Indoles 2-propiolic acids

Indole reacts with propiolic acid to form l,l-bis(3-indolyl)ethane via the 1 1 Michael adduct (74MI30503). 

Scheme 2.95 CuI/L-proline-catal) ed s aithesis of indoles from propiolic acids.

Indolenines exemplified by compound (336) can be converted into the related 3/f-indole 1-oxide (338) by a sequence involving reduction to the dihydro compound (337) followed by oxidation with /M-chloroperbenzoic acid. Compound (338) behaves as a typical cyclic nitrone and undergoes cycloaddition with DMAD and methyl propiolate. The initially formed cycloadduct (339) rearranges via an aziridine intermediate to an oxazolo[3,2-a]indole (340) (Scheme 107) <93JCS(Pl)939>. 

In contrast to the alkynylation of acidic C-H bonds which can also be achieved using alkynyliodonium salts, the direct C-H functionalization of aromatic compounds or olefins has never been realized with this class of reagents so far. However, after several unsuccessful attempts using palladium or copper catalysts and alkynyliodonium salts for the alkynylation of heterocycles, Waser and Brand reported in 2009 the first efficient alkynylation of indoles using TIPS-EBX 52 and AuCl as catalyst (Scheme 18) [117]. With indole, selective C3-aIkynylation was obtained. The reaction was tolerant to many functional groups such as bromides, acids, or alcohols. The method was already used in the synthesis of starting materials for Friedel-Crafts reactions of aminocyclopropanes [118] and for hydroamidation to access indole c -enamides [119]. In 2010, Nevado and de Haro demonstrated that alkynylation was also possible using directly terminal propiolic ester derivatives and (diacetoxyiodo)benzene as co-oxidant [120]. 

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