Palladium mediated stereospecific synthesis of 3-enynyl substituted thioflavones/flavones

Synthesis and reactivity of 3-(1-alkynyl)chromones

For the first time, the literature data on the methods of synthesis and reactivity of 3-(1-alkynyl)chromones are summarized and systematized. The main method for obtaining these compounds is the Sonogashira cross-coupling reaction of 3-halochromones with terminal acetylenes, and their most important chemical properties include the transformation into furans, reactions with dinucleophiles, ambiphilic [4+2]- and [4+3]-cyclizations, and also dimerization and mixed condensation of 2-methyl-3-(1-alkynyl)chromones due to the vinylogous methyl group. Except for the oxacyclization to furans, chemical transformations of 3-(1-alkynyl)chromones are accompanied by pyrone ring transformation, in which not only the carbonyl group but also the triple bond can participate. This significantly increases the synthetic value of these compounds and ensures the production of more complex heterocyclic systems based on them. The mechanisms of the reactions are discussed, the conditions for their implementation and the yields of the resulting products are indicated.

The bibliography includes 80 references.

Halogenated Flavones and Isoflavones: A State-of-Art on their Synthesis

BACKGROUND

Flavonoid is a family of compounds present in the everyday consumption plants and fruits, contributing to a balanced diet and beneficial health effects. Being a scaffold for new drugs and presenting a wide range of applicability in the treatment of illnesses give them also an impact in medicine. Among the several types of flavonoids, flavone and isoflavone derivatives can be highlighted due to their prevalence in nature and biological activities already established. The standard synthetic route to obtain both halogenated flavones and isoflavones is through the use of already halogenated starting materials. Halogenation of the flavone and isoflavone core is less common because it is more complicated and involves some selectivity issues.

OBJECTIVE

Considering the importance of these flavonoids, we aim to present the main and more recent synthetic approaches towards their halogenation.

METHODS

The most prominent methodologies for the synthesis of halogenated flavones and isoflavones were reviewed. A careful survey of the reported data, using mainly the Scopus database and halogenation, flavones and isoflavones as keywords, was conducted.

RESULTS

Herein, a review is provided on the latest and more efficient halogenation protocols of flavones and isoflavones. Selective halogenation and the greener methodologies, including enzymatic and microbial halogenations, were reported. Nevertheless, some interesting protocols that allowed the synthesis of halogenated flavone and isoflavone derivatives in specific positions using halogenated reagents are also summarized.

CONCLUSION

Halogenated flavones and isoflavones have risen as noticeable structures; however, most of the time, the synthetic procedures involve toxic reagents and harsh reaction conditions. Therefore, the development of new synthetic routes with low environmental impact is desirable.

Ionic Liquids and Ohmic Heating in Combination for Pd-catalyzed Cross-coupling Reactions: Sustainable Synthesis of Flavonoids

In order to meet the increasing demand for environmentally benign chemical processes, we developed a Suzuki–Miyaura reaction protocol based on the combination of ohmic heating (ΩH) and supported ionic liquid phase catalysis (SILPC) in aqueous media. This methodology was applied to the synthesis of a series of flavonoid derivatives, including isoflavones, styrylisoflavones, and diarylalkenylisoflavones.

Donor-Acceptor Substituted Benzo-, Naphtho- and Phenanthro-Fused Norbornadienes

The photochromic norbornadiene/quadricyclane (NBD/QC) couple has found interest as a molecular solar thermal energy (MOST) system for storage of solar energy. To increase the energy difference between the two isomers, we present here the synthesis of a selection of benzo-fused NBD derivatives that contain an aromatic unit, benzene, naphthalene or phenanthrene, fused to one of the NBD double bonds, while the carbon atoms of the other double bond are functionalized with donor and acceptor groups. The synthesis protocols involve functionalization of benzo-fused NBDs with bromo/chloro substituents followed by a subjection of these intermediates to a cyanation reaction (introducing a cyano acceptor group) followed by a Sonogashira coupling (introducing an arylethynyl donor group, -CCC₆H₄NMe₂ or -CCC₆H₄OMe). While the derivatives have good absorption properties in the visible region (redshifted relative to parent system) in the context of MOST applications, they lack the ability to undergo NBD-to-QC photoisomerization, even in the presence of a photosensitizer. It seems that loss of aromaticity of the fused aromatics is too significant to allow photoisomerization to occur. The concept of destroying aromaticity of a neighboring moiety as a way to enhance the energy density of the NBD/QC couple thus needs further structural modifications, in the quest for optimum MOST systems.

