Abstract
The sterically congested Lewis acid 1 was
used as a catalyst in the Prins reaction of various styrenes and
formaldehyde. 4-Aryl-1,3-dioxanes 5 were
selectively formed as the exclusive products of the reaction with
styrenes 4a-i and
vinylthiophenes 4j-k.
The reaction proceeded in most cases with good to excellent yields
(55-99%). Styrenes which carried a strongly electron-withdrawing
group (CN, CO2Me) did not react. The reaction with β-alkylstyrenes 6 was successful for the methyl substituted
substrate 6a and yielded (88%) trans-5-methyl-4-phenyl-1,3-dioxane (7a) preferentially (dr = 75:25).
For steric reasons, other β-alkylstyrenes 6b-d did not react. The remarkable stereodiscrimination
attained by catalyst 1 was employed in
the regioselective transformation of 4-propenylstyrene (10) to dioxane 11 (83% yield).
Keywords
carbocations - catalysis - chemoselectivity - electrophilic additions - Lewis acids
