Practical method for increasing optical purity of cis‐verbenol

Abstract

R/S mixture of monoterpene alcohol cis-verbenol can be separated in preparative scale by its conversion into phthalic mono-ester and subsequent crystallization of its diastereomeric salts with (R)-α-methylbenzylamine and (S)-α-methylbenzylamine. Finally, basic methanolysis of the resolved phthalic mono-esters results (S)-cis-verbenol and (R)-cis-verbenol in high enantiomeric and diastereomeric purity.

Resolution by diastereomeric salts: Access to both enantiomers of racemic acid using the same enantiomer of resolving base

“…The behavior of diastereomeric salts we observed here is not common according to our experience 1,11–15 and among a huge number of known resolution procedures. Usually, the treatment of racemate with a resolving reagent results in a pair of salts with different solubility (crystallinity).…”

Planar Chiral Rhodium Complex Based on the Tetrahydrofluorenyl Core for Enantioselective Catalysis

“…20 We initiated our investigation with the synthesis of benzofulvene compound 2 from (S)-verbenone in two steps using modified previously known procedures 19b,21 (Scheme 2). (S)-Verbenone can be presynthesized from enantiomeric pure (−)-α-pinene in a single step in 70% yield (caution: commercially available (S)verbenone usually has an enantiomeric purity of less than 65% ee and must be purified before use; 22 see the Supporting Information). At the first step, with a palladium-catalyzed coupling of (S)-verbenone with 1,2-dibromobenzene, a decrease in the excess of (S)-verbenone to 1.8 equiv and in the reaction time to 16 h allowed us to reach the yield of the target annulated α,β-unsaturated carbonyl compound 1 in 76% instead of 62% as in the original work of Miura et al 21 In the second step, subsequent hydrogenation and dehydration of 1 gave 2 smoothly in 89% yield.…”