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.