Synthesis of the ABC ring system of manzamine A

A synthesis of the core ABC ring system of the manzamine alkaloids is described, starting from arecoline. The key steps involve a Claisen rearrangement to set up a 4-substituted-3-methylenepiperidine and a stereoselective azomethine ylide dipolar cycloaddition reaction. Condensation of the aldehyde 6 and sarcosine ethyl ester hydrochloride salt gives an intermediate azomethine ylide, which undergoes an intramolecular cycloaddition reaction to set up two new rings and three new chiral centers stereoselectively. The aldehyde 6 was not a suitable substrate for related azomethine ylide cycloaddition reactions with other amines. However, the related dimethyl acetal 26 could be condensed with a variety of amines to give the desired tricyclic products. The cycloaddition reaction with N-methyl or N-allyl glycine ethyl ester gave almost exclusively the exo adduct, whereas cycloaddition with glycine ethyl ester gave the endo adduct.     

Cascade cyclization, dipolar cycloaddition to bridged tricyclic amines related to the Daphniphyllum alkaloids

A tandem one-pot reaction of an aldehyde with a primary amine involving condensation and then cyclization (N-alkylation), followed by intramolecular dipolar cycloaddition of the resulting nitrone or azomethine ylide, provides a synthesis of bridged tricyclic amines. The reaction was most successful using hydroxylamine, and when the dipolarophile was an unsaturated ester, subsequent reduction of the N-O bond and cyclization to the lactam provided the core ring system of the yuzurimine, daphnilactone B, and bukittinggine type Daphniphyllum alkaloids.     

Cascade cyclization intermolecular dipolar cycloaddition by multi-component couplings—synthesis of indolizidines and pyrrolizidines

A three-component coupling reaction of a primary amine (an amino-acid or amino-ester or hydroxylamine), an alkene or alkyne dipolarophile and an aldehyde bearing a halide as a leaving group has been developed. Condensation of the amine with the aldehyde and cyclization (intramolecular N-alkylation) provides, after decarboxylation or deprotonation, a cyclic azomethine ylide (or nitrone) that undergoes intermolecular dipolar cycloaddition with the dipolarophile. This sets up, in a single step, the bicyclic indolizidine or pyrrolizidine ring system, depending on the length of the tether between the aldehyde and the halide. The reaction is successful with stabilized and non-stabilized azomethine ylides that result from using primary amino-esters or amino-acids, respectively.     

Competition between alkenes in intramolecular ketene-alkene [2 + 2] cycloaddition: what does it take to win?

In the course of developing a new synthetic methodology using ketenes in sequential cycloaddition steps, we were faced with a competition problem with molecules containing a ketene tethered to more than one reacting partner. To pinpoint the electronic and tethering requirements for a chemoselective reaction, we undertook a series of ketene-alkene [2 + 2] cycloaddition competition experiments. Those experiments were conducted on molecules containing either two identical alkenes having different tether lengths or two alkenes having the same tether length but being electronically different. We demonstrated that the reaction is much faster for forming five-membered rings than six-membered rings and calculated the Hammett constant rho for intramolecular ketene-alkene [2 + 2] cycloadditions to be -1.39.     

Approach to the homoerythrina alkaloids using a tandem N-alkylation/azomethine ylide cycloaddition

Synthetic efforts toward the homoerythrina alkaloids 1-3 are described. Two separate model systems guided the pivotal [3 + 2] azomethine ylide cycloaddition cascade to form the A-C rings of these alkaloids. The cycloaddition precursors 63 and 68, prepared in nine and ten steps, respectively, from alkyne 47, each contain an enolizable ketone, a tethered electrophile, and an electron-poor dipolarophile. Heating 63 and 68 with the stannyl amine 17 generated demethoxyschelhammeridine 65 and demethoxyschelhammericine 70, the products of intramolecular azomethine ylide cycloadditions. Subsequent attempts to install the C-3 methoxy group of 1-3 are also described.     

