Tandem RCM−Isomerization Approach to Glycals of Desoxyheptoses from a Common Precursor

Efficient Synthesis of Cyclohepta[b]indoles and Cyclohepta[b]indole-Indoline Conjugates via RCM, Hydrogenation, and Acid-Catalyzed Ring Expansion: A Biomimetic Approach

Cyclohepta[b]indoles, prevalent in natural products and pharmaceuticals, are conventionally accessed via metal or Lewis acid-mediated cycloadditions with prefunctionalized substrates. Our study introduces an innovative sequential catalytic assembly for synthesizing cyclohepta[b]indoles from readily available isatin derivatives. The process involves three catalytic sequences: ring-closing metathesis, catalytic hydrogenation, and acid-catalyzed ring expansion. The RCM of 2,2-dialkene-3-oxindoles, formed by butenyl Grignard addition to 3-allyl-3-hydroxy-2-oxindoles, yields versatile spirocyclohexene-3-oxindole derivatives. These derivatives undergo further transformations, including dibromination, dihydroxylation, epoxidation, Wacker oxidation at the double bond. Hydrogenation of spirocyclohexene-3-oxindole yields spirocyclohexane-3-oxindoles. Their subsequent acid-catalyzed ring expansion/aromatization, dependent on the acid catalyst, results in either cyclohepta[b]indoles or cyclohepta[b]indole-indoline conjugates, adding a unique synthetic dimension. The utility of this methodology is exemplified through the synthesis of an A-FABP inhibitor, showcasing its potential in pharmaceutical applications.

Ring closing metathesis for the construction of carbazole and indole-fused natural products

The synthesis and functionalization of carbazole ring systems have received considerable attention in organic synthesis due to their widespread occurrence in biologically active compounds. One of the classical methods for the synthesis of carbazoles involves C-C bond formation of a biaryl amine moiety by oxidizing agents. Over the last few years, various new strategies have evolved for the synthesis of carbazole ring systems. During the past two decades, ring-closing metathesis (RCM) based approaches have been efficiently employed for the synthesis of nitrogen containing heteroaromatic systems including carbazoles. Herein, we discuss the construction of carbazole ring systems using RCM and the application of RCM based methods in the preparation of other indole-fused heterocycles. The application of these methods in the synthesis of carbazole alkaloids and bioactive indole-fused natural products has been discussed to highlight the importance of RCM in total synthesis.

2-Methyltetrahydrofuran as a Solvent of Choice for Spontaneous Metathesis/Isomerization Sequence

A new protocol for ring-closing metathesis/isomerization sequence was developed. The reactions of selected dienes were performed in overheated 2-methyltetrahydrofuran at 120 °C and provided a wide range of cyclic vinyl ethers and amides with good yields and selectivities. Computational analysis suggests that the relative yield of products depends on a thermodynamically driven process on the basis of relative stabilities of isomers.

Synthesis of Phenylpropanoids via Matsuda-Heck Coupling of Arene Diazonium Salts

The Pd-catalyzed Heck-type coupling (Matsuda-Heck reaction) of electron rich arene diazonium salts with electron deficient olefins has been exploited for the synthesis of phenylpropanoid natural products. Examples described herein are the naturally occurring benzofurans methyl wutaifuranate, wutaifuranol, wutaifuranal, their 7-methoxy derivatives, and the O-prenylated natural products boropinols A and C.

Cross metathesis of allyl alcohols: how to suppress and how to promote double bond isomerization

Under standard conditions the cross metathesis of allyl alcohols and methyl acrylate is accompanied by the formation of ketones, resulting from uncontrolled and undesired double bond isomerization. By conducting the CM in the presence of phenol, the catalyst loading and the reaction time required for quantiative conversion can be reduced, and isomerization can be suppressed. On the other hand, consecutive isomerization can be deliberately promoted by evaporating excess methyl acrylate after completing cross metathesis and by adding a base or silane as chemical triggers.

Build/couple/pair strategy combining the Petasis 3-component reaction with Ru-catalyzed ring-closing metathesis and isomerization

A "build/couple/pair" pathway for the systematic synthesis of structurally diverse small molecules is presented. The Petasis 3-component reaction was used to synthesize anti-amino alcohols displaying pairwise reactive combinations of alkene moieties. Upon treatment with a ruthenium alkylidene-catalyst, these dienes selectively underwent ring-closing metathesis reactions to form 5- and 7-membered heterocycles and cyclic aminals via a tandem isomerization/N-alkyliminium cyclization sequence.

Synthesis and conformational analysis of locked carbocyclic analogues of 1,3-diazepinone riboside, a high-affinity cytidine deaminase inhibitor

Cytidine deaminase (CDA) catalyzes the deamination of cytidine via a hydrated transition-state intermediate that results from the nucleophilic attack of zinc-bound water at the active site. Nucleoside analogues where the leaving NH(3) group is replaced by a proton and prevent conversion of the transition state to product are very potent inhibitors of the enzyme. However, stable carbocyclic versions of these analogues are less effective as the role of the ribose in facilitating formation of hydrated species is abolished. The discovery that a 1,3-diazepinone riboside (4) operated as a tight-binding inhibitor of CDA independent of hydration provided the opportunity to study novel inhibitors built as conformationally locked, carbocyclic 1,3-diazepinone nucleosides to determine the enzyme's conformational preference for a specific form of sugar pucker. This work describes the synthesis of two target bicyclo[3.1.0]hexane nucleosides, locked as north (5) and south (6) conformers, as well as a flexible analogue (7) built with a cyclopentane ring. The seven-membered 1,3-diazepinone ring in all the three targets was built from the corresponding benzoyl-protected carbocyclic bis-allyl ureas by ring-closing metathesis. The results demonstrate CDA's binding preference for a south sugar pucker in agreement with the high-resolution crystal structures of other CDA inhibitors bound at the active site.