Structural variety of zinc and copper complexes based on a 2,3-disubstituted 1,2,3,4-tetrahydroquinazoline ligand

The ring-chain tautomerism of 2-(3-tosyl-1,2,3,4-tetrahydroquinazolin-2-yl)quinolin-8-ol (H(2)L(ring)) has been exploited to produce mononuclear complexes or, alternatively, dinuclear complexes, as desired, by varying the stoichiometry of the ligand. Cu(2+) and Zn(2+) stabilise the ring tautomeric form of the ligand in their mononuclear complexes M(HL(ring))(2). The structural characterisation of Zn(HL(ring))(2)·2MeOH·0.5H(2)O shows O,N-donor behaviour of the ring tautomer. The 1,2,3,4-tetrahydroquinazoline undergoes a ring-opening reaction upon formation of phenoxo-bridged dinuclear complexes M(2)(L(chain))(2) in which the chain tautomer is acting as O,N,N,N-donor. The crystal structure of Cu(2)(L(amide))(L(quinazoline))(MeOH)·2MeOH evidenced the sensitivity of H(2)L(ring) to the copper-mediated aerobic oxidation, which results in two derivatives of the ligand, a quinazoline and an amide. The quinazoline ligand is acting as monoanionic and mononucleating through its O,N,N binding site, while the amide ligand behaves as a trianionic and binucleating through its O,N,N,N and O,O binding sites in Cu(2)(L(amide))(L(quinazoline))(MeOH)·2MeOH.     

A novel stereocontrolled synthesis of cis-fused bicyclic lactams via [3 + 2]-cycloaddition of alkynyltungsten complexes with tethered aziridines

Treatment of alkynyltungsten complexes with tethered aziridines in the presence of BF(3).Et(2)O led to [3 + 2]-cycloaddition reactions, affording bicyclic tungsten-enamine species stereoselectively. The stereochemistry of the resulting product reveals that ring opening of aziridine is initiated by S(N)2 attack of the tungsten fragment. Decomplexation of these organometallics with I(2) in CH(2)Cl(2), followed by hydrolysis, afforded only cis-fused bicyclic lactams efficiently. [reaction: see text]     

Features of reaction between fluorine-containing glycidyl ethers and alcohols in basic medium

The reaction of fluorine-containing glycidyl ethers with various alcohols (i-PrOH, MeOH, PhOH, 2,2,3,3-tetrafluoropropanol) in basic medium resulted in products of regioselective opening of the oxirane ring. In reaction of 2,2,3,3-tetrafluoropropyloxymethyloxirane with 2-propanol under conditions of phase-transfer catalysis the main product was the corresponding 1,2-diol (yield 42%).     

Pyridinium salt photochemistry in a concise route for synthesis of the trehazolin aminocyclitol, trehazolamine

[reaction: see text] A strategy for the concise synthesis of trehazolamine, the aminocyclitol core of the potent trehalase inhibitor trehazolin, has been developed. The methodology takes advantage of photocyclization reaction of 1-methoxyethoxymethyl-3-pivaloxymethylpyridinium perchlorate to generate a bicyclic-aziridine intermediate, which is transformed under aziridine ring opening conditions to the key intermediate, 3,5-diacetoxy-3-pivaloxymethyl-4-(N-acetylamino)cyclopentene. In addition, the strategy is used in an enantio-divergent sequence for preparation of the natural (+)-trehazolamine and its unnatural (-)-enantiomer. In this route, the chiral auxiliary containing 1-(tetracetyl-alpha-D-glucosyl)-3-pivaloxymethylpyridinium perchlorate undergoes photocyclization to generate separable, diastereomeric bicyclic-aziridines, which are then independently transformed to enantiomeric 3,5-diacetoxy-3-pivaloxymethyl-4-(N-acetylamino)cyclopentenes.     

