Synthesis of the phytoalexin (±)-phaseollin: 3-phenylthiochromans as masked 2H-chromenes and o-prenyl phenols

Phenylthiyl radicals are shown to add regiospecifically to 2H-chromenes to afford 3-phenylthiochromans (8), (10), (13), (14), and (15). The sulphide (15), as equivalent to a chromene protected against acid and oxidation, has been used in two syntheses of (±)-phaseollin, a major phytoalexin of beans and other legumes, via the sequence (17)→(18)→(19)→(20)→(21)→(±)-(1) or (19)→(22)→(23)→-(±)-(1). Also the 3-phenylthiochromans, on electron transfer from metal naphthalenide or a mercury cathode, open to o-prenylphenols, providing a two step route to biogenetically important phenols from chromenes which is tolerant of free phenol and carbonyl functions and trisubstituted double bonds.