Palladium-Catalyzed Modular Assembly of Electron-Rich Alkenes, Dienes, Trienes, and Enynes from (E)-1,2-Dichlorovinyl Phenyl Ether

Design, synthesis and evaluation of bioactivity of peptidomimetics based on chloroalkene dipeptide isosteres

Ample biologically active peptides have been found, identified and modified for use in drug discovery to date. However, several factors, such as low metabolic stability due to proteolysis and non-specific interactions with multiple off-target molecules, might limit the therapeutic use of peptides. To enhance the stability and/or bioactivity of peptides, the development of "peptidomimetics," which mimick peptide molecules, is considered to be idealistic. Hence, chloroalkene dipeptide isosteres (CADIs) was designed, and their synthetic methods have been developed by us. Briefly, in a CADI an amide bond in peptides is replaced with a chloroalkene structure. CADIs might be superior mimetics of amide bonds because the Van der Waals radii (VDR) and the electronegativity value of a chlorine atom are close to those of the replaced oxygen atom. By a developed method of the "liner synthesis", N-tert-butylsulfonyl protected CADIs can be synthesized via a key reaction involving diastereoselective allylic alkylation using organocopper reagents. On the other hand, by a developed method of the "convergent synthesis", N-fluorenylmethoxycarbonyl (Fmoc)-protected carboxylic acids can be also constructed based on N- and C-terminal analogues from corresponding amino acid starting materials via an Evans syn aldol reaction and the Ichikawa allylcyanate rearrangement reaction involving a [3.3] sigmatropic rearrangement. Notably, CADIs can also be applied for Fmoc-based solid-phase peptide synthesis and therefore introduced into bioactive peptides including as the Arg-Gly-Asp (RGD) peptide and the amyloid β fragment Lys-Leu-Val-Phe-Phe (KLVFF) peptide, which are correlated with cell attachment and Alzheimer's disease (AD), respectively. These CADI-containing peptidomimetics stabilized the conformation and enhanced the potency of the cyclic RGD peptide and the cyclic KLVFF peptide.

Selective catalytic synthesis of new terpenic chlorides using NaDCC as an eco-friendly and highly stable FAC agent

A simple, mild and efficient scope pathway for a selective catalytic chlorination of terpenic olefins is investigated in the presence of a highly efficient chlorination agent and a readily available Lewis acid catalyst. The sodium dichloroisocyanurate (NaDCC) used in the present work as an easy handling, sustainable and cost-effective chlorine donor due to its high free available chlorine (FAC), exhibits a high efficiency for selective catalytic chlorination. Herein, we report for the first-time the FeCl2/NaDCC combination system for the selective catalytic chlorination towards new functionalized terpenic olefins. In order to examine the general features of this catalytic reaction, the effects of pH, solvent, dilution, chlorination agent nature, stoichiometry and reaction kinetics are optimized using carvone as a model substrate. Among the studied parameters, catalyst stoichiometry was found to be determinant for highly controllable chlorination selectivity towards new allylic and vinylic chlorides. Indeed, the oxidation state, ligand and metal effects of the catalyst are examined using various Lewis acids, where the chlorinated ones (MClx), such as FeCl2, FeCl3 and SnCl2, exhibit a comprehensive approach for a controllable chlorination reaction. In addition, the homogeneous catalytic system shows good reusability with significant catalytic conversion depending on the FAC content in the reaction medium. The reaction proceeds under mild conditions with shorter reaction time and high selectivity towards new high added value allylic and vinylic chlorinated derivatives of naturally occurring terpenic olefins in good to excellent yields.

Chloroalkene dipeptide isosteres as peptidomimetics

To date various biologically active peptides have been discovered, characterized and modified for drug discovery. However, the utilization of peptides as therapeutics involves some limitation due to several factors, including low metabolic stability owing to proteolysis and non-specific interactions with multiple off-target molecules. Hence, the development of "peptidomimetics," in which a part or whole of a molecule is modified, is a desirable strategy to enhance the stability or bioactivity of peptide-based drugs. In this situation, we have designed and developed a synthetic method for chloroalkene dipeptide isosteres (CADIs), which involves replacement of an amide bond in peptides with a chloroalkene structure and are classified as peptidomimetics. By a developed synthetic method, an N-tert-butylsulfonyl protected CADI can be obtained utilizing diastereoselective allylic alkylation with organocopper reagents as a key reaction. This CADI can be transformed into an N-fluorenylmethoxycarbonyl protected CADI in short steps. In addition, CADIs are used in Fmoc-based solid-phase peptide synthesis and introduced into a bioactive peptide. Protocols for practical preparation of some CADIs and peptide mimetics containing a CADI are described as detailed recipes.

