Photochemische Umsetzung von optisch aktiven 2‐(1′‐Methylallyl)anilinen mit Methanol

Photochemical Reaction of Optically Active 2‐(1′‐Methylallyl)anilines with Methanol It is shown that (−)‐( S )‐2‐(1′‐methylallyl)aniline ((−)‐( S )‐ 4 ) on irradiation in methanol yields (−)‐(2 S , 3 R )‐2, 3‐dimethylindoline ((−)‐ trans ‐ 8 ), (−)‐(1′ R , 2′ R )‐2‐(2′‐methoxy‐1′‐methylpropyl)aniline ((−)‐ erythro ‐ 9 ) as well as racemic (1′ RS , 2′ SR )‐2‐(2′‐methoxy‐1′‐methylpropyl) aniline ((±)‐ threo ‐ 9 ) in 27.1, 36.4 and 15.7% yield, respectively (see Scheme 3 ). By deamination and chemical correlation with (+)‐(2 R , 3 R )‐3‐phenyl‐2‐butanol ((+)‐ erythro ‐ 13 ; see Scheme 4 ) it was found that (−)‐ erythro ‐ 9 has the same absolute configuration and optical purity as the starting material (−)‐( S )‐ 4 . Comparable results are obtained when (−)‐( S )‐ N ‐methyl‐2‐(1′‐methylallyl)aniline ((−)‐( S )‐ 7 ) is irradiated in methanol, i.e. the optically active indoline (+)‐ trans ‐ 10 and the methanol addition product (−)‐ erythro ‐ 11 along with its racemic threo ‐isomer are formed ( cf. Scheme 3 ). These findings demonstrate that the methanol addition products arise from stereospecific, methanol‐induced ring opening of intermediate, chiral trans , ‐(→(−)‐ erythro ‐compounds) and achiral cis ‐spiro [2.5]octa‐4,6‐dien‐8‐imines (→(±)‐ threo ‐compounds; see Schemes 1 and 2 ).