Regioselective cyclization

Because of the extremely low solubility of the cyclized products, only the 8-acetyl-9-X oxo-6,9-dihydrothiazolo[5,4-g]quinoline was isolated. On the other hand, 6-substituted derivatives having the same possibility to produce linearly or angularly annelated thiazoloquinolines cyclize regioselectively. 

Special steric or electronic influences may thwart the expected reaction pathway, as in the regio- and stereo-selective conversion of (86) to the pyrrolizidine (87 Scheme 42). This surprising formation of a five- instead of a six-membered ring is interpreted in terms of an aza-Cope equilibrium between the expected iminium salt (88) and its isomer (89 The latter finally cyclizes regioselectively in an exo-type mode to give (87). Further support for this assumption was obtained from quenching experiments with related compounds. ... 

With the replacement of one ester grouping by a ketone, the ketone enolate acts as the nucleophile. Thus (66) cyclizes regioselectively to (67) and (68) cyclizes to (69 Scheme 29). This reaction is described more fully in Section 3.6.4. 

Regioselective cyclization of (118) via the ketone enolate occurs to give (119). Zinc is necessary for a reasonable yield, and longer heating rearranges (119) to (120). Similarly, but under standard conditions, the 4-ketopyran (121) cyclizes regioselectively to (122) and the anthraquinone (123) cyclizes to (124 Scheme 41). ... 

Carbohydrates, which have recently been used extensively as chiral components in syntheses, have been transformed into diesters that, on cyclization, lead to precursors of polycyclic natural products. The tetrahydrofuran diester (135) was cyclized with potassium r-butoxide to an isomeric mixture (136) that was decarboxylated to (137). The related diester (138) was cyclized to a mixture (8 1) of (139) and (140), whereas the lactone (141) cyclized regioselectively to (142) and this was in turn methylated with iodomethane to the trans-syn-cis isomer (143 Scheme 43). ... 

Allyl ethers and allylamines undergo dimerization, whereas 1,6-dienes are cyclized regioselectively according to the titanium complexes used. ... 

Majumdar KC, Debnath P, Alam S, Maji PK (2007) Thiol-mediated radical cyclization regioselective formation of indole-annulated sulfur heterocycles by tandem cyclization. Tetrahedron Lett 48 7031-7033... 

An intramolecular reaction was carried out using this procedure (Eq. 67). The unsaturated aldehyde cyclizes regioselectively, contrasting with the nonselectivity of conventional methods (p. 333). The monocyclic reduction product was formed in 30% yield, however. 

---