Anodic intramolecular

Scheme 8 Anodic intramolecular coupling of phenols with olefins.

The oxadiazoles 77 were also obtained by anodic intramolecular cyclization of 1-arylmethylenesemicarbazides 76 in CH3OH-O.I mol/1 Et4C104 solution in 51-65% yield [55]. The aromatic methyl esters were also formed either as a minor or major product in 55-80% yield (Scheme 38). The selectivity of the... 

In connection with the anodic intramolecular coupling of 1-benzyltetrahydroiso-quinoline derivatives, the anodic oxidation of 4-(3,4-dimethoxybenzyl)-6,7-dimethoxy-isochromanone 15 has been studied rather extensively. The main product obtained from the isochromanone is a y-lactone, 7a,8-dihydro-3,10,ll-trimethoxy-2 H-phenanthro[9.8a, b]furan-2,7(5 H)-dione 16 13 ... 

Moeller and coworkers [484] found that electrochemical cyclization (anodic intramolecular alkoxylation) of optically active dipeptides proceeded highly diasteroselectively. 

Dimethoxyphenols such as 156 bearing a double bond at the side chain undergo anodic intramolecular cycloaddition resulting in the formation of three possible compounds 157, 158 and 159 (Scheme 31). These compounds are promising synthetic intermediates for a variety of sesquiterpenes. 

Despite the fact that the electrochemical oxidation of most of the nonconjugated dienes generally does not give products which result from interaction of the double bonds with one another, the anodic oxidation l-acetoxy-l,6-heptadienes gives intramolecularly cyclized products, that is, the cyclohexenyl ketones (equation 15)13. The cyclization takes place through the electrophilic attack of the cation generated from enol ester moiety to the double bond. 

Somewhat surprisingly, no spin adduct was seen from the oxidation of Ph4B ( pa = 0.92 V) under similar conditions, the anode potential being varied between 0.5 and 2.2 V. Since Ph-PBN could be independently formed in a thermal reaction and was stable under the anodic conditions used, and Ph was judged to be electroinactive, it was concluded that Ph4B decomposed intramolecularly with direct formation of biphenyl. 

The intramolecular coupling of enolethers with enolethers, styrenes, alkyl-substituted olefins, allylsilanes, and vinylsilanes was systematically studied by Moeller [69]. Many of these coupling reactions turned out to be compatible with the smooth formation of quaternary carbon atoms (Eq. 11) [70], which were formed diastere-oselectively and led to fused bicyclic ring skeletons having a ds-stereochemistry [71]. The cyclization is compatible with acid-sensitive functional groups as the allylic alkoxy group. Moeller has demonstrated in some cases that these reactions can be run without loss of selectivity and yield in a simple beaker with either a carbon rod or reticulated carbon as anode without potential control and a 6-V lantern battery as power supply [71]. 

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]. 

Nitrogen-Oxygen Bond Formation The anodic oxidation of vicinal dioximes gives 3,4-diphenylfurazan-2-oxide by intramolecular ring formation (Scheme 42) [59]. 

Intramolecular anodic olefin coupKng reactions involving heteroatomic aromatic rings were used to prepare fused, bicyclic ring skeletons (Scheme 122) [159]. 

Anodic C, C-coupling is a very powerful tool to synthesize cyclic compounds with high regio- and stereoselectivity. It involves inter- and intramolecular coupling of arylolefins, dienes, enolethers, phenol ethers, and aromatic amines and often opens a quick entry into complex natural products in a few steps. Although the mechanism is fully established in only a few cases, it does appear to involve the coupling of two radical cations at the site of their highest radical density and is further controlled by steric constraints. This important type of reaction is reviewed in Chap. 5 and in Refs. [89, 90]. 

Chemoselective anodic methoxylation at a distinct carbon atom in the a-position to an amino group in a polypeptide was achieved by prior introduction of a silyl group as an electroauxiliary at this carbon atom [156]. Amide oxidation in A-acetylpyrrolidines substituted with electron-rich phenyl rings led to either methoxylation a to the nitrogen atom or in the benzylic position. Mechanistic studies indicate that both the amide and the phenyl oxidation compete, but intramolecular electron transfer leads to... 

Fig. 50 Stereoselective intramolecular addition of anodically generated phenoxy cations [257-259],...

Intramolecular Carbon-Carbon Bond Forming Reactions at the Anode... 

One of the most synthetically useful anodic C-C bond forming reactions developed to date involves the intramolecular coupling of phenol derivatives with olefins. Yamamura has demonstrated that these reactions tend to lead to three classes of products (Scheme 22) [36]. The type of product generated depended strongly on both the nature and stereochemistry of the functional groups attached to the olefin moiety. For example, consider the two cyclization reactions illustrated in Scheme 23 [37]. In this experiment, the stereochemistry of the initial olefin substrate completely dictated the ring skeleton of the product. 

---