Nickel peroxide dehydrogenation of oxygen-, sulfur-, and nitrogen-containing heterocycles

Multicomponent synthesis of chromophores – The one-pot approach to functional π-systems

Multicomponent reactions, conducted in a domino, sequential or consecutive fashion, have not only considerably enhanced synthetic efficiency as one-pot methodology, but they have also become an enabling tool for interdisciplinary research. The highly diversity-oriented nature of the synthetic concept allows accessing huge structural and functional space. Already some decades ago this has been recognized for life sciences, in particular, lead finding and exploration in pharma and agricultural chemistry. The quest for novel functional materials has also opened the field for diversity-oriented syntheses of functional π-systems, i.e. dyes for photonic and electronic applications based on their electronic properties. This review summarizes recent developments in MCR syntheses of functional chromophores highlighting syntheses following either the framework forming scaffold approach by establishing connectivity between chromophores or the chromogenic chromophore approach by de novo formation of chromophore of interest. Both approaches warrant rapid access to molecular functional π-systems, i.e. chromophores, fluorophores, and electrophores for various applications.

A Novel preparation of 2-Aryl-5-Methoxyoxazoles

2-Aryl-5-methoxyoxazoles can be synthesized by oxidation of Schiff's bases with (diacetoxyiodo) benzene in good yields under mild conditions.

Heterogeneous mesoporous manganese oxide catalyst for aerobic and additive-free oxidative aromatization of N-heterocycles

Herein, we report a heterogeneous, aerobic, additive-free and environmentally benign catalytic protocol for oxidative aromatization of saturated nitrogen-heterocycles using a mesoporous manganese oxide material.

Insights into the Pummerer synthesis of oxazolines

A rapid and simple method to access unnatural 2-substituted 5-thio oxazolines has been developed. This methodology is based on a Pummerer reaction followed by an intramolecular nucleophilic substitution, which changes the paradigm for the normal use of a base in Pummerer chemistry. We also provide a useful two-step method for the synthesis of the starting material and a mechanistic proposal based on experimental observations, which contests the previously proposed reaction pathway. The reaction proved to be general, and different substituents, such as alkyl, aryl, alkenyl and functionalized groups, can be used without a significant decrease in efficiency.

Total synthesis of the azolemycins

The first total syntheses of newly isolated polyazole natural products azolemycins A–D, along with the synthesis of the tetra-oxazole non-natural analogue, are described.

Concise synthesis of 2,4-disubstituted thiazoles from β-azido disulfides and carboxylic acids or anhydrides: asymmetric synthesis of cystothiazole C

A novel and efficient method for the one-pot synthesis of 2,4-disubstituted thiazoles from carboxylic acids or anhydrides is presented. Based on this new method, the total synthesis of the bis-2,4-disubstituted bis(thiazoles) natural product cystothiazole C is also presented.

Copper-catalyzed oxidation of azolines to azoles

We report herein convenient, aerobic conditions for the oxidation of thiazolines to thiazoles and data regarding the oxidation mechanism. These reactions feature operationally simple and environmentally benign conditions and proceed in good yield to afford the corresponding azoles, thus enabling the inexpensive preparation of valuable molecular building blocks. Incorporation of a novel diimine-ligated copper catalyst, [((Mes)DAB(Me))Cu(II)(OH(2))(3)](2+) [(-)OTf](2), provides increased reaction efficiency in many cases. In other cases copper-free conditions involving a stoichiometric quantity of base affords superior results.

A direct synthesis of oxazoles from aldehydes

An expedient method for the direct conversion of aldehydes to 2,4-disubstituted oxazoles is presented. The method relies on the oxidation of an oxazolidine formed from the condensation of serine with an aldehyde and proceeds through a 2,5-dihydrooxazole intermediate. In contrast to standard methods that start from carboxylic acids, the use of aldehydes as starting materials does not require intermediate purification and affords the oxazoles under relatively mild conditions.

An effective C-C double bond formation via Cu(I)-catalyzed dehydrogenation

A dehydrogenation method using Cu(I) and either N,N-di-tert-butylthiadiaziridine 1,1-dioxide or N,N-di-tert-butyldiaziridinone is reported. The dehydrogenation allows a facile introduction of C-C double bond(s) into various carbo- and heterocycles such as oxazolines and thiazolines.

Total Synthesis of Potent Antifungal Marine Bisoxazole Natural Products Bengazoles A and B

The bengazoles are a family of marine natural products that display potent antifungal activity and a unique structure, containing two oxazole rings flanking a single carbon atom. Total syntheses of bengazole A and B are described, which contain a sensitive stereogenic centre at this position between the two oxazoles. Additionally, the synthesis of 10-epi-bengazole A is reported. Two parallel synthetic routes were investigated, relying on construction of the 2,4-disubstituted oxazole under mild conditions and a diastereoselective 1,3-dipolar cycloaddition. Our successful route is high yielding, provides rapid access to single stereoisomers of the complex natural products and allows the synthesis of analogues for biological evaluation.

Selective functionalization in positions 2 and 3 of indole via an iodine-copper exchange reaction

Two selectively successive I/Cu exchange reactions performed with (Nphyl)CuLi allow the functionalization of indoles in positions 2 and 3. The 2,3-diketones prepared by this method can readily be converted to tricyclic heterocycles by standard methods.

Dialkylaminoacetonitrile Derivatives as Amide Synthons. A One-Pot Preparation of Heteroaryl Amides via a Strategy of Sequential SNAr Substitution and Oxidation

Dialkylamino acetonitrile derivatives were utilized as alternative to cyanohydrin synthons for preparation of the corresponding heteroaryl dialkyl amides via a strategy of sequential base-mediated coupling and oxidation. The most advantageous oxidant, NiO(2)-H(2)O, can readily oxidize 2-substituted aminoacetonitriles to the corresponding amides under both basic and neutral conditions by forming cyanohydrins in situ.

Total Synthesis of Dendroamide A, a Novel Cyclic Peptide That Reverses Multiple Drug Resistance

Dendroamide A (1) was isolated from a blue-green alga on the basis of its ability to reverse drug resistance in tumor cells that overexpress either of the transport proteins, P-glycoprotein or MRP1. Because of this activity, methods for the synthesis of analogues of this oxazole- and thiazole-containing cyclic peptide have been developed, and the total synthesis of 1 has been completed. Highlights of the synthetic strategy are as follows: (1) a dicyclohexylcarbodiimide coupling of D-Ala and L-Thr, followed by reaction with Burgess reagent and DBU-assisted oxidation to form D-Ala-oxazole; (2) formation of D-Val-thiazole and D-Ala-thiazole via modified Hantzsch reactions; and (3) use of molecular modeling to select the preferred precursor for the final cyclization of the peptide analogue. Synthetic 1 demonstrated spectral properties identical to those of the natural product and reversed P-glycoprotein-mediated drug resistance more effectively than MRP1-mediated resistance. Certain of the synthetic precursors had biological activity, indicating that cell permeability and peptide cyclization are necessary for optimal activity. Thus, the structure and the biological activities of the natural product are confirmed, and methods for the synthesis of analogues for further structure-activity explorations are defined.