Anti-isohumulone

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. 

De Taeye L., De Keukeleire D., Verzde M. Isolation and identification of the anti-isohumulones and the anti-acetyUiumulinic acids. 

The tautomeric cyclohexane-1,3,5-trione system of humulone contains a crossed acyloin chromophore. Consequently, two alternative acyloin rearrangements with concurrent ring contraction could be possible. In addition to the "normal" series of isohumulones and derivatives (see Chapter 5), an "inverse" isomerization could, therefore, occur leading to the series of the anti-isohumulones and derivatives. 

The existence of the anti-isohumulones and the anti-humulinic acids was advanced at one time in order to rationalize the existence of two isomers of most of the five-membered ring hop derivatives. The possibility of the anti-structures was rejected as soon as it was shown unequivocally that the isolated isomers were in fact epimers and not positional isomers (1,2). The existence of the anti-series of the five-membered hop bitter acids is now, however, firmly established. 

The anti-isohumulones are formed upon boiling humulone in aqueous buffer solutions with pH values ranging from pH 5.4, as in brewery conditions, up to pH 11.0. The best yields, although still modest (see 9.4.), are obtained at pH 11.0 during 1.5 h. The residue, after extraction with iso-octane and removal of the solvent, is separated by CCD in the two-phase system ether aqueous buffer pH 5.5. After 1850 transfers two bands are found with distribution coefficients of 2.3 and 4, which contain cis anti-isohumulone (148, Fig. 69) and trans anti-isohumulone (149, Fig. 69), respectively. According to the lUPAC nomenclature these compounds are the 5-(3-methylbutanoyl)-2-(3-methyl-2-butenyl)-3,4-dihydroxy-4-(4-methyl-3-pentenoyl)-2-cyclopentenones. 

The pK values, determined in methanol water 1 1, are 3.3 and 3.15 for compounds 148 and 149, respectively. The UV absorption maxima are situated around 224-225 nm in acidic methanol and 255 nm in alkaline methanol. The molecular formula Is C21H3QO5. As usual, the chromophore is a 1,3-diketo group in a five-membered ring skeleton (4,5). The H NMR spectra show that the anti-isohumulones occur as a mixture of two enol tautomers, since the methine ring... 

Considering the close analogy with the isohumulones, the formation mechanism should be identical. Consequently, cis anti-isohumulone (148, Fig. 69) would have the (4S,5S)-configuration, while trans anti-isohumulone (149, Fig. 69) would have the (4R,5S)-configuration. 

The pH value of an aqueous solution (21) of potassium humulate (0.055 mol 20 g humulone, 3.09 g KOH) is adjusted to pH 11 with KOH 1 N. After boiling during 1.5 h, cooling and acidification (HCI), the aqueous layer Is extracted with iso-octane (4 x), the extract is dried and the solvent is removed. The residue is distributed in the two-phase system ether aqueous buffer pH 5.5. After 1850 transfers, the K values are 2.3 for cis anti-isohumulone and 4.0 for trans anti-isohumulone. 

The reaction conditions for the formation of the anti-isohumulones from humulone vary from slightly acidic (pH 5.4) to weakly basic (pH 11.0). In aqueous buffer pH 11.0 the yield is 1.5%, or at least 50 times lower than the yield of the isohumulones. The acyloin rearrangement is regioselective in favour of the formation of the isohumulones. The main reason for this should be the fact that the carbonyl group is part of a strongly... 

As studied extensively for the isohumulones, the side chains would be in the trans configuration in the transition state. In the case of the anti-isohumulones, these are the 3-methylbutanoyl group and the 3-methyl-2-butenyl group at C-6 in humulone. Non-stereoselective acyloin ring contraction affords the epimeric anti-isohumulones with identical absolute configuration at C-5 and opposite absolute configuration at C-4 (Fig. 70). 

The isolation and characterization of the anti-isohumulones have proved that the acyloin rearrangement of humulone occurs along dual pathways. The low yield is partly due to the unfavourable regioselectivity, partly to the smooth degradation to the anti-humulinic acids (see 10.1.). On the other hand, the anti-isohumulones are better soluble in water, as evidenced by the need for ether as upper phase in the counter-current distribution. Therefore, the utilization yield in the brewing process will be higher. 

