From a biogenetic scenario to a synthesis of the ABC ring of manzamine A

On the basis of a biogenetic proposal for explaining the biogenesis of manzamine A, the cycloaddition of dihydropyridinium salt 26 with diene derivative 5 leads to adducts 27. These adducts, as well as their related and previously described analogues 9, are now shown to be precursors of diene derivatives such as 10, 13, and 28. Treatment of diene 32 with sodium azide resulted in a one-step formation of the tricyclic imino derivative 34. This key intermediate was further transformed into tricyclic derivative 40, which possesses the essential features of the ABC ring of manzamine A.     

Synthesis of the ABC ring system of Jiadifenin via Pd-catalyzed cyclizations

An efficient route toward the central ABC system of jiadifenin has been developed using two key Pd-catalyzed cyclizations. A protic solvent-activated Mizoroki-Heck reaction was used to construct the C(9) quaternary carbon and the A ring. A cascading Tsuji-Trost cyclization/lactonization sequence was employed to establish the BC ring system and the C(5,6) stereochemistry.     

Synthesis of the ABC ring system of azaspiracid. 1. Effect of D ring truncation on bis-spirocyclization

[reaction: see text] Synthesis of a spirocyclization precursor with a truncated D ring has been accomplished. Subsequent bis-spirocyclization induced the formation of equal amounts of the natural transoidal 10R,13R bis-spirocycle and its cisoidal 10R,13S epimer under an apparent thermodynamically controlled process.     

Synthesis of the zoanthamine ABC ring system: some surprises from intramolecular Diels-Alder reactions

[reaction: see text] In connection with the total synthesis of the marine alkaloids zoanthamine and norzoanthamine, an elaborate model study was conducted starting from (-)-carvone. In nine steps alkenyl iodides corresponding to the C11-C24 fragment of zoanthamine were obtained. The alkenyl iodides were coupled to various stannanes (C6-C10 fragment) via the Corey modification of the Stille reaction, affording a variety of Diels-Alder precursors. An interesting and highly unexpected cascade reaction sequence was observed during the screening of an intramolecular Diels-Alder reaction, generating a novel tetracyclic framework. A slight modification in the Diels-Alder precursor allowed the desired cycloaddition to take place, giving the cycloadduct in 87% yield.     

Stereoselective synthesis of the ABC ring system of norzoanthamine

[reaction: see text] An efficient synthesis of enone 4, representing the ABC ring motif of norzoanthamine, is presented. The crucial C22 quaternary center was introduced via a stereoselective methylation of enone 8. The trans-anti-trans relative configuration of the ABC framework of 4 was installed via a sequence of reactions that included a hydroboration and a modified Robinson annulation.     

Synthesis of the ABC and IJ ring fragments of yessotoxin

Synthesis of a 6/6/6 tricyclic ether system (3) corresponding to the ABC ring fragment of yessotoxin (1) has been achieved via coupling of a triflate and a 2-lithiofuran followed by intramolecular hetero-Michael addition. The IJ ring fragment (4) of 1 was readily synthesized via successive Sharpless epoxidation and 6-endo cyclization of the resulting vinyl epoxide.     

A stannous chloride-induced deacetalisation-cyclisation process to prepare the ABC ring system of 'upenamide

A stannous chloride-induced deacetalisation-cyclisation approach to the ABC core of the macrocyclic alkaloid, 'upenamide is reported. The use of a substituted β-lactone to prepare a C-2 substituted 'upenamide analogue is also disclosed.     

Synthesis and biological evaluation of manzamine analogues

[Structure: see text] The synthesis and biological evaluation of a series of analogues of manzamine A, representing partial structures of the pentacyclic ABCDE diamine core, is described. All new compounds were screened against Plasmodium falciparum and demonstrated attenuated antimalarial activity relative to that of manzamine A.     

