Sulphuric acid tetrahydrofuran

Labelling the aliphatic carbon atom with (53%) allowed comparison of the nuclear magnetic C -shifts of the covalent sp -hybridized triphenyl-C -methanol (in tetrahydrofuran solution) with that of the labelled triphenylcarbonium ions, (C8Hg)3C HS07 (triphenyl-C -methanol in sulphuric acid solution). A shift of 129-6 p.p.m. to lower shielding was observed in the triphenylcarbonium ion, as compared with the covalent triphenylmethanol. 

The method is readily adapted for the preparation of dibromides from diols. Typical examples are provided in Expt 5.54. The cyclic ethers tetrahydrofuran and tetrahydropyran are readily cleaved by the hydrobromic acid-sulphuric acid medium, and this provides an alternative and convenient preparation of the corresponding a, co-dihalides. 

Dibromobutane (from tetrahydrofuran). Place a mixture of 250g (170ml) of 48 per cent hydrobromic acid and 75 g (41 ml) of concentrated sulphuric acid in a 500-ml round-bottomed flask, add 18.1 g (20.5 ml, 0.25 mol) of redistilled tetrahydrofuran (Section 4.1.79, p. 406) (b.p. 65-66 °C), attach a reflux condenser and reflux gently for 3 hours. Separate the lower layer of dibromide and purify as in the previous preparation. The yield of 1,4-dibromobutane, b.p. 83-84 °C/12mmHg is 40g (74%). 

An alternative reagent, which is particularly effective for the conversion of diols into diiodo compounds, is a mixture of potassium iodide and 95 per cent orthophosphoric acid (Expt 5.58). The reagent also cleaves tetrahydrofuran and tetrahydropyran to yield the corresponding a, co-diiodo compounds [cf. the hydrobromic acid-sulphuric acid reagent, Section 5.5.2, p. 559]. 

Formation of aldimines from propanal and octanal. To a stirred solution (0°C) of 10 mmol of the methoxyamine dissolved in 30 ml of benzene (CAUTION) (previously washed with concentrated sulphuric acid and distilled) is added 10 mmol of the pure aldehyde. An immediate cloudiness usually results. The mixture is allowed to warm to room temperature and c. 15 g of anhydrous sodium sulphate added. After stirring an additional 30-40 minutes, it is filtered and the sodium sulphate washed thoroughly with dry ether. The solvent is removed first with aspirator pressure and then with a vacuum pump (0.5 mm) to generally furnish 9.5-10 mmol of the aldimine as a colourless oil. The aldimines are dissolved in tetrahydrofuran (0.4 m) and stored at —20 to — 30 °C. Attempts to store the aldimines as neat liquids result in deterioration. The solutions of aldimines are conveniently transferred via a syringe to reaction vessels. 

Further investigations on electrophilic additions to (65) (Poutsma and Ibarbia, 1971) found that the products of the reaction with tri-fluoroacetic acid in benzene at —15° to — 25° are mainly 66 and 67 (S = CFsCO), but minor amounts of 68 and 69 were also observed. A similar product distribution was obtained from 65 with acetic acid, with water in tetrahydrofuran-sulphuric acid, and with methanol in the presence of acids. The detection of compound 68 indicates that terminal protonation is not exclusive and some central attack also occurs. 

The resultant sulphate ester can be converted to the alcohol by acid hydrolysis. If an acid-sensitive group is present, this hydrolysis is still successful through use of a catalytic amount of sulphuric acid in the presence of 0.5-1.0 equivalents of water with tetrahydrofuran as solvent. The use of base in the formation of the cyclic sulphates themselves can also alleviate problems associated with acid-sensitive groups [117,118]. 

Syntheses of 0-methylthalisopavine (59b) have been recorded. In one of these the construction of the carbon-nitrogen skeleton was effected by oxidative cyclization of the lactam (58) to the lactam (59a) by vanadium oxyfluoride in acetonitrile, in 40% yield. Reduction of (59a) by diborane in tetrahydrofuran gave the tertiary amine (59b). An alternative synthesis involved the cyclization, with sulphuric acid, of the acetal (60), prepared from deoxyveratroin. " ... 

Formic acid Hydrocyanic acid Ethanolamine Formamide Acetamide iV-Methylformamide iV-Methylacetamidie Ethylenediamine n-Hexane Psrridine Sulphur dioxide TetrahydrofUran... 

Agricultural waste products such as corn cobs and oat hulls, which are rich in pentosans (polysaccharides containing pentose residues), are heated with dilute sulphuric acid to effect hydrolysis and cyclization. Furfural is isolated from the mixture by steam distillation and then pyrolysed at about 400°C over a chromium-containing catalyst to give furan. The furan is hydrogenated over nickel at about 125°C and 10 MPa (100 atmospheres) to give tetrahydrofuran. 

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