Syntheses in enantiopure form of four diastereoisomeric naphthopyranquinones derived from aphid insect pigments

Abstract

The first syntheses are described of the four enantiopure naphthopyranquinones (1R,3R,4S)- and (1R,3R,4R)-3,4-dihydro-4,7,9-trihydroxy-1,3-dimethylnaphtho[2,3-c]pyranquinone (quinone A 1 and quinone A' 2) and their two C-3 epimers, the (1R,3S,4S)- and (1R,3S,4R)-diastereoisomers 5 and 6, using enantiopure lactate as the source of asymmetry. Key factors in these syntheses are the maintenance of stereochemical integrity throughout the sequences and intramolecular diastereoselective cyclisations of the titanium phenolates of phenolic lactaldehydes. For these cyclisations the differing degree of diastereoselectivity is explained as are the stereochemistries of the product 2-benzopyran-4,5-diols.

Role of the methoxy group in product formation via TiCl4 promoted 4-phenyldioxolane isomerizationsons

“…Our interest in naphthopyranquinones as potential antimicrobial and antibiotic agents has extended over two decades [1][2][3][4][5][6] as a result of their well documented importance. 7,8 Additionally, the synthesis of benzopyrans as model systems has received attention by ourselves 6,9-13 and others.…”