Radical-mediated nitrile translocation as the key step in the stereoselective transformation of 2-(4-chloro-2-cyanobutyl)aziridines to methyl cis-(1-arylmethyl-4-phenylpiperidin-2-yl)acetates

Non-activated 2-(4-chloro-2-cyano-2-phenylbutyl)aziridines were used as building blocks for the stereoselective synthesis of novel cis-2-cyanomethyl-4-phenylpiperidines via a microwave-assisted aziridine to piperidine ring expansion followed by a radical-induced nitrile translocation through initial formation and subsequent cleavage of intermediate bicyclic iminyl radicals. Furthermore, these 2-(cyanomethyl)piperidines were shown to be eligible substrates for the preparation of methyl cis-(1-arylmethyl-4-piperidin-2-yl)acetates through a Pinner reaction using gaseous HCl in methanol.     

Organolithium-induced alkylative ring opening of aziridines: synthesis of unsaturated amino alcohols and ethers

Organolithium-induced alkylative ring opening of N-sulfonyl-protected aziridinyl ethers is described. The reactions were efficiently carried out with a variety of organolithiums, providing a promising new strategy to unsaturated amino alcohols and ethers. Cis- and trans-1,4-dimethoxybut-2-ene-derived aziridines were prepared, and their propensity to undergo organolithium- induced alkylative desymmetrization is detailed. Use of a single enantiomer of the latter aziridine provides a route to enantiopure unsaturated amino ethers.     

Selective tandem enyne metathesis for the synthesis of functionalized cycloheptadienes

The regio- and site-selective ring expansion of dienes and the regioselective ring expansion of substituted cyclopentenes provide 1,3-cycloheptadienes by enyne metathesis under methylene-free conditions. Site-selectivity results from differential ring strain among two different cycloalkenes in diene reactants. The high regioselectivity found in the ring expansion of tetrahydroindene (THI) is explained on the basis of a selective ring opening by the second generation Grubbs' ruthenium carbene complex. The ring opening of substituted cyclopentenes and cyclopentene contained in a bicyclic ring system was also achieved. The ring expansion of bicyclic dienes provided seven-membered dienes contained in the bicyclo[5.2.0]nonane ring system. Details of the structural analysis are also discussed. A mechanistic analysis is provided to account for the data presented herein.     

A regio- and stereoselective approach to quaternary centers from chiral trisubstituted aziridines

A thorough investigation of a regio- and stereospecific aziridine ring opening reaction presents new synthetic technology for the construction of a variety of quaternary beta-substituted-alpha-amino functional groups. Mild, metal-free reaction conditions allow for application in highly functionalized systems. This reaction has been applied to the challenging stereoselective formation of tertiary alkyl-aryl ethers. The strategy for the formation of these hindered ethers has been investigated using a variety of functionalized aziridines and phenols to determine the scope of the reaction. Other nucleophiles, such as thiolate, azide, and chloride, have also been examined to encompass the synthesis of a broader range of functionalities. This aziridine ring opening reaction manifold has demonstrated utility in assembling: beta-substituted-alpha-amino carboxamides, beta-substituted-alpha-amino esters, beta-substituted-alpha-amino silyl ethers, beta-thio-alpha-amino carboxamides, beta-azido-alpha-amino carboxamides, and beta-halo-alpha-amino carboxamides. Studies to probe the effect of the aziridine substitution patterns show that alkyl aziridines display similar reactivity to alkynyl aziridines, giving insight into mechanistic possibilities.     

Application of the photocyclization reaction of 1,2-cyclopenta-fused pyridinium perchlorate to formal total syntheses of (−)-cephalotaxine

Two strategies for the formal total synthesis of (−)-cephalotaxine, based on pyridinium salt photochemistry, are described. The routes begin with photocyclization reaction of 1,2-cyclopenta-fused pyridinium perchlorate. This process efficiently and regioselectively produces a tricyclic aziridine, which undergoes sequential aziridine ring opening and enzymatic desymmetrization to generate enantio-enriched intermediates that contain the spirocyclic D,E-ring system found in cephalotaxine. These substances serve as precursors to late stage key intermediates used by Mori, Tietze, and Yoshida in earlier syntheses of (−)-cephalotaxine.     

Diastereocontrol by a hydroxyl auxiliary in the synthesis of pyrrolidines via radical cyclization

[reaction: see text] Organoselenium precursors of 3-aza-5-hexenyl radicals carrying a 1-hydroxyalkyl group in the 2-position were prepared by addition of organometallic reagents to N-allyl-2-aziridinecarbonitrile, reduction of the resulting aziridine ketone, and regioselective benzeneselenol ring opening of the aziridine. Reductive radical cyclization was highly selective, affording the corresponding trans-2,4-disubstituted pyrrolidine (cis/trans ca. 1/10) as the major diastereomer. Recrystallization afforded material that was substantially more enriched in the trans isomer (cis/trans < 1/25).     

