Heterocycles from alkylidenecyclopropanes.

The synthesis of isocoumarins over the last decade. a review

The reaction of o-hydroxy benzaldimines with electron-rich alkenes proceeds under mild conditions in the presence of an air-stable chiral catalyst and nicely complements Akiyama′s previously reported Broensted acid catalyzed reaction of aldimines with dihydrofurans and -pyrans leading to tetrahydroquinolines.

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Catalytic Asymmetric Mannich–Ketalization Reaction: Highly Enantioselective Synthesis of Aminobenzopyrans

A simple, efficient and environmentally benign protocol has been developed for the one-pot, multicomponent synthesis of medicinally important tetrahydrobenzo[b]pyrans and pyran-annulated heterocycles using a commercially available, inexpensive, non-toxic, and biodegradable tris-hydroxymethylaminomethane (THAM) as a novel organocatalyst. Ambient reaction conditions, wide scope, avoidance of conventional isolation as well as purification techniques and the reusability of the catalyst for five consecutive runs have improved the practical utility of this multicomponent reaction protocol manifold.

Tris-hydroxymethylaminomethane (THAM): a novel organocatalyst for a environmentally benign synthesis of medicinally important tetrahydrobenzo[b]pyrans and pyran-annulated heterocycles

Alkylidenecyclopropanes are a peculiar class of olefinic compounds characterized by a high ring strain, but at the same time a sufficient stability to permit their use in organic synthesis. The strain associated with the structure of these compounds is in general responsible for the good reactivity of the carbon-carbon double bond, even in sterically encumbered substrates. The key role of these compounds in organic synthesis resides in the exclusive, and by far the most useful, possibility of the introduction of a cyclopropyl ring in a more complex molecule through reactions onto the exocyclic double bond. For this reason the investigations on the synthesis and reactivity of these compounds has exploded in the last decade. This article will deal with all the reactions which involve the exocyclic double bond of alkylidenecyclopropanes in cycloaddition processes. Albeit most of the reactions are true pericyclic cycloaddition processes, also those reactions which lead to cycles by stepwise processes will be reported with comments on the mechanism.

J.D. Hepworth

Papers

2.24 – Pyrans and Fused Pyrans: (iii) Synthesis and Applications