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Zinc chloride promotes the cyclisation of R-citronellal and hydrogenation gives L-menthol.
The red methyl group prefers to be equatorial in the transition state and directs the formation of the two new chiral centres. The transition state is like a trans-decalin with two fused six-membered chair rings. Both new substituents go equatorial in the product while the Lewis acid binds to the oxygen and accelerates the reaction.