Influence of External Ligands and Auxiliaries on the Reactivity of Low-Valent Titanium in McMurry Reaction: Selectivity and Mechanistic Profile

Toward the Synthesis of Paspaline-Type Indole-Terpenes: Stereoselective Construction of Core Scaffold with Contiguous Asymmetric Quaternary Carbon Centers

The core scaffold of paspaline-type indole-terpenes was synthesized by using the House-Meinwald rearrangement as a key step. Rearrangement of the epoxide methyl group in the precursor with methylaluminum bis(4-bromo-2,6-di-tert-butylphenoxide) as a Lewis acid proceeded smoothly to construct contiguous asymmetric quaternary carbon centers by a 1,2-chirality transfer.

Ruthenium(ii)-catalyzed olefination via carbonyl reductive cross-coupling

A catalytic olefination method via carbonyl reductive cross-coupling was achieved by ruthenium(ii) catalysis.

Natural availability of carbonyl groups offers reductive carbonyl coupling tremendous synthetic potential for efficient olefin synthesis, yet the catalytic carbonyl cross-coupling remains largely elusive. We report herein such a reaction, mediated by hydrazine under ruthenium(ii) catalysis. This method enables facile and selective cross-couplings of two unsymmetrical carbonyl compounds in either an intermolecular or intramolecular fashion. Moreover, this chemistry accommodates a variety of substrates, proceeds under mild reaction conditions with good functional group tolerance, and generates stoichiometric benign byproducts. Importantly, the coexistence of KO^tBu and bidentate phosphine dmpe is vital to this transformation.

Generation of TiII Alkyne Trimerization Catalysts in the Absence of Strong Metal Reductants

Low-valent Ti^II species have typically been synthesized by the reaction of Ti^IV halides with strong metal reductants. Herein we report that Ti^II species can be generated simply by reacting Ti^IV imido complexes with 2 equiv of alkyne, yielding a metallacycle that can reductively eliminate pyrrole while liberating Ti^II. In order to probe the generality of this process, Ti^II-catalyzed alkyne trimerization reactions were carried out with a diverse range of Ti^IV precatalysts.

A simple and practical catalytic electron transfer system composed of TiCl4 and metallic Yb: application in carbonyl olefination and insight into the mechanism

A simple and practical catalytic electron transfer system composed of TiCl₄ and Yb, which turned out to be effective for homo- and cross-deoxygenative coupling of aldehydes and ketones was developed. Mechanistic data provide an insight into the rate-determining step and intermediates.

Synthesis and antihistamine evaluations of novel loratadine analogues

A series of loratadine analogues containing hydroxyl group and chiral center were synthesized. The effect of the synthesized compounds on the histamine-induced contractions of guinea-pig ileum muscles was studied. In addition, the in vivo asthma-relieving effect of the analogues in the histamine induced asthmatic reaction in guinea-pigs was determined. Most of the compounds exhibited definite H(1) antihistamine activity. The S-enantiomers, compounds 2, 4 and 8, are more potent than the R-enantiomers, compounds 1, 3 and 7. Compound 6 was the most active one among the eight synthesized compounds.

Unexpected Ti(III)/Mn-promoted pinacol coupling of ketones

Titanocene(III) chemistry has emerged in the last decades as an indispensable tool in C-C bond-forming reactions. In this context, pinacol and related reactions allow the stereoselective synthesis of vicinal diols. In this work, we present new applications of these reactions using as starting materials aromatic ketones. Simple and smooth reaction conditions have been developed and have been applied for inter- and intramolecular processes. We also describe that although Cp(2)TiCl is usually used as a monoelectronic reducing agent, it can be also used as an efficient Lewis acid.

TiCl3 as a New Catalyst for the Imino Diels-Alder Reaction

Titanium(III) chloride was found to be an effective catalyst for the imino Diels-Alder reaction between an imine (generated in situ) and an activated alkene in acetonitrile at ambient temperature. The reaction proceeds smoothly to afford the corresponding adduct as a mixture of chromatographically separable cis- and trans isomers in moderate to good yields.