Pd-mediated synthesis of substituted benzenes fused with carbocycle/heterocycle

A new Pd-catalyzed one-pot multicomponent coupling reaction for the construction of benzene ring fused with carbocycle or heterocycle under a Cu-free condition is described.

Microwave enhanced palladium catalysed coupling reactions: A diversity-oriented synthesis approach to functionalised flavones

Microwave enhanced diversity-oriented synthesis (MEDOS) using palladium catalysed protocols is introduced as a powerful new strategy for the synthesis of systematically modified small molecules and is highlighted by application to functionalised flavones.

Preparation and Reaction of 2-Aryl-3-silyl-1,3-butadiene

[reaction: see text] A conjugated pi-electron compound, 2-aryl-3-silyl-1,3-butadiene, was easily prepared from 1-benzyloxy-3-silyl-2-propyne, bis(iodozincio)methane, and an aryl halide in the presence of nickel catalyst. A subsequent cross-coupling reaction of the product with another aryl halide gave an unsymmetrical 2,3-diaryl-1,3-butadiene efficiently.

Regio- and Stereospecific Synthesis of Novel 3-Enynyl-Substituted Thioflavones/Flavones Using a Copper-Free Palladium-Catalyzed Reaction†

[reaction: see text] A variety of 3-enynyl substituted flavones/thioflavones were synthesized via a sequential one-pot procedure using copper-free palladium-catalyzed cross coupling in a simple synthetic operation. The cross coupling between 3-iodo(thio)flavone and a broad range of terminal alkynes was carried out in the presence of Pd(PPh3)2Cl2 and triethylamine to afford the corresponding 3-enynyl derivatives in a regio- and stereoselective fashion. The best results are obtained by employing 3 equiv of the terminal alkynes. The process worked well irrespective of the substituents present on the (thio)flavone ring as well as in the terminal alkynes except arylalkynes. The reaction is quite regioselective, placing the substituent of the terminal alkyne at the far end of the double bond attached with the (thio)flavone ring. The orientation of the (thio)flavonyl and acetylenic moieties across the double bond was found to be syn in the products isolated. A tandem C-C bond-forming reaction in the presence of palladium catalyst rationalized the formation of coupled product. The catalytic process apparently involves heteroarylpalladium formation, regioselective addition to the C-C triple bond of the terminal alkyne, and subsequent displacement of palladium by another mole of alkyne. The present methodology is useful for the introduction of an enynyl moiety at the C-3 position of flavones and thioflavone rings to afford novel compounds of potential biological interest. In the presence of CuI the process afforded 3-alkynyl (thio)flavones in good yields.

Enynylation of 2-Iodo-4-(phenylchalcogenyl)-1-butenes via Intramolecular Chelation: Approach to the Synthesis of Conjugated Dienynes or Trienynes

[reaction: see text] 2-Iodo-4-(phenylchalcogenyl)-1-butenes 3 and 4, which are derived from methylenecyclopropanes 1, can be enynylated with alkynes catalyzed by Pd(OAc)(2) to give conjugated dienynes 5 and 6 in the absence of any phosphine ligand and copper salt, and trienyne 9a can be obtained by oxidation of compound 5a. A plausible reaction mechanism has been proposed.

Palladium-Catalyzed Intermolecular Three-Component Coupling of Aryl Iodides, Alkynes, and Alkenes To Produce 1,3-Butadiene Derivatives

[reaction: see text] The sequential three-component coupling of aryl iodides, diarylacetylenes, and monosubstituted alkenes effectively proceeds in the presence of Pd(OAc)(2), LiCl, and NaHCO(3) as catalyst, promoter, and base, respectively, in DMF-H(2)O to produce the corresponding 1,3-butadiene derivatives.