Organomanganese complexes in organic synthesis
Abstract (Summary)Tricarbonylcycloheptatrienemanganese underwent stereospecific nucleophilic addition of stabilized enolates to give exo-substituted adducts in good yield. Decomplexation with ceric ammonium nitrate in buffered wet acetone gave a mixture of 3-hydroxycyclohepta-4,6-diene and lactonecyclohepta-3,5-diene products from the stereospecific addition of external water at the remote terminus or the adjacent terminus of the dienyl system respectively. A pendent hydroxyl group added intramolecularly and stereospecifically to the adjacent terminus of the dienyl system in the absence of external nucleophiles during iodine-mediated decomplexation. Tricarbonylchlorobenzenemanganese(1+) underwent aromatic nucleophilic substitution when treated with phenoxides from protected tyrosine and 4-hydroxyphenylglycine amino acid derivatives. In situ decomplexation occurred in the presence of acetonitrile. No racemization of the amino acid moiety was observed in these reactions. This diaryl ether synthesis was envisioned as an entry to the synthesis of models of glycopeptide antibiotics of the ristocetin and vancomycin families. A survey of these families led to a generic model as a synthetic goal. One necessity of this synthesis, the ability to form diaryl ethers and triaryl diethers selectively, was demonstrated. All attempts to synthesize manganese complexes of chlorophenylalanine derivatives, also crucial to the original synthetic plan, failed. However, several novel arenetricarbonylmanganese complexes with functionalized side chains were prepared via Mn(CO)5BF4. Differences in the mode of complexation between the standard Mn(CO)5Br-AlCl3 method and hits new method may explain the inability to synthesize any chloroaromatic complexes via Mn(CO)5BF4. An alternative synthesis of fragments of the target antibiotic molecules through aromatic nucleophilic substitution followed by stereoselective nucleophilic addition was developed. Diaryl ether manganese salts of 1-6-?-benzene and protected derivatives of L-tyrosine, D-tyrosine, and L-leucine-L-tyrosine were prepared. These reacted with the lithium anion of 3R(2,5-dimethoxy-3-isopropyl)3,6-dihydropyrazine to give phenyl glycine equivalents in 70% d.e. Hydrolysis of the dihydropyrazinemoiety gave the respective N-acetyl-4-O-(3-D-phenylglycine methyl ester) -D- and -L-tyrosine methyl esters thereby demonstrating the potential of success for the synthesis of antibiotic fragments by this methodology.
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:organomanganese complexes organic synthesis
Date of Publication:01/01/1990