Ring-Closing Metathesis Approach to Quinolizinium Salts

Significance

Ring-closing metathesis reactions of 1-butenyl-2-vinylpyridinium salts using Grubbs II and Hoveyda-Grubbs catalysts as a synthetic route to dihydroquinolizinium salts are reported. The products were efficiently converted into quinolizinium salts by heating over palladium on charcoal. The yields are reasonably good, and the substrate scope was fairly well studied. The last step (oxidation) was extensively studied and optimized. Interestingly, the use of microwave irradiation (200 ˚C, 100 W, 5 min) resulted in complete conversion but a 1:1 mixture of the desired oxidized product and the fully reduced product (tetrahydroquinolizinium salt).