Cyclization of linalool

Tetrabromocyclohexa-2,5-dienone in methylene chloride promotes the biomimetic cyclization of linalool and dehydrolinalool to (60 R=vinyl or ethynyl) as the major product with minor amounts of (61 X = Br, R = vinyl or ethynyl) in contrast to brominative cyclization with A/ -bromosuccinimide when (61 X = Br,... 

Arigoni and co-workers128 used isotopic labeling experiments to show that the cyclization of linalool to a-terpineol occurs at the center of the double bond rather than at its extremities. In principle, the intermediate cation 30 could adopt an elongated conformation 31 leading to 33 (which is not found) or the folded conformation 32, where attack of the carbocation at the center of the double bond gives the observed product 34. 

Cyclization of Linalool 97 to yield 2-methylcyclopenten-2-ol has also been achieved under solvent-free conditions (with GH2), yielding isobutene as a by-product (Scheme 2.40). Both metathesis products were then converted to useful fuel compounds. 

A remarkable feature of the cyclizations of linaloyl derivatives is the high stereoselectivity in the formation of a-terpineol. Thus, the hydrolysis of ( —) linaloyl phosphate (33-OP) affords (-h )-a-terpineol with an optical purity of 40% (79). The cyclization of linalool to optically active a-terpineol may also be carried out with sulfuric acid in acetic acid (81) and aqueous sulfuric acid (82). The a-terpinyl j9-nitrobenzoate produced in the hydrolysis of linaloyl /7-nitrobenzoate is obtained with an optical purity as high as 87% (80). These results require that cyclization occurs, at least in part, from an asymmetric transition state. 

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