The most important property of the anti-isohumulones is the very high bitterness level, which is twice that of the isohumulones. It appears that the anti-isohumulones are the most bitter hop derivatives known today. There are reasonable indications for the occurrence of the anti-isohumulones in beer, altough the presence has not been proved unambiguously. The concentration must be at least a factor of 10 lower than that of the iso-alpha acids. 

In view of the exceptional bitterness it could be interesting to increase the ratio of the anti-isohumulones relative to that of the isohumulones in the brewery. A ratio of 2 3 can be reached by boiling humulone at pH 5.0 during several hours in water dioxane 1 1 containing catalytic amounts of magnesium salts (8). It is remarkable that in these conditions the least stable trans anti-isohumulone is formed preferentially. Nothing is known yet about other characteristics. This particular aspect of hop chemistry has not been further explored, although it could be worthwhile to do so. 

The weakly basic conditions needed for the formation of the anti-isohumulones may concurrently give rise to hydrolysis. This degradation is known in detail for the isohumuiones. The respective intermediates along the pathway to the formation of humulinic acids are the allo-isohumulones, the hydrated allo-isohumulones and the acetyihumulinic acids (see Chapter 8). 

A totally analogous reaction sequence must take place for the anti-isohumulones. This has been proved by the isolation of cis anti-acetyihumulinic acid (150, Fig. 71) and trans anti-acetyihumulinic acid (151, Fig. 71) or the 5-(3-methylbutanoyl)-2-(3-methyl-2-butenyl)-4-ethanoyl-3,4-dihydroxy-2-cyclopentenones from the CCD bands with distribution coefficients of 0.75 and 1.16, respectively, in the two-phase system ether aqueous buffer pH 4.1 (1,2). The optically active compounds are weak acids with pK values of 3.8 for 150 and 3.7 for 151, respectively. The molecular... 

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. 

The anti-acetyihumulinic acids must be formed from the anti-isohumulones in the same way that the acetyihumulinic acids arise from the isohumulones, i.e. via isomerization and dehydration of the double bond in the 4-methyl-3-pentenoyl side... 

Fig. 73. Structural formulae of deacylated anti-isohumulone (155), compounds 156-160, deacylated anti-acetyihumulinic acid (161) and deacylated anti-humulinic acid (162).

The formation of deacyiated anti-isohumulone can be represented, either via deacylation of anti-isohumuione or via the anti-rearrangement of deacylated humulone. From treatment of cis anti-isohumulone under various pH conditions, it appears that deacylation occurs oniy to a very minor extent (< 2%). The main changes occur in the 4-methyl-3-pentenoyi group, while the 3-methylbutanoyl side chain shows poor reactivity (6). On the other hand, it has been proved that deacylated six-membered ring hop derivatives rearrange very smoothiy to compounds with anti-structure. Thus, deacylated tetrahydrohumulone (156, Fig. 73), obtained syntheticaily, is converted to racemic deacylated tetrahydro-anti-isohumulone (157,... 

Fig. 73) (7). Similarly, racemic deacylated anti-isohumulone is obtained starting from synthetic deacylated humulone (158, Fig. 73) (8). Furthermore, the lower homologue 160 (Fig. 73) of deacylated anti-isohumulone has been isolated via the model compound 159 (Fig. 73) for humulone (9).

Fig. 74. Reaction scheme for the formation of the isohumulones (65,66), the anti-isohumulones (148,149) and deacylated anti-isohumulone (155) via acyloin rearrangements of humulone.

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. 

The total yield of the deacylated derivatives of the anti-series is 8.5%. When the anti-isohumulones and the anti-acetyihumulinic acids are included, the yield even exceeds 10%. This is certainly not negligible in comparison with the most important hop bitter acids, the isohumulones. 

In addition to the anti-isohumulones, the deacylated anti-isohumulone and the corresponding derivatives, two components containing anti-structures have been isolated. The primary anti-components are transformed in the reaction medium to several classes of compounds, to which the two representatives belong. It may be deduced that many more derivatives of the anti-series exist. 

Compound 168 is clearly a derivative of deacylated anti-isohumulone (155, Fig. 73). Upon degradation of deacylated anti-acetyihumulinic acid (161, Fig. 73) 2-methylpropanal is formed (see 8.3.2. and 10.2.2.2.). An aldol condensation between these compounds, followed by dehydration, leads to formation of 168. Similar alkylidene-containing products, derived from deacylated anti-isohumulone itself, are possibly present in the complex reaction mixture, but have not yet been detected. 

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