SYNTHESIS OF PRENYLATED QUINONES BY THE OXIDATIVE-DEGRADATION APPROACH - BIRCH VS VINYLOGOUS BIRCH HYDROGENOLYSIS (BIHY VS VIBIHY) IN CONTROLLING (2)DELTA-STEREOCHEMISTRY OF THE PRENYL CHAIN

Two novel approaches toward prenylated quinones are described. The first (route A) involves the following three basic operations: (a) construction of a 4-hydroxybenzylamine carrying a tertiary allyl alcohol (cinnamyl alcohol) side chain of appropriate length (C5, C-10, etc), followed by (b) vinylogous Birch reductive cleavage with concomitant isomerization of the double bond (vinylogous Birch hydrogenolysis, VIBIHY) and (c) Fremy's salt oxidative degradation of the resulting prenylated phenolic benzylamines to the corresponding quinones. The required phenolic cinnamyl alcohols were successfully synthesized by means of two alternative routes, namely: (1) cyclopalladation of phenolic benzylamines followed by ketovinylation and treatment of the resulting beta-aryl substituted alpha,beta unsaturated ketone with methyllithium and (2) reaction of the dilithio derivatives of the appropriate phenolic benzylamine with the desired alpha,beta unsaturated aldehyde, followed by acid rearrangement. The key feature of this approach, namely, the so-called vinylogous Birch hydrogenolysis (VIBIHY) takes place very efficiently on the phenolic tertiary cinnamyl alcohol stage, provided that the reaction (Li/NH3) is carried out on its bissilylated derivative. Unfortunately, its stereochemistry cannot be properly controlled, as it leads to the formation of ca 2.5:1 (E/Z) mixture of (DELTA2) alkenes. The second generation approach (route B), which solves this problem, requires the following: (a) preparation of a 4-hydroxybenzylamine carrying a 3-methyl-2-buten-1-ol (dimethylallyl alcohol) side chain, or higher homologue, followed by (b) Birch hydrogenolysis (BIHY) and step c above. The key Birch hydrogenolysis takes place with almost complete control of the stereochemically labile DELTA2 double bond, thereby making this approach the one of choice for the synthesis of isoprenyl benzoquinones.