Lead tetraacetate with silyl enol ethers

In the presence of boron trifluoride etherate, 39 (R=H and Ar==C6H5) reacts smoothly with silyl enol ethers (the achiral anion source) at low temperatures ( — 80 to — 30 °C) to provide a chromatographically separable mixture of diastereomeric acids from which either 40 or 41 can be obtained in good yield. Oxidative decarboxylation of the chiral auxiliary with freshly crystallized lead tetraacetate occurs without racemization of the newly formed chiral center to provide either 42 or 43 with 98% ee. Noteworthy is the fact that if pure cis isomers are used, such as 39, they undergo facile isomerization to a 65 35 cis trans mixture at the low reaction... 

Silyl enol ethers can be dimerized to symmetrical 1,4-diketones by treatment with Ag20 in dimethyl sulfoxide or certain other polar aprotic solvents.465 The reaction has been performed with R2, R-1 = hydrogen or alkyl, though best yields are obtained when R2 = R1 = H. In certain cases, unsymmetrical 1,4-diketones have been prepared by using a mixture of two silyl enol ethers. Other reagents that have been used to achieve either symmetrical or cross-coupled products are iodosobenzene-BFy-EtiO.466 ceric ammonium nitrate,467 and lead tetraacetate.m If R1 = OR (in which case the substrate is a ketene silyl acetal), dimerization with TiCL, leads to a dialkyl succinate (32, R1 = OR).4 9... 

Lead tetraacetate was employed by Stoodley and coworkers for an oxidative isomerization in their synthesis of 4-demethoxydaunomycinone (47). The diene (48) reacted with the oxirane dienophile (49) via the least hindered endo transition state to give the cycloadduct (50) in 86% yield. Hydrolysis of the silyl enol ether followed by reduction of the oxirane and introduction of the acetylene moiety gave the compound (51), which was oxidatively isomerized with LTA in acetic acid to give the quinone (52). All that remained now to complete the synthesis was conversion of the acetylene to a methyl ketone and dealkylation of the ether, llie last two steps were accomplished in an over l yield of 38%, the low yield attributable to problems in formation of the hydroxy group from the ether (Scheme 11). Bulman-Page and Ley employed LTA for a similar transformation in their synthesis of demethoxydaunomycinone and related anthracyclinones. 

Reaction of enol silyl ethers with lead tetrabenzoate followed by treatment with triethylammonium fluoride leads to the a-benzoyloxyketones. In both cases, the sequence involves bisoxygenation of the double bond followed by hydrolysis. a-Acetoxylation is also possible, but the benzoate is the reagent of choice for that system. i In the cyclohexanone series, reaction of lead tetraacetate with cyclohexanone trimethylsilyl enol ether afforded also a-acetoxycyclohexanone after hydrolysis." When the same reaction was performed on the triethyltin enol ether, the main product was now a-acetoxycyclohexanone with a small amount of dimeric product. This latter result could be explained by a transmetallation followed by ligand coupling, in the major pathway. 

---