Asymmetric synthesis of the balanol heterocycle via a palladium-mediated epimerization and olefin metathesis

Photo-induced intramolecular dearomative [5 + 4] cycloaddition of arenes for the construction of highly strained medium-sized-rings

Medium-sized-ring compounds have been recognized as challenging synthetic targets in organic chemistry. Especially, the difficulty of synthesis will be augmented if an E-olefin moiety is embedded. Recently, photo-induced dearomative cycloaddition reactions that proceed via energy transfer mechanism have witnessed significant developments and provided powerful methods for the organic transformations that are not easily realized under thermal conditions. Herein, we report an intramolecular dearomative [5 + 4] cycloaddition of naphthalene-derived vinylcyclopropanes under visible-light irradiation and a proper triplet photosensitizer. The reaction affords dearomatized polycyclic molecules possessing a nine-membered-ring with an E-olefin moiety in good yields (up to 86%) and stereoselectivity (up to 8.8/1 E/Z). Detailed computational studies reveal the origin behind the favorable formation of the thermodynamically less stable isomers. Diverse derivations of the dearomatized products have also been demonstrated.

Diastereoselective Alkylations of Chiral Tetrazolo[1,5-a]azepines via Heterobenzylic Anion Intermediates

The alkylations of chiral seven-membered rings fused to tetrazoles are highly diastereoselective. The diastereoselectivity depended on the placement and the size of the substituent on the ring and on the electrophile. Subsequent alkylations occurred with high stereoselectivity, allowing for the construction of quaternary stereocenters. Computational studies revealed that torsional effects are responsible for the observed diastereoselectivities. Substituted products can be reduced to the corresponding secondary amines, thus providing an approach for synthesizing diastereomerically enriched azepanes.

Enantioselective Synthesis of Medium-Sized-Ring Lactones via Iridium-Catalyzed Z-Retentive Asymmetric Allylic Substitution Reaction

Medium-sized rings are important structural units, but their synthesis, especially in a highly enantioselective manner, has been a great challenge. Herein we report an enantioselective synthesis of medium-sized-ring lactones by an iridium-catalyzed Z-retentive asymmetric allylic substitution reaction. The reaction features mild conditions and a broad substrate scope. Various eight- to 11-membered-ring lactones can be afforded in moderate to excellent yields (up to 88%) and excellent enantioselectivity (up to 99% ee). The utilization of both Z-allyl precursors and an Ir catalyst is critical for the medium-sized-ring formation.

Comprehensive Account on the Synthesis of (-)-Balanol and its Analogues

BACKGROUND

A variety of diseases have been associated with hyperactivation of protein kinase C (PKC) enzymes such as cancer, diabetes, asthma, cardiovascular and central nervous system disorders. There is a dire need to selectively inhibit these enzymes by synthesizing new potent inhibitors. Balanol, a fungal metabolite belonging to the PKC inhibitor family, is especially included in this aspect. Tremendous effort has been put towards the synthesis of balanol by different research groups.

OBJECTIVE

The aim of this review is to provide a detailed description of synthetic approaches adopted for the synthesis of key fragments of balanol (azepane and benzophenone). All the factors that resulted in excellent yield and high enantioselectivity have also been mentioned.

CONCLUSION

It has been shown throughout this review that the synthesis of hexahydroazepine and benzophenone cores of balanol was achieved by employing a variety of important key steps with commercially available starting precursors, which make this total synthesis more valuable. Moreover, this article provides ideas to the synthetic as well as pharmaceutical chemists for the synthesis of (-)-balanol and its analogues.

Copper-catalyzed alkynylation/annulation cascades of N-allyl ynamides: regioselective access to medium-sized N-heterocycles

A synthetic strategy based on sequential application of aza-Claisen rearrangement, C–H functionalization, C–N coupling and cyclization as key steps has been developed for the synthesis of 6-, 7-, 8-, and 9-membered rings N-heterocycles.

Synthesis of chiral seven-membered cyclic sulfonamides through palladium-catalyzed arylation of cyclic imines

An enantioselective palladium-catalyzed addition of arylboronic acids to seven-membered cyclic N-sulfonyl aldimines and ketimines has been developed, giving the chiral cyclic sulfonamides with excellent yields and up to 99% ee.

