Synthesis of sulfide-and disulfide-type bisaporphines from thebaine

The rearrangement reaction of thebaine ( 1 ) with methanesulfonic acid, in the presence of hydrogen sulfide, resulted in a sulfide-type bisaporphine 13 , instead of the expected thiol 12 . The acidic hydrolysis of the thiocyanato derivatives 3, 9, 20 and 21 , obtained from thebaine ( 1 ), as well as the reduction of 9 and 21 with sodium borohydride yielded disulfide-type bisaporphines 16 and 23 .


Introduction
The naturally occurring opium alkaloid thebaine (1) is a suitable starting material in the synthesis of several 2-substituted dopaminergic aporphines 1 .The key step for the conversion is the rearrangement of thebaine (1) with methanesulfonic acid that results in 2,10-dimethoxy-11hydroxyaporphine (5), via the formation of a methoxonium ion intermediate (4).In the presence of water, the product of the rearrangement is morphothebaine 2 (6).With the aim of studying the structure-activity relationships of aporphines, three methods were considered for the formation of compounds containing appropriate functional groups.
According to the first possibility 3 , the product 5 of the acid-catalyzed rearrangement of thebaine (1) is converted to 2,10,11-trihydroxyaporphine (7), or to, for example, 2fluoroapomorphine (8) by protection of the hydroxy groups in the 10-and 11-positions.
The third route, based on our experience 6,7 using the acid-catalyzed rearrangement of thebaine (1) in the presence of alcohols and thiols, leads directly to the desired 2-alkoxy-and 2alkylthioaporphines (for example 10), which is the result of the nucleophilic substitution of the methoxonium ion intermediate 4. The excess of thiol results in the cleavage of the 10-methoxy group, where thiols play an important role as methyl acceptors.Thus, from thebaine (1) in the presence of ethanethiol, 2-ethylthioapomorphine (11) can be obtained in one pot, in high yield (Scheme 1).Our recent papers have focused on the synthesis of 2-mercaptoaporphines (12, 22).In the course of our work instead of the desired thiols 12, 22, sulfide-14, 15 and disulfide-type 16, 23 bisaporphines were isolated.

Results and Discussion
The acid-catalyzed rearrangement of thebaine (1) was carried out with methanesulfonic acid, in the presence of hydrogen sulfide, at 100 o C. Instead of the desired 2-mercaptoapocodeine (12), a sulfide-type bisaporphine 13 was isolated.
It is likely that, during the rearrangement, nucleophilic substitution of the 2-methoxy group occurs and the resulting 2-mercaptoapocodeine (12), a good nucleophile, reacts immediately with the methoxonium ion intermediate 4 to afford the sulfide 13.The structure of the product was confirmed by elemental analysis and LC-MS.According to the 1 H-NMR spectrum, the structure of 13 is symmetrical.The O-demethylation of this compound was carried out with methanesulfonic acid, in the presence of methionine, at 100 o C. The reaction proceeded in two steps, and an asymmetrical, partially O-demethylated derivative 14 could be isolated by quenching the process after 30 minutes.The reaction was completed after 2 hours, resulting in the symmetrical bisapomorphine 15 (Scheme 2).

13
H H

Scheme 2
The hydrolysis or reduction of 2-thiocyanatoapocodeine (9), previously synthesized from thebaine in a multi-step process, 5 offered further possibilities for the formation of the thiol function in the 2-position.Hydrolysis with methanesulfonic acid, at 90 o C, gave a low yield product which was shown to be a disulfide-type bisaporphine 16, instead of the expected thiol 12. Surprisingly, the reduction of the thiocyanato derivative 9 with sodium borohydride in ethanol afforded also the disulfide 16.The disulfide was isolated also in the rearrangement reaction of thebaine with methanesulfonic acid, in the presence of potassium thiocyanate.From the multi-component reaction mixture of the latter one-pot reaction, the disulfide could be isolated in low yield (Scheme 3).
The determination of the structure of disulfide 16 was difficult because, according to the elemental analysis, 1 H-NMR and MS spectra, the compound could have a thiol structure 12.To decide the thiol or disulfide question, the results of an acylation reaction were used.During the acylation of morphothebaine (6), which has a hydroxy-group in the 2-position instead of the mercapto-group of 12, the formation of various products was described in the literature depending on the reaction conditions.Refluxing with acetic anhydride, in the presence of sodium acetate formed the ring opened phenanthrene 8 19, while the product with acetic anhydride, in the presence of pyridine, at room temperature, was diacetylmorphothebaine 9 (18).
According to our model experiments, using the selective acylation method of Welsh 10 (20 o C , Ac 2 O, NaHCO 3, H 2 O) from morphothebaine, in addition to the diacetyl derivative 18, the unknown 2-acetoxyapocodeine (17) was isolated also, as the main product (Scheme 4).
From the Welsh acylation reaction of 16, unreacted starting material was isolated, which indicated the disulfide structure for our product since there was no formation of the 11-O-acetyl derivative, presumably due to the steric effect of the bisaporphine.
The 9→16 transformation requires a revision of our former publication. 5The product of the acid-catalyzed rearrangement of 7-thiocyanato-6-demethoxythebaine (20), followed by acidic hydrolysis, had been identified mistakenly as 3-mercaptoapocodeine (22).The product from the reduction of 3-thiocyanatoapocodeine (21) with sodium borohydride and the product of the acidic hydrolysis of 21 were found to be identical, and the disulfide structure of the compound 23 was identified similarly to the above-mentioned acylation experiment (Scheme 5).Experimental Section General Procedures.Melting points were determined with a Kofler hot-stage apparatus and are uncorrected.Column chromatography was performed on silica gel (Merck 60, 70-230 mesh).Thin layer chromatography was performed on precoated Merck 5554 silica gel 60 F 254 foils, using a 9:1 dichloromethane/methanol developing system.The spots were visualized with Dragendorff's reagent.Elemental analyses (C, H, N, S) were obtained on a Carlo Erba 1106 analyser. 1H NMR and 13 C NMR spectra were recorded on a Bruker WP 200 SY spectrometer.Chemical shifts are reported in ppm (δ) from internal CHCl 3 (7.26).The coupling constants (J) are measured in Hz.Mass spectra were measured with a Finnigan LCQ Classic ion trap mass spectrometer.