Selective Cleavage of the Methylenedioxy Substituent

In ethanol-free methylene chloride, bulbocapnine methyl ether (30) with boron tribromide affords a mixture of diphenol 33 and phenanthrene 35. 

The presence of a little ethanol in the solvent leads, upon the addition of boron tribromide, to some hydrobromic acid resulting in formation of the hydrobromide salt of the initial aporphine. No phenanthrene is ultimately obtained under these conditions. Bonding of boron tribromide to the basic nitrogen seems to be the initial step in the opening of ring B. This is supported by the observation that reaction of (-f-)-bulbocapnine methyl ether (30) with boron tribromide in the presence of a strong base such as l,8-bis(dimethylamino)-naphthalene furnishes mainly the phenanthrene derivative 35. On the other hand, the hydrobromide salt of 30 when treated with boron tribromide produces only the diphenol 33.  

Natural (H-)-bulbocapnine can easily be converted into its unnatural ( —)-enantiomer by oxidation to dehydrobulbocapnine, reduction with zinc in hydrochloric acid, and resolution with L-tartaric acid. Sequential treatment of ( —)-buIbocapnine first with boron trichloride and then with boron tribromide affords (-)-1,2,10,11 -tetrahydroxyaporphine.  

In the first authenticated dimerization of an aporphine, oxidation of (+) bulbocapnine methyl ether (30) with iodine provided the dimer 36. Reduction of 36 with zinc in dilute sulfuric acid led to racemic 30, whereas reduction with lithium aluminum hydride supplied a pair of diastereomeric dimers 37.  

Treatment of dehydroglaucine with mercuric acetate in aqueous solution, followed by reduction with sodium borohydride similarly gave a (C-7)-(C-7) glaucine dimer.  

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