Synthesis of the ravidomycin ring system

Synthesis of the tetracyclic aromatic ring system of the ravidomycin family of anti-tumor agents is reported. The synthesis is a sequential Meerwein and Diels-Alder route with the crucial regiochemistry of the Diels-Alder reaction controlled with a chlorine atom.     

A practical synthesis of the ABC ring model of ecteinascidins

A practical synthesis of 1,2,3,4,5,6-hexahydro-1,5-imino-10-hydroxy-9-methoxy-3,8,11-trimethyl-3- benzazocin-4-one (3) as an ABC ring model compound of ecteinascidin 743 and safracins from 3-hydroxy-4-methoxy-5-methylbenzaldehyde (7) is described. The overall yield in 15 steps is 27%.     

Synthesis of the 13,14-diazasteroid ring system

l-Vinyl-6-methoxy-3,4-dihydronaphthalene (1) reacted with azo dienophiles to form cyclo-adducts II, III, IV, and V. The reaction of adduct II with bromine resulted in the aromatization of the B ring to form the cycloadduct VIII.     

Synthesis of the hydroazulene ring system of guanacastepene

[reaction: see text] A 12-step synthesis of 26, the functionalized hydroazulenone ring of guanacastepene (1), has been completed using the EtAlCl(2)-initiated cyclization of gamma,delta-unsaturated ketone 13 to construct 2,2,3-trisubstituted cyclopentanone 14, the palladium-catalyzed coupling of vinylmagnesium bromide with enol triflate 17 to prepare triene 21, and olefin metathesis of triene 21 to form the key hydroazulene 20.     

Controlling influences in bisspiroketal formation: synthesis of the ABC ring system of azaspiracid

Substitution effects on the stereochemical outcome of bisspiroketalization on the C₁-C₁₇ carbon backbone of azaspiracid is presented. A possible explanation is offered to explain the observed stereochemical outcome.     

Synthesis of the stenine ring system from pyrrole

The skeleton of the stemona alkaloid, stenine, has been synthesized starting from pyrrole, employing an asymmetric organocatalyzed cyclization, Sonogashira coupling, a diastereoselective intramolecular propargylic Barbier reaction, cyclocarbonylation, and diastereoselective alkene reduction. Modulation of the electron-rich nature of the pyrrole nucleus by employing an α-trifluoroacetyl group is essential. The α-trifluoroacetyl group may be rapidly removed under carefully defined, mild conditions.     

Development of an approach to the synthesis of the ABC ring system of Hemibrevetoxin B

An efficient approach for the synthesis of a model of the ABC ring system of Hemibrevetoxin B is described. Key features include a ring expansion to yield the ring C oxepane, the reduction of a 2-furyl ketone with high levels of 1,3-stereocontrol, and an Achmatowicz oxidative ring expansion to yield the ring A tetrahydropyran. All seven stereogenic centers present in the model compound were controlled with high levels (>98:<2) of diastereoselectivity.     

Synthesis of the guanacastepene A-B hydrazulene ring system through photochemical ring transposition

The A-B hydrazulene ring system of the guanacastepenes has been synthesised using a photochemical ring transposition of a 6-6 bicycle.     

Application of RCM reaction in the construction of ABC ring of micrandilactone A

[reaction: see text] The functionalized ABC ring system of micrandilactone A was successfully constructed in 14 steps. The key reactions in this synthesis are the intermolecular Diels-Alder reaction (IMDA) and the eneyne ring-closing metathesis (RCM) reaction.     

Synthesis of macrocyclic ligands containing the indole ring system

2,7-Diformyl-3-methyl-4,6-dimethoxyindole (6) has been synthesized and converted by metal template reactions into the macrocyclic complexes (10–12).     

Synthesis of the spirocyclic cyclohexadienone ring system of the schiarisanrins

[structure: see text] Studies on the synthesis of the spirocyclic cyclohexadienone ring system 2 of the schiarisanrin family of natural products 1 are described and were based on the Lewis acid-promoted C-alkylation of the corresponding phenolic precursor.