Optically active a carbon-chiral 1-substituted ethyl and 1, 2-disubstituted ethyl bis (dimethylglyoximato) cobalt complexes were prepared by resolution of chiral axial base-coordinated complexes followed by ligand displacement of the chiral axial base with achiral base. An unprecedented type of chiral octahedral cobalt complexes, trans- [Co (AB) 2 (X) (Y)], have also been synthesized enantioselectively. By taking advantage of the features of these chiral complexes, some precise information was obtained on the mechanism of Co-C bond formation and cleavage of the alkyl cobaloxime complexes, and Co-C bond dissociation energies were estimated which, in turn, made it possible to estimate steric and electronic effects on the Co-C bond cleavage quantita-tively. Unexpectedly, these complexes lead to the findings of X-ray-induced racemization of α carbon-chiral cobalt complexes in the crystalline-state, solid state-specific phenomena in the solid state photoracemization, unidirectional β to α (solid-state specific) photoisomerization. The mechanisms of these reactions are elucidated based on the X-ray structural analyses.
Allenes comprize that class of compounds characterized by a 1, 2-diene grouping with an unique geometrical feature owing to the cumulated diene system. Within the last ten years, allene chemistry has considerable progressed to development of modern synthetic methodologies for a variety of desirable intermediate and natural products. On the other hand, allenes also enjoy a wide variety of reactivities in many types of reactions via ionic, radical or concerted mechanism. Here we describe the review of recent studies on syntheses of chiral allenes and cyclic compounds including heterocyclic compounds as lactones, furans, indoles, pyrrolidines, thiophenes, sulfolenes, macrolides, oxazolidinones using allenes as versatile building blocks (synthons).
A wide variety of bicyclic amino alcohols have been prepared from D-camphor and screened as chiral elements in asymmetric synthesis. Endo-3-amino-endo-2-hydroxybornane and exo-3-amino-exo-2-hydroxybornane were found to be efficient chiral auxiliaries for the synthesis of optically pure cyclopropane derivatives, and [P]- and [M]- heterohelicenes. Both (S)- and (R)-secondary alcohols are obtained with predictable stereochemistry by utilizing these diastereomeric amino alcohols as catalysts in the addition of diethylzinc to aldehydes and in reduction of aromatic ketones by borane. The shortest asymmetric synthesis of (R)-(-)-muscone with essentially 100% enantiomeric excess (ee) was accomplished by employing (1R, 2R, 3S, 4S)-3-[(1-methyl-2-pyrrolyl)methylamino]-1, 7, 7-trimethylbicylo[2.2.1]heptan-2-ol as a non-transferable ligand in the conjugate addition of alkoxydimethylcuprate to (E)-2-cyclopentadecenone. This asymmetric conjugate addition was promoted by use of 0.36 equivalents of the amino alcohol as a chiral catalyst, giving (R)-muscone in 99% ee and 86% chemical yield.
Significant attentions have been focused on the inclusion chemistry of cyclodextrins and the related compounds and synthetic studies of cyclooligosaccharides have been tarring out intensively. For example, Ogawa and coworkers reported the total synthesis of cyclodextrins and a manno isomer of cyclodextrins by means of phenylselenyl triflate promoted cycloglycosylations. Synthesis of 1-3β linked cycloglucohexaose was reported by Collins' group. Modifications of α-cyclodextrin into trimethyl, 1, 3-anhydro, and a chimera analog have also reported recently. However, cyclooligosaccharide available today is only limited to D series. As we have developed a novel thermal glycosylation procedure, which can be applied to rhamnosylation with high α-selectivity, we designed the synthesis of cyclo-L-rhamnohexaose (40), the first cyclooligosaccharide of L series. On the basis of the area where the compound has been prepared, this compound was named α-cycloawaodorin, which would open a new dimension of the inclusion chemistry.
The cleavage reaction of cyclopropane ring is one of the useful method in organic synthesis. Especially, the sequence of cyclopropanation-homoconjugate addition is one of the accessible methods for the synthesis of natural products possessing ring- and side-chain contiguous chiral centers. This report deals with the synthetic studies of biologically active natural products possessing such chiral centers by using the opening reaction of double activated cyclopropanes. Cyclopropane derivatives have been synthesized from methyl acetoacetate by alkylation, diazo transfer followed by copper-mediated cyclopropanation. The various adducts were obtained by homoconjugate addition of nucleophiles (Gilmann reagents, Grignard reagents, cyanide anion, methanol etc.) to cyclopropanes. The opening reaction proceed with inversion of absolute configuration at the apical carbon of the cyclopropane. The adducts were converted effectively into several biologically active natural products and/or known synthetic intermediates.
Various [2.n] cyclophanes, [2.n] naphthalenophanes, and [2.n] phenanthrenophanes having cyclobutane rings were prepared, in most cases stereoselectively, by intramolecular [2+2] photocycloaddition of α, ω-bis(vinylaryl)alkanes. This facile reaction is reviewed. The stereoselectivity is recognized in the syntheses of syn-naphthalenophanes and syn-phenanthrenophanes. The reaction mechanism is discussed with flash-photolysis data and solvent and additive effects. Combined with a cationic cyclocodimerization and Birch reduction, this reaction presents a systematic synthesis of [2.n]-, [3.n]-, and [4.n]-cyclophanes form only one common starting materials, α, ω-bis(vinylaryl)alkanes. Crown compounds were also synthesized by this reaction in excellent yields. They showed an interesting Li+-ion selectivity in solvent extraction and transport experiments.
Intercalation of metal complexes into smectites is one of the useful methods for the preparation of supported metal complex catalysts. Smectites can intercalate metal complexes as well as various organic compounds simultaneously. Multiple modification of clays with these guest compounds (tuning guests) can be applied to the preparation of catalysts for molecular recognition reactions such as shape selective hydrogenation and asymmetric hydrogenation. In this article, clay-supported metal complexes were prepared by various methods such as cation exchange and ligand exchange methods. For the ligand exchange method, clays were modified with tuning guests including alkylammonium ions and alkylsilanes. These clay-supported catalysts were used for catalytic hydrogenations of alkenes, alkynes, aldehydes, and ketones and their shape selectivities were studied