Ether conversion, cation effects

A novel O-deprotection protocol involving anodic cleavage of aromatic ether (LXXXV) was effectively used in the total synthesis of the natural products alkannin and shikonin [Eq. (42)]. The electrolysis was conducted at a carbon anode in MeCN/ H2O with LiC104 as electrolyte, giving an 80% yield at 50% conversion [97]. Although both naphthoquinone tautomers were initially formed after trapping of the radical cation by water and loss of CH2O, the tautomer with alkyl substitution at the quinone double bond was more thermodynamically stable. 

Formaldehyde was polymerized in ether, toluene or methylene chloride with cationic initiators [30]. The activity of the initiators was quite independent of the solvent. The individual initiators showed a remarkable difference [15]. In ether at —78°C BF3 initiated formaldehyde polymerization reached very quickly a final low conversion of 30% while SnCl4 initiation caused a very high conversion to polymer (Fig. 6). Qualitatively the effectiveness of initiators in cationic formaldehyde polymerization at... 

The following example demonstrates the utility of the indirect cation-pool method. The first step is the electrochemical generation of ArS(ArSSAr) which was characterized by NMR and CSI-MS. ArS(ArSSAr) serves as a quite effective chemical reagent for the generation of alkoxycarbenium ions from a-ArS-substituted ethers, presumably because of its high thiophilicity (Scheme 5.25). The conversion is complete within 5 min at —78 °C. The alkoxycarbenium ion pool thus obtained exhibits similar stability and reactivity to that obtained with the direct electrochemical method. Therefore, alkoxycarbenium ion pools generated by the indirect method also serve as powerful reagents for flash chemistry. 

Isomerization. Cationic iridium complex effects selective isomerization of unsymmetrical diallyl ethers and conjugated boronates containing an allylic ether group, including an access to 7-(siloxy)allylboronic esters. The conversion of allyl homoallyl ethers to -y,8-unsaturated carbonyl compounds is promoted by [(cod)IrCl]2. 

Elimination of Thiophenol from Thioacetals. Conversion of thioacetals to vinyl sulfides is accomplished under exceptionally mild conditions by treatment with (CuOTOa-CeHe (eq 45). The reaction involves an a-phenylthio carbocation intermediate. Three factors contribute to the effectiveness of this synthetic method the Lewis acidity of a copperfT) cation that is unencumbered by a strongly coordinated counter anion, the solubility of the copper(I) triflate-benzene complex, and the insolubility of CuSPh in the reaction mixture. An analogous elimination reaction provides an effective route to phenylthio enol ethers from ketones (eq 46). ... 

---