Chiral Halonium Ion X -Induced Polyene Cyclizations

In 2010, Denmark s group demonstrated racemization of the enantiopure (4R,5S)-5-bromo-4-octyl tosylate via bro-miranium ion-to-olefin transfer in acetolysis [41]. While no decrease in enantiospecificity was observed in the absence of the (El-d-octene, in the presence of the ( )-4-octene, the enantiopure bromiraninm ion generated from the (4R,5S)-5-bromo-4-octyl tosylate transferred to the ( )-4-octene, and enantiospecificity was lost at the same time (Table 9.1). Based on these reports, suppression of haliranium ion transfer from the in ifn-generated enantiopure haliranium ion to an umeacted substrate is crucial for the successful catalytic asymmetric halocyclization. 

Kato s group greatly advanced bromonium ion-induced polyene cyclization [44]. In 1986, they developed 2,4,4,6-tetrabromocyclohexa-2,5-diene (TBCO) as a brominating 

TABLE 9.1 Decrease in Enantiospecificity in Acetolysis from Bromiraninm lon-to-Olefln Transfer 

SCHEME 9.27 Bromocyclization of methyl geranyl geranate with TBCO. Br 

SCHEME 9.28 BDSB-promoted bromonium ion-induced polyene eyclization. 

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