SYNTHESIS AND ENATIODIFFERENTIATION OF ISOMERIC A-TETRAHYDRO-2,5,5,8A-TETRAMETHYL-2H-1-BENZOPYRANS (EDULAN-I AND EDULAN-II)

The benzopyran derivatives 1a-d were prepared in a biomimetic-type rea ction from their natural precursor 3-hydroxy-retro-alpha-ionol (6) whi ch was available from alpha-ionol by tert-butyl chromate oxidation, re duction with NaBH4, and subsequent rearrangement of the 7,8 double bon d. The so-obtained geometrical isomers 8-oxo-retro-alpha-ionol 4a/b we re separated by preparative and analytical multilayer coil countercurr ent chromatography. The racemic 3-oxo-retro-alpha-ionol 4a was esterif ied with (R)-(-)-2-phenylpropionic acid, and the resulting diastereome ric esters (5a/b) were isolated in pure form (de 90%) by preparative H PLC. Configuration at C-9 was determined by H-1 NMR spectroscopy. The isomeric diols 6a/b obtained from esters 5a/b by LiAlH4 reductive clea vage were subjected to thermal treatment (simultaneous distillation ex traction), yielding two pairs of diastereomeric edulans (1a/b and 1c/d ) which were subsequently obtained in optically pure form by analytica l HPLC. The absolute configuration at C-8a was established by NOE expe riments. Using on-line coupled multidimensional gas chromatography-mas s spectrometry DB-Wax/heptakis-(2,6-di-O-methyl-3-O-pentyl)-beta- cycl odextrin] with selected ion monitoring mode, enantiodifferentiation of 1a-d in a number of natural sources revealed predominance of the 2S e nantiomers.