Eleuthesides and their analogs: V. Medium- and large-ring lactones based on levoglucosenone

Ozonolysis of the bridging double bond in bicyclic enol ethers obtained by the Michael reaction and subsequent intramolecular etherification afforded chiral decanolides fused to a tetrahydropyran ring. Three-step procedures were developed for the synthesis of chiral lactones with medium and large rings via oxidative cleavage with pyridinium chlorochromate of mixed bicyclic ketals which were prepared by treatment with methanolic HCl of Michael adducts derived from levoglucosenone and cyclopenta-, cyclohexa-, cyclohepta-, and cyclododecanone.     

Development of a [3+3] cycloaddition strategy toward functionalized piperidines

This paper describes a novel route to functionalized piperidines via a formal [3+3] cycloaddition reaction of activated aziridines and palladium-trimethylenemethane (Pd-TMM) complexes. The cycloaddition reaction generally proceeds enantiospecifically with ring opening at the least hindered site of the aziridine. Therefore, readily available enantiomerically pure 2-substituted aziridines can be utilized to prepare enantiomerically pure 2-substituted piperidines in good to excellent yield. The N-substituent on the aziridine proved to be crucial to the success of this reaction with only 4-toluenesulfonyl (Ts) and 4-methoxybenzenesulfonyl (PMBS) aziridines permitting smooth cycloaddition to take place. Additionally, spirocyclic aziridines have been found to participate in the [3+3] cycloaddition reaction, whereas 2,3-disubstituted aziridines can be applied to provide fused bicyclic piperidines, albeit in low yield.     

Investigations on the Reactivity of Fascaplysin,Part II,General Stability Considerations and Products Formed with Nucleophiles

Reversible deprotonation of fascaplysin ( 1 ) was achieved with non‐nucleophilic bases (Scheme 1). Under basic aqueous conditions, opening of ring D of 1 occurred, yielding zwitter‐ionic reticulatine 2a , whereas, in a methoxide‐containing MeOH solution, an unexpected addition of three molecules of MeOH to the pyridinium ring produced an isomer mixture 3 of a trimethoxy‐substituted compound (Scheme 2). Transformation of the keto group of 1 to the oxime 4A took place in the presence of pyridine as base (Scheme 3). Grignard and alkyllithium reagents added as expected to the keto group of 1 , providing tertiary alcohols 5 and 6 (Scheme 4).     

Acetal-vinyl sulfide cyclization on sugar substrates: effect of structure and substituent

A range of 2-deoxyfuranoside and -pyranoside derivatives were fashioned into derivatives that carry a vinyl or propenyl side chain. Extension of the alkene by a Suzuki cross-coupling reaction with 1-bromo-1-(phenylthio)ethene gave thioenol ethers as the cyclization substrates. The treatment of these substrates with BF(3).Et(2)O in tert-butylmethyl ether below 0 degrees C induced cyclization to optically active bicyclic ethers. If the cyclizations are carried out in toluene as the solvent, the isomerization of the terminal thioenol ether to the inner thioenol ether can take place prior to the cyclization. The cyclization reactions can be impeded by steric and electronic factors. The opening of the bicyclic ethers could be illustrated with the base-induced conversion of the ketone 53 to the cyclooctenone 54.     

Nucleophile‐Dependent Regio‐ and Stereoselective Ring Opening of 1‐Azoniabicyclo[3.1.0]hexane Tosylate

1‐[(1R)‐(1‐Phenylethyl)]‐1‐azoniabicyclo[3.1.0]hexane tosylate was generated as a stable bicyclic aziridinium salt from the corresponding 2‐(3‐hydroxypropyl)aziridine upon reaction with p‐toluenesulfonyl anhydride. This bicyclic aziridinium ion was then treated with various nucleophiles including halides, azide, acetate, and cyanide in CH₃CN to afford either piperidines or pyrrolidines through regio‐ and stereoselective ring opening, mediated by the characteristics of the applied nucleophile. On the basis of DFT calculations, ring‐opening reactions under thermodynamic control yield piperidines, whereas reactions under kinetic control can yield both piperidines and pyrrolidines depending on the activation energies for both pathways.     