Access to Chiral Seven-Member Cyclic Amines via Rh-Catalyzed Asymmetric Hydrogenation

A highly efficient asymmetric hydrogenation of azepine/oxazepine-type seven-member cyclic imine hydrochlorides was successfully developed using Rh/bisphosphine-thiourea ligand ZhaoPhos, affording various chiral seven-member cyclic amines with full conversions, high yields, and excellent enantioselectivities (up to 96% yield, >99% ee). Additionally, this asymmetric hydrogenation can proceed well on gram scale with excellent ee value. Moreover, control experimental results displayed that the anion-bonding interaction between the chloride ion of the substrate and thiourea motif of the ZhaoPhos played an important role to obtain excellent enantioselectivity.

A unified approach to the important protein kinase inhibitor balanol and a proposed analogue

A common approach to the important protein kinase inhibitor (−)-balanol and an azepine-ring-modified balanol derivative has been developed using an efficient fragment coupling protocol which proceeded in good overall yield.

Palladium-catalyzed insertion of CO2 into vinylaziridines: new route to 5-vinyloxazolidinones

2-Vinylaziridines undergo a mild Pd-catalyzed ring-opening cyclization reaction with an ambient atmosphere of carbon dioxide to give 5-vinyloxazolidinones. The process is high yielding as well as regio- and stereoselective.

Total synthesis of (-)-balanol, all stereoisomers, their N-tosyl analogues, and fully protected ophiocordin: an easy route to hexahydroazepine cores from garner aldehydes

Total syntheses of (-)-balanol and all of its stereoisomers starting from easily available Garner aldehydes are described. Diastereoselective Grignard reactions on Garner aldehydes and ring-closing metatheses are the key steps for the construction of hexahydroazepine subunits. The benzophenone subunits were constructed through coupling of suitably functionalized aromatic aldehyde and bromo components. The synthetic route constitutes a convenient and scalable reaction sequence to generate all of the stereoisomers of balanol. The methodology is explored further for the synthesis of N-tosyl analogues of balanol and of fully protected ophiocordin.

Chiral version of the Burgess reagent and its reactions with oxiranes: application to the formal enantiodivergent synthesis of balanol

An efficient formal synthesis of a (-)-balanol intermediate ( 25a) from cyclohexadiene oxide was accomplished in eight steps. An asymmetric version of the Burgess reagent allows for an enantiodivergent approach to both enantiomers of balanol from a racemic starting material.

Synthesis and evaluation of novel oxazoline MMP inhibitors

MMP inhibitors with novel oxazoline zinc binding groups have been synthesized and evaluated. Selectivity for the inhibition of MMP-9 over MMP-1, MMP-2, and MMP-12 has been achieved in several cases.

Nitrogen Heterocycles via Palladium-Catalyzed Carbocyclization. Formal Synthesis of (+)-α-Allokainic Acid

[reaction: see text] The Pd-catalyzed carbocyclization of ketoamides was investigated and found to be highly dependent on the phosphine ligand as well as the presence of coordinating counterions. Nitrogen heterocycles were formed without erosion of the stereochemical integrity. The utility of the lactam products was demonstrated by the formal synthesis of (+)-alpha-allokainic acid.

Facile and efficient total synthesis of (+)-preussin

[structure] The enantioselective total synthesis of (+)-preussin, a potent antifungal agent, has been achieved. The key steps are a Pd(0)-catalyzed oxazoline-forming reaction from L-phenylalanine, hydrogenolysis, and subsequent diastereoselective reductive cyclization of the intermediate aminoketone to pyrrolidine using Pearlman's catalyst.

Formal total synthesis of (-)-balanol: concise approach to the hexahydroazepine segment based on RCM

A concise synthesis of the hexahydroazepine moiety 13 of (-)-balanol 1 is described that comprises only eight steps and is distinctly shorter than all previous reported approaches to this particular compound. Sharpless epoxidation of divinylcarbinol 4 and ring closing alkene metathesis (RCM) reaction for the formation of the heterocyclic scaffold 9 constitute the key transformations of this sequence. The latter reaction is best achieved with catalytic amounts of the ruthenium indenylidene complex 18 recently reported. Furthermore, it is demonstrated that RCM can be successfully carried out even in the presence of an azido function provided that Schrock's molybdenum alkylidene complex Mo(=NAr)(=CHCMe2Ph)[OC(Me)(CF3)2]2 (Ar = 2,6-diisopropylphenyl) is used as precatalyst.