Abstract
In this work, we describe a user-friendly procedure for the preparation of all the isomeric forms of the terpene linalool oxide. The presented method is based on the transformation of the linalool enantiomers into the corresponding diastereoisomeric mixtures of the two furanoid oxides and two pyranoid oxides. Taking advantage of the different steric hindrance of the hydroxyl functional groups, the pyranoid forms were separated as a diastereoisomeric mixtures of their benzoate esters. Conversely, the cis- and trans-furanoid isomers were transformed in the corresponding acetates, which were directly separated by chromatography. The hydrolysis of the latter esters afforded cis- and trans-furanoid linalool oxides whereas the same reaction performed on the benzoates mixture afforded a separable mixture of cis- and trans-pyranoid linalool oxide. Overall, the method features, as a unique mandatory requirement, the availability of both linalool enantiomers, and can be conveniently performed from a milligram to a multigram scale.
Highlights
All eight ether stereoisomers with general structure 1 and 2 (Figure 1) are naturally occurring monoterpenes, collectively called linalool oxides [1]
The pyran-based linalool oxide is more obviously floral, with less resemblance to tea. All these monoterpenes are important flavours and are thought to give a relevant contribute to the scent of different foods and beverages of vegetal origin such as grape [5], papaya [2,6], wine [7], and tea [8] where they are often present as glycosides derivatives [9,10] or less frequently as acetate [11]
The removal of the acetate functional group afforded cis- and trans-furanoid linalool oxides whereas the hydrolysis of the benzoate esters afforded a mixture of cis- and trans-pyranoid linalool oxide, which were separated by chromatography
Summary
All eight ether stereoisomers with general structure 1 and 2 (Figure 1) are naturally occurring monoterpenes, collectively called linalool oxides [1]. A given linalool enantiomer was Klein treated all the forms of these monoterpenes Among the latter three approaches, et with peracetic acid to diastereoisomeric mixtureaofgiven the enantiopure furanoid and. Even if the above procedure appears very simple provides all the at the acid-catalyzed cyclization step provided furanoid andand pyranoid oxides in a isomers once,Even their chromatographic separation is troublesome. The removal of the acetate functional group afforded cis- and trans-furanoid linalool oxides whereas the hydrolysis of the benzoate esters afforded a mixture of cis- and trans-pyranoid linalool oxide, which were separated by chromatography. Since the cyclization step did not involve racemization [22], the linalool oxide isomers obtained through our procedure possess the same enantiomeric purity of the starting linalool, namely 97% ee for (R)-linalool and 50% ee for (S)-linalool
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