Multiple Cycloaddition Reactions of Ketones with a β‐Diketiminate Al Compound

A β‐diketiminate Al compound ( 1 ) with an exocyclic double bond reacts with two equivalents each of benzophenone and 2‐benzoylpyridine in a [4+2] cycloaddition to generate bicyclic and tricyclic compounds 2 and 3, respectively. Compound 2 consists of six‐ and eight‐membered aluminium rings, whereas 3 has two five‐ and one eight‐membered ring. Compounds 2 and 3 were characterized by a number of analytical tools including single‐crystal X‐ray diffraction. The quantum mechanical calculations suggest that the dissociation of the solvent molecule from 1 would lead to an active species 1A having two 1,4‐dipolar 4π electron moieties, in which the electrophilic site is the Al atom and the nucleophilic positions are polarized exocyclic and endocyclic C?C π bonds. The detailed mechanistic study shows that the dipolarophiles, benzophenone, and 2‐benzoylpyridine undergo double cycloaddition with two 1,4‐dipolar 4π electron moieties of 1A . Herein, the addition of one molecule of the dipolarophile promotes the addition of the second one.     

Investigation of the halocyclization of S- and N-allyl derivatives of 2-benzothiazolethione

3-Allyl-2-benzothiazolethione reacts with iodine regiospecifically with annelation of the five-membered ring, while with bromine a mixture of the five- and six-membered rings are formed. 2-Allylthioben- zothiazole reacts with halogens with annelation of the five- and six-membered rings, while 2-(2-methyl-2-propenyl)thiobenzothiazole is annelated at the five-membered ring.     

Fused ring aziridines as a facile entry into triazole fused tricyclic and bicyclic heterocycles

The intramolecular dipolar cycloaddition of an azide with an alkyne has provided a useful entry into triazole fused tricyclic heterocycles containing both the triazole ring and the oxazolidin-2-one ring system. The requisite azido-alkynes have been prepared via a two-step sequence from fused ring aziridines. A series of 6-12 membered rings containing both the oxazolidinone and triazole rings have been prepared. These ring systems have been designed as conformationally restrained analogs of RNA-binding oxazolidinones.     

Using the Pummerer cyclization-deprotonation-cycloaddition cascade of imidosulfoxides for alkaloid synthesis

The Pummerer reaction of imidosulfoxides bearing tethered alkenyl groups has been employed for the synthesis of several alkaloids. The required imidosulfoxides necessary for the cascade sequence were easily obtained by heating the appropriate amide with (ethylsulfeny)acetyl chloride followed by sodium periodate oxidation. The initially formed thionium ion, obtained by treating the imidosulfoxide with acetic anhydride and p-toluenesulfonic acid, reacts with the neighboring imido group, and the resulting oxonium ion undergoes subsequent deprotonation to produce an isomünchnone dipole. This mesoionic betaine intermediate undergoes ready intramolecular dipolar cycloaddition across the neighboring pi-bond. Exposure of the resulting cycloadducts to additional acetic anhydride leads to ring opening and formation of a 5-acetoxy-substituted 2(1H)-pyridone. This six-ring heterocyclic system constitutes a valuable building block for the synthesis of a variety of pyridine, quinolizidine, and clavine alkaloids. The cyclization-deprotonation-cycloaddition cascade has been successfully applied to the synthesis of the naturally occurring alkaloids onychnine, dielsiquinone, (+/-)-lupinine, (+/-)-anagyrine, (+/-)-pumiliotoxin C, and (+/-)-costaclavine.     

Asymmetric synthesis of unusual fused tricyclic beta-lactam structures via aza-cycloadditions/ring closing metathesis

Conveniently substituted bis-beta-lactams, pyrrolidinyl-beta-lactams, and piperidinyl-beta-lactams undergo ring-closing methatesis using Grubbs' carbene, Cl(2)(Cy(3)P)(2)Ru=CHPh, to give medium-sized rings fused to bis-2-azetidinone, pyrrolidinyl-2-azetidinone, or piperidinyl-2-azetidinone systems. The diolefinic cyclization precursors can be obtained from optically pure 4-oxoazetidine-2-carbaldehydes bearing an extra alkene tether at position 1 or 3 of the beta-lactam ring via [2 + 2] cycloaddition of imino 2-azetidinones, N-metalated azometine ylide [3 + 2] cycloaddition, and subsequent N-acylation of the pyrrolidinyl nitrogen atom, or through aza-Diels-Alder cycloaddition of 2-azetidinone-tethered imines. Under standard reaction conditions, the combination of cycloaddition reactions of 2-azetidinone-tethered imines with ring-closing methatesis offers an asymmetric entry to a variety of unusual fused tricyclic 2-azetidinones bearing two bridgehead nitrogen atoms.     

