A Modified Batcho-Leimgruber Synthesis

The Batcho-Leimgruber indole synthesis is probably the most important and widely used method for the preparation of lH-2,3-unsubstituted indoles. In an attempt to extend the utility of the process to the preparation of 3-substituted indoles, functionalisation of the specific intermediate enamine 1 was explored. Treatment of 1 with ethyl chloroformate in refluxing chloroform in the presence of AT,JV-diethylaniline failed to give the expected ester. Instead, a product was obtained the spectral data for which suggested that it was a dienamine. The same product could be obtained in 52% yield simply by treatment of 1 with half an equivalent of TsOH in toluene at 90°C, and catalytic hydrogenation of this dienamine gave 2 in 58% yield. 

The highly functionalised pyran 1 can be prepared in a one-pot operation and in yields up to 30% by reaction of the readily available sulfone 2 with methyl coumalate 3, using methylene chloride as solvent and DBU as base. 

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