Helvetica Chimica Acta · 1986 · 18 citations · 13 references
Abstract Hydrogenation of the ketone group in di‐ O ‐benzylderivative ( 8 ) of the known macrocyclic lactone zeralenone ( 7 ) using a novel chiral borane complex 3 . BH 3 , prepared in situ , proceeded at lower temperatures with moderate diastereoselectivity (∼40%, d. e. at −60°). Unsaturated diastereomers 9 and 10 were separated, and 9 converted into zeranol ( 11 ), a known anabolic agent. Restricted conformational mobility at lower temperatures is assumed for the intermediate 8 on the basis of the temperature‐dependent CD spectra of its acetyl congeners 18 and 19 . X‐Ray structure analysis of 7‐ O ‐acetylderivative ( 13 ) of 11 revealed the ( R )‐configuration at C(7). Two crystallographically independent H 2 O molecules are involved in the H‐bonds, one of them (O(21)) rises the helices of the molecules of 13 along b. Small positive torsional angle [C(16)‐]C(161)‐C(1) [=O] (+19.3°), transoid ( E ) conformation of the lactone group, and nearly achiral arrangement of the C(11)‐C(12) bond (torsional angle [C(11)‐]C(12)‐C(121)[C(161)] is −93°) are the main conformational features that differentiate the macrocylic RAL (resorcinic‐acid lactone) derivatives from the 6‐membered lactone derivative 20 , studied earlier by CD. Consequently, the rules developed for the CD effects within conjugation band (around 270 nm), and n→π* band (around 255 nm) of the latter compound, cannot be applied the macrocyclic lactones.
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