminoethylphosphonic acid chiral at C-1 by virtue of deuterium and tritium has to be prepared.