General,Highly Selective Synthesis of 1,3‐ and 1,4‐Difunctionalized Building Blocks by Regiodivergent Epoxide Opening

We describe a regiodivergent epoxide opening (REO) featuring a catalyst‐controlled synthesis of enantiomerically and diastereomerically highly enriched or pure syn‐ and anti‐ 1,3‐ and 1,4‐difunctionalized building blocks from a common epoxide precursor. The REO is attractive for natural product synthesis and as a branching reaction for diversity‐oriented synthesis with epoxides.     

Synthesis of 2‐Ene‐1,4‐diols by a New Cascade‐Opening of 1,3‐Diepoxides: Towards an Efficient Synthesis of Dihydroxytaxoid Derivatives

Taxing taxanes : A new method for the preparation of alk‐2‐ene‐1,4‐diols from 1,3‐dienes, especially for geometrical constrained derivatives, via dioxirane derivative formation followed by Ti^III‐induced oxirane ring opening, is described. The strategy has been very efficiently implemented to the construction of the 11‐ene‐10,13‐diol moiety present in taxol derivatives.

     

Titanocene(III)‐Catalyzed 6‐exo Versus 7‐endo Cyclizations of Epoxypolyprenes: Efficient Control and Synthesis of Versatile Terpenic Building Blocks

In this article, a complete study on the selectivity of titanocene(III) cyclization of epoxypolyprenes is presented. The requirements for the formation of six‐ or seven‐membered rings during these cyclizations are determined, taking into account the different substitution pattern in the epoxypolyprene precursor. Thus, a complete selectivity to 6‐exo or 7‐endo cyclization process has been achieved, yielding mono‐, bi‐, and even tricyclic compounds, constituting a new and efficient access to this type of derivative. Additionally, this procedure opens the possibility to prepare excellent building blocks for the synthesis of polycyclic compounds with a trisubstituted oxygenated function, which is present in several natural terpenes.     

Regiodivergent Synthesis of 1,3‐ and 1,4‐Enynes through Kinetically Favored Hydropalladation and Ligand‐Enforced Carbopalladation

Pd‐catalyzed hydroalkynylations were developed that involve ligand‐enabled regiodivergent addition of an alkyne to an allenamide, giving branched and linear products stereoselectively and facilitated by the neighboring amide group. Regioselectivity was achieved with the use of (o‐OMePh)₃P and BrettPhos, which allowed the functionalization of various alkynes, including steroids, carbohydrates, alkaloids, chiral ligands, and vitamins. Based on the experimental results, it was proposed that hydro‐ and carbopalladation processes operated during the formations of the branched and linear products, respectively.     

A General and Highly Selective Palladium‐Catalyzed Hydroamidation of 1,3‐Diynes

A chemo‐, regio‐, and stereoselective mono‐hydroamidation of (un)symmetrical 1,3‐diynes is described. Key for the success of this novel transformation is the utilization of an advanced palladium catalyst system with the specific ligand Neolephos. The synthetic value of this general approach to synthetically useful α‐alkynyl‐α, β‐unsaturated amides is showcased by diversification of several structurally complex molecules and marketed drugs. Control experiments and density‐functional theory (M06L‐SMD) computations also suggest the crucial role of the substrate in controlling the regioselectivity of unsymmetrical 1,3‐diynes.     

Open Chain Chiral Macrolide Building Blocks by Opening of Deoxygenated 1,6-Anhydrosugars with 1,3-Propanedithiol

Chiral building blocks for macrolides and related natural products are obtained from 1,6-anhydrosugars by conversion of the bicyclic acetals 2 or 12 into the open chain chiral 1,3-dithianes 6 and 13. Branched precursors can be obtained by opening of the ?erný epoxide 1 with the 1,3-dithiane anion to yield 7, followed by ring opening with 1,3-propanedithiol to the bis-1,3-dithiane 8.     

Tandem Three‐Component Synthesis of syn‐1,2‐ and syn‐1,3‐Diol Derivatives

The development of efficient methods for stereocontrolled synthesis of polyol derivatives has been of continuing interest for the synthetic community. We describe herein tandem olefin cross‐metathesis/hemiacetalization/intramolecular oxa‐Michael addition of allylic/homoallylic alcohols, α,β‐unsaturated ketones, and aldehydes, which enabled the synthesis of syn‐1,2‐ and syn‐1,3‐diol derivatives in a step‐economical manner. A series of differentially protected polyol derivatives could be obtained in subsequent transformations via chemoselective/regioselective cleavage of the acetal moiety of the tandem reaction products.     

