The use of organoselenium reagents in organic synthesis is now commonly accepted as introducing new functional groups into organic substrates under extremely mild reaction conditions. One of the useful reaction involving organoselenium reagents is the trans-addition reaction across carbon-carbon double bonds. They are highly stereospecific, and subsequent oxidation normally leads to regioselective and stereospecific elimination of the selenoxide to yield olefins. Recent advances in this field were reviewed in the following three topics; (1) preparation of reactive electrophilic selenium reagent in situ from diphenyl diselenide using easily available oxidants and their applications to the addition reactions across carbon-carbon multiple bond. (2) the design of chiral selenium reagents and their application to the asymmetric methoxyselenenylation and intramolecular oxyselenenylation. (3), catalytic oxyselenenylation- deselenenylation reaction and the other several catalytic approach.
In the experiments of specific deuteration of cytochrome c3 (D. vulgaris, Miyazaki), it was found that the deuterated methyl group of methionine-methyl-d3 in the medium was incorporated not only into the methionine residues of the polypeptide but also into the C-2 and C-7 positions of the heme groups. It shows that methionine is involved in the biosynthesis of porphyrin in this sulfate-reducing bacterium. In the known porphyrin biosynthetic pathway, all four methyl groups of protoporphyrin IX are transformed from acetate groups by uroporphyrinogen decarboxylase. This pathway cannot explain the incorporation of methyl groups into the C-2 and C-7 positions from methionine. Therefore, our result is the strong evidence for the existence of previously unknown pathway of anaerobic porphyrin biosynthesis in D. vulgaris. Intermediates from uroporphyrinogen III to coproporphyrinogen III would be formed through methylation at the C-2 and C-7 positions, followed by decarboxylation of the acetate groups at the C and D rings.
Asymmetric reactions by the use of chiral lithium amide have become one of efficient methods for the preparation of chiral compounds for these ten years. We examined enantioselective deprotonation of meso-epoxides by chiral lithium amide, prepared from chiral diamine, e.g., (S) -2- (pyrrolidin-1-yl) methylpyrrolidine. Chiral allylic alcohols were obtained by the reaction, and especially in the cases of 4-substituted cyclopentene oxides high selectivity was achieved to afford synthetically useful cyclopentenol derivatives in high enantiomeric excesses. Both enantiomers of 4-hydroxy-2-cyclopentenone, (-) -carbovir, (-) -aristeromycin, and (-) -untenone A were synthesized by applying the reaction. The reaction was extended to a catalytic asymmetric reaction by the use of a catalytic amount of the chiral lithium amide and excess of lithium diisopropylamide. The chiral lithium amide was also found to be effective for kinetic resolution of racemic epoxides.
Since exceptionally bulky oxygenophilic organoaluminum reagent, methylaluminum bis (2, 6-di-t-butyl-4-methylphenoxide), MAD was successfully introduced for stereoselective activation of carbonyl compounds almost 10 years ago, it has been recognized widely as a highly promising Lewis acid in selective organic synthesis and also successfully applied to other field of chemistry. The key feature of MAD is the exceptional steric hindrance created by the two bulky 2, 6-di-t-butyl-4-methylphenoxy ligands which prevents it from self-association and hence makes it feasible to exist as a monomer in solution, thereby inducing the inherent oxygenophilicity of aluminum. The unique character of MAD is essentially related to the remarkable ability to recognize oxygen containing organic molecules as a Lewis acid receptor and activate certain functionalities, which could meet versatile synthetic requirements due to continuous developments of selective transformations and catalysts with high efficiency in organic synthesis. In this review, we go through the issue titled above according to the typical reaction patterns, where the crucial role of MAD shall be outlined.
The synthetic strategy of novel heterocyclic compounds from a series of urazoles (key compounds), which were prepared by addition-elimination reactions of benzylidene ketones with 4-phenyl-3H-1, 2, 4-triazole-3, 5 (4 H) -dione (PTAD), was described. Alcoholysis and aminolysis of the urazoles afforded intriguing tricyclic oxazolidinone derivatives (hetero analogues of triquinane sesquiterpenes). The reaction proceeded via Michael addition of nucleophiles to enone substructure, opening of the urazole ring by backbone participation and final skeletal rearrangements. This methodology was extended to one-pot syntheses of [4.3.3] propellane hetero analogues and 3-cyanoindole derivatives. While, oxime ethers of the key compounds were photochemically active to rearrange to triazinoindoles, via cleavage of N-N bond and subsequent skeletal rearrangement from a polar excited state. More active urazoles afforded surprisingly dimers and solvent-incorporated adducts (triazinoindolines) in a stereospecific fashion. On the contrary, sensitized reactions gave cyclobutane derivatives.