Pulse radiolysis studies of ortho-quinone chemistry relevant to melanogenesis.

The contributions of pulse radiolysis towards characterisation of unstable ortho-quinones relevant to melanogenesis are reviewed. The quinones discussed include dopaquinone, the precursor of both eumelanogenesis and phaeomelanogenesis, and 5-S-cysteinyldopaquinone, an early component of the phaeomelanogenic pathway. Redox exchange between dopaquinone and 5-S-cysteinyldopa is shown to be a determinant of the balance between eumelanogenesis and phaeomelanogenesis. Ortho-quinones resulting from the oxidation of tertiary N,N-dialkylcatecholamines cyclise to redox-inactive betaines which fail to autoactivate tyrosinase. This is consistent with the dopa detected during melanogenesis catalysed by tyrosinase being formed indirectly by a combination of dopaquinone intramolecular reductive addition to form leucodopachrome (cyclodopa), followed by redox exchange between remaining dopaquinone and leucodopachrome. Rapid tautomerism of the ortho-quinone of 4-cyanomethylcatechol to a redox-inactive quinomethane likewise inhibits tyrosinase autoactivation. The incorporation of trihydric phenol moieties in melanin is modelled by the reactions of several ortho-quinones with phloroglucinol, which itself is not directly oxidised by tyrosinase due to the meta-positioning of the hydroxyl groups. The importance of a susceptibility towards nucleophilic attack as well as a propensity to undergo redox-exchange, in the chemistry of melanogenic ortho-quinones, is emphasised.     

Synthesis of nitrogen and sulfur macrocycles with cis exogenous oxygen and sulfur donor atoms

A series of new N4 and N8 macrocycles has been prepared, that includes cis-exogenous O2, S2 and S/O atoms to allow chelation to a metal external to the macrocyclic ring. We found that thioamide units within the macrocycles were unstable to attack by secondary amines and thus alkylated precursors containing only tertiary amines could lead to exogenous-S2 macrocycles. Cyclisation of alkylated tetraamine precursors with dimethyloxalate or dithiooxamide led to both N4 and N8 macrocycles via 1 + 1 and 2 + 2 cyclisation reactions with exogenous-O2 or S2 respectively. Alkylation of preformed exogenous-O2 macrocycles was explored and led to alkyl substitution at the secondary amine nitrogens in the ring, however synthesis of these species was overall lower yielding than cyclisation using alkylated tetraamine precursors. Thionation of an exo-O2 macrocycle using an analogue of Lawesson's Reagent led to formation of the analogous exogenous-S2 and exogenous-O,S macrocycles. Related S2N2 macrocycles with exogenous-O2 were prepared by a cyclisation route but could not be isolated free of larger ring analogues.     

Modification of natural coumarins reaction of khellactone esters with amines

The reaction of diesters of khellactone with primary and secondary amines under mild conditions has given derivatives of 4′-aminodihydroseselin. Under more severe conditions, not only the replacement of a 4′-acyloxy group by an amino group but also the opening of the lactone ring with the formation of the corresponding cinnamamide takes place. The ease of hydrolysis of the 3′-acyloxy group and subsequent esterification of the alcohols formed and also the use of various amines makes it possible to obtain very diverse acyloxy and amino derivatives.     

Alpha-oximono-esters as precursors to heterocycles--generation of oxazinone N-oxides and cycloaddition to alkene dipolarophiles

