Glycerol, acetal with benzaldehyde

An acetal of benzaldehyde may become a protective group of polyhydroxy compounds. The hydroxyl groups in the 1 and 3 positions of glycerol may form a cyclic acetal with benzaldehyde. The secondary hydroxyl group left intact is available for esterification. Hydrogenolytic removal of the benzal group allows the synthesis of the P-monoglyceride (eq. 13.21).46... 

Isopropylideneglycerol, a five-membered cyclic hydroxy ketal from acetone and glycerol, is prepared in 90% yield by removing the liberated water by an azeotropic distillation. In another procedure, calcium carbide is added directly to the reaction mixture as a desiccant. Acetaldehyde and benzaldehyde, unlike acetone, react with glycerol to form a mixture of the five- and six-membered cyclic hydroxy acetals. Alkoxy acetals are made by the acetalization of a,/3-olefinic aldehydes in weakly acidic solutions however, the addition of alcohol to the double bond may not go to completion. ... 

Benzaldehyde is condensed with glycerol in the presence of the acid catalyst, p-toluenesulphonic acid. The product is crystallized, collected by filtration and further purified (cf. Jensen and Pitas, 1976). Acylation is carried out with an acid chloride (Section 10.13) and, after removal of pyridine hydrochloride, the acyl-1,3-benzylidene glycerol is crystallized. Purity can be estimated by TLC (Jensen and Pitas, 1976). A critical stage now comes, where the blocking group has to be removed. Cleavage of the benzylidene acetal is carried out by treatment with boric acid and water. The presence of boric acid inhibits isomerization to the more stable 1-monoacylglycerol (the equilibrium ratio is 9 1 in favour of the 1-isomer). 

A powerful oxidizer. Explosive reaction with acetaldehyde, acetic acid + heat, acetic anhydride + heat, benzaldehyde, benzene, benzylthylaniUne, butyraldehyde, 1,3-dimethylhexahydropyrimidone, diethyl ether, ethylacetate, isopropylacetate, methyl dioxane, pelargonic acid, pentyl acetate, phosphoms + heat, propionaldehyde, and other organic materials or solvents. Forms a friction- and heat-sensitive explosive mixture with potassium hexacyanoferrate. Ignites on contact with alcohols, acetic anhydride + tetrahydronaphthalene, acetone, butanol, chromium(II) sulfide, cyclohexanol, dimethyl formamide, ethanol, ethylene glycol, methanol, 2-propanol, pyridine. Violent reaction with acetic anhydride + 3-methylphenol (above 75°C), acetylene, bromine pentafluoride, glycerol, hexamethylphosphoramide, peroxyformic acid, selenium, sodium amide. Incandescent reaction with alkali metals (e.g., sodium, potassium), ammonia, arsenic, butyric acid (above 100°C), chlorine trifluoride, hydrogen sulfide + heat, sodium + heat, and sulfur. Incompatible with N,N-dimethylformamide. 

The condensation of glycerol with aldehydes and ketones was studied with benzaldehyde, a mixture of the cyclic acetals having the five- and the six-membered ring resulted, showing that presence of hydroxyl groups on vicinal carbon atoms is not essential for occurrence of the reaction. On the other hand, on condensation of acetone with various other polyhydric alcohols, the cyclic acetal having the five-membered ring was alway formed exclusively. 

---