Mechanistic Study of the Titanocene(III)‐Catalyzed Radical Arylation of Epoxides

An atom‐economical and catalytic arylation of epoxide‐derived radicals is described. The key step of the catalytic system is a sequential electron and proton transfer for the rearomatization of the radical σ‐complex and catalyst regeneration. Kinetic, computational, spectroscopic, and cyclovoltammetric investigations highlight the key issues of the reaction mechanism and catalyst stabilization by collidine hydrochloride. Studies employing radicophiles rule out the participation of cations as reactive intermediates.     

Synthesis of Extra‐Large‐Pore Zeolite ECNU‐9 with Intersecting 14*12‐Ring Channels

Highly crystalline and (hydro)thermally stable zeolites with extra‐large pores [≥14‐ring (14‐R)] are desirable as catalysts. A novel zeolite, ECNU‐9, with an intersecting 14*12‐R channel system was rationally designed and synthesized by a building block strategy, in which the interlayer expansion of a two‐dimensional silicate structure was realized by combining organic amine assisted layer‐stacking reorganization and subsequent silylation with a square‐shaped single 4‐ring (S4R) silane, 1,3,5,7‐tetramethylcyclotetrasiloxane (TMCS). The PLS‐3 precursor was disassembled into building blocks and then intercalated with flexible and removable organic amine pillars to offer enough interlayer spacing for accommodating TMCS molecules. The additionally introduced building blocks interconnected the neighboring layers to construct new 14‐R and 12‐R pores. ECNU‐9 possesses a well‐ordered structure with a novel topology. The corresponding Ti‐ECNU‐9, with tetrahedral Ti ions in the framework, showed superior catalytic performance in the selective epoxidation of bulky alkenes.     

Synthesis of Bridged Benzazocines and Benzoxocines by a Titanium‐Catalyzed Double‐Reductive Umpolung Strategy

A sequence of two titanium(III)‐catalyzed reductive umpolung reactions is reported that allows the rapid construction of benzazo‐ and benzoxozine building blocks. The first step is a reductive cross‐coupling of quinolones or chromones with Michael acceptors. This reaction proceeds with complete syn‐selectivity for the quinolone functionalization while the anti‐diastereomers are obtained as the major products from chromones. With different reaction conditions, the stereochemical outcome can be altered to afford the syn‐chromanone products as well. A subsequent reductive ketyl radical cyclization forges the tricyclic title compounds in good yields. A stereochemical model explaining the observed stereoselectivities is provided and the product configurations were unambiguously verified by X‐ray analyses and 2D NMR spectroscopic experiments.     

Regiodivergent Copper Catalyzed Borocyanation of 1,3‐Dienes

Copper catalyzed multi‐functionalization of unsaturated carbon‐carbon bonds is a powerful tool for the generation of complex molecules. We report a regiodivergent process that allows a switch between 1,4‐borocupration and 4,1‐borocupration of 1,3‐dienes upon a simple change in ligand. The subsequently generated allyl coppers are trapped in an electrophilic cyanation to selectively generate densely functionalized and synthetically versatile 1,2‐ or 4,3‐borocyanation products.     

Regiodivergent and Stereoselective Hydrosilylation of 1,3‐Disubstituted Allenes

Methods for the highly stereoselective and regiodivergent hydrosilylation of 1,3‐disubstituted allenes have been developed. The synthesis of E allylsilanes is accomplished with palladium NHC catalysts, and trisubstituted Z alkenylsilanes are accessed with nickel NHC catalysts. Unsymmetrically substituted allenes are well tolerated with nickel catalysis and afford Z alkenylsilanes. Evidence for a plausible mechanism was obtained through an isotopic double‐labeling crossover study.     

Manganese‐Corrole Complexes as Versatile Catalysts for the Ring‐Opening Homo‐ and Co‐Polymerization of Epoxide

Manganese‐corrole complexes in combination with a co‐catalyst [PPN]X ([PPN]⁺=bis(triphenylphosphoranylidene)iminium) were found to be new versatile catalysts for the polymerization of epoxides, copolymerization of epoxides with CO₂, and copolymerization of epoxides with cyclic anhydrides affording a wide range of polymeric materials. This work should allow the synthesis of new types of improved innovative (co)polymers with original properties and would clearly increase the number of applications for polyesters, polycarbonates, and polyethers.     

