RESUMEN
A route is developed to a structural analogue of [5]HELOL, a previously reported helically grooved sensor of remote chirality. It gives the material enantiomerically pure and in multigram quantities. The enantiomers of alcohols, phenols, amines, and carboxylic acids, even when their centers of chirality are remote from any functional groups, can be differentiated by 31P NMR spectroscopic analyses of their reaction products with the chlorophosphite of this material.
RESUMEN
Experiments carried out 24 years ago with tantalum carbenes have led to the much cited hypothesis that metals (other than ruthenium) must be in their highest oxidation states for their carbene derivatives to initiate olefin metatheses. The hypothesis legitimizes the uniqueness of high-oxidation-state molybdenum and tungsten carbenes as effective initiators, and it means that the Fischer tungsten carbenes that even earlier were found to initiate olefin metatheses and related transformations must be oxidized before they can be effective. The newer initiators have been termed "well-defined", the older "ill-defined". But what does the evidence show?
RESUMEN
Although the contortions required are unprecedented, the benzynes formed by the fluoride-induced elimination of TMSOTf from o-trimethylsilyl [6]- and [7]-helicenol triflates add to another ring of the helicenes rather than to an external furan molecule.
RESUMEN
We present a new technique, based on regression analysis, to determine the second-order nonlinear optical susceptibility tensor of thin films. The technique does not require the absolute levels or phases of measured signals to be mutually calibrated. In addition it yields indicators that address the quality of theoretical models describing the sample. We use the technique to determine the susceptibility tensor of samples of a nonracemic chiral material which have very low symmetry (both chiral and anisotropic) and have many independent tensor components. The results show the importance of using detailed theoretical models that account for the linear optical properties of the sample.
RESUMEN
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,omega-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.
RESUMEN
Hetero[7]helicenebisquinones can be synthesized easily on a multigram scale by combining the silyl enol ethers of 3,6-diacetyldibenzofuran, 3,6-diacetyldibenzothiophene, and 3,6-diacetylcarbazole with p-benzoquinone. They can be resolved into their enantiomers by a procedure that had previously been used to resolve carbohelicenebisquinones. Their absolute configurations are assigned.
RESUMEN
[7]Helicenebisquinones can be made easily and in quantity by reacting the silyl enol ethers of a 9,10-dialkoxy-, or better a 9,10-disiloxy-, 3,6-diacetylphenanthrene, with p-benzoquinone. If an ethyl enol ether is used in place of the silyl enol ether, the transformation proceeds in much lower yield. The [7]helicenes are efficiently resolved into their enantiomers, and absolute configurations are assigned.