Table 1. Structure of the studied compounds.

Compound	C7–C8 bond	R1	R2	R3
9a	Single	OH	Isonicotinic	H
9b	Single	OH	Nicotinic	H
9c	Double	H	Isonicotinic	H
9d	Double	H	Nicotinic	H
9e	Single	H	Isonicotinic	H
9f	Single	H	Nicotinic	H
9g	Double	H	Isonicotinic	Me