4-Methyl-2-(2-methyl-1-propenyl)tetrahydropyran (cis- and trans- mixture)

Chemical properties

Rose Oxide occurs in small quantities, mainly as the levorotatory cis form, in essential oils (e.g., Bulgarian rose oil and geranium oil). Commercial synthetic products are either optically active or inactive mixtures of the cis- and trans-isomers. Their sensory properties depend on the starting material and the method of synthesis.They are colorless liquids with a strong odor reminiscent of geranium and rose oil.

Chemical properties

Tetrahydro-4-methyl-2-(2-methylpropen-1-yl)pyran has a powerful distinctive geranium top note.

Occurrence

Reported found in the oils of rose (Bulgarian) and geranium (Réunion); both the cis- and the trans-form have been reportedly isolated from geranium oil. Unidentified isomers reported found in black currant berries, passion fruit, some types of Thymus and white wine.

Definition

ChEBI: Rose oxide is a member of the class of oxanes that is tetrahydro-2H-pyran which is substituted at positions 2 and 4 by an isoprop-1-enyl group and a methyl group, respectively. Organic compound of the pyran class and the monoterpene class and a fragrance found in roses and rose oil. All four possible stereoisomers are known; the 2S,4R and 2S,4S diastereoisomers [also known as the (-)-cis- and (-)-trans-isomers, respectively] are the main constituents in several essential oils and are used as a food flavouring and in perfumes and cosmetics. It has a role as a plant metabolite. It is a monoterpenoid, a member of oxanes and an olefinic compound.

Preparation

Rose oxide is usually prepared from citronellol, which can be converted into a mixture of two allyl hydroperoxides by photosensitized oxidationwith singlet oxygen. Reduction of the hydroperoxideswith sodium sulfite yields the corresponding diols . Treatmentwith dilute sulfuric acid results in allylic rearrangement and spontaneous cyclization of one of the isomers; amixture of diastereoisomeric rose oxides is thus formed. The unreacted diol isomer is separated by distillation. (?)- Citronellol as the starting material yields an approximately 7 : 3 mixture of (?)-cisand (?)-trans-rose oxide. Higher proportions of the cis-isomer may be obtained by dehydration with a stronger acid.Other methods for the production of rose oxide starting from citronellol consist of halogenation–dehalogenation reactions of citronellol.
An alternative technical process does not use citronellol but starts from prenal [107-86-8] and 3-methyl-3-buten-1-ol [763-32-6]. Under acidic conditions, the reaction affords a mixture of isomers of dehydro rose oxide (C₁₀H₁₆O6; [1786- 08-9], [92356-05-3], [60857-05-8]). Subsequent hydrogenation using a ruthenium/alumina catalyst leads to rose oxide.The proportion of cis-isomer can be increased by isomerization under acidic conditions.

Taste threshold values

Taste characteristics at 20 ppm: green, vegetative and herbal with a citrus nuance.

Flammability and Explosibility

Non flammable