Bioactive cyclobutane-containing alkaloids

A computational investigation into the substituent effect on the chemo- and stereoselectivity of crossed intermolecular radical anion [2 + 2] cycloadditions of enones

The chemoselectivity of cycloaddition is caused by the bulky group on the C atoms which form σ bond in the first step. The stereoselectivity mainly caused by the difference in steric interaction between the trans and cis transition states.

Stereocontrolled synthesis and functionalization of cyclobutanes and cyclobutanones

In the last decade a certain number of new cyclobutane and cyclobutanone synthesis and functionalization protocols have been published. Organo- and biocatalyzed eco-friendly approaches to cyclobutane-containing molecules have been developed with interesting results. Also, successful new total synthesis of bioactive compounds and drugs have been recently reported where a four membered ring represented the key intermediate. Therefore, the rising interest in this field represents a great point of discussion for the scientific community, disclosing the synthetic potential of strained four membered ring carbocyclic compounds. Herein we report a critical survey on the literature concerning the enantiocontrolled synthesis and functionalization of cyclobutane derivatives, with particular attention to metal-free, low impact methodologies, published during the period 2000-2013.

Enantioselective synthesis of cyclobutanes via sequential Rh-catalyzed bicyclobutanation/Cu-catalyzed homoconjugate addition

Enantiomerically enriched cyclobutanes are constructed by a three-component process in which t-butyl (E)-2-diazo-5-arylpent-4-enoates are treated with Rh2(S-NTTL)4 to provide enantiomerically enriched bicyclobutanes, which can subsequently engage in homoconjugate addition/enolate trapping sequence to give densely functionalized cyclobutanes with high diastereoselectivity. This three-component, two-catalyst procedure can be carried out in a single flask. Rh2(S-NTTL)4-catalyzed reaction of t-butyl (Z)-2-diazo-5-phenylpent-4-enoate gives the Büchner cyclization product in excellent enantioselectivity.

Rhodium-catalyzed intermolecular [2 + 2] cycloaddition of terminal alkynes with electron-deficient alkenes

The first catalytic intermolecular [2 + 2] cycloaddition of terminal alkynes with electron-deficient alkenes is reported. The reaction proceeds with an 8-quinolinolato rhodium/phosphine catalyst system to give cyclobutenes from various substrates having polar functional groups in high yields with complete regioselectivity.

Synthesis of cyclobutane lignans via an organic single electron oxidant-electron relay system

A direct method to synthesize lignan cyclobutanes and analogs via photoinduced electron transfer is presented. A variety of oxygenated alkenes are employed to furnish terminal or substituted cyclobutane adducts with complete regiocontrol, yielding cycloadducts with trans stereochemistry. Key to minimizing competing cycloreversion is the inclusion of an aromatic electron relay (ER). This method has been adapted to the synthesis of the natural products magnosalin and pellucidin A.

Nickel-catalyzed intermolecular [2 + 2] cycloaddition of conjugated enynes with alkenes

A nickel-catalyzed intermolecular [2 + 2] cycloaddition of conjugated enynes with alkenes has been developed. A variety of electron-deficient alkenes as well as electronically neutral norbornene and 1-decene were applicable to this reaction. The use of conjugated enynes circumvented possible side rections, such as oligomerizations and cyclotrimerizations. The isolation of reaction intermediate complexes revealed that the η(3)-butadienyl coordination is the key for the selective formation of cyclobutene.

Ring expansion of alkynyl cyclopropanes to highly substituted cyclobutenes via a N-sulfonyl-1,2,3-triazole intermediate

Regioselective ring expansion of alkynyl cyclopropanes to highly substituted cyclobutenes was developed. The reaction involves a copper-catalyzed cycloaddition of an alkyne with an arylsulfonyl azide and a silver-catalyzed carbene formation followed by ring expansion of a cyclopropyl carbene intermediate.

