Metathetic approach to naphthoxepin and spirocyclic molecular frameworks

The Mizoroki-Heck reaction between in situ generated alkenes and aryl halides: cross-coupling route to substituted olefins

This review covers palladium-catalyzed typical Mizoroki-Heck cross-coupling reactions of aryl halides with in situ generated alkenes, by following a typical Heck coupling mechanism to form substituted olefins unlike direct cross-coupling of alkenes with aryl halides in Heck olefination. These reactions solve the issue of alkenes undergoing polymerization at high temperatures and increase reaction efficiency by reducing the reaction time and purification steps.

Synergistic approach to polycycles through Suzuki-Miyaura cross coupling and metathesis as key steps

This account provides an overview of recent work, including our own contribution dealing with Suzuki–Miyaura cross coupling in combination with metathesis (or vice-versa). Several cyclophanes, polycycles, macrocycles, spirocycles, stilbenes, biaryls, and heterocycles have been synthesized by employing a combination of Suzuki cross-coupling and metathesis. Various popular reactions such as Diels–Alder reaction, Claisen rearrangement, cross-metathesis, and cross-enyne metathesis are used. The synergistic combination of these powerful reactions is found to be useful for the construction of complex targets and fulfill synthetic brevity.

Application of Claisen Rearrangement and Olefin Metathesis in Organic Synthesis

Several new synthetic methods to diverse polycycles and heterocycles on the basis of [3,3]-sigmatropic rearrangement and ring-closing metathesis as key steps are summarized. In this regard, Claisen, aza-Claisen, and Overman rearrangements are used in combination with other popular reactions such as ring-closing metathesis, Suzuki coupling, Diels-Alder reaction, and retro-Diels-Alder reaction. To this end, we prepared polycycles (e.g., cyclophanes, cage molecules), heterocycles (e.g., oxepins, azapins), carbocycles, and spirocycles with the use of the above protocol.

Design and Synthesis of Polycycles, Heterocycles, and Macrocycles via Strategic Utilization of Ring-Closing Metathesis

In this perspective, we summarize new synthetic approaches for the construction of various polycyclic compounds involving ring-closing metathesis as a key step. In this regard, we used ring-closing metathesis in combination with other popular reactions like Suzuki-Miyaura coupling, Claisen rearrangement, Fischer indolization, Grignard addition, Diels-Alder reaction, and [2+2] cycloaddition reaction etc. To this end, a variety of functional molecules such as α-amino acids, cyclophanes, heterocycles, propellanes, spirocycles, and macrocycles have been prepared. The strategies developed and the molecules prepared here play a key role in designing new materials and also act as lead compounds in drug design. The strategies and tactics developed here are useful to design polycycles, macrocycles, and heterocycles of diverse ring systems.

Bridgehead vicinal diallylation of norbornene derivatives and extension to propellane derivatives via ring-closing metathesis

Here, we report a simple synthetic strategy to the bridgehead vicinal diallylation of norbornene derivatives. These substrates are useful to generate propellanes via ring-closing metathesis. Single-crystal X-ray diffraction analysis of four compounds led to the realization of configurational correction of earlier reported molecules.

Highly Efficient Cascade Reaction for Selective Formation of Spirocyclobutenes from Dienallenes via Palladium-Catalyzed Oxidative Double Carbocyclization-Carbonylation-Alkynylation

A highly selective cascade reaction that allows the direct transformation of dienallenes to spirocyclobutenes (spiro[3.4]octenes) as single diastereoisomers has been developed. The reaction involves formation of overall four C–C bonds and proceeds via a palladium-catalyzed oxidative transformation with insertion of olefin, olefin, and carbon monoxide. Under slightly different reaction conditions, an additional CO insertion takes place to give spiro[4.4]nonenes with formation of overall five C–C bonds.

Construction of spirocarbocycles via gold-catalyzed intramolecular dearomatization of naphthols

A highly efficient, gold-catalyzed intramolecular dearomatization reaction of naphthols via 5-endo-dig cyclization is described. This facile and direct approach furnishes spirocarbocycles in excellent yields under mild conditions.

