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-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.
Xia
Wujiong
W
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, B.C., Canada V6T 1Z1.
Yang
Chao
C
Patrick
Brian O
BO
Scheffer
John R
JR
Scott
Carl
C
D018360:Crystallography, X-Ray; D005663:Furans / Q000138:chemical synthesis*; D009636:Norbornanes / Q000737:chemistry* / Q000528:radiation effects; D010777:Photochemistry; D013237:Stereoisomerism
D005663:Furans; D009636:Norbornanes