Asymmetric Synthesis of Dihydrofurans via a Formal Retro-Claisen Photorearrangement

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−Büchi reaction, the first step of the
rearrangement is suggested to involve (n,π*)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.