Inhibitory Effect of Chlorogenic Acid Analogues Comprising Pyridine and Pyrimidine on α-MSH-Stimulated Melanogenesis and Stability of Acyl Analogues in Methanol.

In continuation of studies for α-MSH stimulated melanogenesis inhibitors, we have evaluated the design, synthesis, and activity of a new series of chlorogenic acid (CGA) analogues comprising pyridine, pyrimidine, and diacyl derivatives. Among nineteen synthesized compounds, most of them (fifteen) exhibited better inhibitions of melanin formation in B16 melanoma cells. The results illustrated that a pyridine analogue 6f and a diacyl derivative 13a of CGA showed superior inhibition profiles (IC50: 2.5 ± 0.7 µM and 1.1 ± 0.1 µM, respectively) of α-MSH activities than positive controls, kojic acid and arbutin (IC50: 54 ± 1.5 µM and 380 ± 9.5 µM, respectively). The SAR studies showed that both -CF3 and -Cl groups exhibited better inhibition at the meta position on benzylamine than their ortho and para positions. In addition, the stability of diacyl analogues of CGA in methanol monitored by HPLC for 28 days indicated the steric bulkiness of acyl substituents as a key factor in their stability.

Ethyl 8''-chloro-1'-methyl-2,12''-dioxo-12''H-di-spiro-[indoline-3,2'-pyrrolidine- 3',6''-indolo[2,1-b]quinazoline]-4'-carboxyl-ate.

In the title compound, C29H23ClN4O4, the quinazoline-indole system and the indolin-2-one system are each essentially planar, with maximum deviations from their mean planes of 0.150 (2) and 0.072 (2) Å, respectively. The central pyrrolidine ring adopts a twisted conformation on the C-C bond involving the spiro C atoms. Its mean plane forms dihedral angles of 83.37 (9) and 86.56 (8)°, respectively, with the indole rings of the indolin-2-one and quinazoline-indole systems. In the crystal, mol-ecules are linked via pairs of N-H⋯O hydrogen bonds, forming inversion dimers. The dimers are linked via C-H⋯O hydrogen bonds, forming chains propagating along [001].

Ethyl 1''-benzyl-1'-methyl-2''-oxodi-spiro-[indeno-[1,2-b]quinoxaline-11,3'-pyrrolidine-2',3''-indoline]-4'-carboxyl-ate.

In the title compound, C36H30N4O3, the quinoxaline-indene system is roughly planar, with a maximum deviation from the mean plane of 0.218 Šfor the C atom shared with the central pyrrolidine ring. This latter ring forms dihedral angles of 84.54 (7) and 83.91 (8)° with the quinoxaline-indene system and the indole ring, respectively. The central pyrrolidine ring has an envelope conformation with the N atom as the flap, while the pyrrolidine and five-membered rings of the indole group adopt twisted conformation and envelope (with the C atom bearing the quinoxaline-indene system as the flap) conformations, respectively. In the crystal, mol-ecules are linked via weak C-H⋯N hydrogen bonds, forming a chain running along [100].

Ethyl 1''-benzyl-2''-oxo-2',3',5',6',7',7a'-hexa-hydro-1'H-di-spiro-[indeno-[1,2-b]quinoxaline-11,2'-pyrrolizine-3',3''-indoline]-1'-carboxyl-ate monohydrate.

In the title compound, C38H32N4O3·H2O, the quinoxaline-indene and pyrrolizine systems are essentially planar, with maximum deviations from their mean planes of 0.162 and 0.563 Å, respectively. The pyrrolizine ring forms dihedral angles of 88.53 (5) and 89.95 (8)° with the quinoxaline-indene system and the indoline ring, respectively. The central pyrrolidine ring has an envelope conformation with the C atom bearing the quinoxaline-indene system as the flap. The pyrrolidine ring of the indole system adopts an envelope conformation with the C atom bonded to the pyrrolizine ring N atom as the flap. The five-membered ring attached to the central pyrolidine ring adopts a twisted conformation. In the crystal, O-H⋯N and O-H⋯O hydrogen bonds between water mol-ecules and pyrrolizine N and carbonyl O atoms together with C-H⋯O inter-actions result in chains along [100].