Hemi- and Monoterpenes

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. 

Terpene compounds have natural occurrence in plants found as major components of most of the plant essential oils. Based on their structural and functional properties, terpene compounds have been classified according to their basic structural unit isoprene containing five carbons, fii the formation of terpenes, prenyldi-phosphate serves as a precursor. The terpene compounds exist in the form of mono-, sesqui-, hemi-, di-, tri-, and tetraterpenes. The mmioterpenes containing two isoprene units are responsible to cmistruct the major portion of all the essential oils. These compounds work as carbure, alcohol, aldehyde, ketone, ester, ether, peroxyde, and phenols [58]. The sesquiterpene compounds contain three isoprene units, and the functional properties are very close to monoterpene compounds. 

Croteau and Ronald (1983) and Coscia (1984) reviewed chromatographic studies on terpinoids. Those authors considered terpinoids as ubiquitous, structurally diverse, lipophilic compounds. Terpinoids exist in various polymeric forms of the unsaturated branch-chain pentane, isoprenol. Terpenes are characterized as hemi-prenes (C5), monoterpenes (Cio), sesquiterpenes (C15). diterpenes (C20), and triterpenes (Cjo)- A representative hemiprene (hemiterpene) is mevalonic acid, a precursor in the biosynthesis of isoprenoids. Monoterpenes contain numerous... 

The functional isoprene units in biological systems are DMAPP and its isomer IPP. Depending on the number of isoprene subunits, one differentiates between hemi- (C5), mono- (C q), sesqui- (Cu), di- (C20), sester- (C25), tri- (Cjg), tetrater-penes (C g), and polyterpenes (Cg) (Figure 16.7). Condensation of DMAPP with IPP by geranyl-diphosphate synthase (EC 2.5.1.1) produces geranyl-pyrophosphate (GPP) as a monoterpene. Subsequent condensation of GPP with IPP by farnesyl-diphosphate synthase (EC 2.5.1.92) yields farnesyl-diphosphate... 

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