pi-aromatic and sulfur nucleophilic partners in cationic pi-cyclizations: Intramolecular amidoalkylation and thioamidoalkylation cyclization via omega-carbinol lactams

NaBH4 reduction of imides 1 and 6a,b,c followed by a pi -cyclization of the resultant N-acyliminium ions, generated in trifluoroacetic acid conditions , afforded two positional isomers, isoindolobenzothiazolinones 4 and 8, res pectively. These ring closures proceeded via an intramolecular alpha -amido alkylation with the classical pi -aromatic or the atypical sulfur atom as a n internal nucleophile. A ready access to the related six-membered N,S-hete rocyclic compounds such as isoindolobenzothiazinones 20a and 21a is also de scribed. During this reaction, we have shown that tu-carbinol lactam precur sor 14a led to endocyclic and exocyclic N-acyliminium ions 18a and 19a in e quilibrium via the cyclic aza-sulfonium ion A. The latter furnished the exp ected products 20a and 21a in good yields. Similarly, different omega -carb inol lactams 14b-e substituted at C-angular position afforded the correspon ding isoindolobenzothiazinones 20b-e and 21b-e bearing an angular alkyl, ar alkyl, or aryl group. In the case of methyl 14b and benzyl 14e groups, an a dditional amount of the dehydration products 16b and 31 was isolated. These results indicate that the isomerization-pi -cyclization takes place via th e cleavage of the thioether linkage in acidic medium.