Terpenoids, symmetrical

The result of the retrosynthetic analysis of rac-lO is 2-hydroxyphenazine (9) and the terpenoid unit rac-23, which may be linked by ether formation [29]. The rac-23 component can be dissected into the alkyl halide rac-24 and the (E)-vinyl halide 25. A Pd(0)-catalyzed sp -sp coupling reaction is meant to ensure both the reaction of rac-24 and 25 and the ( )-geometry of the C-6, C-7 double bond. Following Negishi, 25 is accessible via carboalumination from alkyne 27, which might be traced back to (E,E)-farnesyl acetone (28). The idea was to produce 9 in accordance with one of the methods reported in the literature, and to obtain rac-24 in a few steps from symmetrical 3-methyl-pentane-1,5-diol (26) by selective functionalization of either of the two hydroxyl groups. 

In the absence of carbonyl compounds, allylic bromides treated with Cp2TiCl underwent the expected dimerization process, which has been exploited for the synthesis of symmetrical terpenoids such as squalene and [1-onocerin [82]. In contrast, non-allylic olefinic iodoethers and a-bromo carbonyl compounds underwent 5-exo cyclization to the corresponding tetrahy-drofuran derivatives [83,84]. 

Symmetric and asymmetric aliphatic, alicyclic and aromatic ethers and ethers with the oxygen atom bound in the ring can be found in foods. Volatile dialkyl ethers are virtually absent from foods, but some are synthesised and used as flavourings, especially for cosmetic purposes. Odour- and taste-active ethers are mainly terpenoid ethers, derived from monoterpenes and sesquiterpenes, and aromatic ethers. 

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