| Summary: | 博士 === 國立臺灣大學 === 化學研究所 === 87 === A novel photochemical rearrangement of this work is from a series of 2-(4’-alkylstyryl)furan or thiophene to 5-(3-alkylbutadienyl)benzo[b]-furan or thiophene with good yields. The starting materials were prepared from 2-furaldehyde and the corresponding benzyl chloride or bromide using Wittig reaction, the UV absorption is at 330 nm. The styrylfuran or styrylthiophene is known to undergo photochemical isomerization and in the presence of oxygen or iodine to afford cyclized products, but the skeletal rearrangement is occurred only when photolysis is carried out in the absence of oxygen or iodine.
This novel and efficient photochemical rearrangement is both mechanistically and synthetically interesting. The mechanism consists of a conrotatory photocyclization, a novel 1,9-hydrogen shift and a lateral ring opening. To study this process, we have prepared deuterated compounds with similar photorearrangement reaction. On the another hand, the semiempirical calculations (PM3) for the geometry of a series of intermediates have indicated that there are heat of formation and electrostatic charges.
For both alkyl substituent and weak electron donating substituent, the reaction can occur in dichloromethane at similar conditions. But for strong electron withdrawing substituents, the reaction can not occur for prolonged photolysis.
For alkoxy substituents, the reaction can occur to obtain enone products in the dichloromethane. But the reaction obtain alkoxy ethyl ether derivative in benzene, the intermediates are unstable. At prolonged photolysis or separated by silica gel to obtain enone derivative. For hydroxy substituent, the reaction occurs 1,9-hydrogen shift to undergo photorearrangement, then again undergo 1,3-hydrogen shift to obtain another compound as a major product. However, the enone product is minor product.
We used the above benzo[b]furan product to synthesize some natural products egonol and demethoxy egonol etc.
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