Scheme 1. Synthesis of enantiomerically enriched g-lactams through auto-tandem asymmetric catalysis using 1.

Enantioselective allylic substitution catalyzed by transitionmetal complexes is an important process in organic synthesis.[1] For many years, mainly palladium complexes that contain chiral ligands have been employed as efficient catalysts in these reactions. Recent studies have demonstrated that chiral catalysts based on other transition metals show different regioselectivity in the synthesis of branched allylic products via monosubstituted p-allyl intermediates. [2] Although a variety of carbon and nitrogen nucleophiles can be used in those reactions, applicable oxygen nucleophiles are still limited to phenols and alcohols, which produce allylic ethers.[3] Thus, enantioenriched branched allylic alcohols, which serve as useful chiral building blocks, are often synthesized by other processes, such as the hydrogenation of a,b-unsaturated ketones, [4] the nucleophilic addition of vinylmetal reagents to aldehydes and ketones, [5] and the kinetic resolution of racemic allylic alcohols.[6] Recently, new ways to access these compounds have been developed, and they involve allylic substitution by a two-step conversion involving allylic esters and silyl ethers (Scheme 1; OPG = ester or silyl ether). [7,8] In the reaction of allylic chlorides with boronic acids in the presence of a ruthenium catalyst, the allylic alcohols were synthesized but high regio-and enantioselectivities were not achieved.[9] Herein, we describe the direct synthesis of chiral allylic alcohols by the regio-and enantioselective allylic substitution using water as the nucleophile.[10]Previously, we reported the enantioselective allylic substitution of 1,3-disubstituted allylic carbonates with amine and carbon nucleophiles catalyzed by planar-chiral cyclopentadienyl ruthenium (Cp'Ru) complexes (1; see Table 1). [11] This system was successfully extended to the regio-and enantioselective allylic substitution of monosubstituted allylic halides with oxygen and carbon nucleophiles. [12] From those studies, we found that Cp'Ru catalysts showed characteristic reactivity towards the less reactive nucleophiles. Thus, we started an examination of the enantioselective allylic substitution with water using Cp'Ru catalysts.Treatment of cinnamyl chloride (2 a) with water in the presence of 1 mol % of the Cp'Ru complex (S)-1 a resulted in the selective formation of the branched allylic alcohol (3 a; see equation in Table 1). After optimization of the reaction conditions, we found that the reaction in a mixture of THF/ water (8:1) at 25 8C with sodium hydrogen carbonate (1.2 equiv), produced 3 a after 4 hours in 99 % yield with 81 % ee.[13] Notably, the linear allylic alcohol was not formed at all. To improve the enantioselectivity, we examined the effect of the aryl groups on the phosphine ligand of the Cp'Ru catalyst (Table 1). Replacement of the phenyl groups with 3,5-dimethylphenyl and 4-methoxy-3,5-dimethylphenyl groups led to an increase in enantioselectivity to 88 % ee and 90 % ee, respectively (Table 1, entries 2 and 3). Complexes 1 d and 1 e, which have 3,...

Molecular design involving the incorporation of an α-amino acid residue into the side chain or main chain of a polymer is often used to stabilize artificial molecular architectures through intramolecular hydrogen bonding. However, this molecular design strategy rarely considers the importance of interactions between substituents at the α-position of amino acid moieties, as found in nature. Herein, we report the synthesis of a novel series of π-stacked helical poly­(quinolylene-2,3-methylene) with amino acid derivatives bearing different substituents at the α-position. We found that the thermal stability of π-stacked helical poly­(quinolylene-2,3-methylene) is significantly improved by packing the substituents in the empty spaces between the side chains. In particular, when a bulky cyclohexyl alanine derivative was used as the side chain, the π-stacked helical structure maintained its stability even in dimethylsulfoxide, a hydrogen bond competitor. The stabilization of the π-stacked structure by the amino acid substituents resulted in a unique polymerization behavior involving nucleation–elongation steps. In the case of derivatives with leucine and cyclohexyl alanine, which form stable π-stacked helical structures, metastable structures with entangled main chains were formed in the initial polymerization stage. These structures subsequently underwent an irreversible structural change to achieve a thermodynamically stable helical π-stacked conformation as a nucleus for subsequent polymerization. Thereafter, the polymerization reaction proceeded with the elongation of the π-stacked helical structure. Differences in the stability of these systems indicated that the amino acid substituents on the side chains determine the most thermodynamically stable π-stacked helical structure.

An asymmetric allylic substitution with sodium carboxylate using a planar-chiral cyclopentadienyl ruthenium complex has been developed. Optically active allylic esters were prepared in good yields with high regio- and enantioselectivities.

No abstract

π‐Stacked polymers, which consist of layered π‐electron systems in a polymer, can be expected to be used in molecular electronic devices. However, the construction of a stable π‐stacked structure in a polymer is considerably challenging because it requires sophisticated designs and precise synthetic methods. Herein, we present a novel π‐stacked architecture based on poly(quinolylene‐2,3‐methylene) bearing alanine derivatives as the side chain, obtained through the living cyclo‐copolymerization of an o‐allenylaryl isocyanide. In the resulting polymer, the neighboring quinoline rings of the main chain form a layered structure with π–π interactions, which is stabilized by intramolecular hydrogen bonds. The vicinal quinoline units form two independent helices and the whole molecule is a twisted‐tape structure. This structure is established on the basis of UV/CD spectra, theoretical calculations, and atomic‐force microscopy.

Herein, we present a novel synthetic strategy, to amino-acid-free peptide synthesis based on postpolymerization conversion, for chiral nonnatural polypeptides. Optically active poly-N-alkoxyamides were prepared by our asymmetric polymerization as precursors of polypeptides, and the reductive cleavage of N–O bonds using a SmI2–THF complex was carried out. The reaction proceeded smoothly with quantitative conversion to afford a nonnatural polypeptide. The resulting polypeptide adopted a one-handed stable helical structure in solution, which was established by circular dichroism (CD) and theoretical calculations using density functional theory (DFT). The simulated spectra by time-dependent (TD) DFT methods clearly indicated the validity of the proposed structure. The synthetic approach is a promising candidate for the synthesis of nonnatural polypeptides.

We, herein, present a novel synthesis of responsive helical poly(aryl isocyanide)s bearing aza‐crown ethers as pendant groups. Chiral aryl isocyanide monomers bearing an aza‐crown ether as a pendant were designed and synthesized, piror to polymerization using a Pd‐Pt µ‐ethynediyl complex as an initiator to give the corresponding polymers in good yield. The resulting polyisocyanides adopted a stable helical structure in solution, as confirmed by circular dichroism spectroscopic analysis. In addition, the polymers were soluble in various solvents. Furthemore, the addition of suitable alkali metal ions to the crown ether of the sidechain on the helical polyisocyanide to form host‐gest complexes resulted in deformation of the helix due to electrostatic repulsion, and these phenomena depended on the size of metal cations. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 496–504.