Current Organic Chemistry - Volume 22, Issue 9, 2018

Background: The term chlorophyll c encompasses a small but heterogeneous group of algal pigments, characterized for being porphyrins rather than chlorins. Chlorophylls c are protochlorophyllide-type compounds that differ in their peripheral substituents. Mass spectrometry has contributed to the structural elucidation of chlorophylls c, often in parallel with the development of soft ionization techniques. Objective: To provide comprehensive information on the structural characterization of chlorophylls c by mass spectrometry. Method: A detailed exam of the existing bibliography from both the chemical and marine science literatures followed by the analysis of data on molecular masses and product ions. Previous tandem mass spectra of chlorophyll c pigments are reproduced employing high resolution mass spectrometry. Results: This article reviews the application of direct and tandem mass spectrometry techniques to the analysis of acidic and esterified chlorophylls c, summarizes their fragmentation patterns and enumerates some experimental fragmentation rules for their identification. The high resolution mass spectra of acidic and esterified chlorophyll c compounds are shown as characteristic examples. Conclusion: The combined information on the molecular mass and diagnostic fragmentation allows the unambiguous characterization of the known acidic chlorophylls c, including the discrimination of isomeric pigments. Tandem mass spectrometry is a technique of choice for the study of galactolipid-esterified chlorophylls c, as the systematic fragmentation of successive parts in the molecules allows good tentative approaches to their structures.

Background: Non-allomerized chlorophylls a and b are part of a great diversity of environments: senescent and ripened vegetal tissues, processed foods, sediments, fecal pellets, etc. Consequently, the articles characterizing their mass spectrometry behavior belong to a variety of disciplines. Objective: To compile in a single review all the information published related to mass spectrometry of 26 chlorophyll derivatives, including chlorophyllides, pheophorbides, pheophytins, and hydroxy-, lactone-, and pyro-chlorophyll derivatives. Method: A detailed and extensive bibliographic search through the Web of ScienceTM followed by a comprehensive and critic discussion. Results: This review gathers all the information concerning mass spectroscopic characteristics published for each non-allomerized chlorophyll of a and b series. This means that for each one of the 26 chlorophyll derivatives, it has been compiled the complete information published of mass (or exact mass), mass spectrometry technique and equipment (ionization method and mass analyzer), product ions, structural assignments and proposed mechanisms (if available), detailing the references. Conclusion: The evaluation of referenced data in literature has allowed to obtain common characteristics in the mass spectrometry behavior between the different chlorophyll derivatives. In addition, specific product ions have been reported, revealing specific structural modifications in the chlorophyll molecule, facilitating the structural characterization. This review embraces the application of predictive software tools, which improve accuracy of predictions and make straightforward the structural assignments.

Background: Phototrophic prokaryotes inhabit a diverse array of environments on our planet and also contain a wide range of chlorophylls and bacteriochlorophylls, encompassing both chlorin and bacteriochorin structures. Objective: This review considers the ecological niches of the phototrophic prokaryotes, together with the mass spectrometric study of their chlorophyll and bacteriochlorophyll pigments. Methods: A thorough review of the mass spectrometry literature on chlorophylls from phototrophic prokaryotes has been carried out. Results: Mass spectrometry, particularly coupled with liquid chromatography and used in conjunction with soft ionization techniques, has been pivotal in the assignment of closely related chlorophyll and bacteriochlorophyll structures from these organisms. Conclusion: Exploration of our natural environment continues to find previously uncharacterized photosynthetic prokaryotes, utilizing subtle but significant modifications to the chlorophyll and bacteriochlorophyll structures in our current knowledge.

Objective: This mini-review highlights the highly sensitive ¨soft¨ ionization mass spectrometric methods applied to chlorophyll chemistry. Method: This is a first-hand review of the important developments made over the mass spectrometry of chlorophyll a and chlorophyll b derivatives. Results: The review covers the capacity of the mass spectrometric techniques to solve problems, e.g., assignment of stereochemical configuration at a chiral center, determination of the structures of the chlorophyll b allomers, which are difficult to prepare and purify in larger amounts, and elucidation of the mechanism of Willstatter's allomerization of chlorophylls. Only one example, in which the conventional electron impact mass spectrometry was used, is provided by the characterization of the chlorophyll derivatives found as retinal pigments in the eyes of the remarkable deep-sea fish Malacosteus niger. Other mass spectrometric investigations, dealing with structural properties of chlorophylls, are considered briefly in an end paragraph of this review. Conclusion: Different mass spectrometric techniques have been shown to be very useful in the structural assignments of chlorophyll derivatives.

