Yoko Saikawa

Pigment chemistry: The red sweat of the hippopotamus

Within a few minutes of perspiration, the colourless, viscous sweat of the hippopotamus gradually turns red, and then brown as the pigment polymerizes. Here we isolate and characterize the pigments responsible for this colour reaction. The unstable red and orange pigments turn out to be non-benzenoid aromatic compounds that are unexpectedly acidic and have antibiotic as well as sunscreen activity.

Identification of the toxic trigger in mushroom poisoning

We have isolated the small, highly strained carboxylic acid cycloprop-2-ene carboxylic acid from the Asian toxic mushroom Russula subnigricans. This compound is responsible for fatal rhabdomyolysis, a new type of mushroom poisoning that is indicated by an increase in serum creatine phosphokinase activity in mice. We found that polymerization of the compound at high concentrations via ene reaction abolishes its toxicity.

Absolute configuration and tautomeric structure of xylindein, a blue–green pigment of Chlorociboria species

The S absolute configuration of both chiral centers of xylindein was assigned using X-ray crystallographic heavy atom analysis after its conversion to a synthetic derivative. Crystallographic analysis of xylindein crystallized with phenols revealed that the proposed structure is the proper tautomer in the crystals.

Total synthesis of kendomycin featuring intramolecular dötz benzannulation

One-step formation of the ansa-skeleton realized the synthesis of kendomycin, an ansa-type quinone methide. The Fischer carbene complex derived from the ansa-chain portion was subjected to the intramolecular Dotz benzannulation to afford the desired oxametacyclophane with exclusive regioselectivity. Subsequent Claisen rearrangement, ortho oxidation of the resulting phenol derivative, and mild transformation from p-quinone to p-quinone methide on a silica gel plate furnished kendomycin.

Smooth Isoindolinone Formation from Isopropyl Carbamates via Bischler–Napieralski-Type Cyclization

Isopropyl carbamates derived from benzylamines provide isoindolinones by treatment with phosphorus pentoxide at room temperature. Utility of this Bischler-Napieralski-type cyclization and a new mechanism involving a carbamoyl cation for rationalization of this smooth conversion are discussed.

Total syntheses of lactonamycin and lactonamycin Z with late-stage A-ring formation and glycosylation.

and inhibition of drug-resistant influenza virus neuraminidase using anthraquinone-sialic acid hybrids, photodegradation of HIV-1 protease and inhibition of HIV-1 replication in living cells by designed fullerene-sugar hibrids, evaluation of molecular probes based on the 9-methylstreptimidone derivative DTCM-glutarimide, a boron-doped diamond electrode mediated by methoxy radicals, Angew. iodination and one-pot arylation by on/off switching of electric current, Application of electrochemically generated hypervalent iodine oxidant to natural products synthesis, Electrochemistry, in press. a toxic matalloendopeptidase from the tropical toadstool,

Toxic isolectins from the mushroom Boletus venenatus.

Ingestion of the toxic mushroom Boletus venenatus causes a severe gastrointestinal syndrome, such as nausea, repetitive vomiting, diarrhea, and stomachache. A family of isolectins (B. venenatus lectins, BVLs) was isolated as the toxic principles from the mushroom by successive 80% ammonium sulfate-precipitation, Super Q anion-exchange chromatography, and TSK-gel G3000SW gel filtration. Although BVLs showed a single band on SDS-PAGE, they were further divided into eight isolectins (BVL-1 to -8) by BioAssist Q anion-exchange chromatography. All the isolectins showed lectin activity and had very similar molecular weights as detected by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis. Among them, BVL-1 and -3 were further characterized with their complete amino acid sequences of 99 amino acids determined and found to be identical to each other. In the hemagglutination inhibition assay, both proteins failed to bind to any mono- or oligo-saccharides tested and showed the same sugar-binding specificity to glycoproteins. Among the glycoproteins examined, asialo-fetuin was the strongest inhibitor. The sugar-binding specificity of each isolectin was also analyzed by using frontal affinity chromatography and surface plasmon resonance analysis, indicating that they recognized N-linked sugar chains, especially Galbeta1-->4GlcNAcbeta1-->4Manbeta1-->4GlcNAcbeta1-->4GlcNAc (Type II) residues in N-linked sugar chains. BVLs ingestion resulted in fatal toxicity in mice upon intraperitoneal administration and caused diarrhea upon oral administration in rats.

Synthetic studies on lactonamycins: synthesis of the model BCDEF aglycon.

The lactonamycin model aglycon 4 was synthesized from the trihalogenated benzene derivative 10. Ethynyltetraol 6 was prepared from 10 via carbon elongations, oxidative demethylation, a cycloaddition reaction with the diene derived from homophthalic anhydride, and dihydroxylation. Final E- and F-ring constructions from 6 were realized via a palladium-catalyzed cyclization-methoxycarbonylation, a stereoselective methanol addition, and lactonization, leading to the production of 4.

Synthetic studies on polymaxenolides: synthesis and structure elucidation of nominal epoxyafricanane and other africane-type sesquiterpenoids.

A racemic total synthesis of the sesquiterpenoid unit of the hybrid marine natural product polymaxenolide has been achieved based on a three-component assembly followed by ring-closing metathesis as the key steps. However, the spectral data of our product synthesized from Δ(9(15))-africanene by epoxidation were not identical with those of the natural product named epoxyafricanane. The structure confirmation of the synthetic nominal epoxyafricanane is described.

Diastereoselective synthesis of useful building blocks by crotylation of β-branched α-methylaldehydes with potassium crotyltrifluoroborates

The diastereoselective construction of stereotriads having consecutive methyl, hydroxy, and methyl substituents was realized by the substrate-controlled crotylation of beta-branched alpha-methylaldehydes with potassium crotyltrifluoroborates. Especially, crotylation of 2-(1,3-dithian-2-yl)propanal with potassium ( E)-crotyltrifluoroborate afforded, in good yield and with excellent diastereoselectivity, a useful building block that has different and potential functional groups on both ends.