Preparation
The Petasis reaction proceeds under mild conditions, without the use of strong acids, bases, or metals. Unlike the Strecker and Ugi syntheses, the Petasis reaction avoids the use of cyanide and isocyanide reagents. The amine is mixed with the carbonyl substrate using either dioxane or toluene as a solvent at 90 °C for 10 minutes. Subsequently, the boronic acid is added to the mixture and product is generated, either after 30 minutes at 90 °C, or after several hours at 25 °C. In α-amino acid synthesis, α-keto acids, such as glyoxylic and pyruvic acid, are stirred in ethanol, toluene, or dichloromethane with amines and vinyl boronic acids at 25–50 °C for 12-48 h to give the corresponding β,γ-unsaturated compounds.
One of the most attractive features of the Petasis reaction is its use of boronic acids as a nucleophilic source. Unlike most vinyl substrates, vinyl boronic acids are stable to air and water and can be removed during workup with a simple extraction. Many boronic acid derivatives are easy to prepare and with the advent of the Suzuki coupling, a larger number of them are now commercially available. In the seminal report of the reaction, the organoboronic acids were prepared by hydroboration of terminal alkynes with catecholborane.
Other methods of generating boronic acids were also reported.
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