Synthetic experiments related to the indole alkaloids V. mercuric acetate oxidation of 2‐[2‐(3‐indolyl)ethyl]‐1,2,3,4‐tetrahydroisoquinolines and the formation of benz[a]indolo[3,2‐h]quinolizine derivatives

Abstract

Oxidation of 2-[2-(3-indolyl)ethyl] -1,2,3,4tetrahydroisoquinoline (I) with mercuric acetate gave 5,6,8,9,14,14b-hexahydrobenz[a]indolo[3,2-h]quinolizine (IV) and 8,9-dihydro-14H-benz[a]indolo[3,2-h]quinolizin-7-ium iodide (VI) , as well as starting material.The base (PI) was oxidized with iodine and potassium acetate to VI and on PalladiumHeating the iodide (VI) with Palladium-carbon brought about an irreversible rearrangement to VII and both these salts with base yielded the red anhydro base 8,g-dihydro- benz[a]indolo[3,2-h]quinolizine (VIII). This base was also obtained from N by oxidation in air.The corresponding 8,9-dehydroanhydro base (XI), benz[a]indolo[3,2-h]quinolizine, was readily obtained from X and alkali. The quinolizinium salts (VI), (VII), and (IX), on catalytic, zinc dust and acetic acid, o r sodium borohydride reduction, regenerated the base (IV). Selenium degradation of TV gave, among other products, l-(a-ethylphenyl)-P-carboline. An analogous series of products was obtained with the 6,7-dimethoxy derivative of.1. Various other aspects of these and related transformations a r e described. I 1In conjuction with efforts to extend the reductive cyclization ring closure to the formation of PP-disubstituted indolenines (strychnine types), we have also investigated the use of mercuric acetate in generating the iminium salt. This communication describes the results obtained when this oxidation procedure was applied to 2 -[2 -(3indolyl)ethyl]-1,2,3,4-tetrahydroisoquinolines (I). This procedure has been utilized recently (6a) to prepare indolo-[2,3 -a]quinolizine derivatives from the 1 -[2 -(3indolyl)ethyl] -1 , 2 , 3 , 4 -tetrahydropyridines and the method shows considerable potential for these types of syntheses, illustrated by the preparation of Flavopereirine (6a) and dl-Eburnamonine (6b). Another application of interest is its use in the formation of a Vincadifformine -type skeleton and CoronaridineThe oxidation of (I) could yield two different iminium salts: the 1,4dihydroisoquinolinium salt (111), and the 3,4 -dihydroisoquinolinium salt ( J I ) .These isomeric salts, upon cyclization, would yield ( 6~) . 395 396