Stereoselective preparation of cyclobutanes with four different substituents: total synthesis and structural revision of pipercyclobutanamide A and piperchabamide G

A general strategy was developed for the diastereo- and enantioselective synthesis of cyclobutanes with four different substituents. It consists of three transition metal-catalyzed reactions — a RhII-catalyzed cyclopropanation, a AgI-catalyzed regioselective and stereospecific ring expansion, and a RhI-catalyzed addition reaction. Structures of pipercyclobutanamide A and piperchabamide G were synthesized and revised.

Crossed Intermolecular [2+2] Cycloaddition of Styrenes by Visible Light Photocatalysis

We report a method for the crossed [2+2] cycloaddition of styrenes using visible light photocatalysis. Few methods for the synthesis of unsymmetrically substituted cyclobutanes by photochemical [2+2] cycloaddition are known. We show that careful tuning of the electrochemical properties of a ruthenium photocatalyst enable the efficient crossed [2+2] cycloaddition of styrenes upon irradiation with visible light. We outline the logic that enables high crossed chemoselectivity, and we also demonstrate that this reaction is remarkably efficient; gram-scale reactions can be conducted with as little as 0.025 mol% of the photocatalyst.

Photocatalytic [2 + 2] cycloadditions of enones with cleavable redox auxiliaries

α,β-Unsaturated 2-imidazolyl ketones undergo [2 + 2] cycloaddition with a variety of Michael acceptors upon irradiation with visible light in the presence of Ru(bpy)(3)(2+). Cleavage of the imidazolyl auxiliary from the cycloadducts affords cyclobutane carboxamides, esters, thioesters, and acids that would not be accessible from direct cycloaddition of the corresponding unsaturated carbonyl compounds.

Asymmetric organocatalytic formal [2 + 2]-cycloadditions via bifunctional H-bond directing dienamine catalysis

A new concept in organocatalysis allowing for the construction of cyclobutanes with four contiguous stereocenters with complete diastereo- and enantiomeric control by a formal [2 + 2]-cycloaddition is presented. The concept is based on simultaneous dual activation of α,β-unsaturated aldehydes and nitroolefins by amino- and hydrogen-bonding catalysis, respectively. A new bifunctional squaramide-based aminocatalyst has been designed and synthesized in order to enable such an activation strategy. The potential and scope of the reaction are demonstrated, and computational studies which account for the stereochemical outcome are presented.

Formal intermolecular [2 + 2] cycloaddition reaction of allenamide [corrected] with alkenes via gold catalysis

An efficient method was developed to construct the densely functionalized cyclobutane[corrected] adducts through formal intermolecular cycloaddition of allenamides [corrected] with electron-rich olefins via gold catalysis, in which vinyl ethers/amides and electron-rich styrenes worked very well. In addition, a series of allenamide [corrected] dimerization products were prepared from the same allenamide [corrected] substrates.

Total synthesis and structural revision of the piperarborenines via sequential cyclobutane C-H arylation

A strategy for the construction of unsymmetrical cyclobutanes using C-H functionalization logic is demonstrated in the total synthesis of piperarborenine B and piperarborenine D (reported structure). These syntheses feature a new preparation of cis-cyclobutane dicarboxylates from commercially available coumalate starting materials and a divergent approach to the controlled cis or trans installation of the two distinct aryl rings found in the natural products using the first example of cyclobutane C-H arylation. The structure of piperarborenine D is reassigned to a head-to-head dimer, which was synthesized using an intramolecular [2+2] photocycloaddition strategy.

Grahamines A-E, cyclobutane-centered tropane alkaloids from the aerial parts of Schizanthus grahamii

