Functionalizations of [6]- and [7]Helicenes at Their Most Sterically Hindered Positions

Although the reactions of enol ethers of aryl methyl ketones with benzoquinone make it easy to prepare nonracemic helicenes that are substituted by hydroxyl groups at their 1,ω-positions, the hydroxyl groups fail to facilitate the introduction of electrophiles ortho to them. However, ethers of [6]- and [7]helicenols prepared in this way, seemingly because of the activation by the alkoxyl groups at the 6-positions, combine with electrophilic reagents to introduce bromines and acyl groups exactly into these positions. Moreover, these bromine and acyl groups can be transformed into other functional groups (including phosphine oxides and acetylenes), the ether functions adjacent to these functional groups can then be removed, and the phenols can be oxidized to quinone-acetals. An alternative way to introduce functional groups next to the phenols is to rearrange their phosphate esters. Two reactions that differentiate the ends of the helicenes are also described.