Recent studies of the synthesis of oligonucleotides incorporating conformationally locked nucleoside or nucleotide building blocks have been comprehensively reviewed in connection with the antisense stratagy as well as creaction of new functions of DNA/RNA. The synthesis, and conformational properties of a wide varaiety of nucleoside derivatives locked by introduction of three, four, and five membered-ring systems to the deoxyribose or ribose moiety have been described. The hybridization abilities of oligonucleotides incorporating these locked-nuucleosides are also discussed. In addition, this review deals with the synthesis and properties of conformationally fixed cyclouridylate derivatives bridged between the uracil and the 5'-phosphate of 5'-uridylic acids and their successful application to the construction of bent DNA and RNA.
In this review, the hypercoordination of boron and aluminum as typical main group elements is embodied, and their synthetic utility is demonstrated with several synthetic examples. B and Al Lewis acids are found to be successfully utilized in several chelation-controlled reactions of various substrates (alkoxy carbonyl compounds, fluoro carbonyl compounds, fluoro epoxides, and alkoxy epoxides) via unprecedented pentacoordinate chelate-type complex formation by taking advantage of the exceedingly high affinity of B and Al to fluorine and oxygen, and hence, contrary to the previous observations, can be classified as chelating Lewis acid reagents rather than non-chelating Lewis acids. In addition to the experimental demonstration, such pentacoordinate complex formation of B and Al Lewis acids with various bidentate substrates is also characterized by low-temperature 1H, 13c, 11B, 27AlNMR spectroscopy.
Syntheses of multi-functional natural and unnatural compounds have been of great significance in organic chemistry not only as the target molecules but also as leading field for conceptual and methodological developments. Particularly those compounds in relation to biology have collecting attentions from various field such as structure-activity relationship, asymmetric synthesis, drug design for chemotherapy and agricultural control, etc. We have to keep looking for new chemistry for these purposes; thus, we became interesting in the acetylene biscobalthexacarbonyl complex from its synthetic point of views. It started from our previous C-glycosidation with silylacetylenes providing sugar acetylenes, in which the alkynyl group at the anomeric position of a pyranose ring can be epimerized through a biscobalthexacarbonyl complex by treatment with trifluoromethanesulfonic acid. Three steps involving cobalt complexation, acidic transformation and decomplexation afford overall epimerization and thus one can obtain either the α- or β-alkynyl C-glycoside as desired. Ring opening of a dihydropyran derivative using Nicholas-type cation intermediates was also an accomplished part of this study. Several decomplexation conditions of endo-type acetylenecobalt complexes provide various olefins possessing potential utilities for synthesis. These methodologies have been utilized for the synthesis of polyoxygenated natural products and derivatives. Basis of these chemistry is described in this issue, and the application and more examples are to be described in the next issue.
With its excellent color and tone reproduction, thermal dye transfer printing produces continuous-tone color images that rival silver halide photography. However, conventional thermal dye transfer images fade easily when subjected to light or to heat. Many approaches have been taken to solve this serious problem, including the addition of a protective layer over the dye receiver sheet, the addition of UV-light-absorbing agents and antioxidizing agents to the dye receiving layer, the addition of dye adsorbents to the dye receiving layer, the hardening of the dye receiving layer after thermal printing, and the employment of chemical reactions between reactive dyes and chemical agents in the dye receiving layer. Each of these approaches has had some success, but the last approach has been most promising. In this paper, we describe the various methods to improve image stability and our study of the chelate system providing exceptionally high image stability.
Our longstanding interest in the Diels-Alder reactions led us to examine lithium perchlorate in diethyl ether (LPDE) as a medium for effecting [4 + 2] cycloadditions, despite the general view that the rate of the Diels-Alder reaction is essentially independent of solvent polarity. This article describes an effecient, synthetically useful protocol for the construction carbocyclic systems via intramolecular cationic Diels-Alder reactions of in situ-generated heteroatom-stabilized allyl cations in highly polar media. We also present that use of catalytic acid in LPDE to promote intramolecular Diels-Alder reactions of conformationally restricted substrates possessing terminal dienes results in acid catalyzed migration of the diene prior to [4 + 2] cycloaddition.
Cyclization reactions using vinyl sulfides and vinyl selenides have recently attracted much attention. One of the major characteristics of sulfur-and selenium-substituted donor olefins is a large HOMO coefficient on the hetero-atom. Due to such specific character, sulfur and selenium-substituents can be used as selectivity-controlling elements in cyclizations. Novel reactions resulting from a combination with other elements, such as [2+1] cycloadditions of 1-seleno-2-silylethenes, are also described. After the cyclization steps, sulfur and selenium are useful for various functional group transformations. In this review, [2+1], [2+2], [3+2], and [4+2] cycloaddition reactions employing vinyl sulfides and selenides in organic synthesis are outlined.