Naphthopyranquinone Antibiotics: Novel enantioselective syntheses of frenolicin B and some of its stereoisomers

Two new enantioselective syntheses of the naphthopyranquinone antibiotic frenolicin B (1), of its enantiomer 2, and of its diastereoisomers 3 and 4 were accomplished using two different routes from optically active β‐Hydroxy esters (R)‐ and (S)‐11 and 18. β‐Hydroxy esters (R)‐ and (S)‐11 were prepared stereoselectively from optically active sulfenylacetates (S)‐ and (R)‐10, respectively (Scheme 2, Method A). Alternatively, compound 18 was obtained in excellent yield by enantioselective hydrogenation of the corresponding β‐keto ester 17, using a chiral ruthenium‐complex catalyst (Scheme 3, Method B). Subsequently, compounds (S)‐11 and 18 were transformed into frenolicin B (1). In analogy, Stereoisomers 2–4 were prepared from (S)‐ and (R)‐11 in good yields.