Rearrangement reactions are ubiquitous and they can occur both in the ground state as well as in the excited state. [1] The photochemical properties of stilbene-type compounds are well-documented [2±4] and major reactions include cis ± trans isomerization, exciplex reactions, addition, and oxidative cyclization to phenanthrene. The mechanism for the oxidative cyclization involves a photochemically allowed six-electron conrotatory process to form a trans-dihydrophenanthrene intermediate; [4b] oxidation of this intermediate then affords the phenanthrene product. Styrylfuran is also known to undergo photochemical isomerization, and in the presence of oxygen or iodine it affords cyclized products [Eq. ( 1)]. [5] We have prepared several styrylfurans 1 ± 11 with substituents in the para position of the benzene ring and now report a novel skeletal rearrangement of the styrylfurans (Scheme 1). The starting materials were prepared from 2-furaldehyde and Scheme 1. Photochemical rearrangement of the styrylfuran derivatives 1 ± 11.
the corresponding benzyl chloride through a Wittig reaction. When N 2 -degassed p-methylstyrylfuran 1 (1 Â 10 À2 m in CH 2 Cl 2 ) was irradiated (350 nm) in a Rayonet reactor for 4 h the only product isolated was the 5-(3-methylbuta-1,3dienyl)benzo[b]furan 12 (96 % yield). [6] The presence of a 3-substituted 1,3-butadienyl group could be clearly identified candidates for dynamic resolution, yet to our knowledge this is the first general synthesis of a class of atropisomeric compounds by a dynamic resolution using a recyclable resolving agent.