Preparation of a series of terminally and internally substituted delta-alkenyl and delta-alkynyl esters 6, 7 and 9, potential precursors to oxazin-2-one nitrones, has been attempted. Condensation between pyruvic or benzoylformic acid and the appropriate alcohol proceeded smoothly in some cases whilst allylic transposition was a major feature in other cases--most especially during reactions with alpha-vinylbenzyl alcohol. Oximation of pyruvic acid derivatives furnished E-oxime isomers whilst benzoylformic acid derivatives afforded mixed geometrical isomers. The E-oxime of 4a1 carrying an internal Me group undergoes facile thermal cyclisation affording nitrones 1c and 1d in good yield. Oximes E-5a,b with a terminal methyl substituent on the alkene moiety furnish nitrone only under the influence of an external electrophile [PhSeBr/AgBF4]. A terminal Ph substituent on 5c,d prohibits formation of the cyclic dipole irrespective of reaction conditions, and whilst 5d reacts to afford a bicyclic isoxazolofuranone 13 by an IOOC reaction (intramolecular oxime olefin cyclisation) 5c remains thermally inert. Finally delta-alkynyl oximes 9c,d also failed to cyclise. The regio- and stereochemical characteristics of the cycloadditions between the new dipoles and electron poor olefinic dipolarophiles have been investigated. The conditions needed for reaction were rather forcing since the dipoles are somewhat stabilised by the adjacent alkoxycarbonyl group. All reactions proceeded regiospecifically to give adducts with 5-substituted isoxazolidine rings whilst diastereoselectivity varied with the choice of dipolarophile and the steric demands of the nitrone substituents. The phenylselenyl dipole 10a could not be trapped by any dipolarophiles bar dimethyl acetylenedicarboxylate.     

X=Y-ZH systems as potential 1,3-dipoles: Part 17. Sequential michael addition-5-endo-trig cyclisation of arylidene imines of α-amino acid esters

Imines of α-amino acid esters undergo regiospecific Michael addition to methyl acrylate or acrylonitrile in good yield in benzene at 25°C catalysed by benzyItrimethyl ammonium methoxide (BTAM). The Michael adducts cyclise to a mixture of two stereoisomeric polysubstituted proline ester derivatives in the presence of 1 mol. of BTAM. Mechanistic studies, involving chiral intermediates, show this cyclisation to be an example of a disfavoured 5-(enolexo)-endo-trig process.     

Functionalization of the pyridazin-3(2H)-one ring via palladium-catalysed aminocarbonylation

5-Iodo- and 4,5-dibromo-2-methylpyridazin-3(2H)-ones were aminocarbonylated in the presence of various amines including amino acid methyl esters in a palladium-catalysed reaction. The iodo derivative afforded the corresponding amides with complete conversion and high isolated yields. The dibromo derivative has shown unexpectedly high reactivity in this reaction, resulting in 4,5-dicarboxamides using primary amines as N-nucleophiles. Monoaminocarbonylation has not been observed, i.e., neither 4-bromo-5-carboxamide nor 4-carboxamido-5-bromo derivatives have been formed. However, the use of secondary amines such as piperidine and morpholine resulted in the formations of mixtures of amino-substituted bromopyridazinones. That is, no carbon monoxide insertion took place in these cases. Some mechanistic details of the formation of aminocarbonylation and amination products are also discussed.     

N-alkoxy-n-acylnitrenium ions in intramolecular aromatic addition reactions

N-alkoxy-N-acylnitronium ions are generated by treatment of N-alkoxy-N-cohloroamides with silver ions in ethereal solvents. These intermediates readily cyclise onto aromatic nuclei on alkozy side-chains to give benzoxazines and benzoxazepines and on the acyl side-chains to give γ, δ and ϵ benzolactams. Spirane products are formed by ipso addition When a 4-methoxy substituent ia present on the side-chain aromatic rings. The yields and regioselectivities of these reactions have been ascribed to different transition structures for cyclisation onto the acyl and alkoxy side-chains which involve respectively an exocyolic and endocyclic N-0 π-bond. Evidence for this exeptionally high π-bond character has been obtained from MNDO calculations which predict a π-bond order of 0.9 and a rotational barrier of 29.7 kcalmol^-1     

Transformations of 5-methyl-6-carbethoxy-3,4-dihydrothieno-[2,3-d]pyrimidine for synthesis of 4-methoxy-, 4-alkylamino-, and other derivatives of thieno[2,3-d]pyrimidine

