Synthesis of Racemic and - -Menthol

It is worth of noting that rose oil contains a mixture of enantiomers consisting of up to 80 % —)-menthol. Since natural sources of (-)-menthol are limited and the requirements of the fragrance and pharmaceutical industries are enormous, industrial methods for the production of —)-menthol have been developed over the years. 

Example 9.1 Complete the retrosynthesis and then propose the synthesis of ( )-menthol TM 9.1, without considering the stereostructure of this target molecule. 

we observe a trisubstituted cyclohexane ring, whose construction by the Diels-Alder reaction in the absence of an electron-accepting group is rather 

Note The reader is prompted to consider a retrosynthesis that includes the retro-Diels-Alder disconnection of the precursor of TM 9.1 to isoprene and nitro-alkene as a dienophile. The retrosynthesis presented in Sect. 2.5, Example 2.20, provides a hint. 

For the next generation aromatic target molecule TM 9.1a, preferably disconnected, is at an isopropyl unit, affording easily available meta-methylphenol (meta-cresol) and propylene as convenient Friedel-Crafts alkylating agents. 

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