Current Organic Chemistry - Volume 23, Issue 3, 2019

Background: Molecular organic photochemistry is concerned with the description of physical and chemical processes generated upon the absorption of photons by organic molecules. Recently, it has become an important part of many areas of science: chemistry, biology, biochemistry, medicine, biophysics, material science, analytical chemistry, among others. Many synthetic chemists are using photochemical reactions in crystals to generate different types of organic compounds since this methodology represents a green chemistry approach. Objective & Method: Chemical reactions in crystals are quite different from reactions in solution. The range of organic solid state reactions and the degree of control which could be achieved under these conditions are quite wider and subtle. Therefore, for a large number of molecular crystals, the photochemical outcome is not the expected product based on topochemical principles. To explain these experimental results, several physicochemical factors in crystal structure have been proposed such as defects, reaction cavity, dynamic preformation or photoinduced lattice instability and steric compression control. In addition, several crystal engineering strategies have been developed to bring molecules into adequate orientations with reactive groups in good proximity to synthesize complex molecules that in many cases are not available by conventional methods. Some strategies involve structural modifications like intramolecular substitution with different functional groups to modify intermolecular interactions. Other strategies involve chemical techniques such as mixed crystal formation, charge transfer complexes, ionic and organometallic interactions. Furthermore, some examples of the single crystal to single crystal transformations have also been developed showing an elegant method to achieve regio and stereoselectivity in a photochemical reaction. Conclusion: The several examples given in this review paper have shown the wide scope of photochemical reactions in organic molecular crystals. There are several advantages of carrying photochemical reaction in the solid state. Production of materials unobtainable by the traditional solution phase reactions, improved specificity, reduction of impurities, and enhancement in the yields by the reduction of side reactions. These advantages and the multidisciplinary nature of solid-state photochemistry make this discipline quite likely to develop a lot in the future.

The first attempts at microwave-assisted (MW) syntheses of bile acid derivatives were performed in domestic MW appliances. However, the reproducibility of these syntheses, which were performed in uncontrolled conditions, was very low. In the first part of this overview, compounds synthesized under such conditions are presented. Consequently, with the development of MW technology, MW-assisted reactions in MW reactors became reproducible. Thus, in the second part of this review, syntheses of bile acidsbased compounds in MW reactors are presented. Among others, publications dealing with the following topics will be covered: − Chemical transformations of hydroxyl and/or carboxyl functions of bile acids into esters or amides, − Hydroxyl group oxidations, − Derivatization of oxo-compounds with different nitrogen-containing compounds (e.g. 4-amino-3- substituted-1H-1,2,4-triazole-5-thiones, thiocarbohydrazides and thiosemicarbazides) Bile acid-based molecular tweezers, capable of stereospecific molecular recognition Reactions of hydroxyl functions to give chlorine derivatives, presenting reactive intermediates in substitution reactions with N- or O-containing nucleophilic arylhydrazides, urea derivatives, substituted thiadiazoles or triazoles or amino acid methyl esters, mainly in solvent-free conditions. Some of the synthesized compounds expressed antimicrobial potential and/or good recognition properties as artificial receptors for specific amino acids or anions. Detailed comparisons between conventional and MW-assisted procedures for chemical transformations of bile acids are given in most of the presented publications. Based on these results, MW irradiation methods are simpler, more efficient, cleaner and faster than conventional synthetic methods, meeting the requirements of green chemistry.

Diactivated cyclopropanes containing two geminal electron withdrawing groups, commonly called as ‘Doubly Activated Cyclopropanes’ are useful synthons for the synthesis of many interesting natural products and functionalized molecules. These geminal electron withdrawing groups (EWG’s) facilitate the regioselective ring opening of cyclopropanes by polarizing the C-C bond adjacent to it. This polarization also allows them to undergo 1,3 dipolar cycloaddition reactions when substituted with a suitable electron donor substituent at the adjacent carbon (donor-acceptor cyclopropanes) in the presence of suitable dipolarophiles. In this review, we discuss the recent advances in the chemistry of doubly activated cyclopropanes: their synthesis, reactions and applications in total synthesis.

Magnetically separable Brønsted acidic catalysts combine the advantages of high efficiency of homogeneous Brønsted acidic catalyst with the ease of magnetic separation from the reaction medium. In addition to their ease of separation, the magnetically separable Brønsted acidic catalysts also possess high stability towards air and moisture, facile functionalization and tunable hydrophobic properties. This review portrays the applications of sulfonic acid anchored γ -Fe2O3 or Fe3O4 nanoparticles, magnetic core encapsulated acid functionalized silica or mesoporous nanoparticles, functionalized ionic liquid coated acidic magnetically separable nanoparticles and miscellaneous magnetically separable Brønsted acidic nanoparticles in diverse organic transformations. In addition, the merits of magnetically separable Brønsted acid nanocatalyst are also summarized and compared with the traditional homogeneous/heterogeneous Brønsted acidic catalysts.

Neurodegenerative diseases (NDDs) are characterized by a progressive deterioration of the motor and/or cognitive function, that are often accompanied by psychiatric disorders, caused by a selective loss of neurons in the central nervous system. Among the NDDs we can mention Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), amyotrophic lateral sclerosis (ALS), spinocerebellar ataxia 3 (SCA3), spinal and bulbar muscular atrophy (SBMA) and Creutzfeldt-Jakob disease (CJD). AD and HD are characterized mainly by massive neuronal loss. PD, ALS, SCA3 and SBMA are agerelated diseases which have characteristic motor symptoms. CJD is an NDD caused by prion proteins. With increasing life expectancy, elderly populations tend to have more health problems, such as chronic diseases related to age and disability. Therefore, the development of therapeutic strategies to treat or prevent multiple pathophysiological conditions in the elderly can improve the expectation and quality of life. The attention of researchers has been focused on bioactive natural compounds that represent important resources in the discovery and development of drug candidates against NDDs. In this review, we discuss the pathogenesis, symptoms, potential targets, treatment and natural compounds effective in the treatment of AD, PD, HD, ALS, SCA3, SBMA and CJD.