4-Chloro derivatives of thieno[2,3-d]pyrimidine are formed by the action of phosphorus oxychloride on 5-methyl- and 5-methyl-6-carbethoxythieno[2,3-d]pyrimidin-4-ones. Action of nucleophilic reagents (methanol, sodium methylate, primary and secondary amines) on these chloro derivatives gave 4-methoxy-, 4-alkylamino-, and 4-dialkylamino substituted thieno[2,3-d]pyrimidines. It was found that 4-methoxy derivatives of thieno[2,3-d]pyrimidines undergo a thermal rearrangement into 3-methyl-substituted thieno[2,3-d]pyrimid-4-ones. In the bromination of 5-methyl-4-chloro- and 5-methyl-4-methoxy-substituted thieno[2,3-d]pyrimidines by N-bromosuccinimide, 5-bromomethyl derivatives of thieno[2,3-d]pyrimidine are formed, from which, by the action of primary and secondary amines, 5-aminomethyl-substituted thieno[2,3-d]pyrimidines were obtained. A synthesis of 1,2,3,4-tetrahydro-1,3-diazepino[4a,10-d,e] thieno[2,3-d]pyrimidines was also carried out.Deceased.Translated from Khimiya Geterotsilicheskikh Soedinenii, No. 7, pp. 925–928, July, 1985.     

Halogenures de tellurenyle—iii: Coupure carbone aromatique-tellure par protodetelluration et carbodetelluration intramoleculaire

In the course of electrophilic cyclisation attempts, o-phenyltellurobenzoyl-chloride did not cyclise to telluroxanthone. but isomerised to 2-chloro tellurobenzophenone through intramolecular carbodetelluration. A breaking of the same C_aromatic-Te bond is also realized by protonolysis. This last reaction leads to a preparative method of synthesis of ditellurosalicylic acid, a starting material for synthesis of benzocondensed tellurium heterocycles.     

Heterocyclisations des hydroxy-6 methoxy-2 hexene-2 et heptene-2 oates de methyle.

Tetrahydropyran derivatives – and – are formed in good yields by cyclisation of methyl-6-hydroxy-2-hexenoate or 2-heptenoate mediated by various electrophilic reagents (mCPBA, benzeneselenyl chloride, N-bromosuccinimide, iodine). Cyclisations of Z and E isomers are stereospecific. The diastereoselectivity of cyclisation of the secondary alcohol varies with the nature of the electrophilic reagent.     

Stereoselective synthesis and cyclisation of the acyclic precursor to auripyrone A and B

The acyclic precursor to the auripyrones has been synthesized by a stereoselective aldol strategy. This compound fails to undergo cyclisation to form the spiroacetal dihydropyrone ring system found in auripyrone A and B; instead, it forms a dihydropyrone ring by cyclisation of the C11 hydroxyl onto the C15 carbonyl with subsequent dehydration. In contrast, a model compound was prepared and shown to cyclise to both the spiroacetal dihydropyrone ring system and the dihydropyrone ring.     

Reactivity of the 1,2-Diphenytethyl Radical in a Solvent Cage

The 1,2-diphcnylethyl radical and derivatives were generated from photolysis of tran-f-stilbene and its derivatives 1–13 with secondary amines as quenchers. The 1,2-diphenylelhyl radicals that escaped from the solvent cage were trapped by 2-methyl-2-nitrosopropane (MNP) and were detected by the HPLCEPR method. The yield of the spin adduct is greater for tertiary amines. The smaller yields of the spin adduct formed from secondary amines are ascribed to greater reactivities of the 1,2-diphenylethyl and dialkylamino radicals within the solvent cage.     

