Sesquiterpenoids 2£,6£ -Farnesol

Siccanin (50) also contains a sesquiterpenoid portion. The immediate precursors of siccanin were shown to be siccanochromene-A (48) and -B (49). When a cell-free system of Helminthosporium siccans was used, monocyclo-farnesol (51) was labelled by [2- C]mevalonate. 

This compound must arise from bromonium ion cyclization of the 6,7-bromohydrin of farnesol acetate. Hydrolysis of the acetate group in (47) and sequential treatment with phosphorus tribromide and water gave obtusenol (46). Other syntheses of marine sesquiterpenoids include those of pallescensin-E (48),30 furoventalene (49),81 and dactyloxene-B (50) and -C (51).32 The last mentioned established the absolute configurations of the two dactyloxenes. A second synthesis of ancistrofuran (53) and its C-2 epimer has been recorded starting from the lactone (52) which is derived from homogeranic acid (Scheme 3).33... 

Some sesquiterpenoids function as essential oils in plants, while others act as fungal antibiotics.The acyclic compound farnesol is widely distributed in nature, being found in many plants and in the chlorophyll of some bacteria (see Section 2.4.4). Mono- and dicyclic sesquiterpenoids are common in plants. Some examples are given in Fig. 2.17b. 

The Carroll, or an equivalent, reaction between linalool and an ace-toacetic ester gives geranylacetone. Further elaboration as shown in Figure 4.32 produces sesquiterpenoids such as nerolidol, farnesol and bisabolol, which have some perfumery use although their odours are... 

Recent biosynthetic investigations have revealed that cantharidin (635), a well-known vesicant compound isolated from dried beetles (Lytta vesicatoria L.), can be regarded as a degraded sesquiterpenoid. Thus the labelling pattern in cantharidin (635) after incorporation of [2- C]mevalonic acid (35) and (E,E)-[ir,12- H2]farnesol (16) [but not the E,Z)- or (Z,Z)-isomers ] is consistent with the biosynthetic sequence outlined in Scheme 67. ... 

According to the biogenetic isoprene rule, sesquiterpenes arise from a linear precursor such as farnesol. There is considerable biosynthetic evidence to support this, and farnesyl pyrophosphate is considered as the immediate biosynthetic precursor of almost all sesquiterpenoids (110, 263). Conceptually, cyclization of farnesyl pyrophosphate can proceed from either the tail end (by ionization of the pyrophosphate moiety) or the head (by a or OH" attack on A ), Both... 

Sesquiterpenoids, similar to monoterpenes, very often occur as constituents of volatile oils, for example, zingiberene in ginger oil, (-)-tra i-/i-caryophyllene in clove leaf oil, and farnesol in rose oil. 

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