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Selective Hydrogenolysis of Benzyl and Carbobenzyloxy Protecting Groups for Hydroxyl and Amino Functions
Published in Dale W. Blackburn, Catalysis of Organic Reactions, 2020
Louis S. Seif, Kenneth M. Partyka, John E. Hengeveld
Synthetic organic chemistry abounds with examples of hydrogenolysis reactions. In the synthesis of multifunctional compounds, reactive sites must be temporarily blocked. Benzyl ethers, benzylamines, benzyl carbamates, and benzyl carbonates are several protecting groups which are commonly used.
Radiochemistry for Preclinical Imaging Studies
Published in George C. Kagadis, Nancy L. Ford, Dimitrios N. Karnabatidis, George K. Loudos, Handbook of Small Animal Imaging, 2018
HYNIC (2-hydrazino nicotinamide (Meszaros et al. 2010)) is a bifunctional chelator. First, it can be conjugated to nucleophilic groups in a biomolecule, such as the amine groups of lysine residues, via its carboxylic acid group. The reactivity of the carboxylic acid can be increased, for example, by preparing the ester with N-succinimidyl as leaving group. Furthermore, when incorporating HYNIC during the peptide synthesis, the hydrazine functional group may need to be protected because it itself is reactive toward activated carbonyl functions, forming a hydrazone. This may include bond formation between the unprotected hydrazine of one HYNIC molecule and the ester function of another HYNIC unit, leading to oligomerization (Meszaros et al. 2010). A popular protecting group is N-tert-butoxycarbonyl. In the subsequent 99mTc-labeling step, the hydrazido ligand binds to the radiometal and the pyridyl nitrogen usually coordinates with the radiometal as well forming a bidentate chelate complex, at least in the crystalline state (Meszaros et al. 2010). To complete the coordination complex, additional coligands are required. A range of mono- and oligodentate coligands have been studied for this, and among them are aminocarboxylates such as ethylenediamine-N,N′-diacetic acid (EDDA, see Figure 16.2e) and tricine, phosphines, N-heteroaromatics, polyalcohols, and combinations thereof yielding ternary complexes (Meszaros et al. 2010). The HYNIC approach has been developed for larger proteins (e.g., antibodies and serum albumin) and smaller proteins (e.g., annexin V). For a protein, 99mTc labels may be distributed between several potential coupling sites, but site-specific labeling can be achieved in certain cases as in the case of annexin V (Meszaros et al. 2010).
Recent developments in the greener approaches for the dithioacetalization of carbonyl compounds
Published in Journal of Sulfur Chemistry, 2023
The protection and deprotection of carbonyl functional groups are the most common and vital synthetic steps in multistep organic synthesis required to synthesize complex molecules like natural products and pharmaceuticals [1,2]. The reactions involved in these critical steps are really challenging due to a multitude of factors, such as the chemical nature of reactants and reagents used, necessary reaction conditions, and the type of other functional groups present in the multifunctional organic compounds [1,2]. Moreover, the stability of the protecting group during the synthesis, deprotection, and purification steps plays a crucial role in selecting a suitable protecting group. Cyclic and acyclic dithioacetals are widely recognized as the most convenient protecting groups in carbonyl chemistry. They have gained significant attention due to their inherent stability under various conditions, including acidic and basic environments, the presence of mild oxidants, nucleophiles, and hydride reagents [1]. Furthermore, dithioacetals are versatile building blocks widely employed in synthesizing bioactive natural products, fine chemicals, and pharmacologically active compounds [3–12]. Equally, 1,3-dithianes have garnered considerable interest due to their versatile utility in diverse synthetic processes [13–16]. They perform as masked acyl anions or masked methylene functions in the C–C and C–N bond-forming reactions, as well as in many fundamental organic transformations [13–29]. As a result, enormous effort has been made to develop highly efficient methods for synthesizing dithioacetals [1,30,31]. The most straightforward and traditional way to synthesize dithioacetals is the condensation reaction of aldehydes and ketones with thiols or dithiols in the presence of strong Brønsted or Lewis acids [1]. However, the traditional methods for dithioacetalization suffer from several drawbacks, including the toxicity of reagents and metal catalysts, harsh reaction conditions, moisture sensitivity, tedious work-up and purification process of the target products [1,30,31]. The use of hazardous and conventional organic solvents in traditional methods also has a negative impact on the environment and health [32]. Over the past couple of decades, many alternative greener synthetic strategies have emerged to overcome the above-mentioned drawbacks of conventional methods and reduce the environmental impact of industrial chemical processes [30,31,33–64].