Enol-allyl-ethers

Silicon-based Lewis acids have been known for some time, and the related chemistry in catalysis has recently been reviewed [24]. Most examples in the literature are mainly based on achiral species and will be discussed only briefly in this section. In general, a broad variety of reactions can be catalyzed with compounds like MejSiOTf, MejSiNTf or MOjSiClO. One advantage over some metal Lewis acids is that they are compatible with many carbon nucleophiles like silyl enol ethers, allyl organometallic reagents and cuprates. 

Scheme 27 Diastereoselective alkylation of propargyl alcohols with silyl enol ethers, allyl silanes and electron-rich arenes...

The success of the Claisen-Cope rearrangement need not be limited to the production of aldehydes via enol ethers. Allylic alcohol (58) is successively transposed into a mixture of allylic isomers (59 Scheme 4), and is subjected to an orthoester Claisen rearrangement at 150 "C to provide ester (61). The moderate temperature of the Claisen step permits the isolation of an intermediate (c/. Scheme 3) prior to the final Cope rearrangement (195 C) to. y-unsaturated esters (60). The esters (60) are a 55 45 mixture of ( )- and (Z)-double bond isomers owing to the near equal steric bulk of the methyl and acetic acid residues in the transition state for the Cope rearrangement. ... 

BF3 Et20 has been shown to mediate and catalyze the substitution of ferf-alkyl fluorides with various nucleophiles including silyl enol ethers, allyl silanes, hydrosilanes and phosphines (eq 53). 5... 

The CO ligands are electron withdrawing and these allyl complexes are electrophilic. The iron complexes, being cationic, are the more reactive. Very weak nucleophiles such as enol ethers, allyl silanes and electron-rich aromatics may be used (Scheme 9.6). Malonate anions, amines and organozinc reagents have also been used. The resulting t -complexes are usually quite unstable 9.18 and decomplex on exposure to air to give the allylation product 9.19. 

The 7, i5-unsaturated alcohol 99 is cyclized to 2-vinyl-5-phenyltetrahydro-furan (100) by exo cyclization in aqueous alcohol[124]. On the other hand, the dihydropyran 101 is formed by endo cyclization from a 7, (5-unsaturated alcohol substituted by two methyl groups at the i5-position. The direction of elimination of /3-hydrogen to give either enol ethers or allylic ethers can be controlled by using DMSO as a solvent and utilized in the synthesis of the tetronomycin precursor 102[125], The oxidation of the optically active 3-alkene-l,2-diol 103 affords the 2,5-dihydrofuran 104 in high ee. It should be noted that /3-OH is eliminated rather than /3-H at the end of the reac-tion[126]. 

Silyl enol ethers are other ketone or aldehyde enolate equivalents and react with allyl carbonate to give allyl ketones or aldehydes 13,300. The transme-tallation of the 7r-allylpalladium methoxide, formed from allyl alkyl carbonate, with the silyl enol ether 464 forms the palladium enolate 465, which undergoes reductive elimination to afford the allyl ketone or aldehyde 466. For this reaction, neither fluoride anion nor a Lewis acid is necessary for the activation of silyl enol ethers. The reaction also proceed.s with metallic Pd supported on silica by a special method[301j. The ketene silyl acetal 467 derived from esters or lactones also reacts with allyl carbonates, affording allylated esters or lactones by using dppe as a ligand[302]... 

Another preparative method for the enone 554 is the reaction of the enol acetate 553 with allyl methyl carbonate using a bimetallic catalyst of Pd and Tin methoxide[354,358]. The enone formation is competitive with the allylation reaction (see Section 2.4.1). MeCN as a solvent and a low Pd to ligand ratio favor enone formation. Two regioisomeric steroidal dienones, 558 and 559, are prepared regioselectively from the respective dienol acetates 556 and 557 formed from the steroidal a, /3-unsaturated ketone 555. Enone formation from both silyl enol ethers and enol acetates proceeds via 7r-allylpalladium enolates as common intermediates. 

The cyclic enol ether 255 from the functionalized 3-alkynoI 254 was converted into the furans 256 by the reaction of allyl chloride, and 257 by elimination of MeOH[132], The alkynes 258 and 260, which have two hydroxy groups at suitable positions, are converted into the cyclic acetals 259 and 261. Carcogran and frontalin have been prepared by this reaction[124]. 

This effect is the basis of the synthetic importance of ester enolate Claisen rearrangements in which enolates or silyl enol ethers of allylic esters are rearranged into 4-pentenoate esters. 

Generally, isolated olefinic bonds will not escape attack by these reagents. However, in certain cases where the rate of hydroxyl oxidation is relatively fast, as with allylic alcohols, an isolated double bond will survive. Thepresence of other nucleophilic centers in the molecule, such as primary and secondary amines, sulfides, enol ethers and activated aromatic systems, will generate undesirable side reactions, but aldehydes, esters, ethers, ketals and acetals are generally stable under neutral or basic conditions. Halogenation of the product ketone can become but is not always a problem when base is not included in the reaction mixture. The generated acid can promote formation of an enol which in turn may compete favorably with the alcohol for the oxidant. 

Claisen rearrangement of the allyl enol ether of tnfluoroacetylacetone gives a C-allylated derivative [123] (equation 106)... 

The 2,2 -bis(phenylthiomethyl) dispiroketal (dispoke) derivative is cleaved by oxidation to the sulfone, followed by treatment with LiN(TMS)2. The related bromo and iodo derivatives are cleaved reductively with LDBB (lithium 4,4 -di- -butylbiphenylide) or by elimination with the P4- -butylphosphazene base and acid hydrolysis of the enol ether. The 2,2-diphenyl dispiroketal is cleaved with FeCl3 (CH2CI2, rt, overnight)." The dimethyl dispiroketal is cleaved with TFA, and the allyl derivative is cleaved by ozonolysis followed by elimination. ... 

Silyl enol ethers and ketene acetals derived from ketones, aldehydes, esters and lactones are converted into the corresponding o/i-unsaturated derivatives on treatment with allyl carbonates in high yields in the catalytic presence of the palladium-bis(diphenylphosphino)ethane complex (32). A phosphinc-free catalyst gives higher selectivity in certain cases, such as those involving ketene acetals. Nitrile solvents, such as acetonitrile, are essential for success. 

Direct treatment of TIPS enol ethers of a variety of cyclic and acyclic ketones with the strong-base combination of n-BuLi/KO-t-Bu leads to /3-ketosilanes (2) after aqueous work-up. In contrast with the earlier method, this rearrangement appears to proceed through allylic, rather than vinylic, metallation, since enol ethers lacking an allylic a-proton are unreactive. 

Allenyl Silyl enol ethers, 86 Allyl alcohol trimethylsilyl ether, 84 Allyl carbonates, 114-15 9 Allyl-ay 2 octalone, 34-5 2-Allyl-2 methylcyclohexanone, 106 (Allyldimethylsilyl)methyl chloride, 58, 59 (AUyldimethylsilyl)methylmagnesium chloride, 59... 

---