Minoru Isobe

An increase in cytosolic calcium ion concentration precedes hypoosmotic shock‐induced activation of protein kinases in tobacco suspension culture cells

Hypoosmotic shock induced a transient increase in cytosolic free calcium concentration ([Ca2+]cyt) and subsequent activation of 50‐, 75‐ and 80‐kDa protein kinases in tobacco (Nicotiana tabacum L.) suspension culture cells. Depletion of external calcium suppressed both the elevation of [Ca2+]cyt and the activation of protein kinases in response to hypoosmotic shock, indicating that the elevation of [Ca2+]cyt is prerequisite for the activation of protein kinases. Pharmacological analysis indicated that the hypoosmotic shock‐activated protein kinases were activated by phosphorylation, suggesting that the activities of these protein kinases are regulated by putative protein kinases. These results suggest that the hypoosmotic signal is transduced to protein kinase cascades which are triggered by [Ca2+]cyt elevation.

Low-temperature photooxygenation of coelenterate luciferin analog synthesis and proof of 1,2-dioxetanone as luminescence intermediate☆

Coelenterate luciferin analog having bulky tert-butyl group at the 2-position was suitable for studies on chemiluminescence under various conditions. Photooxygenation of the analog(s) at low temperature (−78°C) afforded luminous intermediates which were proved as peroxides by reduction with PPh3 with resultant loss of luminescence ability. In order to clarify these structures of accumulated luminous intermediates by means of 13C NMR, three 13C enriched analogs were synthesized at the 2, 3 and 5 positions of 3,7-dihydroimidazo[1,2-a]pyrazin-3-one skeleton in 99% enrichment with site-specificity. These 13C-enriched coelenterate luciferin analogs were photooxygenated at −78°C to form two peroxidic products as luminescent intermediates. Structures of these unstable intermediates were deduced by means of 13C NMR spectra at low temperatures using substrates enriched at three sites by 13C. Photooxygenation in a mixture of CF3CD2OD and CD3OD as highly protic solvents afforded the dioxetanone and 2-hydroperoxide. These two peroxides emitted light independently at different temperatures either at 400 nm (neutral species) and/or 475 nm (anionic species) after diluting to 10−5 M in diglyme (DGM) containing acid or base.

Total Synthesis of Ciguatoxin

Something fishy: Ciguatoxin (see structure) is one of the principal toxins involved in ciguatera poisoning and the target of a total synthesis involving the coupling of three segments. The key transformations in this synthesis feature acetylene-dicobalthexacarbonyl complexation.

PARTIAL SYNTHESIS OF CIGUATOXIN (5R)-ABC SEGMENT

Abstract An ABC segment of ciguatoxin has been synthesized in a correct enantiomeric form from a D-glucose derivative based on the route for the opposite enantiomer by switching enantiomerism of a pseudosymmetric intermediate. This route, however, has been improved in several steps and ended up with a vinylthioether group for future extension toward the D ring of ciguatoxin molecule.

One-step recyclization of sugar acetylenes to form medium ether rings via dicobalthexacarbonyl complexes☆

Four C-1 alkynylated D-glucals were converted into the corresponding dicobalthexacarbonyl complexes. All of them were recyclized upon treatment with acid to form the medium size (7, 8, 9 and 10 membered) ether rings. The crucial mechanism was cis-trans double bond isomerization of allylic cation connected to dicobalthexacarbonyl complexes. Decomplexation was successfully achieved under high pressure hydrogenation by using rhodium catalyst.

Synthetic studies on antibiotic Dynemicin A. Synthesis of cyclic enediyne model compound of Dynemicin A

Abstract Dynemicin A, a potent antitumor antibiotic has novel 10-membered cyclic enediyne moiety. The bicyclo[7.3.1]-tridecadiyne system having aniline moiety o

Stereocontrolled Synthesis of (−)‐5,11‐Dideoxytetrodotoxin

New derivatives of an intriguing marine natural product are now accessible. The first asymmetric synthesis of the simple tetrodotoxin analogue, 5,11-dideoxytetrodotoxin (3), was achieved. Hydroxylation at position C8 of the key intermediate 1 relied on the neighboring trichloroacetamide group, and stereoselective elaboration of the vinyl group gave alpha-hydroxylactone 2, which was transformed into the title compound through a new guanidylation method.

First Identification of 5,11-Dideoxytetrodotoxin in Marine Animals, and Characterization of Major Fragment Ions of Tetrodotoxin and Its Analogs by High Resolution ESI-MS/MS

Even though tetrodotoxin (TTX) is a widespread toxin in marine and terrestrial organisms, very little is known about the biosynthetic pathway used to produce it. By describing chemical structures of natural analogs of TTX, we can start to identify some of the precursors that might be important for TTX biosynthesis. In the present study, an analog of TTX, 5,11-dideoxyTTX, was identified for the first time in natural sources, the ovary of the pufferfish and the pharynx of a flatworm (planocerid sp. 1), by comparison with totally synthesized (−)-5,11-dideoxyTTX, using high resolution ESI-LC-MS. Based on the presence of 5,11-dideoxyTTX together with a series of known deoxy analogs, 5,6,11-trideoxyTTX, 6,11-dideoxyTTX, 11-deoxyTTX, and 5-deoxyTTX, in these animals, we predicted two routes of stepwise oxidation pathways in the late stages of biosynthesis of TTX. Furthermore, high resolution masses of the major fragment ions of TTX, 6,11-dideoxyTTX, and 5,6,11-trideoxyTTX were also measured, and their molecular formulas and structures were predicted to compare them with each other. Although both TTX and 5,6,11-trideoxyTTX give major fragment ions that are very close, m/z 162.0660 and 162.1020, respectively, they are distinguishable and predicted to be different molecular formulas. These data will be useful for identification of TTXs using high resolution LC-MS/MS.

Hydrosilylation of acetylenes with catalytic biscobalthexacarbonyl complex and its application to heteroconjugate addition methodology

Abstract The acetylene biscobalthexacarbonyl complex can catalyze hydrosilylation of acetylenes to produce vinylsilanes. The regioselectivity is extremely high with phenylthioacetylenes, while it is hardly selective depending upon the substituents other than the sulfide group. High selectivity with dihydropyranyl-phenylthioacetylene prompted us to establish a practical convenient route to obtain the electrophilic precurosr of vinylsulfones in heteroconjugate addition. A new empirical rule for the assignment of synlanti stereochemistry of the products can be formulated, the 13C NMR chemical shifts of the anti-isomer being higher than those of the syn-isomer.

Synthesis of the JKLM-ring fragment of ciguatoxin

A stereoselective synthesis of the LM-ring fragment has been achieved starting from a sugar derivative. A stereoselective synthesis of the JKLM-ring fragment has been achieved through a coupling between two segments via heteroconjugate addition, seven-membered ether ring formation mediated by an acetylene cobalt complex, and spiroketalization reaction.