Acidified Alcohols as Agents to Introduce and Exchange Alkoxyls on the Periphery of Helicenes
摘要:
Alcohols containing HCl transform the hydroquinone reduction-products of helicenebisquinones into p-alkoxyphenols that have the alkoxyls specifically on the peripheries of the helices. The reactions are quick, and the yields are high. Alkoxyls at the 6-position are replaced also, but only after 2 h at 60 degrees C, not after the 5 min at 25 degrees C sufficient to replace the peripheral hydroxyls of the hydroquinones. After the remaining inside hydroxyls of a dihydroxytetramethoxy[6]helicene prepared by this procedure have been converted into camphanates, one diastereomer crystallizes from solution, allowing an enantiomer resolution to be carried out on a large scale. By then simply reapplying the procedure with alcoholic acid, a variety of resolved dihydroxytetraalkoxy[6]helicenes can be prepared in excellent yields without resolution procedures having to be developed for each, Similar procedures are effective when applied to a [7]carbohelicene.
Application of the Russig−Laatsch Reaction to Synthesize a Bis[5]helicene Chiral Pocket for Asymmetric Catalysis
作者:Spencer D. Dreher、Thomas J. Katz、Kin-Chung Lam、Arnold L. Rheingold
DOI:10.1021/jo991498u
日期:2000.2.1
The enantiomers of a bis[5]helicenediol Ligand ([5]HELOL) have been synthesized in appreciable amounts by a procedure in which key steps are the union of p-benzoquinone with an enol ether of 3-acetylphenanthrene and the displacement of phenol and phenol ether functions by alcohols (the Russig-Laatsch reaction). This diol catalyzes the addition of diethylzinc to aldehydes and gives nonracemic alcohols with enantiomeric excesses as high as 81%. The stereoselectivities and yields are much greater than when the catalyzing diol is BINOL. The enantioselectivities are greater also than those of other reactions catalyzed by helicene ligands.