SINGLE-STEP REMOVAL OF THE ALLYL ETHER PROTECTING GROUP WITH (PH3P)4RHH AND TRIFLUOROACETIC-ACID

During a study directed toward the synthesis of the highly functionalized trichothecene anguidine and its congeners,2 we were confronted with the problem of removal of the allyl protecting groups in tetracycle la. A number of procedures for this transformation have been reported,3 but only the method of Corey and Suggs3c was deemed mild enough for our substrate. This procedure effects isomerization of the double bond of the allyl group to the vinyl ether with Wilkinson’s catalyst [(Ph3P)3RhCl] in refluxing ethanol in the presence of diazabicyclooctane (DABCO). The base prevents premature liberation of propionaldehyde, the byproduct of hydrolysis which undergoes decarbonylation in the presence of the catalyst and thereby generates a less active catalyst.3c Although isomerization of the double bonds of la proved successful, acidic hydrolysis of the vinyl ethers or ozonolysis followed by deformylation (K2CO3/MeOH) gave complex mixtures of products. © 1990, American Chemical Society. All rights reserved.