Chain elongation by use of an iron carbonyl reagent: A facile synthesis of 6-deoxyheptosiduronic acids

Abstract

A method of chain elongation at the nonreducing terminal of 6-deoxy-6-halo- and 6- O-tosyl-hexopyranosides, to give methyl (methyl 6-deoxyheptopyranosid)uronates was developed on the basis of organo-iron chemistry. Thus, methyl 2,3,4-tri- O-acetyl-6- O- p-tolylsulfonyl-β- d-glucopyranoside reacted almost instantly under mild conditions with sodium dicarbonyl-η 5-cyclopentadienyliron (NaFp) in oxolane, to form an isolable, well-characterized iron derivative ( 2) in which the tosyloxy group was replaced by the Fp group. Oxidative carbonyl insertion in 2 could be induced by a variety of oxidants, but was best accomplished by bromine in the presence of methanol, at low temperatures. The process caused removal of the iron moiety, and led, in 80% overall yield, to the methyl ester of the corresponding, 6-deoxyheptosid-uronic acid. Similarly, the methyl 4- O-benzoyl-6-bromo-6-deoxy-α- and -β- d-glucopyranosides, their 2,3-diacetates, and 2,3-dimethyl ethers, as well as the methyl 6-bromo-6-deoxy-α- d-glucopyranoside and 6-deoxy-6-iodo-α- d-mannopyranoside 2,3,4-tribenzoates furnished high yields of 6-deoxyheptosiduronic esters by application of this reaction sequence, although the intermediate sugar-iron compounds were not normally isolated.