Enol sulfonates rearrangements

Epoxysilanes, 218-219 o,0-Epoxy sulfones, 219-220 Ergosterol, 110-111 Erysodienone, 419 Eschenmosei fragmentation, 283 Ester cleavage, 208-209 Ester enolate Claisen rearrangement, 209-210... 

Benzoyl peroxide Ketosulfones from enol sulfonates Radical rearrangement... 

In the first step an S03 molecule is inserted into the ester binding and a mixed anhydride of the sulfuric acid (I) is formed. The anhydride is in a very fast equilibrium with its cyclic enol form (II), whose double bonding is attacked by a second molecule of sulfur trioxide in a fast electrophilic addition (III and IV). In the second slower step, the a-sulfonated anhydride is rearranged into the ester sulfonate and releases one molecule of S03, which in turn sulfonates a new molecule of the fatty acid ester. The real sulfonation agent of the acid ester is not the sulfur trioxide but the initially formed sulfonated anhydride. In their detailed analysis of the different steps and intermediates of the sulfonation reaction, Schmid et al. showed that the mechanism presented by Smith and Stirton [31] is the correct one. 

Bromination of the enol ether product with two equivalents of bromine followed by dehydrobromination afforded the Z-bromoenol ether (Eq. 79) which could be converted to the zinc reagent and cross-coupled with aryl halides [242]. Dehydrobromination in the presence of thiophenol followed by bromination/dehydrobromination affords an enol thioether [243]. Oxidation to the sulfone, followed by exposure to triethylamine in ether, resulted in dehydrobromination to the unstable alkynyl sulfone which could be trapped with dienes in situ. Alternatively, dehydrobromination of the sulfide in the presence of allylic alcohols results in the formation of allyl vinyl ethers which undergo Claisen rearrangements [244]. Further oxidation followed by sulfoxide elimination results in highly unsaturated trifluoromethyl ketonic products (Eq. 80). 

Carbon nucleophiles which do not readily trigger the rearrangement of epoxides include lithiated dithianes [295, 304], lithiated sulfones [238], lithiated diarylphos-phine oxides [240, 305], lithium enolates [306], and allylic organolithium or organo-magnesium compounds [298, 307-310] (Scheme4.67). 

The Beckmann rearrangement is used in a similar way to produce the lactam 32, an intermediate in the synthesis of swainsonine 33. Stereoselective addition of dichloroketene to the enol ether 30 gave one isomer ( 95 5) of cyclobutanone 31. Beckmann rearrangement with a sulfonated hydroxylamine and dechlorination gave the lactam 32 in 34% yield over five steps7 from a precursor of 30. Note that the m-alkene 30 gives the trans cyclobutanone selectively. 

Another regiospecific preparation of trimethylsilyl enol ethers involves treatment of acyltrimethylsilanes with the lithium anions of alkyl sulfones or nitriles. In this case, the sulfone or nitrile group is eliminated during the silyl alkoxide rearrangement (e.g., 5 — 6). Mixtures of olefin stereoisomers are obtained. Note that 4 and 8 give complementary regiochemical results. 

Combination of silyl enol ethers with the organoaluminum-promoted Beckmann rearrangement of oxime sulfonates resulted in a novel reaction system that leads to the formation of enaminones [44]. Treatment of a mixture of anfr-2-methylcyclohexa-none oxime sulfonate (33) and 2-(trimethylsiloxy)-l-octene in dry CH2CI2 with Et2AlCl at -78 °C for 30 min, and at 20 °C for additional 1 h resulted in formation of the enaminone 34 in 90 % yield (Sch. 21). 

A route to ( , )-2,4-alkadienoic esters involves the Johnson-Claisen rearrangement of y-hydroxy-a,P-unsaturated sulfones and subsequent elimination of PhSO H with DBU. Stereoselective access to trisubstituted enol ethers initiated by iodoalkoxylation of disubstituted alkenes (with PyjIBF, 2HBF, ROH) is concluded by treatment with DBU.5... 

It is possible to enhance the rate of a Claisen rearrangement, especially in the enolate Claisen reaction. Denmark et al. showed that other carbanionic centers accelerate the Claisen rearrangement (as in Table 11.23).467 Generation of the anion of sulfone 631 (sec. 8.6.A) with various bases led to acceleration of the reaction relative to the thermal reaction of 631 and also influenced the syn/anti ratio (632/633). In general, a donor group at the allyl position accelerates the rate and the presence of an amino stabilizing group increases the rate even more. 

Universal methods for coupling of the synthetic building blocks are the Wittig reaction, the Horner-Wadsworth-Emmons reaction, the sulfone coupling by Julia s procedure, the enol ether condensation (Miiller-Cunradi-Pieroh reaction), and the Saucy-Marbet rearrangement. Since in very many cases mixtures... 

The concept of silyl enol ether synthesis via / -elimination from a Brook rearrangement-derived carbanion also appeared in Wicha s studies on additions of 1-phenyl-l//-tetrazol-5-yl (PT) sulfonyl anions to acyl silanes. When PT sulfone 34 was deprotonated in the presence of acyl(triphenyl)silane, ketone 36 was isolated in good yield after hydrolysis of the silyl enol ether intermediate 35. The mechanism involved addition of the... 

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