Rosacea: An Introduction

Compound Synthesis

The compounds of the invention can be prepared in accordance with well-known synthetic procedures, for example, using the general synthetic procedures outlined in U.S. Pat. No. 3,890,319 (issued Jun. 17, 1975) and U.S. Pat. No. 4,029,792 (issued Jun. 14, 1977) both of which patents are hereby incorporated herein by reference. Scheme 1 below illustrates one method to synthesize compounds of Formula I.

Compounds of the invention can be synthesized by reaction of the appropriate quinoxalines 15 with thiophosgene 20 to form corresponding isothiocyanates 25. The reaction with thiophosgene can be carried out in aqueous solution or in dilute aqueous hydrochloric acid at room temperature in a period of about 2 hours. Alternatively, the thiophosgene 20 dissolved in a water-immiscible solvent, such as chloroform, can be added to a basic aqueous solution (sodium carbonate) of quinoxalines 15 and stirred for about two hours. In the first alternative, isothiocyanates 25 precipitate from the reaction mixture. Precipitation can be completed by neutralization with excess aqueous base. Precipitated isothiocyanates 25 are recovered by filtration and dissolved in a suitable solvent, e.g., chloroform, to form a solution. The solution is dried (e.g., MgSO.sub.4), filtered, and concentrated to yield the isothiocyanates 25.

Isothiocyanates 25 are treated with an excess of the appropriately substituted ethylene diamine to form the corresponding 3-quinoxalin-6-yl-thioureas 35. Isothiocyanates 25 are reacted with an excess (e.g., 5 moles to 1 mole) of ethylene diamine 30 in a suitable solvent, e.g., diethyl ether, benzene, chloroform or dioxane. The reaction is carried out at room temperature for about 2 hours. 3-Quinoxalin-6-yl-thioureas 35 precipitate and are recovered by filtration and washing the filter cake with solvent.

Cyclization of 3-quinoxalin-6-yl-thioureas 35 to afford compounds of the invention 10 is effected by heating a suspension of thioureas 35 with mercuric or cupric oxide in a suitable organic solvent, e.g., ethanol. The mercuric or cupric oxide can be replaced by an organic soluble mercuric or cupric salt, e.g., mercuric or cupric acetate. The reaction mixture is filtered, to remove the mercuric or cupric sulfide by-product, and the filtrate is concentrated to give compounds 10 in crude form. Compounds 10 are recrystallized as the free base or converted to an acid-addition salt by conventional reaction with a suitable acid. In certain cases, cyclization can be effected by simply refluxing the thioureas 35 in a suitable organic solvent, e.g., methanol, in the absence of mercuric or cupric oxide.

Quinoxalines 15 are synthesized by well-known synthetic procedures, for example, the procedures disclosed in J. A. JOULE ET AL., HETEROCYCLIC CHEMISTRY 189-224 (3rd ed. 1995), hereby incorporated herein by reference.


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