An intramolecular, alkyne iminium ion cyclization of vinylogous carbamates derived from o-alkynyl anilines and N-protected homopropargyl amines is developed for the stereoselective construction of trans-2,3-disubstituted indolines and pyrrolidine derivatives, respectively. The regioselectivity of the alkyne iminium ion cyclization could be switched using a hydroxy group as an internal nucleophile resulting in cyclic ether-fused 1,2-dihydroquinolines. The entire process of nitrogen heterocycle formation can also be carried out in a 'one-pot' manner starting from o-iodo aniline derivatives.

An intramolecular, alkyne Prins type cyclization of vinylogous carbonates derived from o-alkynyl phenols is developed for the stereoselective construction of trans-2,3-disubstituted dihydrobenzofuran derivatives. Strong Lewis acids like TMSOTf catalyse this reaction efficiently. The presence of mildly electron donating groups on aryl rings increases the efficiency of the reaction.

A single alkynyl vinylogous carbonate was elaborated to tetrasubstituted furan or dihydrofuran via a cascade inter-intramolecular radical reaction by changing the radical being added. The strategy could be used in the synthesis of polycyclic heterocycles as well as bis-furan exhibiting atropisomerism. Installation of a new furan motif on the existing one was feasible by iteration. Stannyl dihydrofuran derivative was used in Stille coupling, whereas intramolecular Friedel–Crafts acylation on the furan gave furanonaphthol.

C-fused pyranoheterocycles can be readily assembled using an intramolecular oxa-Pictet-Spengler type reaction of vinylogous carbonates in a highly stereoselective manner. Required indole and benzofuran rings tethered to vinylogous carbonates are prepared by a tandem Sonogashira coupling-nucleopalladation reaction. The entire process can also be carried out in a 'one-pot' manner starting from homopropargyl alcohol. The C-fused pyranoindoles could be converted to spirooxindoles as well as to ring expanded products under oxidative conditions.

GHARPURE*, S. J.; PRASATH, V.; Org. Biomol. Chem. 12 (2014) 37, 7397-7409, http://dx.

Internal Hydroxy Group as a Nucleophile. -The trimethylsilyl triflate-mediated cyclization of alkynes (I) and (IV) leads to trans-2,3-disubstituted indolines (II) and pyrrolidines (V), resp., whilst the cyclization of alcohols (VI) and (VIII) gives rise to isochromene-and pyran-fused dihydroquinolines (VII) and (IX), resp., as a result of an intramolecular hydroalkoxylation. -(GHARPURE*, S. J.; PRASATH, V.; KUMAR, V.; Chem. Commun. (Cambridge) 51 (2015) 71, 13623-13626, http://dx.doi.org/10.1039/C5CC05292K ; Dep. Chem., Indian Inst. Technol. Bombay, Mumbai 400 076, India; Eng.) -R. Staver 01-037

Stereoselective Synthesis of 2,3-Disubstituted Dihydrobenzofuran Using Alkyne Prins Type Cyclization to Vinylogous Carbonates. -trans-2,3-Disubstituted dihydrobenzofuran derivatives (VI) (13 examples) are stereoselectively obtained in an intramolecular, alkyne Prins-type cyclization of vinylogous carbonates obtained from o-alkynylphenols. Tms-O-Tf catalyzes the reaction efficiently. -(GHARPURE*, S. J.; PRASATH, V.; J. Chem. Sci. (Bangalore, India) 123 (2011) 6, 943-949 ; Dep. Chem., Indian Inst. Technol., Chennai 600 036, India; Eng.) -H. Toeppel 22-096