An efficient method for the preparation of 3-hydroxyl-5-substituted 2-pyrrolidones and application in the divergent synthesis of (−)-preussin and its analogues

AgSbF6-catalyzed C3 aza-Friedel-Crafts alkylation of N,O-acetals with indoles for the synthesis of N-α indole substituted pyrrolidine and piperidine derivatives

An efficient approach to access chiral N-α indole substituted pyrrolidine and piperidine skeletons has been developed through a AgSbF₆-catalyzed N-α aza-Friedel-Crafts alkylation of N,O-acetals 6a, 6b, 9, and 11a-11d with indoles. As a result, a series of 2,3-trans N-α indole substituted pyrrolidines 8a-8x and piperidines 10a-10j were prepared in moderate to excellent yields and with excellent diastereoselectivities (dr up to 99 : 1). Moreover, several 2,5-cis-N-α indole substituted pyrrolidine derivatives 12a-12k were synthesized according to this strategy with moderate to good yields and diastereoselectivities (dr up to 99 : 1).

SmI2‑Mediated Intermolecular Addition-Elimination of Piperidine and Pyrrolidine N-α-Radicals with Arylacetylene Sulfones

An efficient approach to access α-arylacetylene substituted pyrrolidine and piperidine derivatives has been developed through a samarium diiodide mediated addition-elimination process of pyrrolidine and piperidine N-α-radicals with arylacetylene sulfones.

Synthetic Approaches Towards the Total synthesis of tubulysin and its fragments: A review

BACKGROUND

Tubulysins, linear tetrapeptides show extraordinary cytotoxicity against various cancer cells, with IC50 values in nano or picomolar range. Due to their extremely vigorous anti-proliferative and antiangiogenic characteristics, tubulysins exhibit captivating prospects in the development of anticancer drugs. This review focuses on diverse routes for the total synthesis of natural and synthetic tubulysins as well as their fragments.

OBJECTIVE

The purpose of this review is to present the synthetic strategies for the development of antitumor agents, tubulysins.

CONCLUSION

A range of synthetic pathways adopted for the total synthesis of tubulysins and their fragments have been described in this review. Synthesis of fragments, Tuv, Tup, and Tut can be accomplished by adopting appropriate strategies such as Manganese-mediated synthesis, Ireland-Claisen rearrangement, Mukaiyama aldol reaction, and Mannich process etc. Tubulysin B, D, U, V, and N14-desacetoxytubulysin H have been prepared through Mitsunobu reaction, tert-butanesulfinamide method, Tandem reaction, aza-Barbier reaction, Evans aldol reaction, and C-H activation strategies etc. The remarkable anticancer potential of tubulysins toward a substantiate target make them prominent leads for developing novel drugs against multidrug-resistant cancers.

Diastereocontrol in Radical Addition to β-Benzyloxy Hydrazones: Revised Approach to Tubuvaline and Synthesis of O-Benzyltubulysin V Benzyl Ester

Radical addition to chiral N-acylhydrazones has generated unusual amino acids tubuphenylalanine (Tup) and tubuvaline (Tuv) that are structural components of the tubulysin family of picomolar antimitotic agents and previously led to a tubulysin tetrapeptide analog with a C-terminal alcohol. To improve efficiency in this synthetic route to tubulysins, and to address difficulties in oxidation of the C-terminal alcohol, here we present two alternative routes to Tuv that (a) improve step economy, (b) provide modified conditions for Mn-mediated radical addition in the presence of aromatic heterocycles, and (c) expose an example of double diastereocontrol in radical addition to a β-benzyloxyhydrazone with broader implications for asymmetric amine synthesis via radical addition. An efficient coupling sequence affords 11-O-benzyltubulysin V benzyl ester.

N-tert-Butanesulfinyl imines in the asymmetric synthesis of nitrogen-containing heterocycles

The synthesis of nitrogen-containing heterocycles, including natural alkaloids and other compounds presenting different types of biological activities have proved to be successful employing chiral sulfinyl imines derived from tert-butanesulfinamide. These imines are versatile chiral auxiliaries and have been extensively used as eletrophiles in a wide range of reactions. The electron-withdrawing sulfinyl group facilitates the nucleophilic addition of organometallic compounds to the iminic carbon with high diastereoisomeric excess and the free amines obtained after an easy removal of the tert-butanesulfinyl group can be transformed into enantioenriched nitrogen-containing heterocycles. The goal of this review is to the highlight enantioselective syntheses of heterocycles involving the use of chiral N-tert-butanesulfinyl imines as reaction intermediates, including the synthesis of several natural products. The synthesis of nitrogen-containing heterocycles in which the nitrogen atom is not provided by the chiral imine will not be considered in this review. The sections are organized according to the size of the heterocycles. The present work will comprehensively cover the most pertinent contributions to this research area from 2012 to 2020. We regret in advance that some contributions are excluded in order to maintain a concise format.

