Addition of formaldehyde Prins reaction

Formaldehyde can be added at an ethylenic bond under the influence of acidic reagents.88 Reaction of styrene with formaldehyde in sulfuric acid yields 4-phenyl-/w-dioxan as follows 89 

A mixture of 37% aqueous formaldehyde solution (675 g), sulfuric acid (d 1.84 48 g), and styrene (312 g) is heated cautiously for 7 h with stirring and refluxing. After this has cooled, benzene (500 ml) is stirred in, the layers are separated, and the aqueous layer is extracted with benzene (500 ml). The benzene layers are united, washed twice with water (750-ml portions), and freed from benzene by distillation. Fractionation of the residue gives the product, b.p. 96-103°/2 mm (353-436 g, 70-88%). 

In acetic acid the product is mainly 1-phenyltrimethylene diacetate 90 

Other olefins can also be used in such reactions, but arylalkenes give the best yields.89,91 

The stereochemistry of the Prins reaction has been discussed by Bernadi and Leone.92 

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