Iterative synthesis of nitrogen-containing polyketide oxime intermediates.

Typical polyketides consist of C, H, and O atoms, whereas several types of N-containing polyketides are known to show intriguing properties. Because conventional synthetic approaches for such compounds focus on only specific structures, a more general method is desirable. Here, we have developed an iterative synthesis of nitrogen-containing polyketide.

Solid-Phase Biomimetic Synthesis of Polyketide.

Solid-phase biomimetic polyketide synthesis has been developed. This method is composed of (i) carbon chain elongation of resin-bound carboxylic acid via decarboxylative Claisen condensation with malonic acid half thioester, (ii) stepwise transformation of the resulting β-ketothioester, and (iii) hydrolysis of thioester to regenerate the carboxylic acid for the next iteration cycle. Colorimetric tests were available for convenient monitoring of the solid-phase reactions; malachite green (basic dye) and iron(III) chloride successfully detected the carboxylic acid and the β-ketothioester, respectively.

Iterative Polyketide Synthesis via a Consecutive Carbonyl-Protecting Strategy.

To address the difficulty in protecting a β-polycarbonyl compound, a method for the sequential protection of elongating carbonyl groups was demonstrated. The iterative chain elongation of a carboxylic acid with malonic acid half thioester followed by the protection of the resulting β-ketothioester was performed via the stepwise formation of an isoxazole ring using an O-protected oxime functionality. Yangonin and isosakuranetin were synthesized according to this procedure.

Development of Selective Peptide Catalysts with Secondary Structural Frameworks.

Enzymes are biogenic catalysts that enable the vital activity of organisms. Enzymes promote reactions in a selective manner with a high level of substrate recognition ability. The development of such a sophisticated catalyst has been one of the goals for chemists.

Biomimetic iterative method for polyketide synthesis.

An iterative method for synthesizing polyketides was demonstrated, in which the chain elongation of a carboxylic acid was performed by decarboxylative dehydration condensation with a malonic acid half thioester. After transforming the resulting β-ketothioester into an appropriate form, the carboxylic acid functionality was regenerated for the next elongation step.

Exploration of Structural Frameworks for Reactive and Enantioselective Peptide Catalysts by Library Screenings.

By screening large-scale N-terminal l-prolyl peptide libraries, we explored efficient catalysts for asymmetric Michael addition of a malonate to an enal. The catalytically active peptides obtained by the screening could be categorized into two groups based on the similarity of amino acid sequences. One group of the peptides selectively gave an S-product, whereas the other gave an R-product, despite all of the peptides having a common N-terminal sequence, Pro-d-Pro.

Determination of the Absolute Configuration of Side Chains of Basic Amino Acid Residues Using the Water-Soluble Porphyrin-Based Exciton Chirality Method.

We demonstrated that the circular dichroism (CD) exciton chirality method, based on the supramolecular interactions of meso-tetra(4-sulfonatophenyl)porphyrin (MTPPS, M = Zn or H), was applicable for the determination of the absolute configuration between the side chains of two basic amino acid residues of stable monomeric β-hairpin peptides (tryptophan zipper: Trpzip). When MTPPS was added to an aqueous solution containing Trpzip, a bisignate CD signal was detected in the Soret band region in addition to a decrease in absorbance. These spectral changes indicated the formation of a supramolecule consisting of Trpzip and MTPPS via electrostatic interactions between the positively charged lysine residue of Trpzip and the negatively charged sulfonate group of MTPPS.

Helical-Peptide-Catalyzed Enantioselective Michael Addition Reactions and Their Mechanistic Insights.

Helical peptide foldamer catalyzed Michael addition reactions of nitroalkane or dialkyl malonate to α,β-unsaturated ketones are reported along with the mechanistic considerations of the enantio-induction. A wide variety of α,β-unsaturated ketones, including β-aryl, β-alkyl enones, and cyclic enones, were found to be catalyzed by the helical peptide to give Michael adducts with high enantioselectivities (up to 99%). On the basis of X-ray crystallographic analysis and depsipeptide study, the amide protons, N(2)-H and N(3)-H, at the N terminus in the α-helical peptide catalyst were crucial for activating Michael donors, while the N-terminal primary amine activated Michael acceptors through the formation of iminium ion intermediates.

Histidine-containing peptide catalysts developed by a facile library screening method.

Although peptide catalysts have a high potential for the use as organocatalysts, the optimization of peptide sequences is laborious and time-consuming. To address this issue, a facile screening method for finding efficient aminocatalysts from a peptide library has been developed. In the screening for the Michael addition of a malonate to an enal, a dye-labeled product is immobilized on resin-bound peptides through reductive amination to visualize active catalysts.