Chiral N-tert-Butylsulfinyl Imines: New Discoveries

In this account the reactions of chiral N-tert-butylsulfinyl imines with organometallic reagents such as organoalkaline (lithium, sodium, potassium and cesium derivatives), organomagnesium, organozinc, organoboron, organoaluminium, organoindium and organosilicon compounds is comprehensively described. The reactivity in all cases is derived to synthetic applications in order to prepare interesting organic nitrogenated molecules, especially in the field of alkaloid compounds.

1,2-Addition versus homoconjugate addition reactions of indoles and electron-rich arenes to α-cyclopropyl N-acyliminium ions: synthetic and computational studies

α-Cyclopropyl-N-acyliminium ions towards reaction with indoles to give 5-(3-indoyl)-5-cyclopropylpyrrolidin-2-ones and, in the case of highly electron deficient indoles and electron rich arenes, spiroheterocycles.

Alkoxyallene-based syntheses of preussin and its analogs and their cytotoxicity

Short syntheses of oxa-preussin, racemic preussin and (-)-preussin are reported. Starting from a racemic 3-nonyl-substituted methoxyallene derivative, its lithiation and addition to phenylethanal provided the corresponding allenyl alcohol that was converted into two diastereomeric dihydrofuran derivatives by silver nitrate-catalyzed 5-endo-trig cyclization. The acid hydrolysis of the enol ether moiety gave heterocyclic ketones and subsequent highly stereoselective reductions with l-selectride furnished 2-benzyl-5-nonylfuran-3-ol derivatives in good overall yield. The major all-cis-diastereomer has the skeleton and relative configuration of preussin and is hence called oxa-preussin. An analogous sequence with the same allene, but an N-sulfonyl imine as the electrophile, finally led to racemic preussin. The stereoselectivities of the individual steps are discussed in detail. With an enantiopure 2-benzyl-5-nonylpyrrolidin-3-one intermediate the preparation of (-)-preussin with an enantiomeric ratio of >95 : 5 could be accomplished in a few steps. The sign of the optical rotation of this product finally proved the absolute configurations of its precursors and demonstrated that our chiral auxiliary-based route led to the antipode of the natural product. The cytotoxicity of several of the prepared heterocycles against MCF-7 tumor cells was investigated and five compounds, including racemic and enantiopure (-)-preussin, were identified as highly cytotoxic with IC50 values in the range of 3-6 μM.

Recent advances on the total synthesis of alkaloids in mainland China

Alkaloids are a large family of natural products that mostly contain basic nitrogen atoms. Because of their intriguing structures and important functions, they have long been popular targets for synthetic organic chemists. China's chemists have made significant progress in the area of alkaloid synthesis over past decades. In this article, selected total syntheses of alkaloids from research groups in mainland China during the period 2011–16 are highlighted.

An efficient approach to trans-4-hydroxy-5-substituted 2-pyrrolidinones through a stereoselective tandem Barbier process: divergent syntheses of (3R,4S)-statines, (+)-preussin and (−)-hapalosin

Natural products of (+)-preussin and (−)-hapalosin have been asymmetrically synthesized through the stereoselective tandem Barbier process.

Asymmetric syntheses of epohelmins A and B by In-mediated allylation

A diastereoselective new approach for the synthesis of trans-4-hydroxy-5-allyl-2-pyrrolidinone 9 has been developed through In-mediated allylation of α-chiral aldimine 8 with allyl bromide. The stereochemistry at the C-2 stereogenic center of 9 was controlled by both the α-OTBS substitution and the sulfinamide moiety. The utility of this asymmetric allylation is demonstrated by the asymmetric syntheses of epohelmins A (4) and B (10).

Enantioselective 1,4-addition of cyclopropylboronic acid catalyzed by rhodium/chiral diene complexes

Rhodium-catalyzed asymmetric addition of cyclopropylboronic acids to electron-deficient alkenes proceeded to give high yields of the corresponding 1,4-addition products with high enantioselectivity.