A mechanistic study on the nuclease activities of some hydroxystilbenes

The nuclease activities of a series of hydroxystilbeneoids have been studied so as to establish a structure-activity correlation and deduce the mechanistic pathway of the process. Although the test compounds could nick plasmid DNA, only three of these including resveratrol produced double strand breaks in DNA. Amongst these new stilbenes, compound 2e containing a partially methylated catechol and a C-4 hydroxy moieties was equally potent as resveratrol. The activities of the unprotected catechol-derivatives were less than those of the resorcinol-derivatives, which were, however, compensated by partial methylation of the former. The presence of Cu(2+) and O(2) and the participation of a Cu(+)-oxo intermediate were obligatory in the process which did not require addition of any external reducing agent. Overall, the differential nuclease activities of the compounds could be explained primarily with their superoxide anion generation abilities, and to a lesser extent with their DNA binding and Cu(2+) reducing capacities. The amount of superoxide anion produced by the compounds depended strongly on their Cu(+)-complexation abilities, which again, was decided by the pattern and nature of the oxygenated substituents in the aromatic rings.

Reductive Amination of Aliphatic Carbonyls using Low–valent Titanium Reagent: A Convenient Route to free Primary Amines

A simple procedure for reductive amination of aliphatic carbonyls, giving direct access to free primary amines using low–valent titanium (LVT) reagent has been reported.

Dual enantioselective control in asymmetric synthesis

It is desirable and practical to produce both enantiomers of a target from the same chiral starting material by stereodifferentiation of prochiral compounds, for instance utilizing a chiral ligand derived from a natural (L) amino acid. During the past 7 years, excellent results have been achieved in several cases by multiple stereodifferentiation of chiral ligands derived from (S)-indoline-2-carboxylic acid: highly diastereo- and enantioselective pinacol coupling reactions of chiral alpha-ketoamides gave both (S,S)- and (R,R)-quaternary tartaric acid for the first time; asymmetric Diels-Alder cyclization of chiral acrylamides in the presence of Lewis acid afforded extremely high diastereoselectivities of both opposite configurations of the cyclized diastereomers depending upon the structures of chiral ligands and Lewis acids; and asymmetric alkylation of aldehydes to both enantiomers of secondary alcohols and asymmetric hydrogenation of ketones to both enantiomers of chiral secondary alcohols have been achieved using catalysts derived from (S)-indoline-2-carboxylic acid.

Generation of reactive low-valent titanium species using metal--arenes as efficient organic reductants for TiCl(3): applications to organic synthesis

A comprehensive study on the use of metal-arene systems as organic reductants for TiCl(3) has resulted in an efficient method for the generation of highly reactive low-valent titanium (LVT) reagents. The activated titanium species could be prepared by refluxing a mixture of substoichiometric amounts of arenes, TiCl(3), and Li/Mg in THF or DME. Among the LVT reagents screened, TiCl(3)--Li--naphthalene--THF (reagent I) was the best for coupling of carbonyls to olefins. The reagent could carry out the McMurry olefination of both aromatic and aliphatic substrates at a lower temperature and in a much reduced time as compared to the conventional procedures. Subtle changes in the method of preparation of the LVT reagents influenced the stereoisomeric ratio of the olefins. The reagent was also useful for the synthesis of O- and N- heterocycles and vicinal diamines via intramolecular carbonyl coupling and reductive duplication of imines, respectively.

Efficient method for the preparation of pinacols derived from aromatic and aliphatic ketones by using low-valent titanium reagents in dichloromethane-pivalonitrile

The reductive coupling reaction of aldehydes and ketones, including unsymmetrical aliphatic ketones, proceeded smoothly to give the corresponding pinacols in good to high yields under mild conditions by using combination of titanium(II) chloride and zinc or titanium(IV) chloride and zinc in dichloromethane-pivalonitrile. Meso-selective formation of the coupling products was observed in the cases of some aliphatic ketones. The diastereoselectivities of coupling products depend on both difference of bulkiness of 2-, and 2'-substituents of carbonyl group of the reactant, and overall steric effect around the carbonyl groups.Key words: diastereoselective pinacol reaction, dichloromethane-pivalonitrile, titanium(II) chloride, titanium(IV) chloride, zinc.