5-Norbornene-2-carboxylic acid, Butyl ester synthesis

Yield:37981-18-3 85%

Reaction Conditions:

with tert-butylammonium hexafluorophosphate(V);calcium(II) trifluoromethanesulfonate in dichloromethane at -20; for 4 h;Diels-Alder Cycloaddition;

Steps:

Diels-Alder Reactions with calcium-based Lewis acids system Ca(OTf)₂ and Bu₄NPF₆:

General procedure: 10 mol% of the catalyst system (Ca (OTf)₂/Bu₄NPF₆) were taken in a flask containing DCM (2.5 mL per mmol of dienophile) and equipped with a magnetic bead. The vessel was purged with N2 gas and was stirred for 30 min to 1 h until most of the solids dissolved. Following this, the dienophile (2 mmol; 1,4-naphthoquinone) was added and the stirring was continued for 30 more minutes. Upon thorough dissolution of the dienophile, the reaction vessel was cooled to -20 °C for 15 min. To this solution was added diene (4 mmol, 2 equivalents; cyclopentadine) dropwise over 30 min via addition funnel at -20 °C. The mixture was stirred for 4 h at -20 °C and allowed to warm to 0 °C. The reaction was quenched at 0 °C by addition of cold aqueous citric acid solution (10 mL). The aqueous layer was extracted with CH₂Cl₂ (3 x 20 mL). The combined organic layers were washed with cold 50% saturated aqueous NaHCO₃ solution (20 mL) and cold saturated aqueous brine solution (100 mL). The organic layer concentrated in vacuo. This crude product was purified on column chromatography using EtOAc/hexanes.

References:

Kalepu, Rishir;Mishra, Satyendra [Journal of Chemical Sciences,2020,vol. 132,# 1]