STEREOCHEMICAL ASSIGNMENT OF THE PYOCHELINS

摘要

The synthesis of pyochelin has been much improved but gives four stereoisomers. The stereochemistry of the two naturally occurring pyochelins I and II has been assigned, based on (1) the similarity of the NMR spectra of pyochelin I methyl ester to those for 4-methylpyochelin I methyl ester, whose X-ray structure has been determined and relative stereochemistry assigned; (2) comparison of the rotation of the natural material to that of pyochelin I methyl ester synthesized from N-methyl-L-cysteine methyl ester, which is not expected to epimerize during the synthesis and thus assigns pyochelin I methyl ester as 4'R,2 '' R,4 '' R; and (3) the known facile epimerization at C-2 '' of 2 ''-substituted thiazolidine-4-carboxylic acids, which assigns the other (unfavored) naturally occurring isomer, pyochelin II, as 4'R,2 '' S,4 '' R. The remaining two synthetic products, neopyochelin I methyl ester and neopyochelin II methyl ester, were assigned the stereochemistry 4'S,2 '' S,4 '' R and 4'S,2 '' R,4 '' R, respectively, based on (1) the use of N-methyl-L-cysteine methyl ester in their synthesis, which establishes C-4 '' as R; (2) the known instability at C-2 '', which favors neopyochelin II (2 '' R) over neopyochelin I (2 '' S); and (3) the requirement of nonidentity with pyochelins I and II, which requires the S configuration at C-4'. [References: 28]