Tohru Oishi

Asymmetric Baylis-Hillman reactions using chiral 2,3-disubstituted 1,4-diazabicyclo[2.2.2]octanes catalysts under high pressure conditions

Abstract Chiral C 2 -symmetric 2,3-disubstituted 1,4-diazabicyclo[2.2.2]octanes (DABCOs) ( 1 ) have been utilized as catalysts for asymmetric Baylis-Hillman reactions. Optically active α-methylene-β-hydroxyalkanone was obtained in up to 47% ee. Under high pressure conditions, a remarkable enhancement of both reaction rate and enantioselectivity has been observed.

EXPEDITIOUS TANDEM-METATHESIS ROUTE TO THE AB-RING FRAGMENT OF CIGUATOXIN

Abstract The AB-ring fragment of ciguatoxin was synthesized in ten steps from tri-O-benzyl- d -glucal based on a highly diastereoselective ring-closing metathesis and subsequent cross metathesis.

Convergent synthesis of the ABCDE ring system of ciguatoxin CTX3C

Ciguatoxin CTX3C is a representative congener of the ciguatoxins, which are known to be the principal causative-agents of ciguatera seafood poisoning. The structure of CTX3C spans over three nanometers and is characterized by thirteen ether rings. To attain a practical construction of this molecule, efficient supplies of the structural fragments are crucial. Herein we report the convergent synthesis of the ABCDE ring fragment featuring (i) alkylative coupling of the AB ring and E ring, and (ii) ring-closing olefin metathesis.

Direct Interaction between Amphotericin B and Ergosterol in Lipid Bilayers As Revealed by 2H NMR Spectroscopy

Although amphotericin B (AmB) is thought to exert its antifungal activity by forming transmembrane ion-permeable self-assemblies together with ergosterol, no previous study has directly proven AmB-ergosterol interaction. To establish the interaction, we measured (2)H NMR using deuterium-labeled sterols and AmB. The (2)H NMR spectra of deuterated ergosterol in palmitoyloleoylphosphatidylcholine (POPC) bilayers showed that fast axial diffusion of erogosterol was almost completely inhibited by the coexistence of AmB. Conversely, cholesterol mobility in POPC membrane was essentially unchanged with or without AmB. These results unequivocally demonstrate that ergosterol has significant interaction with AmB in POPC bilayers. In addition, we examined the mobility of AmB using deuterium-labeled AmB, and found that, although AmB is almost immobilized in sterol-free and cholesterol-containing POPC membranes, a certain ratio of AmB molecules acquires mobility in the presence of ergosterol. The similar mobility of AmB and ergosterol in POPC bilayers confirmed the idea of the direct intermolecular interaction between ergosterol and AmB.

Combinatorial Synthesis of the 1,5-Polyol System Based on Cross Metathesis: Structure Revision of Amphidinol 3

Combinatorial synthesis of a 1,5-polyol system corresponding to the C1-C14 unit of amphidinol 3 (AM3) and its diastereomers was achieved via chemoselective cross metathesis as the key step. Comparison of (13)C NMR data of the synthetic specimens with that of AM3 led to a controversy regarding the originally proposed structure. From GC-MS analysis of the degradation product, the absolute configuration at C2 of AM3 has been revised to be R.

Synthesis of chiral 2,3-disubstituted 1,4-diazabicyclo [2.2.2] octane. New ligand for the osmium-catalyzed asymmetric dihydroxylation of olefins

Abstract Chiral 2,3-disubstituted 1,4-diazabicyclo[2.2.2]octane (DABCO) derivatives have been synthesized and utilized as a chiral ligand for the osmium-catalyzed asymmetric dihydroxylation of olefins. Optically active diols in up to 41%ee are obtained in good yields.

Synthetic study of ciguatoxin. Absolute configuration of the C2 hydroxy group

The absolute stereochemistry of the secondary alcohol of the 1,2-dihydroxybutenyl substituent of ciguatoxin (1) was shown to be S by comparing the split CD curve of ciguatoxin tetra-p-bromobenzoate (2) with those of di- and tri-p-bromobenzoates of AB ring fragments that were synthesized enantioselectively.

Convergent synthesis of the EFGH ring fragment of ciguatoxin CTX3C

Abstract A stereoselective synthesis of the EFGH ring fragment of ciguatoxin CTX3C has been achieved through: (i) selective cleavage of a dioxepane acetal CO bond; (ii) radical cyclization to form the oxepane G ring; and (iii) chemoselective ring-closing metathesis of a triene yielding the hexahydrooxonin F ring.

Complex formation of amphotericin B in sterol-containing membranes as evidenced by surface plasmon resonance

Amphotericin B (AmB) is a membrane-active antibiotic that increases the permeability of fungal membranes. Thus, the dynamic process of its interaction with membranes poses intriguing questions, which prompted us to elaborate a quick and reliable method for real-time observation of the drugs binding to phospholipid liposomes. We focused on surface plasmon resonance (SPR) and devised a new modification method of sensor chips, which led to a significant reduction in the level of nonspecific binding of the drug in a control lane. With this method in hand, we examined the affinity of AmB for various membrane preparations. As expected, AmB exhibited much higher affinity for sterol-containing palmitoyloleoylphosphatidylcholine membranes than those without sterol. The sensorgrams recorded under various conditions partly fitted theoretical curves, which were based on three interaction models. Among those, a two-state reaction model reproduced well the sensorgram of AmB binding to an ergosterol-containing membrane; in this model, two states of membrane-bound complexes, AB and AB*, are assumed, which correspond to a simple binding to the surface of the membrane (AB) and formation of another assembly in the membrane (AB*) such as an ion channel complex. Kinetic analysis demonstrated that the association constant in ergosterol-containing POPC liposomes is larger by 1 order of magnitude than that in the cholesterol-containing counterpart. These findings support the previous notion that ergosterol stabilizes the membrane-bound assembly of AmB.

Concise synthesis of ciguatoxin ABC-ring fragments and surface plasmon resonance study of the interaction of their BSA conjugates with monoclonal antibodies.

Monoclonal antibodies (mAbs), 4H2 and 6H7, were prepared previously using a protein conjugate of a 1:1 epimeric mixture of the synthetic ABC-ring fragments of ciguatoxin (CTX), 3 and 4. Here, the interactions of these mAbs with the fragments of CTX and CTX3C, 3 and 5, were investigated by surface plasmon resonance (SPR) spectroscopy in an attempt to clarify an antigenic determinant. Compared with the previous synthesis, the fragment 3 possessing the 2S configuration was synthesized from tri-O-acetyl-D-glucal much more effectively. The mAb 4H2 was already known to show a dose-dependent binding to the bovine serum albumin (BSA) conjugate of 3, but not to that of 5. The present SPR study of 4H2 demonstrates that the A-ring side chain of 3 plays a decisive role as an epitope. Therefore, SPR can effectively replace the ELISA method for the analysis of mAbs.