P-Benzoquinone monoacetals

A general and efficient synthesis of 5-oxygenated indoles 126 and 128 has been reported by Kita et al. (92H503). The method involves intramolecular imine formation from p-benzoquinones 125 and p-benzoquinone monoacetals 127 bearing the 2-aminoethyl side chain. Compounds 125 and 127 are prepared by the oxidation of 124 with IBTA in acetonitrile in the presence of water and methanol, respectively (Scheme 35). 

In 1987, Kita and co-workers first developed a general and high yielding (59% quant.) route to p-benzoquinone monoacetals (2a) and spirolactones from para-substituted phenols (la) with PIFA in MeCN in the presence of alcohols (R"OH) [27]. Similar methods for preparing quinone monoacetals and quinol ethers have been developed independently by Lewis et al. (PIDA/CH2C12-R"OH (11-65% yields)) [28] and Pelter et al. (PIDA/R"OH (65-99% yields)) [29] [Eq.(l)]. 

Cationic cycloadditions. Ionization of p-benzoquinone monoacetals is induced by MejSiOTf in highly polar media (LiClO /EtOAc) and the carbocations are captured in situ by alkenes to give bicyclo[3.2.1]octene derivatives. A synthesis of 2-(l -phenylethoxy)-8-oxabicyclo[3.2.1]oct-6-en-3-one in good enantiomeric purity from furan and a mixed acetal of 2-(triethylsiloxy)acrolein (prepared from pyruvaldehyde dimethylacetal in three steps) is promoted by MejSiOTf. 

---