Recrystallization lead dioxide

Ethyl 2-(D-amiino-tetrahydroxybutyl)-5-methyl-4-furoate (5.5 g.) is mixed with 80 ml. of dry benzene and 20 ml. of glacial acetic acid, and cooled in ice plus water. While stirring and cooling, 182 g. of lead tetraacetate (purity, 99.7%)62 is added during about sixty minutes stirring is continued until all the oxidant has been consumed. The lead dioxide is then removed by filtration, and the benzene solution is extracted twice with water.58 The benzene layer is dried with calcium chloride and the solvent is evaporated under diminished pressure, giving an oily residue which rapidly crystallizes in colorless plates yield, 3.6 g. (quantitative). The product is purified by recrystallization from dilute acetic acid or by steam distillation m.p., 57°. 

Summary Ammonium azide is easily prepared by mixing solutions of sodium azide, and aqueous ammonia. The mixture is then treated with carbon dioxide gas to precipitate sodium ion as sodium bicarbonate. The remaining ammonium azide stays in solution, and can be recrystallized from the reaction mixture. The collected crystals should then be dried in a desiccator as vacuum drying may result in loss of product from volatization air-drying the solid may also lead to loss in product. 

The main interest in (-)-encycloaddition processes to yield separable mixtures of diastereoisomeric urazoles. The non-destructive resolution of cyclooctatetraenes, which allows direct access to optically pure derivatives, is a typical illustration and has been amply demonstrated. Typically, (-)-enethyl acetate to afford a mixture of diastereoisomeric adducts, which can be separated by fractional recrystallization from ethyl acetate and hexane. HPLC is an alternative separation technique leading to both enantiomerically pure antipodes. The chiral auxiliary is subsequently removed by basic hydrolysis-manganese dioxide oxidation to afford the optically pure cyclooctatetraenes (eq 2). 

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