Trans-diaryl epoxides: asymmetric synthesis, ring-opening, and absolute configuration

Area of Science:

• Organic Chemistry
• Stereochemistry
• Chiral Synthesis

Background:

• Asymmetric synthesis of epoxides is crucial for producing enantiomerically pure compounds.
• Establishing absolute configurations of chiral molecules requires reliable methodologies.

Purpose of the Study:

• To synthesize anthryl-phenyl, phenanthryl-phenyl, and naphthyl-phenyl trans-epoxides with high enantiomeric purity.
• To determine the absolute configurations of these epoxides using the CD exciton chirality method.
• To validate previous findings on asymmetric synthesis and clarify steric interactions in chiral assignment.

Main Methods:

• Synthesis of enantiopure sulfonium salts derived from Eliel's oxathiane.
• Asymmetric synthesis of trans-epoxides.
• Epoxide ring opening with lithium aluminum hydride (LiAlH(4)) to form alcohols.
• Determination of absolute configurations using the Circular Dichroism (CD) exciton chirality method with a Zn-porphyrin tweezer.

Main Results:

• Synthesis of trans-epoxides (1, 2, and 3) with enantiomeric purities of 95%, 99%, and 96%, respectively.
• Determination of (1R,2R) absolute configurations for the synthesized epoxides.
• Confirmation of the R-configuration at C2 for (-)-1, (+)-2, and (-)-3, consistent with prior studies.
• Assignment of (1S,2R)-configuration for the cis isomer when present.
• Clarification of relative steric sizes of phenyl and CH(2)-aryl groups (phenyl=medium, anthracenyl CH(2) and phenanthryl CH(2)=large) through agreement between molecular modeling and CD spectra.

Conclusions:

• The study successfully synthesized and determined the absolute configurations of novel chiral aryl epoxides.
• The CD exciton chirality method, employing a Zn-porphyrin tweezer, proved effective for assigning absolute configurations.
• The findings reinforce the utility of asymmetric synthesis strategies and provide insights into steric factors critical for chiral recognition.