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

Abstract

Two novel approaches toward prenylated quinones are described. The first (route A) involves the following three basic operations: (a) construction of a 4-hydroxybezylamine carrying a tertiary allyl alcohol (cinnamyl alcohol) side chain of appropriate length (C 5 , 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. Unfortunately, its stereochemistry cannot be properly controlled, as it leads to the formation of ca. 2.5:1 (E/Z)mixture of (Δ 2 )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 a 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 Δ 2 double bond, thereby making this approach the one of choice for the synthesis of isoprenyl benzoquinones