The selective [2 + 2 + 2] cycloaddition of 1, 6-diynes takes place to give polycyclic 1, 3-cyclohexadienes or arenes using ruthenium or palladium complex catalysts. The rhodium carbonyl cluster is an efficient catalyst for the three-component coupling between carbon monoxide, hydrosilanes, and 1, 6-diynes or 1, 6-enynes. Organotitanium (III) complexes are excellent reagents for the intramolecular carbon-carbon bond formation between the nitrile and carbonyl groups as well as the intramolecular pinacol coupling of dials.

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Dendrimers are nanosized hyperbranched molecules of well-defined three-dimensional architecture, which are morphologically similar to biological macromolecules such as proteins. We have synthesized several core-functional dendrimers, which are intended to realize certain functions of biological events. The present paper highlights the molecular design and functions of bioinspired dendrimers, with special attention to their high potential as light-harvesting antenna and O₂ carrier.

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Trinuclear ruthenium pentahydride complex (Cp 'Ru) ₃ (μ-H) ₃ (μ₃-H) ₂ (1 : Cp ' = η-C₅Me₅) reacts with a series of acyclic conjugated dienes such as butadiene, isoprene, or penta-1, 3-diene to form trinuclear μ₃-1, 3-dimetalloallyl complexes 2 by way of an intermediary μ₃-η² : η²-s-cis-1, 3-diene complex. In the reactions with butadiene and isoprene, an agostic intermediate, in which three ruthenium centers concertedly activate a 1, 3-diene by taking the role of coordination sites and an activation site, is detected by ¹H and ¹³C NMR. The intermediary 1, 3-diene complexes with agostic interaction undergo C (sp²) -H bond cleavage at room temperature to generate 1, 3-dimetalloallyl complexes. The reaction of 1 with cyclopentadiene generates a trinuclear p₃-2-methylruthenacyclopentadiene complex 4 as a result of cleavage of the unactivated C (sp³) -C (sp²) bond. In the reaction of 1 with 1, 3-cyclohexadiene, cleavage of an allylic C-H bond followed by the β-H elimination affords α μ₃-η² : η² : η²-benzene complex.

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The intramolecular cyclization of silacyclooligoynes with [Mn (CO) ₃ (Me-Cp)] produced a variety of silyl-substituted π-electron systems based on four- and five-membered rings. The silyl-substituted fulvene, dimethylenecyclobutene, [4] radialene derivatives afforded the corresponding dianion species by reduction with lithium metal in polar solvents, whereas the trimethylenecylopentene derivative gave a tetraanion. These anion species were isolated as dilithium or tetralithium salts of the corresponding π-electron systems. The structures both in crystals and in solution reveal that the silicon atoms attached to the π-electron systems play an important role for the stabilization of the resulting polyanions.

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Efficient methodology for the preparation of two enantiopure cyclohexanoids having a bicyclo [2.2. 1] octene background has been established starting from the same meso ene-1, 4-diol by employing enzymatic and catalytic asymmetric desymmetrization procedures. On the basis of their inherent convex-face selectivity and thermal fragility, we utilize these chiral tricycles as synthetic equivalents of chiral p-benzoquinone and its hydro derivatives whose potential uses have been demonstrated by diastereo- and enantio-controlled synthesis of a wide variety of biologically active natural products.

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This account describes our syntheses and conformational analyses of the bicyclic nucleoside analogues, 3'-O, 4'-C-methyleneribonucleosides (4-BC), 3'-amino-3'- deoxy-3'-O, 4'-C-methyleneribonucleosides (aza 4-BC), 2'-O, 4'-C-methyleneribonucleosides (5-BC), and 3'-amino- and 3'-azido-3'deoxy-2'-O, 4'-C-methyleneribonucleosides (aza 5-BC). The nucleoside analogues (4-BC and 5-BC) were effectively introduced into oligonucleotides by using a DNA synthesizer. Furthermore, unprecedented hybridizing ability towards complementary RNA and DNA, potent triplex-forming ability, and sufficient enzymatic stability of these modified oligonucleotides were also confirmed. From these studies, we believe that these conformationally restrained nucleoside analogues are good candidates for practical antisense/antigene molecules.

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New strategies for the syntheses of the pumiliotoxin (PTX) A dendrobatid alkaloids have been developed which are based on palladium-catalyzed carbonylation, palladium-catalyzed cross-coupling reaction, and nickel-chromium-mediated cyclization. Application of these methodologies to the asymmetric total syntheses of (-) -PTX 209F, (-) - PTX 225F, (+) -PTX A, (+) -allo-PTX 267A, (+) -allo-PTX 339A, and (+) -homo-PTX 223G is described.

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The first total synthesis of polycavernoside A (1), which was isolated from the red alga Polycavernosa tsudai as a causative toxin of fatal human intoxication in Guam in 1991, is described. The synthetic key steps involve the connection of the acyl anion equivalent (9) derived from 3 with aldehyde (8), the stereoselective formation of 6-membered cyclic methyl acetal (7) by lactonization from the requisite secoic acid (22), the stereoselective hydrolytic reconstruction of the acetal moiety of 7 leading to 5-membered cyclic hemiacetal (6), the glycosylation of 6 with 4 derived from L-fucose and D-xylose, and the final triene formation using cross-coupling reaction.

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New approaches to total asymmetric synthesis of ircinal A and manzamine A are described. The synthesis utilizes a highly efficient Diels-Alder reaction of the suitably functionalized dihydropyridinones and siloxydienes as a key step, followed by expedient conversion to manzamine A.

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Two distinct synthetic approaches to the biologically active and structurally unique endothelin antagonist J-104132 (1) have been developed Each synthesis involves a highly selective intramolecular cyclization of a late stage intermediate (bottom to top vs top to bottom) produced from a common early intermediate. Both routes initially yielded multi-gram quantities of the desired product, with the former ultimately developed to pilot scale readiness. Several novel reactions have been developed throughout the course of our studies. These involve a mild bromination of a pyridone to form a bromopyridine, a mild and efficient TEMPO catalyzed oxidation of an alcohol to give a carboxylic acid, as well as a novel stereoselective samarium iodide mediated deoxygenation reaction. These reactions have proved to be quite general and have been applied to a wide variety of substrates.

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