First total synthesis of phenylpyridine analogues of the antimitotic rhazinilam

Rhazinilam-leuconolam family of natural products: a half century of total synthesis

Covering: 1972 to 2021The rhazinilam family of natural products exhibits a main structure with a stereogenic quaternary carbon and a tetrahydroindolizine core imbedded within a 9-membered macrocycle, imposing axial chirality. This unique architecture combined with their taxol-like antimitotic activities have attracted various attention, especially from synthetic chemists, notably in the past decade. The present review describes the known total and formal syntheses of the members of the rhazinilam family (rhazinilam, rhazinal, leuconolam and kopsiyunnanines), according to the strategy developed.

C-H activation: a complementary tool in the total synthesis of complex natural products

The recent advent of transition-metal mediated C-H activation is revolutionizing the synthetic field and gradually infusing a "C-H activation mind-set" in both students and practitioners of organic synthesis. As a powerful testament of this emerging synthetic tool, applications of C-H activation in the context of total synthesis of complex natural products are beginning to blossom. Herein, recently completed total syntheses showcasing creative and ingenious incorporation of C-H activation as a strategic manoeuver are compared with their "non-C-H activation" counterparts, illuminating a new paradigm in strategic synthetic design.

Ring closing and opening reactions leading to aza-polycyclic aromatic compounds

A series of functionalized aza-polycyclic aromatic compounds were prepared by a superacid-promoted ring closing and opening reaction cascade. A reaction mechanism is proposed, which involves reactive dicationic intermediates. A key step in the conversions involves ipso protonation of an aryl group and elimination of an alkyl phenyl group.

A new synthetic approach to biaryls of the rhazinilam type. Application to synthesis of three novel phenylpyridine-carbamate analogues

The synthesis of three novel racemic phenylpyridine-carbamate analogues of rhazinilam and their biological evaluation as inhibitors of microtubule assembly and disassembly by interaction with tubulin are described. The sterically hindered ortho-disubstituted biaryl unit as the challenging key structural element is first obtained by a sequential regiocontrolled nucleophilic addition of a lithium ortho-lithiohomobenzylic alkoxide species to 3-bromo-5-oxazolyl pyridine as the electrophile and a subsequent oxidation step. The incorporation of the amino group by replacement of the bromide has been achieved using a Buchwald-Hartwig amination coupling. Ultimate deprotection steps furnished free-amino and free-hydroxyl appendages which were connected by phosgenation to furnish the nine-membered median carbamate ring.

A Review on Recent Developments in Syntheses of the post-Secodine Indole Alkaloids. Part II: Modified Alkaloid Types

The second part of the planned review on developments in the field of total and formal total synthesis of the post-secodine indole alkaloids concentrates on modified alkaloid types, i.e. those skeletons derived from primary types by formation of additional and/or rupture of existing bonds, while connectivities next to indol(e)ine moiety remain intact. It thus reviews the synthesis of alkaloids of quebrachamine/cleavamine type including VLB-bis-indoles, rhazinilam type, aspidofractinine/kopsane and kopsifoline type, as well as kopsijasminilam alkaloids, lapidilectine B and danuphylline. It covers the literature of from 1991-1992 up to approximately end 2006. A review with 174 references.

Synthesis and biological evaluation of B-ring analogues of (−)-rhazinilam

Three macrocyclic analogues of rhazinilam 1 having a 11- or 12-membered B-ring with an endocyclic carbamate group or an amino-acid residue were synthesized from the natural product. These analogues 3 and 4 displayed a very low activity on tubulin. Thirty N-1 and C-16 substituted analogues of rhazinilam were also synthesized regioselectively from rhazinilam. Stereochemical analyses showed that N-1 and C-16alpha analogues have the same conformation as rhazinilam, whereas C-16beta analogues adopt a different conformation for rings B and D. All N-1 and C-16 analogues were less active than rhazinilam on tubulin, though analogues 5a, 6aalpha, 6balpha, and 6f having the less bulky substituents retained close affinities. A few analogues either active (like 6f) or inactive (like 5o) on tubulin showed significant inhibition of the growth of KB cancer cells.

Convenient Access to New Chiral Ferroceno-(iso)quinolines

[Chemical reaction: see text] New chiral pyridine derivatives possessing the planar chirality of the ferrocene have been prepared by means of an aldolization-crotonization reaction. This very simple reaction has been applied to the synthesis of isomers 1-4 that differ in the position of the nitrogen atom on the pyridine ring. Following the same procedure, asymmetric synthesis of 1 has been achieved using an enantiopure ferrocenylzinc intermediate. This method has also allowed the preparation of a chiral analogue of 2,2'-bipyridine.

Conformational analysis of rhazinilam and three-dimensional quantitative structure–activity relationships of rhazinilam analogues

3D QSAR of rhazinilam (1), an alkaloid isolated from Rhazya stricta (Apocynaceae) with an activity involving disassembly of microtubules and its derivatives was investigated by using the comparative molecular field analysis (CoMFA). In an effort to get a better understanding of the correlation between conformation and antitubulin activity of 1, most probable minimum energy conformation in solution of 1 was analyzed on the basis of NMR data of 1 in solution. The results indicated a correlation between the antitubulin activity of these alkaloids and the steric and electrostatic factors, which modulate their biological activity, and accounted for the potent activities of 1 with suitable relationship for the overall conformation.

Aminoalkoxide-mediated formation and stabilization of phenylpyridyllithium: straightforward access to phenylpyridine derivatives

It is shown that lithium aggregates promoted the efficient metalation of phenylpyridines and stabilization of phenylpyridyllithium. The BuLi-LiDMAE superbase prevented dimerization or nucleophilic addition encountered with t-BuLi or n-BuLi. The reported selective pyridine ring lithiation of 2-, 3-, and 4-phenylpyridine alpha to nitrogen opens a straightforward access to their derivatives.

Application of the palladium-catalyzed borylation/Suzuki coupling (BSC) reaction to the synthesis of biologically active biaryl lactams

The palladium-catalyzed, two-step, one-pot borylation/Suzuki coupling (BSC) reaction was developed to synthesize sterically hindered 2,2'-disubstituted biphenyl and phenyl-indole compounds in a short, simple, and efficient manner from two easily accessible aryl halides. High yields can be obtained by choosing properly both components according to their rough electronic properties. The illustration of the utility of this method was provided by the solution and solid-phase synthesis of seven- or eight-membered biphenyl lactams 5a-e, as well as paullone 3a. These compounds exhibit moderate albeit significant cytotoxicities and may serve as structural models for future medicinal chemistry developments.