Catalytic Reduction of Cyclic Ethers with Hydrosilanes

Catalytic reductive transformations of ethers as a synthetic building block are an important class of chemical reactions because a range of essential chemical feedstocks and fuels in contemporary life can be prepared through the key step of ethereal C?O bond cleavage of cellulosic biomass. Although conventional stoichiometric and catalytic methods for sp²‐ and sp³‐C?O bond cleavage of linear ethers and alcohols with hydrosilanes are well established, silylative ring opening of cyclic ethers has been less highlighted in this context. This review outlines catalytic systems for the silylative reduction of a range of cyclic ethers, including epoxides and sugars, leading to the corresponding alcohols and/or hydrocarbons. The chemical reactivity and selectivity of these ring‐opening catalytic processes are discussed with respect to the type of substrates; the representative catalytic working modes are also described.     

Heteroatomic derivatives of aziridine. 15. Reaction of 1-(triethylsilyl)- and 1-[2-(trialkylsilyl)ethyl]-aziridines with thiols

The reaction of 1-(triethylsilyl)aziridine with alkanethiols proceeds with splitting out of aziridine and the formation of (alkylthio)triethylsilanes. The reaction of 1-(triethylsilyl)aziridine with 2-mercaptoethanol leads to 2-(triethylsilyloxy)ethanethiol; the same reaction in a closed system leads to [2-(2-aminoethylthiol)ethoxy]triethylsilane. 1-[2-(Trialkylsilyl)ethyl]aziridines react with 2-mercaptoethanol and with mercapto carboyxlic acids with opening of the aziridine ring.See [1] for Communication 14.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 891–893, July, 1988.     

1-(Phenylethynyl)aziridine in reactions with amines

The reaction of 1-(phenylethynyl)aziridine with a primary or secondary amine gave 2-benzylimidazoline-2 or N-aminoethyl substituted phenylacetamidine via nucleophilic attack on the aziridine ring. The mechanism of aziridine ring opening by an amine was studied.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 600–603, May, 1989.     

Rhodium(III)-catalyzed ring-opening of strained olefins through C–H activation of O-acetyl ketoximes: an efficient synthesis of trans-functionalized cyclopentenes and spiro[2.4]heptenes

An efficient strategy for the stereoselective synthesis of functionalized cyclopentenes and spiro[2.4]heptenes from strained olefins via C–H activation of aryl ketone O-acetyl ketoximes using [RhCl₂Cp^∗]₂ catalyst is described. The results revealed that a wide range of readily accessible aryl and heteroaryl ketoximes are compatible in this method for the ring opening of bicyclic and spirotricyclic olefins.     

Rhodium(II)-catalyzed aziridination of allyl-substituted sulfonamides and carbamates

Several unsaturated sulfonamides underwent intramolecular aziridination when treated with PhI(OAc)(2), MgO, and catalytic Rh(2)(OAc)(4) to give bicyclic aziridines in excellent yield. Treatment of the resulting azabicyclic sulfonamides in methanol in the presence of p-TsOH resulted in exclusive opening of the aziridine ring at the most substituted position affording six- and seven-membered ring products in high yield. In contrast, the intramolecular aziridination of several cycloalkenyl-substituted carbamates did not require a Rh(II) catalyst and proceeded via an iminoiodinane intermediate. The resulting tricyclic aziridines underwent ring opening when treated with various nucleophiles to give anti-derived products as expected for nucleophilic attack at the three-membered ring. The iodine(III)-mediated reaction of a 3-indolyl-substituted carbamate, however, required a Rh(II) catalyst. The expected aziridine was not observed, but rather simultaneous spirocyclization of C(3) and stereoselective syn-acylation at C(2) occurred to give compound 41, whose structure was unequivocally established by an X-ray crystallographic study. The reaction proceeds in a stepwise manner via a metal-free zwitterionic intermediate which is attacked by a nucleophilic reagent on the same side of the amide anion. Related reactions occurred with both a 2-indolyl- and 3-benzofuranyl-substituted carbamate but with lower stereoselectivity.