2-Ethynyl-4,5,6,7-tetrahydroindoles

Tetrahydroindole reacts with ethyl bromopropynoate on all the mentioned active surfaces to deliver the corresponding ethynyl derivative (Scheme 2.55) [521]. The reaction mixtures obtained on some of these surfaces, namely, CaO, BaO, MgO, and ZnO contain several times more 2-ethynyl-4,5,6,7-tetrahydroindole than a mixture of products formed on AI2O3. The conversion of 4,5,6,7-tetrahydroindole on all the studied surfaces is close to 100%. 

The reaction is general in character. Apart from 2-ethynyl-4,5,6,7-tetrahydroindoles, 5-aryl-2-ethynylpyrroles and 3-ethynyIindoles also undergo [2+2]-cycloaddition. This is the first example of nonphotochemical [2+2]-cycloaddition of DDQ to the triple bond. The mechanism of the reaction involves one-electron transfer to generate ion-radical pair containing ethynylpyr-role radical cation (detected in the UV spectra of the reaction mixture at -30°C) and DDQ radical anion (identified by ESR). 

Tetrahydroindoles underwent a rapid, facile selective ethynylation with propynoate 146 on solid K2CO3 at position 2 to afford ethyl 3-(4,5,6,7-tetrahydroindol-2-yl)-2-propynoates 152 in good to excellent yields. The reaction did not take place in solvents (ether, CHCI3) both with and without K2CO3 (Equation (43)) (08TL3946). 

SCHEME 2.56 Selective ethynylation of 4,5,6,7-tetrahydroindole with ethyl bromopro-pynoate on the surface of KjCOj. 

Ethynylation of 4,5,6,7-tetrahydroindole also proceeds on surfaces of oxides (Si02, Ti02, Zr02) and salts (CaC03, ZrSi04), though the yields of the products are... 

Trofimov, B.A., L.N. Sobenina, Z.V. Stepanova et al. 2008. Chemo- and regioselective ethynylation of 4,5,6,7-tetrahydroindoles with ethyl 3-halo-2-propynoates. Tetrahedron Left 49 3946-3949. 

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