Carene, or δ-3-carene, is a bicyclic monoterpene which occurs naturally as a constituent of turpenti — Organic Chemistry Chemistry Question
(-)-Menthol from (+)-δ-3-carene
Carene, or δ-3-carene, is a bicyclic monoterpene which occurs naturally as a constituent of turpentine. (+)-δ-3-Carene has the required structural features to serve as a starting material for (-)-menthol. The procedure of preparation of (-)-menthol from (+)-δ-3-carene was described as follows. Catalytic isomerization of δ-3-carene provides the needed (+)-δ-2-carene (A) which then was pyrolysed to cleave the cyclopropane ring. The resultant 2,8-menthaldien (B) had the right stereochemistry at C1 and C4 of (–)-menthol. Treatment of the unconjugated diene B with 1 equivalent mole of HCl gives C and then, dehydrochlorination leads to a conjugated diene (D). [VISUAL] Treatment of D with hydrogen chloride affords 8-chloro-3-p-menthene (E) which then reacted with sodium acetate and acetic acid to give mixed (cis/trans) pulegol esters (F) via allylic displacements. Hydrolysis of F affords (-)-cis and (+)-trans-pulegol (G). Because the absolute configuration of C-1 is fixed in this system, reduction of either pulegol isomer provides menthol isomers which can be readily equilibrated to predominently (-)-menthol. [VISUAL]
Write down the structural formulae of the compounds A to G.
Model Answer
The structures of compounds A to G are shown in the synthetic steps below:
Catalytic isomerisation of δ-3-Carene provides (+)-δ-2-Carene (A) which then was pyrolysed to cleave the cyclopropane ring forming diene (B):
[VISUAL]
Treatment of the unconjugated diene (2,8-menthadiene, B) with HCl to give C and then, dehydrochlorination led to a conjugated diene ((+)-2,4(8)-p-menthadiene, D). Treatment of (+)-2,4(8)-p-menthadiene with hydrogen chloride affords 8-chloro-3-p-menthene (E):
[VISUAL]
E reacted with sodium acetate and acetic acid to give mixed (cis/trans) pulegol esters (F) via allylic displacements. Hydrolysis F affords (-)-cis and (+)-trans-pulegol (G). Reduction of either pulegol isomer provides menthol isomers which can be readily equilibrated to predominently (-)-menthol.
[VISUAL]