Fig. 4: Structure elucidation of 1,6-addition products with N-alkylpyridinium reagents.

a Model reaction between 1 and 7 to form two different regioisomers: 9 and 9’ in acetonitrile (ACN):phosphate buffer (PB) (2:3) solution. b HMBC spectrum of the obtained thiol-addition product 9. c Gibbs free energies of activation for 1,6-addition of thiols using methanethiolate at C5 and C6 of compound 1 and for 1,4-addition reaction with maleimide (most favored is in bold).