Anti-acetylhumulinic acid

In the normal isomerization of humulone (70) the bond between C-1 and C-6 is broken and a new bond is formed between C-1 and the carbonyl group at C-5 (Fig. 14.6). The reversed isomerization of humulone (Fig. 14.8), in which the bond between C-5 and C-6 is broken and the new bond formed between C-5 and the carbonyl on C-1, has now been found to occur and the unti-products account for about 10% of the isomerization mixture [93]. By boiling humulone in a buffer solution at pH 11-0 the cis- and tmn.r-isomers of both acetylhumulinic acid (88) are formed. The bitter tasting hop acids known [94]. In the mixture of isomerization mixture are deacylated anti-derivatives deacyl-ated anti-isohumulone (90), deacylated anti-acetylhumulinic acid (91), and deacylated onti-humulinic acid (92) [93]. Deacylated humulone (89) is readily isomerized to these products whereas the deacylation of anti-isohumulone only occurs to a limited extent (< 2 %) so it has been proposed [95] that (90), (91) and (92) are formed via deacylated humulone (89). However, the relative ease of deacylation of humulone and Deacylated derivatives of isohumulone have not been characterized in isomerization mixtures. 

The anti-acetyihumuiinic acids are found in the same reaction mixture from which the anti-isohumulones are isolated. The two-phase system is ether aqueous buffer pH 4.1. Cis anti-acetylhumulinic acid is found in the band with K 0.76, trans anti-acetylhumulinic acid has a K value of 1.16. 

Since only very small quantities of deacylated antihumulinic acid or 2-(3-methyl-2-butenyl)-3,4-dihydroxy-2-cyclopentenone (162, Fig. 73) are present in the reaction mixture of humulone, this compound can be prepared more efficiently by boiling deacylated anti-acetylhumulinic acid in NaOH 50%. Racemization occurs along with this quantitative conversion. Compound 162 is isolated from the CCD band with K 0.32 in the two-phase system ether aqueous buffer pH 5.0 or directly by recrystallization. 

The formation of deacylated anti-acetyihumulinic acid from deacylated anti-isohumulone must proceed in an analogous way as for the formation of acetylhumulinic acids from isohumulones (see 8.3.2.). The intermediates, such as the deacylated anti-allo-isohumulones and the dehydrated derivative, have not yet been isolated. 

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