Light as energy source in chemical reaction. New synthesis of valuable dithienylacetylenes
Photochromism (from Greek φωζ photo “light” and χρωμα chroma “colour”) is determined as reversible transformation between two chemical species, induced by action of light [1]. Herewith, initial form and photoinduced isomer have different physical and chemical properties. The phenomenon is attractive for the design of hi-tech materials, including optical memory elements and molecular switches. Diarylethenes is the most promising class of organic photochromic compounds due to outstanding thermal stability of both isomers and high photostability [2]. The size of so-called ethene bridge significantly affects the photochromic reaction. The photochromic diarylethenes with 4-, 5-, and 6-membered cyclic ethene bridge are known, but there is no example with 3-membered bridge. In this study we report a new approach towards dithienylacetylenes 3 that include the synthesis of diarylcyclopropenones 2 via Friedel-Crafts alkylation of heterocyclic compounds 1 with tetrachlorocyclopropene and following UV-irradiation. It was found that the diarylethenes 2 do not display photochromic properties, but they undergo quantitative photoelimination of carbon monoxide upon UV-irradiation resulting in dithienylacetylene 3. Thus, we have proposed a new synthetic two-step approach to dithienylacetylenes 3 [3], which could be useful synthons in synthesis of photochromic diarylethenes with various ethene bridges.
New approach to the synthesis of functionalized fluoroalkenes
Fluorine has a big influence on physical, chemical and biological properties of organic structures. Organofluorine compounds are widely used in modern medical chemistry to develop new drugs. Insertion of fluorine atom into organic molecules can improve their reactivity in biological systems, increase their metabolic stability, lipophilicity and permeability through membranes. As a consequence, in recent years, the percentage of drugs containing one or more fluorine atoms has increased rapidly up to 40%. The fluoroallylic fragment is also able to change properties of bioactive molecules. Its introduction into such structures as inhibitors of histonedeacetylase, inhibitors of matrix metalloproteinase, asparagine, glutamine, etc. increases their biological activity and electronic properties. We propose a new method for the synthesis of functionalized fluoroalkenes, based on the generation of fluoroallyl nucleophiles from silyl- and boronyl-substituted fluorocyclopropanes and their further usage in the allylation of carbonyl compounds or their derivatives. Due to the fact that the cyclopropanation of alkenyl boronates is not possible under conditions of alkaline dehydrohalogenation of dibromofluoromethane, we have developed a new method for the preparation of silyl- and boronyl-substituted cyclopropanes, which consist of carbene cyclopropanation of multiple C=C bonds by sodium dibromofluoroacetate catalyzed by (IPr)AgCl. The new method is effective for the cyclopropanation of not only boronyl- and silyl-substituted olefins, but also for low-reactivity alkenes, such as monoalkyl substituted alkenes, allyl alcohol ethers and α,β-unsaturated carbonyl compounds. The conditions for isomerization of silyl- and boronyl-substituted fluorohalocyclopropanes in the presence of catalytic amounts of copper (I) bromide in acetonitrile was selected. It was shown that the regioselectivity of the process is determined by the thermodynamic control. Thus, the formation of fluorovinylsilanes or fluorovinylboranes in the isomerization of α-silyl- or α-boronyl-gem-bromofluorophenylcyclopropanes and fluoroallylsilanes upon isomerization of β-silyl-gem-bromofluorophenylcyclopropanes was observed. Thus, new types of fluorinated reagents were obtained that are not previously described in the literature (...)
Reactivity of styrylmalonates as synthetic equivalents of Donor–acceptor cyclopropanes with aldehydes in the presence of BF3•Et2O
Donor–acceptor cyclopropanes (DACs), which can act as sources of 1,2- and 1,3-zwitterions in the presence of Lewis acids, are widely used in organic synthesis for the preparation of various carbo- and heterocyclic compounds, including natural compounds and their analogues. To date, many types of DACs reactivity have been identified. However, the chemistry of styrylmalonates (isomers of DACs, which can be easily generated from DACs) is almost undescribed and has a powerful synthetic potential. The use of styrylmalonates as synthetic equivalents of DACs allows us cardinally change the known reaction pathways of DACs. In this work, a new strategy for cascade assembly of substituted pyrenes based on the reactions of styrylmalonates with aldehydes in the presence of BF3•Et2O has been developed. Generation of formal 1,2-zwitterionic intermediates owing to complexation of dicarboxylate groups with BF3•Et2O is the driving force of the reaction discovered. This method makes it possible to assemble pyrenes or 5,6-dihydro-2H-pyran-2-ones in one synthetic stage from readily available starting compounds with high regio- and diastereoselectivity, and use these pyrenes in futher reactions. We’ve optimized conditions of the reaction and synthesized a number of various substituted pyrenes. Moreover, the reaction shows good results with various aromatic and heteroaromatic substituents. Pyrenes can be easily purified by crystallization. Every product was obtained selectively and determined by full set of physical-chemical methods, including X-ray analysis. 5,6-dihydro-2H-pyran-2-one skeleton is found in various natural compounds demonstrating a broad spectrum of biological activity, such as antiviral and antineoplastic.