Current Organic Synthesis - Volume 12, Issue 1, 2015

Hydroxycarbazoles are versatile and widely used starting materials capable of react with a variety of synthons. This review provides a source which pulls together the known one-pot and multistep synthesis of carbazole heterocycles such as, furocarbazoles, indolocarbazoles, indoloisoflavones, indoloxanthanes, isochromenocarbazolones, pyranocarbazoles, pyridocarbazoles using hydroxycarbazoles as precursors.

The multicomponent reaction (MCR) is an important research field in the organic synthetic methodology. In 1956, Professor Friedrich Asinger reported a method for synthesizing thiazoline scaffold, in which a ketone was treated by sulfur and ammonia in one-pot operation. In 1998, Professor Katrin Groebke, Hugues Bienayme, and Christopher Blackburn reported a method for preparing imidazo[1,2-a]pyridine, in which an annulation took place among α- aminopyridine, aldehyde, and isocyanide in one-pot operation, and then the reaction was entitled as Groebke reaction. Comparing with the bloom in the research of other MCRs such as Strecker (found in 1850), Hantzsch (1882), Biginelli (1891), Mannich (1912), Passerini (1921) and Ugi (1959) MCR, as newcomers in the family of MCR, Asinger and Groebke reactions are not reported as usually as other MCRs. The aim of the present review is to emphasize the importance of these two MCRs in the synthesis of thiazolines and imidazolines. Although the Asinger protocol is innovated as the reaction of α-halogensubstituted ketone with thioamide, or, the C-S bond in thiazoline is contributed from mercapto or thiocarbonyl groups, as well as β- aminothiol as the synthon reacts with carbonyl compounds to form the thiazoline scaffold, the advantage of Asinger reaction is still worthy to be noted because four bonds in thiazoline are produced simultaneously from the reaction of ketone with the simple reactants, sulfur and ammonia. On the other hand, the review on the Groebke reaction is herein catalogued as substrates and catalysts. The 2- aminopyridine-type substrates are the necessary reagents for the Groebke reaction and therefore limit the applicability of the reaction, but some efforts succeed in applying catalysts for enlarging the suitable substrates or agitating the following reactions based on the Groebke adducts. To sum up the present achievements on Asinger and Groebke reaction, it can be concluded that the exploration on catalysts will break the limitation of the substrates and bring them with wide application in the synthesis of thiazolines and imidazolines.

The synthesis of a new non-cross-linked polystyrene supported cyclosulfamide chiral auxiliary is described. Asymmetric aldol reactions using this auxiliary proceeded with good yields and excellent stereoselectivities, which were comparable with those observed by using non-supported cyclosulfamide chiral auxiliary. Furthermore, recovery and recycling of this polymer supported chiral auxiliary have been successfully achieved without appreciable reduction in the yield or stereoselectivity.

An efficient methodology for the synthesis of alcohols in high yields with the selective deprotection of methoxy methyl (MOM) ethers using Zinc(II) trifluoro methane sulfonate (Zn(OTf)2) in isopropanol as a solvent under reflux conditions was reported.

A new family of bipolar hosts TPA-DAF, TPA-(DAF)2 and TPA-(DAF)3 based on triphenylamine and 4,5- diazafluorene moiety were synthesized through Friedel-Crafts reaction, and all of the three prepared hosts showed excellent thermal stability and exhibited blue fluorescence in solution.

An efficient, three component synthesis of novel class of mono, bis and trisdihydropyridines which are fused with pyrazole rings from reaction between pyrazolecarbaldehyde, dimedone and ammonium acetate was described. In this research, mono, bis and trisdihydropyridines were synthesized in the presence of a catalytic amount of recyclable catalyst, nano particle Fe3O4.

A novel and flexible synthetic approach to enureas is described. This convenient method has been developed for the Pd-catalyzed C-N coupling reaction of alkenyl nonaflates with a variety of ureas. A ferrocene based phosphine ligand, Joshiphos appeared noticeable as the most dynamic catalyst and K3PO4 in t-BuOH was established to be the most favorable base-solvent combination for the reaction. This catalyst system displayed amazing functional group tolerance and assigned quick synthesis of the entire series of enureas in excellent yields.

The domino Knoevenagel/Diels-Alder condensation is a highly efficient strategy for the preparation of heterocyclic compounds. In order to study this reaction with an imine catalyst, a three-component reaction involving aromatic aldehydes, benzylideneacetone and meldrum acid catalyzed by a series of new organocyclic amino acid catalysts was developed. This simple experiment and environmentally friendly method is beneficial for us to get numerous pharmacological multi substituted spiro[5,5]undecane-1,5,9-triones in moderate to good yields (up to 81%) with excellent diastereolectivities (>99:1 dr). Moreover, we expanded the reaction substrates, such as barbituric acid, and the catalytic activity was compared with L-proline that is one of the most commonly secondary amine organocatalysts.

A direct and efficient approach for the synthesis of new series of pyrido[3,4-c][1,5]benzothiazepine, pyrido[4,3-d]triazolo[1',5'-a]pyrimidines, pyrido[4,3-d]tetrazolo [1',5'-a]pyrimidine and pyrido[4,3-4',5']pyrimido[1',2'- a]benzimidazoles has been developed via condensation of o-aminothiophenol and heterocyclic amines each with 1-ethyl- 4-piperidinone and the appropriate aldehyde. The reactions were performed in refluxing acetonitrile in the presence of molecular iodine as a catalyst. Also, the 1,3-dipolar cycloadditions of nitrilimines to the latter products were examined and were found to be site- and regioselective. The structures of the new synthesized compounds were established on the basis of spectral data (Mass, IR, 1H and 13C NMR) and elemental analyses.

A diastereoselective synthesis of functionalised tetrahydroquinolines was performed using a one-pot imino Diels-Alder cycloaddition reaction of anilines, benzaldehydes and isoeugenol at room temperature using propylene carbonate as an environmentally friendly solvent and molecular iodine as an efficient catalyst. This new protocol has the advantages of higher yields, mild conditions, simple methodology, a short reaction time and a straightforward work-up. Propylene carbonate served as an alternative solvent to the organic solvents commonly used for this reaction, e.g., CH2Cl2, DMF, and MeCN.