Isao Ichinose

Cyclization of polyenes: XII. Direct brominative ring closure of polyenes

Abstract In connection with a program directed toward the biogenetic-type synthesis of bromine-containing terpenoids, we have developed a new reagent system which provides for selective brominative cyclization of polyenes. The mechanism of Br + -formation in nature is discussed on the basis of the new reagent system.

Cyclization of polyenes. Biogenetic-type synthesis of snyderols

α- and β-Synderols (1) and (2) have been synthesised by the reaction of nerolidol (3) with 2,4,4,6-tetrabromocyclohexa-2,5-dienone (4).

Selective bromination of polyenes by 2,4,4,6-tetrabromocyclohexa-2,5-dienone

2,4,4,6-Tetrabromocyclohexa-2,5-dienone (TBCO) liberates bromonium ion when treated with polyenes to form brominated products. The results of the reaction of TBCO with simple olefins are presented. The analogous bromo-ketones, 4-bromo-2,4,6-trichloro- and 2,4,6-tribromo-4-methyl-cyclohexa-2,5-dienone, (4) and (5) respectively, afford the same products (3a), (6), and (7) when treated with geranyl cyanide (1; R = CN). The evidence suggests that formation of the dibromide (3a) may be due to sequential reactions. TBCO in the presence of cetyltrimethylammonium bromide serves as an excellent reagent for selective bromination of polyenes under very mild conditions.

Brominative cyclisation of nerolidol and geranyl-linalool

(+)-Nerolidol was treated with 2,4,4,6-tetrabromocyclohexa-2,5-dienone to afford the brominative cyclisation products α- and β-snyderols and 3-bromocaparrapi oxide and its 8-epimer; (±)-geranyl-linalool gave the analogous tricyclic ethers.