Allyl acetoacetates, Carroll rearrangement

However, if only two carbon atoms are present (15) they may be disconnected to give an allylic alcohol 16 and the acetoacetic ester, through a retro-Carroll rearrangement [6] (Scheme 7.6). 

The method of preparation of 5-dodecen-2-one presented here is a version of the literature procedure published earlier. It offers several advantages over existing methodology (1) The ester enolate modification of the Carroll rearrangement provides the allylic acetoacetates via a mild, fast, and high yield synthesis. This procedure represents a significant Improvement over... 

The Carroll rearrangement, a variation on the ester Claisen rearrangement, is a useful method for the preparation of y, 8-unsaturated ketones from allyl acetoacetates, and has been adapted to provide a method for the synthesis of a number of specific arylacetones. Thus, treatment of the p-quinol 1 with diketene and a catalytic amount of DMAP at room temperature gave a 72% yield of the arylacetone 2 together with 5% of the benzofuran 3. 

Among the most favorable features of the Carroll variant of the Claisen rearrangement are the relative ease of preparation of the parent system by condensation of allylic alcohols with acetoacetic estet or di-ketene and the defined configuration of the intermediately generated double bond. The Carroll rearrange-... 

An improved version of the Carroll reaction, the ester enolate Carroll rearrangement, was reported in 1984 by Wilson and Ptice. Dianions of allylic acetoacetates, generated by treatment with 2 equiv. of LDA at -78 °C in THF, were rearranged at room temperature or 65 C to yield >keto acids in 40-80% yield (equation 12). In the course of a synthesis of the sesquiterpene isocomene, Snider and Beal used this method for the rearrangement of acetoacetate (73), prepared in 83% yield from reaction of cyclopen-tene (72) with diketene and a catalytic amount of DMAP (Scheme 11). The ( )-isomer of ketone (74) is obtained stereospecifically, since there is a severe steric interaction between the methyl groups in the Carroll rearrangement transition state leading to the (Z)-isomer. 

Carroll reaction. Preparation of y,A-unsaturated ketones by base-catalyzed reaction of allylic alcohols with (1-ketoesters or thermal rearrangement of allyl acetoacetates. 

CARROLL Rearrangement of Allyl Acetoacetic Esters Thermal condensation of allyl alcohols with ethyl acetoacetate in the presence of a catalyst, with loss of C02 a one pot ester exchange-Claisen-lreland rearrangement with loss of C02 (see 1st edition). 

The rate of the Carroll rearrangement has been increased by formation of a dianion on treatment of allylic acetoacetates with two equivalents of lithium diisopropylamide in tetrahy-drofuran at — 78 CC followed by heating to reflux l0°. With one equivalent of base no reaction occurs. 

A further improvement is the ester enolate Carroll rearrangement of the dianion of allylic acetoacetates. generated by treatment with two equivalents of lithium diisopropylamide at — 78 C in tetrahydrofuran100. The dianions rearrange at 20 C to 65 C in 40-80% yield. For an example, see p 3320. 

This observation led to the proposal of an alternative mechanism similar to the Claisen mechanism involving aUyl enol ethers. They proposed that, in the case of Carroll s observations, the first step was a trans-esterification of the allylic alcohol, catalyzed by NaOAc, to the allyl acetoacetate, followed by a Claisen-type rearrangement to the ) -keto acid, which subsequently decarboxylated under the elevated reaction temperature (Scheme 8.4). 

The accessibility of nonracemic allyl acetoacetates makes the Carroll rearrangement an attractive strategy to assemble molecular complexity with adjacent tertiary and quaternary stereocenters. In the late 1950s, researchers at Hoffmann-La Roche utilised propargylic acetoacetate 194 to synthesise pseudoinone 195. Pyrolysis in the presence of Bronsted acid initiated the Carroll rearrangement to yield allenyl ketone 198 which tautomerised to pseudoinone 195. The minor product 196 resulted from Bronsted acid-mediated r-cation cyclisation of 199. 

An alternative route to y,5-unsaturated ketones is via the Carroll-Claisen rearrangement, which uses allylic esters of 3-keto acids (which exist mainly in the enol form) as substrates. These are readily prepared by condensation of allylic alcohols with acetoacetic esters or diketene. Following rearrangement, the intermediate keto acid undergoes in situ decarboxylation on heating. 

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