Chemical synthesis of tetracyclic terpenes and evaluation of antagonistic activity on endothelin-A receptors and voltage-gated calcium channels

Abstract

A class of tetracyclic terpenes was synthesized and evaluated for antagonistic activity of endothelin-1 (ET-1) induced vasoconstriction and inhibitory activity of voltage-activated Ca²⁺ channels. Three repeated Robinson annulation reactions were utilized to construct the tetracyclic molecules. A stereoselective reductive Robinson annulation was discovered for the formation of optically pure tricyclic terpenes. Stereoselective addition of cyanide to the hindered α-face of tetracyclic enone (−)-18 was found and subsequent transformation into the aldehyde function was affected by the formation of bicyclic hemiiminal (−)-4. Six selected synthetic tetracyclic terpenes show inhibitory activities in ET-1 induced vasoconstriction in the gerbil spiral modiolar artery with putative affinity constants ranging between 93 and 319 nM. Moreover, one compound, (−)-3, was evaluated further and found to inhibit voltage-activated Ca²⁺ currents but not to affect Na⁺ or K⁺ currents in dorsal root ganglion cells under similar concentrations. These observations imply a dual mechanism of action. In conclusion, tetracyclic terpenes represent a new class of hit molecules for the discovery of new drugs for the treatment of pulmonary hypertension and vascular related diseases.

Introduction

Endothelin-1 (ET-1), a potent 21-residue peptide produced mainly by vascular endothelial cells, binds to endothelin A receptors (ET_A) expressed in vascular smooth muscle cells and causes blood vessels to constrict.¹ Overproduction of this peptide is believed to be a key factor in pulmonary arterial hypertension² and pathological vascular spasms (vasospasms) that compromise the function of various organs³ and cause failure of vein grafts in coronary artery bypass surgery.⁴ Hence, ET-1 receptor antagonists may serve as hit compounds for the development of drugs that are useful for the treatment of various diseases including pulmonary arterial hypertension, vasospasms, renal failure, sudden hearing loss, and asthma⁵ as well as being useful in support for surgical procedures such as coronary bypass surgery. In search of compounds that possess ET-1 antagonistic activity, we investigated a number of tetracyclic terpenes, whose structures are derived from the A–B–C–D ring of myriceric acid A (Fig. 1), a pentacyclic triterpene isolated from the crude extract of southern bayberry, Myrica cerifera twigs.⁶ Myriceric acid A binds selectively to ET_A receptor, antagonizes specific binding of [¹²⁵I]endothelin-1 (ET-1) to rat cardiac membranes, and inhibits ET-1 induced increase in cytosolic free Ca²⁺ concentration (IC₅₀ = 11 ± 2 nM) and ET-1 induced contraction of rat aortic strips (K_i = 66 ± 15 nM).6, 7, 8, 9 A partial synthesis of (+)-myriceric acid A starting from a pentacyclic triterpene, oleanolic acid, involving 14 steps, has been reported.¹⁰ Its structure and absolute configuration was determined from a single-crystal X-ray analysis of its chemically modified 27-O-acetylmyricerone methyl ester.⁶ Despite these encouraging inhibitory results, only limited follow up studies were reported on myriceric acid A and its analogs.6, 7, 8, 9, 10 We envision that a reduction of the size and complexity of the structure would facilitate the synthesis of a library of molecules for the discovery of hit compounds. Hence, we report herein an asymmetric synthesis of optically pure tetracyclic terpenes (+)-1–(−)-6 (Fig. 1), evaluation of antagonistic activity on ET_A receptor mediated vasoconstriction of the spiral modiolar artery,11, 12 and evaluation of inhibitory activity of voltage-activated Ca²⁺ currents.