The investigation of elimination reaction is rewarding from both a mechanistic and a synthetic point of view. This paper addresses our own approach focused on these three issues : (1) The prerequisites for and limits of concertedness ; (2) The effect of ion-incompatible reaction condition ; (3) The synthetic potential of dehydrohalogenation or ether cleavage.
Recent developments of organic transformations using lanthanoid elements are described. This review deals also with novel reactions of organolanthanoid complexes which have potential utility in organic synthesis.
Reductive polymerization of carbon monoxide on heterogeneous catalysts provides hydrocarbons, and is well known as Fischer-Tropsch synthesis. Interest in the mechanism is focused on an initial stage of the reaction, i.e. how to hydrogenate carbon monoxide by metallic species. Two mechanisms have been proposed on the reduction of CO. One involves dissociative cleavage of CO by metals to metal carbide and metal oxide. Another is through stepwise reduction of CO by hydride reduction followed by protonation. Many model reactions have been provided using mononuclear metal complexes or small metal clusters, contributing to an understanding of the mechanism. This article reviews the recent advances.
Biopterin cofactor [(6 R) -5, 6, 7, 8-tetrahydrobiopterin] in biological samples was measured by radioimmunoassay of biopterin. The results showed that this compound deeply concerned with neurological diseases, such as Parkinson disease. The structure of biopterin cofactor was determined by CD spectra and X-ray crystalographic analysis. Synthesis of biopterin cofactor was achieved by stereoselective hydrogenation of biopterin. Biopterin and related pteridines were synthesized by condensation of a pyrimidine derivative whith a sugar or an epoxyaldehyde. Some natural occurring pteridines were synthesized by the introduction of side chains to pteridine ring under radical conditions. Quinonoid dihydrobiopterin was prepared by oxidation of biopterin cofactor, and decomposed through different pathways depending on the acidity of solution. By using various pteridine derivatives, the relationship between structure and biological activity was studied.
Divinyl monomers like 1, 3-bis (p-vinylphenyl) propane were treated by trifluoromethanesulfonic acid in dry benzene to generate cyclic cations which were trapped in situ by a comonomer and subsequently deprotonated to afford cyclocodimers or 1-methyl-3-styryl [3.n] cyclophanes (n>2). The process, so-called cationic cyclocodimerization, is reviewed here. Several cyclophanes such as [3.n] paracyclophane, [3.n] metacyclophane, [3. n] biphenylophane, [3. n] paracyclo (1.4) naphthalenophane, [3.n](1, 4)-, (1, 5)-, and (2, 6) naphthalenophane derivatives were prepared by this method. Syntheses of monomers, reaction conditions, comonomer properties, and its extension to cationic cyclization toward [3.n.3.n] paracyclophanes are outlined. Cyclophanes obtained can be converted easily to several cyclophanes possessing a functional group at a bridge. Using those cyclophane derivatives, acetolysis of cyclophanylmethyl tosylates and rearrangement of cyclophanylcarbenes were studied. The former showed the strong participation of neighboring [3.3] paracyclophanyl group, because the group has the highest electron donating nature among [n.m] paracyclophanes with the almost ideal structural environment for solvolysis. The latter showed the strain effect of [3.n] paracyclophanyl groups on the fates of the carbenes. Consequently, the cationic cyclocodimerization can give a series of [3.n] cyclophanes which are useful for systematic investigations of reaction intermediates with a stereochemical guide namely a methyl group at C-1 position.
The following new synthetic methodologies involving electron transfer process have been developed. (1) Cleavage of the carbon-silicon bond : The carbon-silicon bonds in organopentafluorosilicates are cleaved by various oxidizing agents via initial electron transfer. The carbon-silicon bonds in allylsilanes and α-heteroatom substituted organosilicon compounds are also cleaved by electrochemical oxidation. (2) Indirect electrochemical oxidation using polymeric electron carrier such as poly (vinylpyridinium) bromide. (3) Cycloaddition of 1, 3-dicarbonyl compounds : The radical species generated by electrochemical oxidation of 1, 3-diketones adds to the olefin to give the formal [3 + 2] cycloadducts, dihydrofurans or tetrahydrofurans. In the presence of molecular oxygen the formal [2 + 2 + 2] cycloaddition of 1, 3-diketone, olefin and molecular oxygen takes place. The electroinitiated radical chain mechanism is proposed.
Several synthetic methods of 1, 3-dioxin-4-one and its derivatives are described. These dioxinones undergo cycloreversion to formyl- or acylketenes, whose inter- or intramolecular trappings provide efficient routes to a variety of heterocycles. Use of 6-unsubstituted dioxinones in de Mayo reaction provides an efficient method for simultaneous introduction of acetic acid and formyl appendages at the vicinal position of alkanes. Recent synthesis of chiral spirocyclic dioxinones as well as their use in asymmetric synthesis are also described.