Allylic 1, 3-strain influences the conformational preferences of acyclic ester enolate having the allylic chiral center. In the most stable conformer, the allylic C-H bond is disposed on the enolate plain and two other more sterically demanding substituents are disposed such that they may differentiate the diastereotopic faces of the enolate. Subsequently, the facial selectivity is determined by the steric balance between these two substituents. Extention of this concept to a cyclic enolate provides the rational explanation and development of the conceptually new conformationally controled stereoselective alkylation of the cyclic enolate. The chiral catalysts are also designed on the basis of the conformation control and applied to the novel ligand-mediated catalytic asymmetric carbon-carbon bond forming reactions.
Methods for preparation of organofluorine compounds are reviewed from the view point of the reagent system consisting of fluoride ion and an oxidant. Reagents like N-halo imides are often used as the oxidant, which may be replaced by electrochemical oxidation. These methods provide us with various types of fluorinated compounds efficiently and selectively under mild conditions.
Several carbon-carbon bond forming reactions useful for organic synthesis are achieved by electrochemical reduction. (1) Electroreduction of ketones in isopropyl alcohol containing a tetraalkylammonium salt generates anion radical species which reacts with aromatic rings, nitriles, and O-alkyl oximes intra- and intermolecularly. Some ring-fused sesquiterpenes and α, β-disubstituted cyclopentanones such as dihydrojasmone are synthesized by electroreduction of γ-cyano ketones. (2) Electroreduction of a system comprising CX3Y, CHX2Y, and an aldehyde leads to an anionic chain reaction and gives the corresponding coupling products, RCH(OH)CX2Y, in high current efficiencies and yields. The addition of -CCl2CO2Me to α-branching aldehydes shows high diastereoselectivity. (3) Electroreduction of aromatic imines in the presence of chlorotrimethylsilane generates anionic species which reacts with electrophiles to give the corresponding inter- and intramolecular cross-coupling products. (4) Electroreduction of aromatic diimines under acidic conditions gives intramolecular coupling products stereoselectively. This reaction is useful for the synthesis of substituted piperazines and 1, 4-diaza-crown ethers.
This article deals with a recent development of selective reactions using zirconocene-alkene complexes in our group. The following reactions are reviewed. (i) Preparation of zirconocene-alkene complexes. (ii) Alkene-alkene coupling reactions on zirconocene. (iii) Reactions of alkene complexes with ketones or aldehydes. (iv) Selective skeleton transformation via C-C bond activation. (v) Selective functionalization of zirconacyclopentenes. (vi) Allylation reaction of alkynes using a zirconocene complex and allylic compounds. (vii) Zirconium catalyzed C-C bond formation reactions of alkenes on zirconocenes. (viii) Zirconocene catalyzed hydrosilation reactions using zirconocene-alkene complexes. (ix) Zirconocene catalyzed C-C bond formation reactions of alkynes on zirconocenes. (x) Zirco-nocene-catalyzed allylation reactions.
The chemical functions of novel heterocyclic o-quinone cofactors, PQQ (pyrroloquinolinequinone) and TTQ (tryptophan tryptophylquinone), and their derivatives were investigated from the viewpoints of synthetic organic chemistry. PQQ was found to be a good catalyst in the aerobic oxidation of biologically important substances such as amines, amino acids, thiols, glucose, NADH, and so on. Studies on the structure-reactivity relationships and product analyses provided a lot of important information on the structural importance of PQQ and on the oxidation reaction mechanisms. In particular, the reaction of PQQ and methanol was examined in detail to suggest that the enzymatic oxidation of methanol proceeds via an ionic mechanism through the C-4 acetal intermediate. PQQ and its related compounds were also found to act as an efficient mediator in the electrochemical oxidation of NADH and the electrochemical reduction of O2, which could be coupled with enzymatic redox reactions. An efficient enzymatic redox reaction in organic solvents was also constructed by applying PQQ as an electron transfer catalyst. The model compound of the newly found cofactor TTQ was synthesized for the first time and its physicochemical properties and chemical functions were found to be very close to those of the native enzyme.
Synthetic studies on the clerodane diterpenoids, a rapidly growing group of natural products, are reviewed from the standpoint of stereocontrols for the construction of a unique four contiguous stereogenic centers on their bicyclic skeleton. Strategies for the clearance of the stereochemical problems are described according to the types of major stereochemical features-namely trans and cis with respects to the ring fusion, and cis and trans disposition of C1-unit at C-8 and C-9 positions. The description involves the methods for enantioselective synthesis. Brief mentions are made on the synthetic methods for the related natural products (sesqui-, di- and sesterditerpenoids) with “clerodane” skeleton or substitution (hence with common problems of the stereocontrol) and on the investigations for the construction of the specific groupings in bioactive clerodane diterpenoids. The preliminary approaches as well as the methods used in the successful total synthesis are introduced in the modern light of stereocontrols.