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A new class of fluorescent dyes based on 1,3-benzodioxole and [1,3]-dioxolo[4.5-f]benzodioxole
(2011)
We report on synthesis and photophysical properties of a new class of fluorescent dyes. They are characterized by large Stokes-shifts, long fluorescence lifetimes in organic solvents and a pronounced dependency of the fluorescence lifetime on the solvent polarity. Also worthy of note is the high bleaching stability. To provide access to biochemical and medical applications a series of derivatives were prepared, which exhibit specific reactivity towards different biologically relevant functional groups (carboxylic acids, amines, maleimides, N-hydroxysuccinimide esters). Furthermore, two alkynes were prepared, which could be used in 'Click' chemistry.
The nucleophilic thiol-ene (thia-Michael) reaction between molecular rods bearing terminal thiols and bis-maleimides was investigated. The molecular rods have oligospiroketal (OSK) and oligospirothioketal (OSTK) backbones. Contrary to the expectations, cyclic oligomers were always obtained instead of linear rigid-rod polymers. Replacing the OS(T)K rods with a flexible chain yielded polymeric products, suggesting that the OS(T) K structure is responsible for the formation of cyclic products. The reason for the preferred formation of cyclic products is due to the presence of folded conformations, which have already been described for articulated rods.
A new concept of a stereoselective synthesis of axially chiral biaryls, formed in the course of the dehydro-Diels-Alder (DDA) reaction, has been disclosed. It is based on asymmetric induction of the newly formed chirality axis by a chirality center, which is present in the two synthesized DDA reactants. Depending on the different length of the linkers joining the alkyne moieties the DDA reaction may be triggered photochemically or thermally, where only the thermal variant was stereoselective.
Five alkynyl pyridines were prepared and cyclized to naphthylpyridines as the main products in the course of a Photo-Dehydro-Diels-Alder reaction. Four of the final products are axially chiral and the determination of the rotational barrier by DFT calculations, dynamic NMR and H PLC experiments is demonstrated. (C) 2011 Elsevier B.V. All rights reserved.
An efficient route from myo- to neo-inositol is described. The key steps of the sequence are oxidation of the hydroxy group at C-5 to the corresponding ketone, followed by a highly (dr = 7.8:1) stereoselective reduction. The route includes nine steps with an overall yield of 51% and is therefore superior to all hitherto reported methods for the preparation of neo-inositol.
Building blocks for oligospiroketal (OSK) rods and evaluation of their influence on rod rigidity
(2012)
We report on the synthesis of three new sleeves and their incorporation in OSK rods. The structures of these sleeves are based on neo-inositol, terephthalaldehyde diacetals, and indacene. To quantify the influence of the sleeves on rod rigidity, we applied the worm-like chain (WLC) model on the new rods and found that this approach is rather disappointing. As the chief cause of this result, we assume that the rigidity of typical molecular rods largely exceeds the rigidity of polymers, which were successfully described by the WLC model. Alternatively, we suggest quantifying the rigidity of molecular rods by fitting an empirical function on the end-to-end distance distribution curve obtained by MD simulations. After checking various function types, the Levy-Martin function proved to be most suitable for this purpose. On the basis of this function, we defined the Levy-Martin parameter and suggest using this parameter for the characterization of the rigidity of molecular rods.
We developed a new type of molecular rods consisting of two (or more) rigid units linked by a flexible joint. Consequently we called these constructs articulated rods (ARs). The syntheses of ARs were carried out by a flexible and modular approach providing access to a number of compounds with various functionalizations in terminal positions. First applications were presented with pyrene, cinnamoyl and anthracenyl labelled ARs.
An asymmetric variant of the dehydro-Diels-Alder (DDA) reaction has been developed and applied in the atropselective synthesis of various (1,5)naphthalenophanes. Whereas the suitability of the photochemically induced DDA (PDDA) was limited, the thermally induced DDA provided the desired product, depending on the chiral auxiliary used and the length of the linker, with nearly perfect stereoselectivity. Furthermore, the mechanism of the DDA was investigated by means of DFT calculations, and a stepwise mechanism involving 1,4-biradicals was suggested.