Synthesis of oligoenes that contain up to 15 double bonds from 1,6-heptadiynes

This paper reports the synthesis of polyene oligomers ("oligoenes") that contain up to 15 double bonds that are identical to the "all five-membered ring" species formed through cyclopolymerization of diisopropyldipropargylmalonate. The oligoenes contain an isopropylidene unit at each end. The isolated oligoenes range from the "dimer" (a pentaene, (E)-di-1,2-[1-(2-methyl-propenyl)-4,4-di-iso-propyl-carboxy-cyclopent-1-enyl]-ethene (3b2)) to the "heptamer" (3b7, a pentadecaene). Oligoenes 3b2, 3b3, 3b4, 3b5, and 3b7 were prepared through Wittig-like reactions between aldehydes and the appropriate monometallic Mo alkylidene or bimetallic Mo bisalkylidene species whose alkylidene is derived from an identical five-membered ring monomeric unit. Compounds 3b2, 3b4, and 3b6 were prepared through McMurry coupling reactions of aldehydes. A representative aldehyde (the "monomeric" aldehyde) is diisopropyl-3-formyl-4-(2-methylprop-1-enyl)cyclopent-3-ene-1,1-dicarboxylate (2b), McMurry coupling of which yields 3b2. A heptaene that contains a six-membered ring in the central unit also was prepared in a Wittig-like reaction involving a bimetallic Mo alkylidene; this species is a model for oligoenes that contain both six-membered and five-membered rings. X-ray structures of two bimetallic species that are employed in the synthesis of the oligoenes are reported.     

On the 1H-NMR spectra of highly charged multi-triphenylmethylium ions

Relative to their neutral precursors, higly charged multi-tritylium ions show typical downfield ¹H-NMR shifts, whereas the corresponding phenylfluorenylium ions show upfield shifts. A Hückel based model is presented that accounts for this contrasting behavior in terms of antiaromatic ring current in the five-membered ring of the fluorenyl system with essentially null ring currents in the two six-membered rings.     

The 2,2′-Coupled Pyrrolidine-Phospholane Ring System: A Highly Enantioselective Synthesis and Kinetic Resolution of the Phosphorus Centre

Abstract

Enantiopure five-membered ring nitrones derived from L-tartaric acid and from L-malic acid undergo highly regio- and stereoselective cycloaddition reactions with an excess of racemic 2,3-dihydro-1-phenyl-1H-phospholes producing two readily separable tricyclic cycloadducts and concomitantly effecting kinetic resolution of the dihydrophosphole derivative (diastereomeric ratio up to 10:l; stereoselectivity factor s = kS/kR = 14). The tricyclic cycloadducts feature 2,2′-connection of pyrrolidine and phospholane rings and five to seven contiguous stereogenic centers of which three are induced and one or two are kinetically resolved during the cycloaddition process.     

Hétérocyclisation de la nor-3 ar-himachalone-9

We report the facile formation of several different three- and five-membered rings fused to the seven-membered ring of 3-nor-arylhimachal-9-one. We also report the formation of several six-membered rings fused to the seven-membered ring of 3-nor-arylhimachal-9-one. However, in these latter instances, the cyclizations were more difficult to accomplish. In fact the yields were generally low. Similar cyclizations to produce heterocyclic seven-membered rings fused to the seven-membered ring of 3-nor-arylhimachal-9-one have not been accomplished.     

Efficient synthetic access to the hetisine C20-diterpenoid alkaloids. A concise synthesis of nominine via oxidoisoquinolinium-1,3-dipolar and dienamine-Diels-Alder cycloadditions

A concise synthetic approach to the hetisine C20-diterpenoid alkaloids is reported. The total synthesis of (+/-)-nominine was accomplished in a 15-step sequence employing a dual cycloaddition strategy. Key features of the synthesis include a reversible intramolecular 4-oxidoisoquinolinium betaine dipolar cycloaddition in conjunction with a pyrrolidine-induced dienamine isomerization/Diels-Alder cascade.     

