Thio effect

The first published crystal structure of the full length HHR [126] in which there was no solvent or ions resolved showed A9 and the scissile phosphate in close proximity, consistent with the interpretation of thio effect measurements [130], and the G8 02 and G12 Ni poised to act as a general acid and base, respectively, as proved in previous photocrosslinking [131] and mutation experiments [132], Given the strong evidence that Mg2+ participates directly in the catalytic process together with the spatial proximity of the A9 and scissile phosphate, made the placement of an Mg2+ ion in bridging position a reasonable assumption. 

Analyses of both the thio effect and of soft acid rescue effects, such as the rescue effects of Cd, Mn, and Zn ions, have contributed significant-... 

It is clear that the analysis of thio effects, rescue experiments and other experiments with derivatives have contributed significantly to our understanding of the mechanism of the action of the large group I intron ribozyme of Tetmhymena. All the available data appear to support the Lewis acid catalysis for activation of the attacking nucleophile and enhancement of the leaving group that is shown in Fig. IIB. 

Thio-Claisen rearrangement. In chiral bicyclic thiolactams via a thio effected with assistance of (MeCN) PdC... 

The mechanism of ribonuclease A (RNase A) activity has been widely studied by evaluating different aspects such as roles of the catalytic amino acids, different substrates, thio effects, organic solvents, and pH and temperature dependence [79]. The usually accepted mechanism is the general acid/base pathway where a... 

The pA of 1,3-dithiane is 36.5 (Cs" ion pair in THF). The value for 2-phenyl-1,3-dithiane is 30.5. There are several factors which can contribute to the anion-stabilizing effect of sulfur substituents. Bond dipole effects contribute but carmot be the dominant factor because oxygen substituents do not have a comparable stabilizing effect. Polarizability of sulfur can also stabilize the carbanion. Delocalization can be described as involving 3d orbitals on sulfur or hyperconjugation with the a orbital of the C—S bond. MO calculations favor the latter interpretation. An experimental study of the rates of deprotonation of phenylthionitromethane indicates that sulfur polarizability is a major factor. Whatever the structural basis is, there is no question that thio substituents enhance... 

Me3SiCH2CH=CH2, TsOH, CH3CN, 70-80°, 1-2 h, 90-95% yield. " This silylating reagent is stable to moisture. Allylsilanes can be used to protect alcohols, phenols, and carboxylic acids there is no reaction with thio-phenol, except when Cp3S03H is used as a catalyst. The method is also applicable to the formation of r-butyldimethylsilyl derivatives the silyl ether of cyclohexanol was prepared in 95% yield from allyl-r-butyl-dimethylsilane. Iodine, bromine, trimethylsilyl bromide, and trimethylsilyl iodide have also been used as catalysts. Nafion-H has been shown to be an effective catalyst. ... 

The position of substitution in disubstituted thiophenes can, in most cases, easily be deduced from the directing effect of each substituent. Thus with a - -M-substituent in the 2-position and a —M-substituent in the 5-position, both substituents direct the entering group to the 3-position as is exemplified by the nitration of methyl 2-bromo-5-thiophenecarboxylate to methyl 2-bromo-3-nitro-5-thio-phenecarboxylate (109) or in the chlororaethylation of methyl 2-methyl-5-thiophenecarboxylate to methyl 2-methyl-3-chloromethyl-5-thiophenecarboxylate (110). °... 

Competitive metalation of thiophene and 2-methylthiothiophene with a deficiency of n-butyllithium gave only 2-methylthio-5-thio-phenecarboxylic acid, showing the activating effect of the methylthio group. ... 

Alkylation in position 3 has a still more pronounced effect. The thiosemi carbazones (101) obtained here were prepared either from 3-methylisothiosemicarbazide hydroiodide or by methylation of thio-semicarbazones. Their cyclization was performed either by boiling in alcohol or by heating to the melting point (102). The presence of... 

The assumed transition state for this reaction is shown in Scheme 5.5. The two bulky t-butoxy groups are expected to locate at the two apical positions. One of the 3,3 -phenyl groups would effectively shield one face of an imine, and consequently, a diene attacks from the opposite side. Judging from this model, similar selectivities were expected in the Mannich-type reactions of imines with silyl eno-lates. Actually, when ligand 10 was used in the reaction of imine la with S-ethyl-thio-l-trimethylsiloxyethene, the corresponding / -amino thioester was obtained in 84% ee (Scheme 5.6). As expected, the sense of the chiral induction in this case was the reverse of that observed when using catalyst 6 [12, 25]. 

D) Preparation of 4-[1 -Methyl-Piperidyl-(4 )]-9,10-Dihydro-4H-Benzol4,5]Cyciohepta[1,2-b] Thiophen-(4)-ol 0.94 g of magnesium filings which have been activated with iodine are covered with a layer of absolute tetrahydrofuran and etched with a few drops of ethylene bromide. A solution of 5.0 g of 1-methyl-4-chloropiperidine in 5 ml of tetrahydrofuran is then added dropwise and boiling then effected for a further hour under reflux. After cooling to room temperature, the solution of 4.5 g of 9,lO-dihydro-4H-benzo[4,5] cyclohepta[1,2-b] thio-phen-(4)-one in 5 ml of tetrahydrofuran is added dropwise. 

Rat MOT DOT thio- glycolates (67 33) Gestation days 6-15 at 0,20, 60, and 120 mg/kg body weight No adverse effects NOAEL= 120 Ciba-Geigy Ltd (1983)... 

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