The Prins Reaction is the acid-catalyzed of addition aldehydes to alkenes, and gives different products depending on the reaction conditions. It can be thought of conceptually as the addition of the elements of the gem-diol carbonyl hydrate of the aldehyde across the double bond.
An excess of aldehyde and temperatures < 70 °C lead to the formation of acetals. When one equivalent of aldehyde is used and temperatures are > 70 °C diols or allylic alcohols may be isolated.
Although the mechanism is different, a Prins allylic alcohol product is equivalent to the result of an Ene Reaction.
Mechanism of the Prins Reaction
Lewis Acidic Chloroaluminate Ionic Liquids: Novel Reaction Media for the Synthesis of 4-Chloropyrans
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