title: Asymmetric synthesis of dihydrofurans via a formal retro-Claisen photorearrangement.
abstract: Solution-phase irradiation of a series of syn-7-benzoylnorbornene derivatives is shown to lead to cis-fused dihydrofuran derivatives in low quantum but excellent chemical yields in what is formally a retro-Claisen rearrangement. In analogy to the well-known Paterno-Buchi reaction, the first step of the rearrangement is suggested to involve (n,pi)(3)-mediated addition of the carbonyl oxygen to the norbornene double bond, producing a triplet 1,4-biradical. This intermediate, rather than closing to the oxetane, undergoes cleavage accompanied by intersystem crossing to form the dihydrofuran. To determine whether the retro-Claisen photorearrangement could be carried out enantioselectively, the 7-benzoylnorbornene reactant was equipped with a para-carboxylic acid substituent to which a series of optically pure amines was attached ionically via salt bridges. Irradiation of these salts in the crystalline state followed by diazomethane workup (the solid-state ionic chiral auxiliary method) was shown to afford the corresponding dihydrofuran in optical yields as high as 93% at 95% conversion. X-ray crystallography revealed that the enantioselectivity arises from crystallization of the reactant in a conformation in which the carbonyl oxygen is more favorably oriented for bond formation to one end of the norbornene double bond than the other, thus leading to a predominance of a single enantiomer.
authors.affiliation: Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, B.C., Canada V6T 1Z1.
authors.forename: Wujiong
authors.initials: W
authors.lastname: Xia
authors.forename: Chao
authors.initials: C
authors.lastname: Yang
authors.forename: Brian O
authors.initials: BO
authors.lastname: Patrick
authors.forename: John R
authors.initials: JR
authors.lastname: Scheffer
authors.forename: Carl
authors.initials: C
authors.lastname: Scott
mesh_terms: D018360:Crystallography, X-Ray; D005663:Furans / Q000138:chemical synthesis*; D009636:Norbornanes / Q000737:chemistry* / Q000528:radiation effects; D010777:Photochemistry; D013237:Stereoisomerism
chemical_list: D005663:Furans; D009636:Norbornanes