Methanolic solutions of trans-isohumulone (2), a major bitter flavouring component in beer, were irradiated with UV light of 313 nm wavelength and yielded four primary products containing an enolized cyclic β-triketone moiety: cis-isohumulone (3), humulone (1), dehydro-isohumulone (7), and dehydro-humulinic acid (5). The last of these products results from loss of the 4-methyl-3-pentenoyl side chain of trans-isohumulone. Nine volatile products derived from this side chain were identified and quantitated. The identifications of all photoproducts were confirmed by independent preparation of authentic samples. No evidence of either intramolecular or intermolecular 2+2 cyclo-addition was observed. This work clarifies previous contradictory reports of the products of isohumulone photolysis and provides an example of unexpected photochemistry of an alkenyl-substituted enolized cyclic β-triketone.Key words: trans-isohumulone, photo-degradation, cyclic β-triketone.
反式-异胡麻酮(2)是啤酒中的一种主要苦味香料成分,其甲醇溶液经波长为 313 纳米的紫外线照射后,产生了四种含有烯醇化环β-三酮分子的初级产品:顺式-异胡麻酮(3)、胡麻酮(1)、脱氢-异胡麻酮(7)和脱氢-胡麻酸(5)。最后一种产品是反式-异胡柚酮的 4-甲基-3-戊烯酰侧链脱落后产生的。从该侧链衍生出的九种挥发性产物已被鉴定和定量。所有光反应产物的鉴定结果都已通过独立制备真实样品得到证实。没有观察到分子内或分子间 2+2 环加成的证据。这项研究澄清了之前关于异胡麻酮光解产物的相互矛盾的报道,并提供了一个烯基取代烯醇化环β-三酮的意想不到的光化学实例。