Prenylflavonoids from Humulus lupulus

Five flavonoids were isolated from the resin part of the female inflorescences of Humulus lupulus, together with four known hop flavonoids, i.e. xanthohumol, 2′,4′,6′,4-tetrahydroxy-3′-C-prenylchalcone, iso-xanthohumol and 6-prenylnaringenin. The new hop compounds were identified as 2′,4′,6′,4-tetrahydroxy-3′-C-geranylchalcone, 5′-prenylxanthohumol, 6″,6″-dimethylpyrano (2″,3″: 3′,4′)-2′,4-dihydroxy-6′-methoxychalcone, its hydrate and 8-prenylnaringenin; apart from 8-prenylnaringenin, these are new flavonoids. Their mass fragmentation patterns were studied by mass spectrometry using atmospheric pressure chemical ionization in combination with collision-activated decomposition. Loss of the isoprenoid substituent in the positive ion mode and retro Diels-Alder fission in both the positive and negative ion modes provided useful information on the substitution patterns of the A and B rings. Nine hop varieties were qualitatively and quantatively characterized by HPLC-mass spectrometry. The flavonoid profiles of the samples examined were uniform and proved to be of little value in hop variety identification. Xanthohumol was the principal flavonoid in all samples (80–90% of the total of flavonoids) and was accompanied by minor amounts of the other eight flavonoids in virtually all samples.