The bifunctional organocatalyst promoted, relaycascade catalysis (use of a single catalyst capable of promoting each catalytic cycle in the designed cascade process) has been the hot area of research in recent times, as it enables the green synthesis of complex molecular architectures in an atom, cost, and step-economic fashion. In this context we summarized the recent developments in the area of the thiourea-tert-amine based cascade catalysis in the current review article. The discussion was divided into three subsections. double-, triple-, and quadruple cascades depending on the number of individual [a] Dr.
Acid-promoted
synthesis of cyclopenta[b]indole
frameworks from 3-indolylmethanols and alkynes has been reported.
The overall transformation represents a formal [3 + 2] annulation
via rearrangement. This protocol showed good generality for the carbinol
substrates as well as alkynes and allowed the generation of structurally
diverse cyclopenta[b]indoles. Terminal alkynes, dialkyl-substituted
internal alkynes, and alkynes with electron-deficient substituents
were found to be not suitable for this transformation. Similarly, N-Ts and N-Boc groups were compatible with
reaction conditions, whereas N-Ac and N-Tf failed to undergo this reaction. Isolation of vinyl chloride
intermediate suggested the involvement of a vinylic carbocation intermediate.
A mechanism has been proposed involving a ring-opening–ring-closing
cascade followed by a 1,3-indole migration process via a spirocyclobutene
intermediate.
We developed a mild strategy for the synthesis of 2‐acylfuran derivatives via cyclizative hydration of propargylic alcohols and alkynes, under Ag(I) catalysis. This is the first report employing the propargylic alcohols and their derivatives as alkyne partners in the cyclizative hydration process. This process found very broad scope for propargylic alcohols, alkynes and tethers. The highly chemo‐selective cyclizative hydration of 6‐acetoxy‐hex‐2‐en‐4‐ynal substrates suggests that, the acetoxy directed hydration of alkyne is not operable under Ag‐catalysis conditions and hence propargylic acetates can also be conveniently employed for the synthesis of 2‐(acetoxyacyl)furans.
FeCl3 catalyzed, highly regioselective cyclizative coupling of internal alkynes with alcohols has been reported for the rapid synthesis of structurally divergent, complex isocoumarins and phthalides respectively in intermolecular and intramolecular fashion. This strategy exhibited very high substrate scope and efficiency and proceeds through the simultaneous formation of C−O and C−C bonds. Observations from a series of control experiments supported a) the mechanism as Lewis acid catalyzed dual activation of ester and alcohol, b) the role of carbocation for the enhanced rates of cyclization, i. e., activation of alkyne by carbocation, and c) no role of HCl in the reported cascade process.magnified image
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