Enol acetates anodic oxidation

Anodic oxidation reactions have been utilized to reverse the polarity of enol ethers and to initiate radical cation cyclizations. As shown below, the ketene acetal 97 is oxidized on a... 

Different nucleophiles such as methanol, allylsilanes, silyl enol ethers, trimethylsilyl-cyanide, and arenes can be used in this process [62]. When the sulfide itself contains an unsaturated or aromatic fragment and the process is carried out in the absence of a nucleophile, an intramolecular anodic sub-stitution/cyclization might occur [61-63]. Methyl esters of 2-benzothiazolyl-2-alkyl or aryl-acetic acid, oxidized in MeOH/Et4 NCIO4 or H2SO4 in the presence of CUCI2, form 2,2-dimethoxy products (Eq. 7) [64]. 

Intramolecular coupling reactions between nucleophilic olefins have also proven to hold potential as synthetically useful reactions. The first example of this type of reaction was reported by Shono and coworkers who examined the intramolecular coupling reaction of an enol acetate and a monosubstituted olefin (Scheme 41) [50]. This reaction was conducted in an effort to probe the nature of the radical cation intermediate generated from the anodic oxidation of... 

Again, the exclusive formation of six-membered rings indicates that the cyclization takes place by the electrophilic attack of a cationic center, generated from the enol ester moiety to the olefinic double bond. The eventually conceivable oxidation of the terminal double bond seems to be negligible under the reaction conditions since the halve-wave oxidation potentials E1/2 of enol acetates are + 1.44 to - - 2.09 V vs. SCE in acetonitrile while those of 1-alkenes are + 2.70 to -1- 2.90 V vs. Ag/0.01 N AgC104 in acetonitrile and the cyclization reactions are carried out at anodic potentials of mainly 1.8 to 2.0 V vs. SCE. 

Moeller has carried out an extensive series of studies of the electrochemical oxidation of electron-rich w-alkenes. One olefinic component is an enol ether, which is converted into an electrophilic center upon oxidation this center then attacks the other site intramolecu-larly. The anodic oxidation of the bis-enol ethers 21 in methanol25 exemplifies the course of such reactions (Scheme 4). The products are w-acetals (22), formed in 50-70% yield in many cases. The cyclization can be used to produce quaternary25 and angularly fused26 bicyclic and tricyclic structures (equation 11). In its original form, this work involved oxidation of a mono-enol ether bearing a nearby styrene-type double bond27. 

Alicyclic enol acetates can lead to the corresponding enones by anodic oxidation. The electrolysis of d-menthenyl acetate 74 in an AcOH—Et4NOTs—(C) system by passing 2.5 F/mol of electricity provides the d-menthenone 75, a precursor for d-menthone 76 and /-citronellol 54 syntheses, in 97% yield (Scheme 3-28)76a). 

Scheme 8). For instance, the fluorination at the position a of ketones has been realized by the anodic oxidation of enol acetates (78) in an MeCN/Et3N-3HF/(Pt) system under potential control, giving a-fluoro ketone (79 equation 38). ... 

The anodic oxidation of enol ethers at a graphite anode in methanol containing 2,6-lutidine and sodium perchlorate results in the dimerization of the enol ethers to acetals of 1,4-dica nyl compounds (equation 22). The mechanism of dimerization is thought to involve a tail-tail coupling of the cation radicals generated by the one-electron oxidation of the enol ethers. 

The anodic oxidation of enamines in methanol containing sodium methoxide as the supporting electrolyte shows a reaction pattern different from that of enol ethers or enol acetates. The main products are mixtures of isomeric methoxylated enamines, (18) and (19), with yields in the range 74-76% (equation... 

The reactions of enol ester radical cations formed in anodic oxidations were pioneered by Shono [220-225] almost two decades ago. A reaction mode was identified that formally corresponds to that of enol cation radicals. Depending on the electrolysis conditions enol acetates were either converted to a-acetoxy ketones (high concentration of acetate) or to enone products (absence of acetate). Similarly, a-methoxy ketones were obtained through electrolysis in methanol-Et4NOTs. Yields for additional reactions not listed here varied between 29% and 90% [222,223]. 

If the anodic oxidation of N-alkylanilines is performed in the presence of nucleophiles like enol ethers, nucleophilic substitution in the of-position to nitrogen by the enol ether can be observed in low yields. The electrophilic intermediate is the N-aryl iminium ion or the N-aryl imine after loss of two electrons and one or two protons. These intermediates add to the enol ether to give acetals (up to 26%) as addition products, or the first addition step is followed by an electrophilic aromatic substitution to form tetrahydroqui-nolines (13-39%) [47]. It should be noted at this point that better results for the nucleophilic a-substitution to nitrogen can be obtained with N,N-dialkylanilines (see next subsection). Optimum results, however, are obtained with N-acylated compounds via the intermediate N-acyl iminium ions (see Ref. 8). 

The coupling of an allyl or acyl moiety onto carbon atoms is achieved by anodic oxidation of a-heteroatom substituted organostannanes or Oj -acetals in the presence of allylsilanes or silyl enol ethers. The reaction probably involves carbocations as intermediates that undergo electrophilic addition to the double bond [245c]. 

Tatsuya Shono, Shigenori Kashimura and Naoki Kise TABLE 4. Anodic oxidation of enol acetates... 

A simple one-step synthesis of (-)-A -tetrahydrocannabinol from chrysanthenol and olevitol has been reported. The anodic oxidation of the enol acetate of iso-pinocamphone (613), obtained in three steps by standard methods from l-oc-pinene, provides a novel synthesis of /-carvone (614 34% overall) in essentially quantitative optical yield. The choice of solvent (CH2CI2-ACOH) and supporting electrolyte (Et4NOTs) is important. Treatment of a-pinene with Bu OK/Bu"Li gave (615) which... 

Vicinally tri- or tetramethoxy compounds may also be cleaved anodically. Thus 1,1,2-trimethoxycyclohexane obtained on electrooxidation of cyclohexanone enol ether may be oxidized to the acetal ester, (CH30)2CH(CH2)4 COOCH3 [34]. 

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