Iridium-Catalyzed Hydrochlorination and Hydrobromination of Alkynes by Shuttle Catalysis

Described herein are two different methods for the synthesis of vinyl halides by a shuttle catalysis based iridium-catalyzed transfer hydrohalogenation of unactivated alkynes. The use of 4-chlorobutan-2-one or tert-butyl halide as donors of hydrogen halides allows this transformation in the absence of corrosive reagents, such as hydrogen halides or acid chlorides, thus largely improving the functional-group tolerance and safety profile of these reactions compared to the state-of-the-art. This method has granted access to alkenyl halide compounds containing acid-sensitive groups, such as tertiary alcohols, silyl ethers, and acetals. The synthetic value of those methodologies has been demonstrated by gram-scale synthesis where low catalyst loading was achieved.

Silver catalyzed proto- and sila-Nakamura-type α-vinylation of silyl enol ethers with dichloroacetylene. Divergent formation of stereochemically pure tri- and tetrasubstituted olefins

The silver-catalyzed reaction of silyl enol ethers with dichloroacetylene (DCA) is described. When DCA was used as a solution in diethyl ether, we found that the silyl group was transferred to the vinyl group, resulting in stereochemically pure tetrasubstituted olefins. However, when DCA was used as a solution in the more polar acetonitrile, protonation was the major pathway, and trisubstituted olefins were the dominant products.

Metal-free POCl3 promoted stereoselective hydrochlorination of ethynylated azaheterocycles

An efficient stereoselective hydrochlorination of ethynylated azaheterocycles was achieved using POCl₃ as a chlorinating agent under metal-free reaction conditions.

Chlorination of phenylallene derivatives with 1-chloro-1,2-benziodoxol-3-one: synthesis of vicinal-dichlorides and chlorodienes

Allyl and vinyl chlorides represent important structural motifs in organic chemistry. Herein is described the chemoselective and regioselective reaction of aryl- and α-substituted phenylallenes with the hypervalent iodine (HVI) reagent 1-chloro-1,2-benziodoxol-3-one. The reaction typically results in vicinal dichlorides, except with proton-containing α-alkyl substituents, which instead give chlorinated dienes as the major product. Experimental evidence suggests that a radical mechanism is involved.

E-Z isomerization in Suzuki cross-couplings of haloenones: ligand effects and evidence for a separate catalytic cycle

Suzuki cross-coupling of haloalkenes is generally assumed to occur with retention of the alkene stereochemistry. While studying Suzuki cross-couplings on E-1,2-dichlorovinyl phenyl ketone, we were surprised to observe extensive isomerization. More surprisingly, the ligand employed strongly influenced the degree of isomerization: DPEphos and Xantphos led to 96% isomerized cross-coupled product whereas reactions in the absence of a phosphine ligand, or reactions employing t-BuXantphos, gave 94% retention of stereochemistry. While E-Z isomerization in Pd-catalyzed vinylic couplings has previously been attributed to events within the cross-coupling catalytic cycle, we present experimental and computational evidence for a separate Pd-catalyzed isomerization process in these reactions.

Synthesis of aryl α,α-difluoroethyl thioethers a novel structure motif in organic chemistry, and extending to aryl α,α-difluoro oxyethers

A method for the preparation of aryl α,α-difluoroethyl thioethers (ArSCF₂CH₃) is reported and the synthesis approach is extended to aryl α,α-difluoroethyl oxygen ethers.

Trihaloethenes as versatile building blocks for organic synthesis

Trihaloethenes are versatile C₂-building blocks that can be simply modified via addition, elimination and transition metal-catalyzed reactions.

Selective ruthenium-catalyzed hydrochlorination of alkynes: one-step synthesis of vinylchlorides

An unprecedented ruthenium-catalyzed direct and selective alkyne hydrochlorination is reported and leads to vinylchlorides in excellent yields with atom economy. The reaction proceeds at room temperature from terminal alkynes and provides a variety of chloroalkenes. Only the regioisomer resulting from the formal Markovnikov addition is selectively formed. Mechanistic studies show the stereoselective syn addition of HCl to alkynes at room temperature and suggest a chloro hydrido Ru(IV) species as a key intermediate of the reaction.

Facile preparation of indoles and 1,2-benzothiazine 1,1-dioxides: nucleophilic addition of sulfonamides to bromoacetylenes and subsequent palladium-catalyzed cyclization

Bromine as a double agent: The bromine atom in 1-bromo-1-alkynes works as an electron-withdrawing group to effect the nucleophilic addition of sulfonamides. It again plays a pivotal role in the palladium-catalyzed cyclization of the resultant (Z)-2-(sulfonylamino)-1-bromoalkenes into nitrogen heterocycles (see scheme).

Suzuki-Miyaura coupling of heteroaryl boronic acids and vinyl chlorides

A protocol for the Suzuki-Miyaura coupling of heteroaryl boronic acids and vinyl chlorides that minimizes protodeboronation is described. A combination of catalytic amounts of Pd(OAc)(2) and SPhos in conjunction with CsF in isopropanol effectively affords a variety of coupled products. Surprisingly, a dramatic temperature dependence in product selectivity was observed.