X=y-zh systems as potential 1,3-dipoles—5 : Intramolecular cycloadditions of imines of α-amino acid esters

Azomethine ylides are readily generated from imines of α-amino acid esters by a formal 1,2-H shift. A suitably positioned unactivated double or triple bond in either of the two precursors of the imines (aldehyde or α-amino ester) leads to an intramolecular cycloaddition generating fused ring systems in good yield. Cis stereochemistry is assigned to the newly created ring junction of the cycloadducts based on NOE difference spectroscopy and, in the case of 8a, by a single crystal X-ray structure. Equilibration of the kinetically formed dipole leads to mixtures of epimeric cycloadducts for imines of phenylglycine methyl ester but equilibration is not observed for other imines. Reasons for this are discussed. The intramolecular cycloaddition is sensitive to ring size with 6/5 and 5/5 cis-fused systems being most easily formed depending in which moiety (aldehyde or amino acid) the dipolarophile is located. Intramolecular trapping of the azomethine ylide by an alkyne is accompanied by variable amounts of aromatized pyrrolic products.     

Synthetic procedure for various selenium-containing electron donors of the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) type

Six selenium variants of BEDT-TTF have been successfully synthesized by a newly developed synthetic method that involves a combination of two key reactions for the construction of two kinds of heterocyclic rings: the first is a one-pot formation of 1,3-dichalcogenole-2-chalcogenones from a common starting material, THP-protected 2-(ethynylthio)ethanol, leading to the inner five-membered rings, and the other is the annelation of the outer six-membered heterocyclic ring onto the inner ring by an intramolecular transalkylation reaction on a chalcogen atom. This method turned out to be widely applicable to the syntheses of the electron donors of bis(ethylenedithio)- and bis(ethyleneselenothio)-substituted types. However, synthetic attempts to form analogous donors of the bis(ethylenediseleno)-substituted type from THP-protected 2-(ethynylseleno)ethanol were unsuccessful. This is attributable to the predominance of side-reactions via a seleniranium (episelenonium) salt over the desired six-membered ring formation by transalkylation via a seleninium salt.     

Cyclization and Isomerization Reactionsin Silylhydrazine Chemistry

 This review describes the synthesis and isomerization reactions of cyclic silylhydrazines. Topics of discussion are the ring expansion of the three-membered Si(SiN₂) to the four-membered (SiN)₂ ring by lithiation of the (SiN₂) ring and by thermal silyl group insertion into the N*N bond, the ring expansion of a three-membered (SiN₂) to a five-membered (CSi₂N₂) ring by SiCH₂ insertion into the Si*N bond, the formation of isomeric four- and six-membered (SiN₂)₂ rings, the synthesis of five- and six-membered silylhydrazine rings, and the expansion of a five-membered (N₂Si₂N)N ring to the isomeric six-membered (SiNN)₂ ring. The mechanisms of the isomerizations are explained by quantum chemical calculations, and the results are verified by crystal structure determinations.     

Synthesis of conformationally restricted nicotine analogues by intramolecular [3+2] cycloaddition

We describe the synthesis of a series of conformationally constrained nicotine analogues 2-5 from appropriate pyridine-containing enals, featuring an intramolecular azomethine ylide-alkene [3+2] cycloaddition. The objective of the current project is to develop new selective nAChRs-targeting ligands. Of the nicotine analogues that we have studied, the conformation-restricting ring B unit can be either a five-membered carbocycle, or a six-membered carbocycle or heterocycle. The present work constitutes a general method for rapid assembly of other related tricyclic nicotine analogues.     

Transannular dipolar cycloaddition as an approach towards the synthesis of the core ring system of the sarain alkaloids

Intramolecular transannular dipolar cycloaddition was investigated as a key step in a synthetic approach to the core of the sarain alkaloids; although the use of an azomethine ylide was unsuccessful with the chosen aldehyde substrate, cycloaddition with a nitrone did give the alternative regioisomeric bridged cycloadduct.