The In-Situ Removal of Menthofuran from Peppermint Oil and Subsequent Reactions of the Solid-Supported Adduct
5th Advisor: Dr. Todd J. Barkman
Menthofuran is an aromatic heterocyclic compound found naturally in the essential oil of Mentha piperita L. or peppermint. Natural peppermint oil may contain between approximately 2 to 8% menthofuran.
Peppermint oil is a complex mixture of over 350 identified chemical compounds; market demands dictate the extent of processing the oil undergoes prior to being introduced into consumer goods. Menthofuran has long been known to readily autoxidize, either alone or within the peppermint oil matrix, leading to a discolored oil with negative organoleptic consequences that adversely affects its commercial viability. Therefore, a facile method for selective removal of menthofuran was developed around 1950. This method has gone largely unchanged for almost 60 years, and is labor-, resource-, and time-intensive.
The purpose of this study was to determine the feasibility of an alternate method of removing menthofuran from peppermint oil which would utilize a new development in the original Diels-Alder (D-A) methodology: a tethered- or anchoreddienophile. The work encompassed in this dissertation demonstrates that an Ntethered maleimide is an excellent alternative to the maleic anhydride methodology developed, and in commercial use, since 1950. A homologous series of maleimidebased dienophiles was employed to demonstrate the efficacy of the iV-tethered dienophile tactic.
A further objective of this study was to investigate the concomitant formation of highly-desirable, economically-valuable derivatives of menthofuran occurring during the Retro-Diels-Alder (RDA) phase of this work. A 23 full-factorial experimental design was employed whereby a model Diels-Alder adduct of menthofuran and yV-methylmaleimide was broken with a strong base at variable concentrations and temperatures. Through the use of this methodology, three factors (base [NaOH, Na2CC>3], concentration [2% aqueous, 10% aqueous], and temperature [room temperature, 100°C]) are studied at two different levels or conditions in the course of eight experiments. It was shown that Na2CC>3, aq. 2% at 100°C resulted in the highest concentration of (-)-mintlactone product at >30% of the total yield. The alternative base, NaOH, did not give a lactone product in these trials. In addition, a thermal RDA process was demonstrated as plausible, resulting in quantitative release of the menthofuran with parallel regeneration of the TV-tethered dienophile.