Synthesis and Silica Gel Promoted Rearrangement of 5-Methylthio-3-carbo-r-butoxy-pent-3-en-2-one

Since the discovery by Knoevenagel [1] that doubly activated carbonic acids (CH-acids) react with carbonyl compounds to give the corresponding conjugated olefins, this reaction has been widely used in organic synthesis as an important method for carbon —carbon bond formation [2, 3). The conden­ sation of 1,3-dicarbonyl compounds with monocar­ bonyl compounds usually leads to a,/3-unsaturated products [4], However, it has recently been de­ scribed that the reaction of alkoxyethanal with methyl or ethyl acetoacetate gives a mixture of a ,ß and ß, y-unsaturated carbonyl compounds [5] or re­ sults in the formation of a ß ,y -unsaturated carbonyl compound and 2,3,5-trisubstituted-4,5-dihydrofuran [6 ], In both cases ß, y-unsaturated carbonyl conden­ sation products were enolized to some extent (15-45% ) [5, 6 ], In the course of our studies on the synthesis of methylenomycin A and other cyclopentanoid anti­ biotics we selected 5-methylthio-3-carbo-f-butoxypent-3-en-2-one (3) as a potential Michael reaction acceptor which should allow us to construct the 1,4-di­ carbonyl skeleton according to the Umpolung con­ cept. In this paper we would like to report the syn­ thesis of 3 from commercially available r-butyl acetoacetate ( 1 ) and methylthioethanal (2 ) as well as its isomerization to the corresponding ß, y-isomer. Although the synthesis of 2 has been described in our recent paper [7] dealing with the synthesis of methyl-