Die durch Lewis-Säuren katalysierte Cyclisierung von Geranylaceton

Geranylacetone ( 1a , 1b ) in the presence of a Lewis acid, undergoes a novel cyclization reaction to yield the two diastereomers 9-oxabicyclo[3·3·1]nonene 2 and 3 as main products. The formation of 2 and 3 is not stereospecific, i.e. both isomers 1a and 1b produce the same product mixture ( 2 / 3 ～ 2:1 ratio). This is in contrast to the known Brønsted-acid catalyzed cyclization of 1 , leading exclusively to the chromenes 4 and 5 in a stereospecific reaction. The reaction mechanisms and the structural assignments are discussed.