1.3- dioxane, basicity

Glass Calomel (in MeOH) Ar(R)(OH)(CH2) NR2 HCIO4 (ill dioxane) Basicity studies 75... 

For example, 2-phenyl-5-fonnylthiazole treated with AgjO in water and dioxane under basic conditions gives 2-phenyI-5-thia201ecarboxylic acid (29). 

Because lactic acid has both hydroxyl and carboxyl functional groups, it undergoes iatramolecular or self-esterificatioa and forms linear polyesters, lactoyUactic acid (4) and higher poly(lactic acid)s, or the cycUc dimer 3,6-dimethyl-/)-dioxane-2,5-dione [95-96-5] (dilactide) (5). Whereas the linear polyesters, lactoyUactic acid and poly(lactic acid)s, are produced under typical condensation conditions such as by removal of water ia the preseace of acidic catalysts, the formation of dilactide with high yield and selectivity requires the use of special catalysts which are primarily weakly basic. The use of tin and ziac oxides and organostaimates and -titanates has been reported (6,21,22). 

Hydrolysis of TEOS in various solvents is such that for a particular system increases directiy with the concentration of H" or H O" in acidic media and with the concentration of OH in basic media. The dominant factor in controlling the hydrolysis rate is pH (21). However, the nature of the acid plays an important role, so that a small addition of HCl induces a 1500-fold increase in whereas acetic acid has Httie effect. Hydrolysis is also temperature-dependent. The reaction rate increases 10-fold when the temperature is varied from 20 to 45°C. Nmr experiments show that varies in different solvents as foUows acetonitrile > methanol > dimethylformamide > dioxane > formamide, where the k in acetonitrile is about 20 times larger than the k in formamide. The nature of the alkoxy groups on the siHcon atom also influences the rate constant. The longer and the bulkier the alkoxide group, the lower the (3). 

Bu4N F , THF, "2 min. The TBDS group is less reactive toward tri-ethylammonium fluoride than is the TIPDS group. It is stable to 2 M HCl, aq. dioxane, oyemight. Treatment with 0.2 MNaOH, aq. dioxane leads to cleavage of only the Si—O bond at the 5 -position of the uridine derivative. The TBDS derivative is 25 times more stable than the TIPDS derivative to basic hydrolysis. 

Monoesterification of a symmetrical dihydroxy aromatic compound can be effected by reaction with polymer-bound benzoyl chloride (Pyr, benzene, reflux, 15 h) to give a polymer-bound benzoate, which can be alkylated with diazomethane to form, after basic hydrolysis (0.5 M NaOH, dioxane, H2O, 25°, 20 h, or 60°, 3 h), a monomethyl ether. ... 

The picolyl ester has been prepared from amino acids and picolyl alcohol (DCC / CH2CI2, 20°, 16 h, 60% yield) or picolyl chloride (DMF, 90-100°, 2 h, 50% yield). It is cleaved by reduction (H2/Pd-C, aq.= FtOH, 10 h, 98% yield Na/NH3, 1.5 h, 93% yield) and by basic hydrolysis (1 NaOH, dioxane, 20°, 1 h, 93% yield). The basic site in a picolyl ester allows its ready separation by extraction into an acidic medium. ... 

These rate constants are for the basic hydrolysis of methyl 4-substituted 2,6-dimethylbenzoates at 125°C in 60% dioxane. 

During the next fifty years the interest in derivatives of divalent carbon was mainly confined to methylene (CHg) and substituted methylenes obtained by decomposition of the corresponding diazo compounds this phase has been fully reviewed by Huisgen. The first convincing evidence for the formation of dichlorocarbene from chloroform was presented by Hine in 1950. Kinetic studies of the basic hydrolysis of chloroform in aqueous dioxane led to the suggestion that the rate-determining step was loss of chloride ion from the tri-chloromethyl anion which is formed in a rapid pre-equilibrium with hydroxide ions ... 

