We have found that the thermolysis of acylpolysilanes [pivaloyl-, adamantoyl-, benzoyl- and mesitoyltris (trimethylsilyl) silane] offers a convenient route to the synthesis of various types of silenes. The silenes thus formed reacted with many substrates, such as olefins, dienes, carbonyl compounds, and alkynes to give the respective adducts. The reactions of the silenes produced from acylpolysilanes with mono-silyl substituted acetylenes and tert-butylacetylene gave [2+2] cycloadducts, silacyclobutene derivatives, with high regiospecificity in high yields. The thermolysis of the resulting silacyclobutenes at high temperature afforded the isomers arising from a 1, 2-trimethylsiloxy shift from the sp3-carbon to silicon in the silacyclobutene ring.
Pauson-Khand reaction (PKR) is a useful reaction for formation of cyclopentenones from alkyne-cobalt complex, which is obtained from alkynes and Co2 (CO) 8, and alkenes by heating the mixture or by treating the mixture with amine N-oxide at room temperature. Until now, there are many reports on PKR of both endo-cyclic and acyclic olefins, however, only a few reports on exo-olefins have appeared. Thus, the authors investigated intra- and inter-molecular PKR of exo-olefins. The intramolecular reaction of cyclic and acyclic enynes bearing exo-olefins was performed to give corresponding bi- and tri-cyclic compounds. Also, intermolecular PKR of exo-cyclic olefins with alkynes furnished spiro-bicyclic compounds. Furthermore, the present methodology was developed to construct tricyclic skeleton of Lycopodium alkaloid, magellanine family and formal total synthesis of magellanine was accomplished.
Recent studies on the developments of the metal-catalyzed new borylation reactions by bis(pinacolato) diboron (1) are described. The 1, 4-addition of 1 to α, β-unsaturated carbonyl compounds and nitriles proceeds in the presence of a catalytic amount of CuCl and KOAc in DMF. Subsequent treatment of the reaction mixture with water produces β-boryl ketones, aldehydes, esters, and nitriles. Vinyl- and allylboron compounds are also obtained by the reactions of 1 with alkynes or allyl halides when using a stoichiometric amount of CuCl and KOAc. The aromatic C-H borylation of arenes or heteroarenes by 1 is effectively catalyzed by a combination of 1/2 [Ir (OMe) (COD)]2 and 4, 4'-di-tert-butyl-2, 2'-bipyridine in hexane. The high activity of the catalyst allows the formation of aryl- and heteroarylboron compounds in high yields at room temperature from equimolar equivalents of 1 and substrate. The direct borylation of alkylarenes by 1 catalyzed by Pd/C exclusively occurs via benzylic C-H activation to give benzylboron compounds.
Highly diverse organic reactions with hydroxylamine-based compounds have been developed during past ten decades or more and are now playing an important role in organic synthesis in a myriad of contexts. Nevertheless, we expect that there still remains useful and promising functionality for the compounds of this class. Hence, we have been making extensive efforts to disclose hidden potential of such compounds. In this report we will present some interesting and powerful organic transformations becoming available as the results of such our activities, which covers the following five novel processes or reactions : (1) silicon-tether strategy for controlling the regioselectivity and diastereoselectivity of intramolecular nitrone cycloadditions; (2) intramolecular Diels-Alder reactions employing hydroxamate tethers; (3) novel [2, 3] -sigmatropic rearrangement for carbon-nitrogen bond formation; (4) dicobalt octacarbonyl-promoted rearrangement of 4-isoxazolines to acylaziridines; (5) novel ene-like cycloisomerization reaction of nitrile oxides with a tethered allyltrimethylsilyl group.
α-Amino acids are the most important naturally occurring amino acids assembling into polypeptides with a wide range of vital biological functions, and hence the development of an efficient and reliable method for the preparation of optically active α-amino acids and their derivatives with sufficient structural diversity has been a field of considerable interest. Main purpose of this review is to illustrate the range of asymmetric approaches available for the laboratory preparation of optically active α-amino acids by using chiral phase-transfer catalysts. The key process is revealed to be stereoselective functionalization (alkylation, Michael addition, aldol reaction) of the Schiff base of α-amino acid esters including glycine tert-butyl ester with either cinchona alkaloid-derived quaternary ammonium salts or purely synthetic chiral ammonium salts as catalyst, providing a practical route to a variety of optically active α-alkyl-α-amino acids, α, α-dialkyl-α-amino acids and β-hydroxy-α-amino acids. In addition, highly stereoselective N-terminal alkylation of Schiff base-activated peptides by chiral phase-transfer catalysis is also described.
This article describes new methods for architectures of glyco-materials with well-defined structures. Novel enzymatic glycosylations by using glycosyl fluorides and sugar oxazolines as glycosyl donors catalyzed by glycosyl hydrolases such as cellulase and chitinase were disclosed. The glycosylation reactions were applied to a selective elongation of a monosaccharide unit by utilizing two enzymatic processes. The first step is a hydrolase-catalyzed lactosylation or lactosaminylation toward appropriate glycosyl acceptors, giving oligosaccharides having a D-galactose unit at the non-reducing end. The second step involves β-galactosidase-catalyzed cleavage of the terminal D-galactose unit to produce the desired oligosaccharides. A facile method for synthesis of sugar oxazoline promoted by potassium fluoride will also be mentioned. A new method for construction of higher-ordered structures of polysaccharide complexes during polymerization process was developed.