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Asymmetric synthesis of dihydrofurans via a formal retro-Claisen photorearrangement

TitleAsymmetric synthesis of dihydrofurans via a formal retro-Claisen photorearrangement
Publication TypeJournal Article
Year of Publication2005
AuthorsXia, WJ, Yang, C, Patrick, BO, Scheffer, JR, Scott, C
JournalJournal of the American Chemical Society
Volume127
Pagination2725-2730
Date PublishedMar
Type of ArticleArticle
ISBN Number0002-7863
KeywordsFURANS, GENERAL-APPROACH, INDUCTION, PHENYL KETONES, PHOTOCHEMISTRY, PHOTOCYCLIZATION, REARRANGEMENT, SOLID-STATE
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.

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