Abstract(−)‐Menthol is one of the most popular aroma compounds worldwide. While in the past mostly extracted from mint plants, today (−)‐menthol synthesis from other raw materials is becoming more relevant. Common starting materials for menthol synthesis are m‐cresol, citral and myrcene, but also substrates like menthone, mono‐ and bicyclic terpenes and terpenoids have been used for this purpose in the past. As for many applications (−)‐menthol of high purity is required, asymmetric syntheses and enantiomeric resolution of obtained raw products are applied for menthol production. This review gives an overview on the most important synthetic menthol production processes of the companies Symrise, Takasago and BASF and relevant literature in the field of menthol synthesis with a focus on the last 20 years.
The aldol reaction of bio acetone in presence of a strongly basic ion exchange resin was carried out with and without the addition of water in a temperature range between − 30 °C and 45 °C. The conversion, selectivity and service time of the ion exchange resins were investigated in a stirred batch reactor and a continuous fixed bed reactor. For the batch experiments, both conversion and selectivity increased with decreasing temperature. Furthermore, the addition of water to the reaction medium has a positive effect on selectivity and catalyst service time of the resins. For the continuous flow experiments carried out in a fixed bed reactor, the selectivity towards diacetone alcohol is higher than in a batch reactor. This high selectivity is favored by a short contact time which inhibits as expected most of the consecutive reactions.
The singlet oxygen reaction with trisubstituted E‐allylic alcohols resulted in the unselective formation of regioisomeric secondary and tertiary allylic hydroperoxides. Increasing the steric demand at the allylic hydroxymethyl group leads to a moderate large‐group effect that can be strongly enhanced by deprotonation of a dimethylated allylic alcohol and even more pronounced by cation complexation. The presence of alkoxide groups did additionally alter the diastereoselectivity of the singlet oxygen ene reactions with chiral allylic alcohols. The molecular combination of Michael ester and enol ether in α‐alkoxy ester substrates made possible the design of chemoselective substrates for singlet oxygen ene versus [2+2] cycloaddition, e. g. a 3,3‐dimethylated Michael ester as a selective substrate for the ene reaction and an adamantyl analogue as a selective 2+2 substrate. The Z/E‐monomethylated substrates as probes for cis regioselectivity revealed that there is no site preference for Z‐alkoxy hydrogen transfer on the way to the corresponding ene product.
The Front Cover shows a cartoon of the hydrogen‐transfer process initiating the singlet oxygen ene reaction with several strong regio‐ and stereodirecting effects following the rule of thumb: If you give singlet oxygen your little finger it will soon have your whole hand (as adapted from a quote by Sigmund Freud). More information can be found in the Article by M. Kleczka et al on page 964 in Issue 11, 2018 (DOI: 10.1002/cptc.201800133).
The front cover artwork is provided by Axel Griesbeck and Diana But at the University of Cologne (Germany). The image shows a cartoon of the hydrogen‐transfer process initiating the singlet oxygen ene reaction with several strong regio‐ and stereodirecting effects following the rule of thumb: If you give singlet oxygen your little finger it will soon have your whole hand (as adapted from a quote by Sigmund Freud). Read the full text of the Article athttps://doi.org/10.1002/cptc.201800133.
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