π-Aromatic and Sulfur Nucleophilic Partners in Cationic
π-Cyclizations: Intramolecular Amidoalkylation and
Thioamidoalkylation Cyclization via ω-Carbinol Lactams1,2
posted on 2001-05-26, 00:00authored byNicolas 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 14b−e substituted at C-angular
position afforded the corresponding isoindolobenzothiazinones 20b−e and 21b−e 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.