Diversity-oriented approach to pyrrole-imidazole alkaloid frameworks

A Review of the Synthetic Strategies toward Dihydropyrrolo[1,2-a]Pyrazinones

Dihydropyrrolo[1,2-a]pyrazinone rings are a class of heterocycles present in a wide range of bioactive natural products and analogues thereof. As a direct result of their bioactivity, the synthesis of this privileged class of compounds has been extensively studied. This review provides an overview of these synthetic pathways. The literature is covered up until 2020 and is organized according to the specific strategies used to construct the scaffold: fusing a pyrazinone to an existing pyrrole, employing a pyrazinone-first strategy, an array of multicomponent reactions and some miscellaneous reactions.

Pyrrole carboxamide introduction in the total synthesis of pyrrole-imidazole alkaloids

In this review various strategies for the incorporation of the signature pyrrole carboxamide moiety in the total syntheses of pyrrole-imidazole alkaloids (PIA) are discussed. These so-called oroidin alkaloids have a broad range of biological activities and display interesting skeletal diversity and complexity. These alkaloids are sponge-derived secondary metabolites and thus far more than 200 members of the PIA family have been isolated over the past few decades. Methods range from classical amide bond forming processes to non-traditional bond formation including the de novo synthesis of the pyrrole itself.

Total Synthesis of the Nagelamides - Synthetic Studies toward the Reported Structure of Nagelamide D and Nagelamide E Framework

The nagelamides are a small subset of the oroidin family of marine sponge-derived alkaloids and are, for the most part, dimeric in nature. As part of our efforts to develop synthetic access to this family, a Stille cross-coupling strategy is used to construct the bis-imidazolyl core skeleton. Reduction of the bis-vinylimidazole delivered the core framework of nagelamide D. Introduction of the 2-amino groups via the corresponding azides and introduction of the pyrrolecarboxamides through a double Mitsunobu reaction using a pyrrole hydantoin provided the putative structure of nagelamide D. The spectroscopic data for the synthetic and sponge-derived materials did not match well, whereas the spectroscopic data were a good match for closely related oroidin alkaloids, supporting the structure of the synthetic material. The structure of the synthetic material was further corroborated by obtaining an X-ray crystal structure of a derivative. Electrocyclization of an advanced precursor affords a dihydrobenzimidazole, which is expected to serve as a key intermediate en route to nagelamide E and ageliferin.

Divergent gold-catalysed reactions of cyclopropenylmethyl sulfonamides with tethered heteroaromatics

Cyclopropenylmethyl sulfonamides with tethered heteroaromatics have been demonstrated to undergo divergent gold-catalysed cyclisation reactions. A formal dearomative (4+3) cycloaddition takes place with furan-tethered substrates, yielding densely functionalised 5,7-fused heterocycles related to the bioactive curcusone natural products. Indole-tethered substrates display divergent reactivity giving biologically important tetrahydro-β-carbolines via a Friedel-Crafts mechanism.

Two-Step Synthesis of 3,4-Dihydropyrrolopyrazinones from Ketones and Piperazin-2-ones

An expedient two-step synthesis of 3,4-dihydropyrrolopyrazinones has been achieved via a Vilsmeier-Haack reaction of ketones, followed by an annulation of the corresponding chloroaldehydes with commercially available piperazin-2-ones. A variety of cyclic and acyclic ketones and piperazin-2-ones participated in this two-step chemistry, affording the desired 3,4-dihydropyrrolopyrazinones in up to 78% yield.

Preparation and Diels-Alder reactions of 1'-heterosubsituted vinylimidazoles

A Diels-Alder/rearrangement sequence has been pursued in our lab en route to a number of oroidin dimers. In order to access the fully substituted core of these molecules, 1',2'-disubstituted 4-vinylimidazoles were required as dienes. The preparation of a series of a 4-vinylimidazoles containing substituents on the vinyl moiety via hydroalumination/electrophilic trapping or hydrosilylation are described. These derivatives undergo Diels-Alder reactions with N-phenylmaleimide to provide the tetrahydrobenzimidazole derivatives. The cycloadducts derived from halosubstituted systems generally undergo elimination, leading to the corresponding dihydrobenzimidazole, whereas the silyl and stannyl derivatives provide the corresponding 4-substituted tetrahydrobenzimidazole.

Synthesis and anticancer activity of focused compound libraries from the natural product lead, oroidin

Oroidin (1), (E)-N-(3-(2-amino-1H-imidazol-4-yl)allyl)-4,5-dibromo-1H-pyrrole-2-carboxamide, is a pyrrole alkaloid isolated from the marine sponge Agelas oroides. Routine screening in a panel of twelve cancer cell lines revealed 1 to be poorly cytotoxic with the 50% growth inhibition concentration (GI50) of 42 μM in MCF-7 (breast) cells and 24 μM in A2780 (ovarian) cells and >50 μM in all other cell lines tested. The development of eight focused libraries comprising thirty compounds total identified N-(biphenyl-4-ylmethyl)-1H-pyrrole-2-carboxamide (4l), N-benzyl-4,5-dibromo-1H-pyrrole-2-carboxamide (5a) and N-(biphenyl-4-ylmethyl)-4,5-dibromo-1H-pyrrole-2-carboxamide (5l) as potent inhibitors of cell growth in our panel of cell lines. Of these compounds GI50 values of <5 μM were observed with 4l against HT29 (colon) and SW480 (colon); 5a against HT29; and 5l against HT29, SW480, MCF-7, A431 (skin), Du145 (prostate), BE2-C (neuroblastoma) and MIA (pancreas) cell lines. As a cancer class, colon cancer appears to be more sensitive to the oroidin series of compounds, with analogue 5l being the most active.