Über die radikalische Desoxygenierung sekundärer Alkohole

On the Radical Deoxygenation of Secondary Alcohols The derivatives 1c – 1f were prepared from tetramethyl‐(+)‐catechin ( 1b ). Their radical reduction with tri‐ n ‐butyltin hydride or tris(trimethylsilyl)silane in the presence of azobis‐(isobutyronitrile) as initiator led to (2 R )‐(—)‐3′,4′,5,7‐tetramethoxyflavan ( 1g ). The best yield of 1g (about 75% from 1b ) was achieved via the N ‐(imidazolyl)thiocarbonyl derivative 1c . Analogously the enantiomeric (2 S )‐(+)‐3′,4′,5,7‐tetramethoxyflavan ( 2g ) was obtained from tetramethyl‐(+)‐epicatechin ( 2b ) via 2c . Experimental evidence is given which shows that in aqueous solution, with a catalytic amount of potassium hydrogen carbonate, (+)‐catechin ( 1a ) epimerises at the C‐2 position of the flavan skeleton to give (+)‐epicatechin ( 2a ).