PAPER	Synthesis 2002: 2521-2526 DOI: 10.1055/s-2002-35645
© Georg Thieme Verlag Stuttgart · New York
Selective Prins Reaction of Styrenes and Formaldehyde Catalyzed by 2,6-Di-tert-butylphenoxy(difluoro)borane
Thorsten Bach*, Johannes Löbel
Technische Universität München, Lehrstuhl für Organische Chemie I, Lichtenbergstr. 4, 85747 Garching, Germany Fax: +49(89)28913315; e-Mail: thorsten.bach@ch.tum.de;
Received 19 August 2002

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