Current Organic Chemistry - Volume 18, Issue 21, 2014

The possibilities for the synthesis of phosphinic esters (phosphinates) and phosphinic amides, useful starting materials and intermediates, are discussed. The classical method involves the use of phosphinic chlorides in reaction with the alcohol/ phenol or amine as the nucleophile. A “greener” approach may start from phosphinic acids, however, their direct esterification and amidation are not possible on conventional heating. According to a novel observation, the direct derivatizations (especially esterifications) still take place under microwave conditions. Another possibility is the alkylating esterification by alkyl halides to afford phosphinates. Phase transfer catalysis may be a useful tool to make such transformations possible. According to another strategy, the phosphinic acids are converted to more active intermediates that may then react with the O- and N-nucleophile efficiently. Among the activating agents, the T3P® reagent is the most noteworthy one. Among the other methods discussed, Arbuzov reaction and the fragmentation-related phosphinylations of O- and N-nucleophiles are underlined. It is interesting that there have not been real reviews in this field, although phosphinic starting materials and intermediates are utilized commonly in synthetic organic chemistry.

The interest in the chemistry of saturated oxazolones continues unabated in virtue of their usefulness as intermediates in the synthesis of highly substituted heterocycles. This review attempts to present the prolific development of recent years in this area and gives a critical and unified account of azlactones under the following headings: saturated oxazolone synthesis; generation of cyclic azomethine ylide intermediates and relative oxazolone/imine cycloadditions and oxazolone/alkene cycloadditions; general overview of their usefulness in the functionalization of carbon nanomaterials and in the synthesis of pharmaceutically and biologically intriguing compounds. A particular attention is reserved to the enantioselective oxazolone cycloaddition processes for both known synthetic approaches: chiral induction by metal catalysts and asymmetric activation by organocatalysts.

The guanidine functional group is an important structural motif in synthesis with a wide range of interesting properties. Guanidines are frequently found in bioactive compounds; both from natural sources or from synthetic origin, these compounds are considered fundamental entities in medicinal chemistry. The relevance of these compounds fosters the efforts to explore new methodologies for the synthesis of substances based on the guanidine core. The aim of this review is to provide a survey of the types of reactions used to prepare these classes of compounds. Given the vast number of contributions to this topic, we have focused on the achievements made in the last five years and, in particular, in the synthesis of biologically relevant heterocyclic guanidine-derivatives.

Chalcones belong to the flavonoid family which constitutes one of the major classes of naturally occurring oxygen heterocyclic compounds. The α,β-unsaturated carbonyl system of chalcones possesses two electrophilic reactive centers allowing them to participate in addition reactions via attack to the carbonyl group (1,2-addition) or involving the β-carbon (1,4-conjugate addition), leading to the synthesis of promising bioactive heterocyclic compounds. The purpose of this review is to present a systematic survey of the most recent literature that uses chalcones in the synthesis of biologically active 5- and 6-membered nitrogen heterocycles such as pyrroles, indoles, isoxazoles, imidazoles, pyrazoles, indazoles, triazoles, tetrazoles, pyridines and pyrimidines. Efficiency, easy-to-handle and cheap reagents, alternative heating conditions and greener protocols will be highlighted. In this review we will cover the literature since the beginning of the 21st century in more than 400 publications.

Cyclometalation reactions are usually defined as the reactions of metal compounds with conventional substrates, such as benzylamines and 2-phenylpyridines. In contrast, unconventional cyclometalation reactions are the reactions of metal compounds with unconventional base substrates, such as 2-amino-3-picoline, anilines and 2- aminopyridines, and with auxiliary substrates, such as aldehydes and alkynes. Other unconventional cyclometalation reactions also exist, examples include the cyclocarbonylations of metal carbonyl compounds with unsaturated hydrocarbons, such as alkynes, alkenes and aryl compounds; reactions with 2-vinylpyridines; and rollover cyclometalation reactions.