Biginelli compounds are one of the most important heterocyclic systems that play an important role in medicinal chemistry. It has been proved that the biological activities of these compounds are dependent on the absolute configuration of the C4 atom of the heterocyclic ring, and thus the oxidative dehydrogenation reaction can affect their biological and pharmaceutical activities. In this review, the oxidation methods of various Biginelli compounds have been summarized with a focus on the conditions of the reactions, the proposed mechanisms, and the effect of the substituents on the reaction.

Formerly, an unexpected acyl migration of 1-methylidene-3,4-dihydroisoquinoline- 2(1H)-yl-methanone derivatives was observed, and on this basis a microwave assisted synthetic method was developed for the preparation of members of the 2'-(3,4- dihydroisoquinoline-1-ylidene)-1-ethan-1-one family, potential CB2 receptor agonists and PDE7 inhibitors. The reaction mechanism was studied with experimental and theoretical methods in order to propose intra- and intermolecular radical pathways.

Background: In the past decades, mechanochemical organic synthesis has become an attractive technique for solvent-free synthesis, which avoids the usage of harmful solvents and production of toxic wastes. This method has been utilized to promote the efficient formation of carbon–carbon and carbon–heteroatom bonds. The mechanochemical synthesis of macrocyclic molecules, rotaxanes and covalent organic frameworks has been reported recently. However, the mechanochemical construction of purely organic cage compounds is scanty and challenging. Objective: The primary objective of this study was to synthesize boronic ester cage compounds under high-speed vibration milling conditions and investigate their properties. Method: A mixture of triboronic acids 3 (0.04 mmol) and pentaerythritol (0.06 mmol) was vigorously shaken under high-speed vibration milling (HSVM) conditions at 58 Hz for 40 min. The resulting solid was dissolved in chloroform (5 mL) and then filtered. Cage compounds 4 were obtained by removal of the solvent, and dried under vacuum at 100 °C for 12 h. Cage compounds 4 were subjected to thermogravimetric analysis and chemical stability experiments. Result: Boronic ester cage compounds 4 were synthesized in nearly quantitative yields under high-speed vibration milling conditions. Furthermore, thermogravimetric analysis and chemical stability experiments showed that cage compounds 4 possessed high thermal stability (Tdec up to 320 °C) and good chemical stability in aqueous media. Conclusion: We have described an efficient solvent-free synthesis of boronic ester cage compounds 4 under the HSVM conditions. Furthermore, cage compounds 4 possessed high thermal stability and good chemical stability in aqueous media.

Background: Neoponkoranol is one of de-O-sulfonated sulfonium salts isolated from Genus Salacia showing potent inhibitory activities against α-glucosidases. In previously reported synthetic strategies, the two key coupling partners should be firstly constructed from different starting materials and the following coupling reaction is usually time-consuming. Thus, a more efficient and scalable synthetic protocol would be necessary for further SAR studies of this bioactive natural product. Objective: Based on our previous synthetic studies on this series of natural products, the aim of the present study is to develop a novel total synthesis of neoponkoranol in a divergent and efficient way starting from inexpensive commercially available D-glucose. This is the first report of gram-scale synthesis of neoponkoranol. Method and Results: Unlike all other reported strategies, the two coupling partners epoxide (14) and thiosugar (19) were firstly synthesized from an identical intermediate diol (12) which could be easily obtained in 3 steps from D-glucose. Notably, the construction of 14 and 19 could be achieved without any purification process. The subsequent acid mediated coupling reaction between resulted coupling partners was accomplished in only 4 hours while previous strategies usually took several days to complete the process. A gram-scale quantity of coupling reaction was then conducted under the same reacting condition and the desired sulfonium salt was isolated without any loss of the yield. Conclusion: The newly developed scalable synthesis of neoponkoranol provided a good opportunity for further pharmaceutical studies on this bioactive natural product.