Peptide-catalyzed kinetic resolution of planar-chiral metallocenes.

Kinetic resolution of racemic planar-chiral metallocenes was performed through the conjugate addition of a nucleophile to the enal part of substrates. While no enantiomeric discrimination was found with low-molecular-weight organocatalysts, a properly designed resin-supported peptide catalyzed the reaction in a highly selective manner.

Peptide-catalyzed consecutive 1,6- and 1,4-additions of thiols to α,β,γ,δ-unsaturated aldehydes.

Regio- and enantioselective addition of thiols to α,β,γ,δ-unsaturated aldehydes was performed with a resin-supported peptide catalyst. It was shown that a 1,4-adduct was generated mainly at the initial stage of the reaction, and this was eventually converted to a thermodynamically stable 1,6- and 1,4-diadduct through retro-addition/addition reactions.

Peptide-catalyzed regio- and enantioselective reduction of α,β,γ,δ-unsaturated aldehydes.

A resin-supported peptide catalyst (see box in the scheme) was used in the title reaction. The inherent regioselectivity was overcome by the peptide catalyst to promote the 1,6-selective reaction prior to 1,4-reduction. High stereoconvergence was also achieved when using a mixture of geometric isomers of the starting aldehydes.

Peptide-catalyzed diastereo- and enantioselective cyclopropanation of aromatic α,β-unsaturated aldehydes.

Highly diastereo- and enantioselective cyclopropanation of aromatic α,β-unsaturated aldehydes was achieved using a resin-supported peptide catalyst under aqueous conditions. In the peptide sequence, the residue possessing an oxygen atom with the appropriate length of the side chain was essential for attaining good diastereoselectivity.

Asymmetric one-pot sequential Friedel-Crafts-type alkylation and α-oxyamination catalyzed by a peptide and an enzyme.

In the presence of a peptide catalyst and the oxidative enzyme laccase, a one-pot sequential reaction including a Friedel-Crafts-type alkylation of α,β-unsaturated aldehydes followed by an α-oxyamination was realized. The reaction in aqueous solvent to promote the enzymatic oxidation, and the use of a peptide catalyst compatible with such conditions, were essential. The present sequential reaction afforded oxygen-functionalized indole or pyrrole derivatives in a highly enantioselective manner.

Asymmetric Michael addition of boronic acids to a γ-hydroxy-α,β-unsaturated aldehyde catalyzed by resin-supported peptide.

A resin-supported peptide catalyst effective for the asymmetric Michael addition of boronic acids to (E)-4-hydroxybut-2-enal was developed. From a spectral study, it was revealed that the optimum peptide consisted of both a β-turn and helix. Such a combination of secondary structures was essential for achieving a high catalytic ability.

Peptide/laccase cocatalyzed asymmetric α-oxyamination of aldehydes.

An asymmetric α-oxyamination could be successfully performed by a peptide catalyst and laccase. The combination of peptide catalysis and enzymatic air oxidation promoted the reaction smoothly in water without employing a metal reagent. The oxyaminated compounds could be obtained as both aldehyde and carboxylic acid products depending on the reaction conditions.

One-pot sequential alcohol oxidation and asymmetric α-oxyamination in aqueous media using recyclable resin-supported peptide catalyst.

An efficient tandem reaction system was developed, in which primary alcohols were used for the oxidation to the corresponding aldehydes followed by an asymmetric α-oxyamination with a resin-supported peptide catalyst.

Efficient asymmetric alpha-oxyamination of aldehydes by resin-supported peptide catalyst in aqueous media.

The resin-supported peptide catalyst having the terminal five-residue Pro-d-Pro-Aib-Trp-Trp combined with polyleucine successfully catalyzed the asymmetric alpha-oxyamination of aldehydes in aqueous media. The secondary structure and the chirality sense of the hydrophobic polyleucine chain significantly affected both reactivity and enantioselectivity.

Organocatalytic asymmetric transfer hydrogenation in aqueous media using resin-supported peptide having a polyleucine tether.

A resin-supported N-terminal prolyl peptide having a beta-turn motif and hydrophobic polyleucine chain effectively catalyzed the asymmetric transfer hydrogenation under aqueous conditions. The polyleucine tether provides a hydrophobic cavity in aqueous media that brought about a remarkable acceleration of the reaction. In addition, the polyleucine chain also turned out to be essential for high enantioselectivity.