Design and synthesis of propellane derivatives and oxa-bowls via ring-rearrangement metathesis as a key step

Various intricate propellane derivatives and oxa-bowls have been synthesized via a ring-rearrangement metathesis (RRM) as a key step starting from readily accessible starting materials such as p-benzoquinone, 1,4-naphthoquinone and 1,4-anthraquinone.

Hybrid macrocycle formation and spiro annulation on cis-syn-cis-tricyclo[6.3.0.0(2,6)]undeca-3,11-dione and its congeners via ring-closing metathesis

We have developed a simple methodology to transform cis-syn-cis-triquinane derivative 2 into the diindole based macrocycle 6 involving Fischer indolization and ring-closing metathesis (RCM). Various spiro-polyquinane derivatives have been assembled via RCM as a key step.

A synergistic approach to polycyclics via a strategic utilization of Claisen rearrangement and olefin metathesis

Olefin metathesis promoted by a well-defined metal carbene complex has evolved into an efficient method for the construction of a broad range of carbocyclic and heterocyclic rings of varying size. The synthetic potential of the olefin metathesis has been further increased by combining various other C-C bond forming processes either in tandem or in sequence. Herein, application of Claisen rearrangement and olefin metathesis to prepare various intricate and/or biologically important targets has been described.

Studies on the Claisen rearrangements in the indolo[2,3-b]quinoline system

A study of the effect of substrate structure on a Claisen-aza-Cope reaction is presented including a rationalisation of the reaction outcome using DFT calculations. An asymmetric version of the reaction is also described that is of relevance to a proposed approach to the communesin family of natural products.

Total synthesis of the indolizidine alkaloid tashiromine

BACKGROUND

Tashiromine 1 is a naturally occurring indolizidine alkaloid. It has been the subject of thirteen successful total syntheses to date. Our own approach centres on the stereoselective construction of the indolizidine core by capture of an electrophilic acyliminium species by a pendant allylsilane. The key cyclisation precursor is constructed using olefin cross-metathesis chemistry, which has the potential to facilitate both racemic and asymmetric approaches, depending upon the choice of the allylsilane metathesis partner.

RESULTS

The use of the allyltrimethylsilane cross-metathesis approach enables the rapid construction of the key cyclisation precursor 3 (3 steps from commercial materials), which undergoes acid-induced cyclisation to give the desired bicyclic indolizidine skeleton as a 96:4 mixture of diastereomers. Simple functional group interconversions allowed the completion of the total synthesis of racemic tashiromine in six steps (19% overall yield). Three chiral alpha-alkoxyallylsilanes (12,14 and 15) were prepared in enantioenriched form and their cross-metathesis reactions studied as part of a putative asymmetric approach to tashiromine. In the event, alpha-hydroxysilane 12 underwent isomerisation under the reaction conditions to acylsilane 17, while silanes 14 and 15 were unreactive towards metathesis.

CONCLUSION

A concise, stereoselective total synthesis of racemic tashiromine has been developed. Attempts to translate this into an asymmetric synthesis have thus far been unsuccessful.

Diversity-Oriented Approach to Biologically Relevant Molecular Frameworks Starting with β-Naphthol and Using the Claisen Rearrangement and Olefin Metathesis as Key Steps

A diversity-oriented approach for the synthesis of various structurally different molecular frameworks from readily accessible and common precursors is described. A Claisen rearrangement followed by ring-closing metathesis or ethylene-promoted ring-closing enyne metathesis has been utilized as the key synthetic transformation to generate naphthoxepine derivatives. The ring-closing metathesis approach has also been used to generate spirocyclic compounds and the pleiadene framework.

Spiro-annulation of barbituric acid derivatives and its analogs by ring-closing metathesis reaction

Barbituric acid 1 and related beta-dicarbonyl compounds were dialkenylated under the phase-transfer catalyst [e.g., benzyltriethylammonium chloride (BTEAC)] conditions to generate the diallylated products. These diallylated products were subjected to the ring-closing metathesis (RCM) reaction to deliver the corresponding spiro-annulated derivatives.