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π-Aromatic and Sulfur Nucleophilic Partners in Cationic π-Cyclizations:  Intramolecular Amidoalkylation and Thioamidoalkylation Cyclization via ω-Carbinol Lactams1,2

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posted on 2001-05-26, 00:00 authored by Nicolas Hucher, Bernard Decroix, Adam Daïch
NaBH4 reduction of imides 1 and 6a,b,c followed by a π-cyclization of the resultant N-acyliminium ions, generated in trifluoroacetic acid conditions, afforded two positional isomers, isoindolobenzothiazolinones 4 and 8, respectively. These ring closures proceeded via an intramolecular α-amidoalkylation with the classical π-aromatic or the atypical sulfur atom as an internal nucleophile. A ready access to the related six-membered N,S-heterocyclic compounds such as isoindolobenzothiazinones 20a and 21a is also described. During this reaction, we have shown that ω-carbinol lactam precursor 14a led to endocyclic and exocyclic N-acyliminium ions 18a and 19a in equilibrium via the cyclic aza-sulfonium ion A. The latter furnished the expected products 20a and 21a in good yields. Similarly, different ω-carbinol lactams 14be substituted at C-angular position afforded the corresponding isoindolobenzothiazinones 20be and 21be bearing an angular alkyl, aralkyl, or aryl group. In the case of methyl 14b and benzyl 14e groups, an additional amount of the dehydration products 16b and 31 was isolated. These results indicate that the isomerization-π-cyclization takes place via the cleavage of the thioether linkage in acidic medium.