Enantioselektive Terpensynthesen durch Diels-Alder-Reaktion von 1-(1-arylalkoxy)-2-methyl-1,3-butadien mit Isopren
Enantioselective Terpene Syntheses by Diels-Alder Reaction of 1-(1-Arylalkoxy)-2-methyl-1,3-butadiene with Isoprene 
 
 
 
The (R)- and (S)-1-(1-arylalkoxy)-2-methyl-1,3-butadienes 6 undergo [4 + 2] cycloadditions with excess isoprene (7) under pressure to yield not only the expected (4S,6R)- and (4R,6S)-trans-carveol (8) with up to 96% e.e. but also up to 40% of (R)- and (S)-limonene (9) with up to 93% e.e. Asymmetric induction is proposed to arise from a transition state involving definite π stacking of one molecule of isoprenyl ether and two molecules of isoprene.