Reaction of 2-(A -allylamino)-3-formyl-4//-pyrido[l, 2-u]pyrimidin-4-ones 219 in EtOH with HONH2 HCI yielded ( )-oximes 220 at 0°C and 221 (R = PhCH2) under reflux. Heating 220 (R = H) in a boiling solvent afforded cw-fused tetracyclic cycloadducts 221 (R = H). In an aprotic solvent (e.g., benzene or MeCN) the main a>fused cycloadducts 221 (R = H) were accompanied by a mixture of trauA-fused cycloadducts 222, A -oxides 223 and tetracyclic isoxazoline 224 (96T887). The basicity of the 2-allylamino moiety of compounds 219 affected the rate of the conversion. Cycloadditions were also investigated in dioxane and BuOH. 

Early efforts to effect the photoinduced ring expansion of aryl azides to 3H-azepines in the presence of other nucleophiles met with only limited success. For example, irradiation of phenyl azide in hydrogen sulfide-diethyl ether, or in methanol, gave 17/-azepine-2(3//)-thione35 (5% mp 106—107 " O and 2-methoxy-3//-azepine (11 %),2 3 respectively. Later workers194 failed to reproduce this latter result, but found that in strongly basic media (3 M potassium hydroxide in methanol/dioxane) and in the presence of 18-crown-6, 17/-azepin-2(3//)-one was produced in 48% yield. In the absence of the crown ether the yield of azepinone falls to 35%. 

Chloramine-B (CAB, PhS02NClNa) and chloramine-T (CAT, p-Me-C6H4S02NClNa) have also been used for the oxidation of sulphoxides107-115. The required sulphone is produced after initial attack by the sulphoxide sulphur atom on the electrophilic chlorine-containing species, forming a chlorosulphonium intermediate as shown in equation (34). These reactions take place at room temperature, in water and aqueous polar solvents such as alcohols and dioxane, in both acidic and basic media. In alkaline solution the reaction is slow and the rate is considerably enhanced by the use of osmium tetroxide as a catalyst115. 

Removal of the 2 -sulfonyloxy group of 859 in a basic medium, followed by reaction with metal halides (LiBr and Nal) or hydrogen halides (HCl-1,4-dioxane, HBr-acetone, or0.1% HFin l,4-dioxane-AlF3)gave, byway of the 2,2 -anhydro intermediate 861, the 2 -halo derivatives 862-865. The 2 -deoxy analog 866 and l-(6-deoxy-6-fluoro- ff-D-mannopyranosyl)thy-mine were also prepared from 864 (R = H) and 861 (R = H), respectively. l-(4-Deoxy-4-fluoro-y -D-glucopyranosyl)thymine was obtained by the condensation method. A different kind of nucleoside, 5-(5-deoxy-5-fluoro-2,3-0-isopropylidene-a-D-ribofuranosyl)-l,3-dimethyluracil has also been prepared. ... 

In the case of the hexacarbonyls, the rate-expression contains not only the same type of first-order term but in addition one second-order overall. For good entering groups (but not CO, for example) the rate expression contains a term strictly first-order in both the complex and the entering nucleophile. The first-order rates of CO exchange are practically identical with the rates of substitution in hydrocarbon solvents, but there is nevertheless some acceleration in ether (THF, dioxan) solutions. This solvent-dependence is not so well-marked ° as in the case of nickel tetracarbonyl. The second-order rate of substitution very strongly depends upon the basicity of the entering nucleophile... 

Electron-donating groups in the para position of the perbenzoic acid tend to decrease the rate of reaction . The reverse seems to be true when these groups are present in a 4,4 -disubstituted diaryl sulphoxide. The effect of ring size on the oxidation of cyclic sulphoxides is apparently very small in dioxane-water solution under either acidic or basic conditions . This suggests that no major hybridisation change occurs for the sulphur atom in going to the transition state. 

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