The PPA induced Fischer indolization of 4-acetonyl-2,6-piperidinediones 4 takes place both at the methylene and the methyl carbons, although the latter regioisomer (3) undergoes a further cyclization of the imide moiety upon the indole 3position followed by ring-opening of the resulting intermediate 9 to give tetmhydrocarbazolone 8. Fragmentation of the two possible regioisomers 3 and 7 to 2-methylindole occurs at higher temperatures. This process is more pronounced when using 4-acetonyl-3,4-dihydro-2( Iti)-pyridone 13 as the substrate for the indolization. The use of N-acetylphenylhydrazone derivatives leads to similar results as a consequence of the deacylation of the initially formed indole derivatives. In this case, an additional C-acylation of the indole ring also occurs. In a previous paper' we have reported that the Fischer indolization of azabicyclcdiones 1 (R=CHs, CH&H,OAc) affords mixtures of the two possible regioisomers in which the desired tetracyclic lactam 2, possessing four of the five rings of pentacyclic Strychnos indole alkaloids,2 was the minor compound. Our interest in lactam 2 (R=CHzCH20Ac) lies in the fact that it can allow the evaluation of the effect of the endocyclic amide carbonyl group on the cyclization upon the indole 3-position, which would afford a pentacyclic Srrychnos type system. 3 4-(2-Indolylmethyl)-2,6_piperidinediones of type 3 were envisaged as alternative potential precursors of 2. For this reason we decided to investigate the Fischer indolization of 4-acetonyl-2,6_piperidinediones 4.4 We present here the results of this study. SCHEME 1
Results
The required ketones 4a-c (Scheme 2) were obtained in high yields from the corresponding dithioacetals 5a-c' by treatment with HgCl#IgO in aqueous methanolic solution. It is well known5 that PPA is the catalyst 7939