Ester dienolates, rearrangement

J. Rodriguez et al. have investigated the stereoselective ester dienolate Carroll rearrangement of ( )- and (Z)-allylic (3-keto esters and found a new, attractive approach to the synthesis of the Prelog-DJerassi lactone and related... 

Ouvrard, N., Rodriguez, J., Santelli, M. Stereoselective ester dienolate Carroll Rearrangement a New Approach to the Prelog-Djerassi lactone framework. Tetrahedron Lett. 1993, 34, 1149-1150. 

Hiersemann, M., Abraham, L., Pollex, A. The ester dienolate [2,3]-Wittig rearrangement - development, opportunities, and limitations. SynleU 2003,1088-1095. 

Hiersemann has reported the synthesis of 2-hydroxy-y-lactones 77 by the [2,3]-Wittig rearrangement of ester dienolate substrates 75 and 76. The products of the rearrangements were required as substrates for 3-oxy-Cope rearrangements, ultimately leading to the stereoselective synthesis of nonnatural amino acids. ... 

Several examples of the ester dienolate [2,3]-Wittig rearrangement have been investigated in order to gain some insight into the factors that determine the... 

Stereochemical outcome of this rearrangement. Enolization of the regioisomeric esters (127) and (128) has been shown to afford a-allyloxy-substituted ester dienolates (129) which undergo a [2,3]-sigmatropic rearrangement to yield 3-alkoxycarbonyl-3-hydroxy-substituted hexa-1,5-dienes (130). These have been transformed to diastereomerically pure 2,3-dialkenyl-substituted (/-lactones (131). 

Aliphatic 3-aza-4-oxa-Cope rearrangement of ester-amide dienolates increases the synthetic utility of anionic [3,3]-sigmatropic rearrangement initiated by N—O bond cleavage. Treatment of the enehydroxylamine 94 with KHMDS in the presence of TMSCl at —80°C provided a mixture of iV,0-disilylated 95 and 0,0-disilylated 96 derivatives (equation 30). Both of these would, on [3,3]-sigmatropic rearrangement, provide the corresponding silyl ethers 97 and 98, from which 99 and 100 are obtained on workup. 

The plant bufadienolide scillarenin (500) has been synthesized. The starting material was 15a-hydroxycortexone (501), which was converted into the diketone ketal (502) by cupric acetate oxidation at C(21), followed by selective ketalization and tosylate elimination. Protection at C(3) as the dienol ether, oxiran formation at C(20) with dimethylsulphonium methylide, and regeneration of the C(3)- and C(21)-oxo-groups by acid hydrolysis then provided (503). Selective reaction at C(21) with the sodium salt of diethyl methoxycarbonyl-methylphosphonate, and boron trifluoride rearrangement of the epoxide ring to the aldehydo-unsaturated ester (504), was followed by enol lactonization to the bufadienolide (505). This was converted, in turn, to scillarenin (500) via the 14,15-bromohydrin, by standard reactions. Unsubstituted bufadienolides have also been prepared by the same method. 

The [2,3]-Wittig rearrangement of dienolates generated from a-allyloxy-substituted a, - and/or /S.y-unsaturated esters has been found to proceed with high diastereose-lectivity to give hexa-1,5-dienes in high yield. The greater reactivity of dienolate than enolate has been explained in quantum chemical terms. 

Substituted, terpenoid 3,7-dienoic acid esters, and presumably other 3,7-dienoates, can be obtained by consecutive ortho-Claisen and Cope rearrangements of 2,5-dienols (Scheme 47). In the example shown, the starting dienols were obtained from mesityl oxide. 

The rearrangement of metal carbenes can also provide halodienes in a very elegant manner. For instance, Fischer chromium chloro carbenes, generated from CrCl2 and a trihalomethyl group borne by the substrate, were shown to evolve under microwave irradiation to provide stereoselectively (Z, )-l-halo-l,3-dienol esters in... 

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