Synthesis of casimiroin and optimization of its quinone reductase 2 and aromatase inhibitory activities

Abstract

An efficient method has been developed to synthesize casimiroin (1), a component of the edible fruit of Casimiroa edulis, on a multigram scale in good overall yield. The route was versatile enough to provide an array of compound 1 analogues that were evaluated as QR2 and aromatase inhibitors. In addition, X-ray crystallography studies of QR2 in complex with compound 1 and one of its more potent analogues has provided insight into the mechanism of action of this new series of QR2 inhibitors. The initial biological investigations suggest that compound 1 and its analogues merit further investigation as potential chemopreventive or chemotherapeutic agents.

Figure 1

X-ray structure and electron density surrounding inhibitors in the active site of human QR2. In each stereoview, QR2 inhibitor complexes depict all FAD and enzyme carbons in grey and beige, oxygen atoms are red and all nitrogen atoms are blue. Hydrogen bonds between Asn161 and the inhibitors are represented as blue dashed lines. (A) QR2 in complex with compound 1. The 2Fo−Fc electron density surrounding the two orientations of compound 1 is contoured at 1s in black. Site 1, compound 1 binds in two orientations in both Site 1 (green and yellow) of chain A (top panel) and Site 2 (blue and magenta) of Chain B (lower panel). The two binding orientations are rotated by ~180° degrees from each other. Each pose was refined at 50% occupancy. The different orientations in the two separate sites are depicted in different colors for clarity. Ile128 and Met154 have been removed for clarity. (B) QR2 in complex with compound 1l binds in one orientation in both Site 1 (blue) of chain A (top panel) and Site 2 (yellow) of Chain B (lower panel). The 2Fo−Fc electron density surrounding compound 1l is contoured at 1s in black.

Figure 2

Stereoview of a superposition of compound 1 and compound 1l bound within the active sites of human QR2. The two orientations of compound 1 bound within the active sites of chain A (blue) and chain B (magenta) are shown with compound 1l (yellow).

Scheme 1

Retrosynthetic Analysis of Compound 1

Scheme 2^a

^aReagents and conditions: (i) (PhO)₂PON₃, Et₃N, PhCH₂OH, PhH, 3 h; (ii) NaH, MeI, rt, 2 h; (iii) Pd/C, H₂, EtOAc, 30 psi, 1 h; (iv) diethyl malonate, 100–110 °C, 3 h; (v) (a) NaOH, 60 °C, 2 h, (b) HCl; (vi) PPA, 95–100 °C 3 h; (vii) TMSCH₂N₂, MeOH-Et₂O, rt, 24 h

Scheme 3^a

^aReagents and conditions: (i) diketene, PhH, 95–100 °C, 3h, or CF₃CO₂H, rt; (ii) PPA, 95–100 °C, 3 h.

Scheme 4^a

^aReagents and conditions: (i) NaH, MeI, THF, 23 °C, 12 h

Scheme 5^a

^aReagents and conditions: (i) (PhO)₂PON₃, Et₃N, PhCH₂OH, PhH, 3 h; (ii) NaH. MeI, rt, 2 h; (iii) Pd/C, H₂, EtOAc, 30 psi, 1 h.

Scheme 6^a

^aReagents and conditions: (i) HBr, heat, 48 h (ii) K₂Cr₂O₇, H₂SO₄, acetone, 0 °C, 10 min.