Hydroxy‐Directed,Fluoride‐Catalyzed Epoxide Hydrosilylation for the Synthesis of 1,4‐Diols

A novel highly regioselective, fluoride‐catalyzed hydrosilylation of β‐hydroxy epoxides has been developed. The reaction is modular and applicable to the synthesis of a broad range of 1,4‐diols. Fluoride is crucial for two reasons: First, it promotes the formation of a silyl ether (which contains a Si‐H bond) and, second, it enables ring opening by an intramolecular S_N2 reaction through activation of the silane. The reaction can be performed under air.     

Synthesis of Dihydropyrrolizine and Tetrahydroindolizine Scaffolds from Pyrroles by Titanocene(III) Catalysis

A synthetic approach to dihydropyrrolizine and tetrahydroindolizine scaffolds from pyrroles has been developed. The key step, a titanocene(III)‐catalyzed radical arylation that proceeds by C?H functionalization is atom‐economical and tolerates a large variety of functional groups. The reaction is therefore attractive for the swift assembly of functional and structural diversity.     

Synthesis of Structurally Well‐Defined Telechelic Polymers by Organostibine‐Mediated Living Radical Polymerization: In Situ Generation of Functionalized Chain‐Transfer Agents and Selective ω‐End‐Group Transformations

Several organostibine chain‐transfer agents possessing polar functional groups have been prepared by the reactions of azo initiators and tetramethyldistibine ( 1 ). Carbon‐centered radicals thermally generated from the azo initiators were trapped by 1 to yield the corresponding organostibine chain‐transfer agents. The high yields observed in the synthesis of the chain‐transfer agents strongly suggest that distibines have excellent radicophilic reactivity. As the reactions proceeded under neutral conditions, functional groups that are incompatible with ionic conditions were incorporated into the chain‐transfer agents. The chain‐transfer agents were used in living radical polymerization to synthesize the corresponding α‐functionalized polymers. As the functional groups in the chain‐transfer agents did not interfere with the polymerization reaction, well‐controlled polymers possessing number‐average molecular weights (M_ns) predetermined by the monomer/transfer agent ratios were synthesized with low polydispersity indices (PDIs). The organostibanyl ω‐polymer ends were transformed into a number of different functional groups by radical‐coupling, radical‐addition, and oxidation reactions. Therefore, it was possible to synthesize well‐controlled telechelic polymers with the same and also with different functional groups at their α‐ and ω‐polymer ends. Distibine 1 was also found to increase PDI control in the living radical polymerization of styrene and methyl methacrylate (MMA) using a purified organostibine chain‐transfer agent. Well‐controlled poly(methyl methacrylate)s with M_n values ranging from 10 000 to 120 000 with low PDIs (1.05–1.15) were synthesized by the addition of a catalytic amount of 1 . The results have been attributed to the high reactivity of distibine 1 towards polymer‐end radicals, which are spontaneously deactivated to yield organostibine dormant species.     

Preparation and Thermal Properties of Polycarbonates/esters Catalyzed by Using Dinuclear Salph‐Al from Ring‐Opening Polymerization of Epoxide Monomers

A dinuclear Salph‐Al complex/bis(triphenylphosphine)iminium chloride catalyst system was synthesized and employed for cyclohexene oxide (CHO) and CO₂ copolymerization. The catalyst system had an excellent selectivity of 99 % for carbonate linkages and the resultant poly(cyclohexene carbonate) (PCHC) had a high glass transition temperature (T _g) of 123.8 °C and a thermal decomposition temperature (5 % weight loss; T _{d 5 %}) of 265 °C. Furthermore, this catalyst system was active in the polymerization of phthalic anhydride (PA) and epoxides. Poly(CHO‐alt ‐PA) was completely alternating, and had improved thermal properties (T _g=142.7 and T _{d 5 %}=295 °C) compared with PCHC. The T _g values of the polyesters could be adjusted by addition of PO to the CHO/PA reaction system. For the CHO/PO/PA terpolymerization, CHO and PO participated concurrently and proportionally in the chain growth and the obtained terpolyesters had tunable T _g values from 62.8 to 142.7 °C depending on the CHO/PO feed ratio.