An enantio- and diastereoselective approach to indoloquinolizidines in continuous flow.

Merging polymer-supported asymmetric organocatalysis with continuous flow in a packed bed reactor has been used as the key, enantiodetermining step in a short synthesis of indoloquinolizidines. Using this approach, a highly enantioselective, solvent-free and rapid conjugate addition of dimethyl malonate to a diverse family of cinnamaldehydes in continuous flow, allowing the preparation of relevant oxodiesters in multigram amounts has been developed. The obtained Michael adducts have been used to complete an expedient diastereoselective synthesis of indoloquinolizidine via cascade Pictet-Splengler cyclisation-lactamisation in continuous flow. The conversion of enantiopure Michael adducts into δ-lactones via telescoped reduction/cyclisation in continuous flow has also been explored.

Catalytic Acceptorless Dehydrogenation of Amino Alcohols and 2-Hydroxybenzyl Alcohols for Annulation Reaction under Neutral Conditions.

A base-free and acceptorless Ru-catalyzed dehydrogenative approach has been developed for the synthesis of N-heterocycles by using 1,3-dicarbonyls and amino alcohols through a domino sequential enamine formation and intramolecular oxidative cyclization strategy. This unified approach is also applicable for the synthesis of O-heterocycles involving 2-hydroxybenzyl alcohol as a coupling reactant via consecutive C-alkylation and intramolecular cyclization steps. The present protocol is general for the synthesis of varieties of biologically important scaffolds, such as tetrahydro-4H-indol-4-one, 3,4-dihydroacridin-1(2H)-one, and tetrahydro-1H-xanthen-1-ones derivatives using a single catalytic system, viz. RuH2CO(PPh3)3. Environmentally benign H2O and H2 are the only byproducts in this domino process. Moreover, RuH2CO(PPh3)3-catalyzed C3-alkylation of tetrahydro-4H-indol-4-one using alcohol as a alkylating partner is also described in this report. For the first time, a solvent-free gram-scale reaction for the acceptorless dehydrogenative annulation has been demonstrated. A plausible mechanism for the Ru-catalyzed base-free and acceptorless dehydrogenative annulation of amino alcohols or 2-hydroxybenzyl alcohols has been provided with several experimental investigations and spectroscopic evidence.

Manganese-Catalyzed Synthesis of Quaternary Peroxides: Application in Catalytic Deperoxidation and Rearrangement Reactions.

Highly efficient, selective, and direct C-H peroxidation of 9-substituted fluorenes has been achieved using a Mn-2,2'-bipyridine catalyst via radical-radical cross-coupling. Moreover, this method effectively promotes the vicinal bisperoxidation of sterically hindered various substituted arylidene-9H-fluorene/arylideneindolin-2-one derivatives to afford highly substituted bisperoxides with high selectivity over the oxidative cleavage of C═C bond that usually forms the ketone of an aldehyde. Furthermore, a new approach for the synthesis of (Z)-6-benzylidene-6H-benzo[c]chromene has been achieved via an acid-catalyzed skeletal rearrangement of these peroxides. For the first time, unlike O-O bond cleavage, reductive C-O bond cleavage in peroxides using the Pd catalyst and H2 is described, which enables the reversible reaction to afford exclusively deperoxidized products. A detailed mechanism for peroxidation, molecular rearrangement, and deperoxidation has been proposed with preliminary experimental evidences.

Sn-Catalyzed Criegee-Type Rearrangement of Peroxyoxindoles Enabled by Catalytic Dual Activation of Esters and Peroxides.

We report here the Sn-catalyzed mild protocol for ring expansion of peroxyoxindoles to afford the series of substituted-2H-benzo[b][1,4]oxazin-3(4H)-one derivatives. In this protocol, we showed the in situ conversion of tert-butyl peroxy compounds into peresters with the aid of external esters, which then underwent the ring-expansion process, and the incipient carbocation was trapped with the alcohol residue generated from the esters. The reaction is also demonstrated in a continuous flow process to afford the rearranged product in 22 min of residence time.

The Rearrangement of Peroxides for the Construction of Fluorophoric 1,4-Benzoxazin-3-one Derivatives.

An unprecedented skeletal rearrangement of 3-( tert-butylperoxy)indolin-2-one using a tin catalyst has been developed. This rearrangement is highly selective to afford a series of fluorophoric ( Z)-2-arylidene and alkylidene-2 H-benzo[ b][1,4]oxazin-3(4 H)-one derivatives in good to excellent yield. In contrast with Sn(OTf)2, the reaction of 3-( tert-butylperoxy)indolin-2-one derivatives with FeCl3 afforded the Hock fragmentation product via C-C bond cleavage.

Recent Advances in the Metal-Catalyzed Activation of Amide Bonds.

The amide functional group is commonly found in peptides, proteins, pharmaceutical compounds, natural products, and polymers. The synthesis of amides is typically performed by using classical approaches that involve the reaction between a carboxylic acid and an amine in the presence of an activator. Amides are thought to be an inert functional group, because they are unsusceptible to nucleophile attack, owing to their low electrophilicity. The reason for this resistance is clear: the resonance stability of the amide bond. However, transition metal catalysis can circumvent this stability by selectively rupturing the N-C bond of the amide, thereby facilitating further cross-coupling or other reactions. In this Focus Review, we discuss the recent advances in this area and present a summary of methods that have been developed for activating the amide N-C bond by using precious and non-precious metals.

Ru-Catalyzed dehydrogenative synthesis of antimalarial arylidene oxindoles.

Ru(ii)-NHC catalyzes α-olefination of 2-oxindoles using diaryl methanols in the absence of an acceptor. A wide array of symmetrical and unsymmetrical diaryl methanols undergoes dehydrogenative coupling with 2-oxindole selectively to generate various substituted 3-(diphenylmethylene)indolin-2-one derivatives in good yields and produces environmentally benign by-products, H2 and H2O. This methodology was successfully applied for the synthesis of a bioactive drug i.e. TAS-301. The biological activities of the synthesized 3-(diphenylmethylene)indolin-2-one derivatives were screened against the Plasmodium falciparum parasite and found to exhibit a significant activity with IC50 = 2.24 µM.

Iron-Catalyzed Batch/Continuous Flow C-H Functionalization Module for the Synthesis of Anticancer Peroxides.

Iron-catalyzed dehydrogenative cross-coupling of carbonyl compounds with aliphatic peroxide was developed under mild conditions. A library of linear alkylated and arylated peroxides are synthesized in good to excellent yield. This method is highly selective and general for a range of biologically important derivatives of 2-oxindole, barbituric acid, and 4-hydroxy coumarin with a good functional group tolerance and without the cleavage of the peroxide bond. This peroxidation reaction is upscalable to grams and also synthesizable in continuous flow with increased safety in short duration. Mechanistic investigation reveals Fe-(II) undergoes redox type process to generate the radical intermediates, which subsequently recombine selectively to form the stable peroxides. The potential of peroxides is evaluated by cell viability assay and found to exhibit the good anticancer activity with minimum IC50= 5.3 µM.

Transition-Metal-Free C-H Hydroxylation of Carbonyl Compounds.

Transition metal and reductant free α-C(sp3)-H hydroxylation of carbonyl compounds are reported. This method is promoted by commercially available inexpensive KO-t-Bu and atmospheric air as an oxidant at room temperature. This unified strategy is also very facile for hydroxylation of various carbonyl compound derivatives to obtain quaternary hydroxyl compounds in excellent yield. A preliminary mechanistic investigation, supported by isotope labeling and computational studies, suggests the formation of a peroxide bond and its cleavage by in situ generated enolate.