Furan retrosynthetic analysis

An illustrative example for the potency of catalyst C is Paquette s highly efficient total synthesis of the natural products teubrevin G 180 and teubrevin H 181, which feature a cyclooctane core fused and spiroannulated to smaller oxygen containing rings. In the retrosynthetic analysis, the viability of an RCM step for annulation of a cyclooct-enone ring to the furan played a central role. Despite the presence of a conformational constraint by the furan ring in... 

Our retrosynthetic analysis of the Paal-Knorr synthesis leads to a problem when applied to furan, as it implies addition of a water molecule, followed by elimination of two water molecules. In practice, simple dehydration of a 1,4-dicarbonyl compound leads to furans as in the preparation of 2.21. 

Several methods exist for the preparation of furan. In this book, we shall use the method of retrosynthetic analysis (COREY and WARREN [5]) as a guiding principle for the heterocyclic synthesis of the most important target systems. This procedure leads to logical starting materials and methods for constructing the required heterocycles which can then be compared with existing and pre-paratively important methods. 

When furan is considered in the light of a retrosynthetic analysis it can be seen to derive from a double enol ether and can, therefore, be dissected retroanalytically in two ways (I, II) according to the following scheme ... 

In the synthesis of the polyketide metabolite citreofuran (57), RCAM was incorporated into the retrosynthetic analysis by setting an alkyne-one as the precursor for the furan ring. The diyne precursor 55 was rapidly assembled from acid 51 by its reaction with chloroenamine 52 to yield the alkoxyisocoumarin 53 upon treatment with Et3N. The addition of homopropargylic Grignard 54 to unstable 53 resulted in... 

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