The protection of uracil and guanine residues in oligonucleotide synthesis

The protection of uracil and 2-$\text{N}$-acyl guanine residues with 4-$\text{O}$-phenyl [or 4-$\text{O}$-(2,4-dimethylphenyl)] and 6-$\text{O}$-(2-nitrophenyl) groups as in $\text{7a}$ [or $\text{7b}$] and $\text{9}$, respectively, is described. These $\text{O}$-aryl protecting groups, which appear to withstand the usual conditions of oligonucleotide synthesis, may readily be removed by treatment with 2-nitrobenzaldoximate ions.     

Total synthesis of (+)-tubelactomicin A. 2. Synthesis of the upper-half segment and completion of the total synthesis

[reaction: see text]. We have completed the total synthesis of natural (+)-tubelactomicin A (1), a 16-membered macrolide antibiotic. This Letter presents a highly efficient synthesis of the upper-half segment (C14-C24) and the completion of the total synthesis featuring a high-yielding Stille coupling for the connection of the upper-half and lower-half segments and Mukaiyama macrolactonization for the construction of the entire structure of 1.     

Reaction of azole condensed quinoxaline N-oxides with acetic anhydride

The reaction of 7-chlorotetrazolo[1,5-α]quinoxaline 5-oxide 6a with acetic anhydride gave 7-chloro-5-(7-chlorotetrazolo[1,5-α]quinoxalin-4-yl)-4,5-dihydro-4-oxotetrazolo[1,5-α]quinoxaline 7a , while the reaction of 7-chloro-1,2,4-triazolo[4,3-α]quinoxaline 5-oxide 6b with acetic anhydride afforded 7-chloro-5-(7-chloro-1,2,4-triazolo[4,3-α]quinoxalin-4-yl)-4,5-dihydro-4-oxo-1,2,4-triazolo[4,3-α]quinoxaline 7b and 7-chloro-4,5-dihydro-4-oxo-1,2,4-triazolo[4,3-α]quinoxaline 8b . The reaction of compound 6a or 6b with acetic anhydride/acetic acid provided 7-chloro-4,5-dihydro-4-oxo-tetrazolo[1,5-α]quinoxaline 8a or compound 8b , respectively.     

Substituent effects on the tautomer ratios between the hydrazone imine and diazenyl enamine forms in side-chained quinoxalines

The 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 9–11 were synthesized by the reaction of the quinoxalines 6–8 with various p-substituted benzenediazonium salts. Compounds 9–11 showed the tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in dimethyl sulfoxide media. The substituent effect on the tautomer ratios of A to B was studied by the nmr spectroscopy to clarify that the presence of the ester group R² on the hydrazone carbon and electron-donating p-substituent R¹ on the side chain benzene ring exhibited a tendency to increase the ratios of the tautomer A .     

Facile fabrication of composites of platinum nanoparticles and amorphous carbon films by catalyzed carbonization of cellulose fibers

Composites of platinum nanoparticles and amorphous carbon films have been facilely fabricated by catalyzed carbonization of cellulose fibers.     

A study on the conformation-anomeric effect-stereoselectivity relationship in anomeric radical reactions,using conformationally restricted glucose derivatives as substrates

We previously theorized that, since the stereoselectivity of anomeric radical reactions is significantly influenced by the kinetic anomeric effect, which can be controlled by restricting the conformation of the radical intermediate, the proper conformational restriction of the pyranose ring of the substrates would therefore make highly alpha- and beta-stereoselective anomeric radical reactions possible. This theory was based on our previous results of the anomeric radical reactions with d-xylose derivatives as the substrates. We herein report the anomeric radical deuteration reactions with the conformationally restricted 1-phenylseleno-d-glucose derivatives, 2g and 3g, restricted in a (4)C(1)-conformation by an O-cyclic diketal moiety, and 4g, 5g, 6g, 7g, and 8g, restricted in a (1)C(4)-conformation by bulky O-silyl protecting groups. The radical deuterations with Bu(3)SnD, using the (4)C(1)-restricted substrates 2g and 3g, afforded the corresponding alpha-products (alpha/beta = 98:2) highly stereoselectively, whereas the (1)C(4)-restricted substrate 6g, having a trigonal (sp(2)) carbon substituent, i.e., -CHO, at the 5-position, selectively gave the beta-products (alpha/beta = 0:100). Thus, the stereoselectivity was significantly increased by the conformational restriction and was completely inverted by changing the substrate conformation from the (4)C(1)-form to the (1)C(4)-form. On the other hand, the deuterations with the (1)C(4)-restricted substrates 4g and 5g showed that the 1,5-steric effect due to the tetrahedral carbon substituent (-CH(2)OTIPS or -CH(2)OH) at the 5-axial position dominantly prevented the hydride transfer from the beta-face competing with the kinetic anomeric effect. This study suggests that, depending on the restricted conformation of the substrates to the (4)C(1)- or the (1)C(4)-form, the alpha- or beta-products would be obtained highly stereoselectively via anomeric radical reactions of hexopyranoses.