Anacardic acid 8 -diene synthesis

Scheme 3 depicts, for anacardic acid diene, the synthesis of H, by the alkylation method with a Q4 halide and the synthesis of co-hydroxy C14 the side-chains for the 8,11-diene and its stereoisomers namely (A) 7Z,10Z, (B) 7Z,10E, (C) 7E,10E and (D) 7E,10Z.. The 1-iodo derivatives of each were obtained as for route (a). This methodology has been applied to the synthesis of Fig (2)-3, anacardic acid 8(Z),ll(Z),14-triene as shown in Scheme 4. Strategies for obtaining side-chains for the stereoisomers of the triene have been described [2,114].

The initial application to the synthesis of anacardic acid-8Z,l lZ-diene, used the synthons, ArCi — ArC7 - ArCio — ArCis [114,236]. 

Scheme 3. Synthesis of anacardic acid 8(Z),1 l(Z)-diene and C14 reactant stereoisomers...

The first synthesis modelled on biomimetic lines was directed to obtaining anacardic acids by way of polyketides [237] and later to a (17 l)-orsellinic acid [43]. A less complicated approach based on the Michael addition of ethyl acetoacetate and ethyl octadec-2-enoate, has led to a C15 orsellinic acid, Fig (4)-56, 2,4-dihydroxy-6n-pentadecylbenzoic, considered to be the biogenetic precursor of the cashew phenols [238], notably cardol, by decarboxylation. The use of bromine at the aromatisation stage in this synthesis precluded the extension of the method to components with unsaturated side-chains, although bromination with copper(ii)bromide and thermal debromination offers an alternative procedure. In a more recent approach, by the use of an oxazole intermediate and its addition to ethyl acetoacetate, (15 0) and (15 1) anacardic acid have been obtained [239] as shown in Scheme 5a, b. The 8(Z),1 l(Z)-diene and 8(Z),1 l(Z),14-triene have also been synthesised [240] by way of ethyl 6-(7-formylheptyl)-2-methoxybenzoate (C), prepared from acyclic sources, rather than, as in previous work, by semisynthesis from the ozonisation of urushiol. 

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