Schizanthus grahamii is an endemic Chilean plant that is known to contain tropane alkaloids. Five new alkaloids, grahamines A-E (1-5), were isolated and characterized by extensive spectroscopic analysis. Their structures were determined to be 2-{[(3α-hydroxytropo-6β-yl)oxy]carbonyl}-2-methyl-3-{[((6β-angeloyloxy)-3α-yl)oxy]carbonyl}-4-phenylcyclobutanecarboxylic acid (1), 2-{[(3α-hydroxytropo-6β-yl)oxy]carbonyl}-2-methyl-3-{[((6β-tigloyloxy)-3α-yl)oxy]carbonyl}-4-phenylcyclobutanecarboxylic acid (2), 1-methyl-2-{[(3α-hydroxytropo-6β-yl)oxy]carbonyl}-4-{[((6β-angeloyloxy)-3α-yl)oxy]carbonyl}-3-phenylcyclobutanecarboxylic acid (3), 1,2-bis{[(3α-hydroxytropo-6β-yl)oxy]carbonyl}-2-methyl-3-{[((6β-angeloyloxy)-3α-yl)oxy]carbonyl}-4-phenylcyclobutanecarboxylate (4), and 1-{[(3α-mesaconyloxytropo-6β-yl)oxy]carbonyl}-2-{[(3α-hydroxytropo-6β-yl)oxy]carbonyl}-2-methyl-3-{[((6β-angeloyloxy)-3α-yl)oxy]carbonyl}-4-phenylcyclobutanecarboxylate (5).

[2+2] cycloadditions by oxidative visible light photocatalysis

Photochemical reactions are remarkable for their ability to easily assemble cyclobutanes and other strained ring systems that are difficult to construct using other conventional synthetic methods. We have previously shown that Ru(bpy)(3)(2+) is an efficient photocatalyst that promotes the [2+2] cycloadditions of electron-deficient olefins with visible light. Here, we show that Ru(bpy)(3)(2+) is also an effective photocatalyst for the [2+2] cycloaddition of electron-rich olefins. This transformation is enabled by the versatile photoelectrochemical properties of Ru(bpy)(3)(2+), which enables either one-electron reduction or one-electron oxidation of interesting organic substrates under appropriate conditions.

Absolute configuration of tropane alkaloids from Schizanthus species by vibrational circular dichroism

The absolute configuration (AC) of 6beta-hydroxy-3alpha-senecioyloxytropane (1), 3alpha-hydroxy-6beta-tigloyloxytropane (2), 3alpha-hydroxy-6beta-senecioyloxytropane (3), and 3alpha-hydroxy-6beta-angeloyloxytropane (4) was assigned as (1R,3R,5S,6R) using density functional theory (DFT) calculations at the B3LYP/DGDZVP level of theory in combination with experimental vibrational circular dichroism (VCD) measurements and comparison with the spectra of similar tropanes. The AC of 1 followed from a sample isolated from Schizanthus grahamii, while those of the mixture of 2 and 3, isolated from the same source, were determined by comparing the VCD measurement to a weighted calculation of the individual VCD spectra according to a 69:31 ratio of 2:3 determined by (1)H NMR signal integration. In turn, Schizanthus pinnatus provided a 7:3 mixture of 1:4 whose AC was determined using the experimental VCD absorptions in the 1150-950 cm(-1) spectral region which were compared with those observed for 1-3 and with those described for other 3alpha,6beta-tropanediol derivatives.

Aziridine alkaloids as potential therapeutic agents

The present review describes research on natural aziridine alkaloids isolated from both terrestrial and marine species, as well as their lipophilic semi-synthetic, and/or synthetic analogs. Over 130 biologically active aziridine-containing compounds demonstrate confirmed pharmacological activity including antitumor, antimicrobial, antibacterial effects. The structures, origin, and biological activities of aziridine alkaloids are reviewed. Consequently this review emphasizes the role of aziridine alkaloids as an important source of drug prototypes and leads for drug discovery.

Cyclobutane-containing alkaloids: origin, synthesis, and biological activities

Present review describes research on novel natural cyclobutane-containing alkaloids isolated from terrestrial and marine species. More than 60 biological active compounds have been confirmed to have antimicrobial, antibacterial, antitumor, and other activities. The structures, synthesis, origins, and biological activities of a selection of cyclobutane-containing alkaloids are reviewed. With the computer program PASS some additional biological activities are also predicted, which point toward new possible applications of these compounds. This review emphasizes the role of cyclobutane-containing alkaloids as an important source of leads for drug discovery.