Asymmetric Synthesis of Dihydrofurans via a Formal Retro-Claisen Photorearrangement

摘要

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.