Humulyl cation

Cyclization modes are even more diverse for the sesquiterpenes than for the discussed hemi- and monoterpenes (Scheme 87.18). Transoid sesquiterpene synthases catalyze the ionization of FPP to the farnesyl cation that can cyclize by attack of the ClO-Cll double bond. A 1,10-ring closure affords the tertiary ( , )-germacradienyl cation in a Markovnikov fashion. Alternatively, the less stable secondary ( , )-humulyl cation can be furnished via an anh-Markovnikov 1,11-ring closure that is, in contrast to the 1,7-cycUzation of the linalyl cation, a well-known reaction. Cisoid synthases encourage the reattack of the diphosphate at C3 of the farnesyl cation to give (/ )- or (S)-nerolidyl diphosphate (NPP). As in LPP this allows for rotation of the newly formed vinyl group into a cisoid conformation. 

The 11-carbon ring of the humulyl carbocation may be retained, as in the formation of humulene (Figure 5.36), or modified to give the caryophyllyl cation containing a nine-membered ring fused to a four-membered ring, as in P-caryophyllene (Figure 5.36). Humulene is found... 

The remaining possible cyclization modes of FPP give rise to the ds-germacryl (10-membered), and the humulyl and ds-humulyl (11-membered), cations as shown in Figure 3.29. a-Humulene 90 is found in the oil of hops Humulus lupulus) and occms as a minor component of many essential oils. fl-Caryophyllene (91) is a humulyl derivative present in clove and cinnamon oil. The ds-germacryl cation is the precmsor of... 

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