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32.3.6.1.2.4 Method 4: Synthesis from α-Hydroxy Ketones

DOI: 10.1055/sos-SD-032-00219

Sainsbury, M.Science of Synthesis, (200832314.

A number of 1,3-dioxoles can be formed by simply heating an α-hydroxy ketone with a non-enolizable aldehyde under azeotropic conditions; for example, 4,5-diphenyl-2-(trichloromethyl)-1,3-dioxole (75) is obtained from benzoin by refluxing it with trichloroacetaldehyde (chloral hydrate) in a benzene solution containing a catalytic amount of 4-toluenesulfonic acid. Chloral hydrate cannot be reacted with 3-hydroxybutan-2-one in this way, but in the presence of pyridine it gives the corresponding 4-hydroxy-1,3-dioxolane 76. This compound is not purified, but it may be O-acetylated in situ by the addition of acetic anhydride to the reaction mixture and affords 4,5-dimethyl-2-(trichloromethyl)-1,3-dioxolan-4-yl acetate. Unfortunately, this compound only gives a very poor yield of 4,5-dimethyl-2-(trichloromethyl)-1,3-dioxole (77) when heated with barium oxide (Scheme 25).[‌40‌] A reaction between benzoin and ethyl orthoformate, catalyzed by acetic acid and carried out at 130°C for 48 hours, gives 2-ethoxy-4,5-diphenyl-1,3-dioxole (78) as colorless crystals in 32% yield. While stable to alkali, this compound is hydrolyzed by aqueous acid to diphenylacetylene (Scheme 25).[‌41‌]

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