Modification of natural coumarins reaction of khellactone esters with amines

The reaction of diesters of khellactone with primary and secondary amines under mild conditions has given derivatives of 4-aminodihydroseselin. Under more severe conditions, not only the replacement of a 4-acyloxy group by an amino group but also the opening of the lactone ring with the formation of the corresponding cinnamamide takes place. The ease of hydrolysis of the 3-acyloxy group and subsequent esterification of the alcohols formed and also the use of various amines makes it possible to obtain very diverse acyloxy and amino derivatives.All-Union Scientific-Research Institute of Medicinal Plants, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 488–499, July–August, 1980.     

Catalyst-free chemoselective N-tert-butyloxycarbonylation of amines in water

[Structure: see text] Catalyst-free N-tert-butyloxycarbonylation of amines in water is reported. The N-t-Boc derivatives were formed chemoselectively without any isocyanate, urea, N,N-di-t-Boc, and O/S-t-Boc as side products. Chiral amines, esters of alpha-amino acids, and beta-amino alcohol afforded optically pure N-t-Boc derivatives. Amino alcohol and 2-aminophenol afforded the N-t-Boc derivative without oxazolidinone formation. Selectivity was observed during competition of aromatic amine vs aliphatic amine, amine vs amino acid ester, amine vs amino alcohol, and primary amine vs secondary amine.     

Oxidation of N-substituted dopamine derivatives: irreversible formation of a spirocyclic product

Oxidation of amide, urea and guanidinium derivatives of dopamine gives relatively stable ortho-quinones whereas oxidation of corresponding thioamide and amidinium derivatives rapidly and quantitatively gives novel bicyclic and spirocyclic products formed via the corresponding ortho-quinone.     

Synthesis and transformations of triphenylpropargylphosphonium bromide

A method of the synthesis of triphenylpropargylphosphonium bromide is developed. Its isomerization and hydration in various solvents are studied, and reactions with secondary amines, triethylamine, and triphenylphosphine are carried out. It is established that secondary amines add to the intermediate allene isomer with subsequent migration of the formed double bond to the phosphorus atom. The reaction of triethylamine with triphenylpropargyl and triphenylethynyl bromides occurs similarly to alkaline hydrolysis involving attack of the amine on the phosphorus atom. Triphenylphosphine forms with triphenylpropargylphosphonium bromide a bis-salt with a terminal methylene group. Experimental evidence is obtained showing that for phosphoxazole derivatives to form from oximes derived from triphenyl(oxomethyl)phosphonium salts that latter should bear an aryl substituent at the keto group.     

Beta-lactams or gamma-lactams by 4-exo-trig or 5-endo-trig anionic cyclisation of lithiated acrylamide derivatives

Substituted acrylamide derivatives of benzylamine are lithiated alpha to nitrogen by LDA. The benzyllithium thus formed undergoes either 5-endo-trig anionic cyclisation, formally by intramolecular conjugate addition to the acrylamide, to yield 5-membered lactams, or, if the acrylamide bears a beta-electron withdrawing group, 4-exo-trig cyclisation to a beta-lactam.     

Reactions of 3-iodo-7-dialkylaminocoumarins with secondary amines

The reaction of 3-iodo-7-diethylaminocoumarin and 2,3,6,7-tetrahydro-10-iodo-1H,5H-quinolizino[9,9a,1-gh]coumarin with secondary amines (diethylamine, piperidine, morpholine, imidazole, and benzimidazole) leads to 4,7-diaminocoumarins. The corresponding 3-iodo-4-chloro-7-dialkylaminocoumarins under similar conditions give 3-iodo-4,7-diaminocoumarins. 4-Aminomethyl derivatives of coumarins are formed in the reactions of 3-iodo-4-methyl-7-diethylaminocoumarin and 2,3,6,7-tetrahydro-9-methyl-10-iodo-1H,5H-quinolizino[9,9a,1-gh]coumarin with these secondary amines.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 609–618, May, 1991.     

Three ways of reactions of 5-(3,5-dimethyl-1<Emphasis Type="Italic">H</Emphasis>-pyrazol-1-yl)-2-phenyl-1,3-oxazole-4-carbonitrile and its analogs with nitrogen-containing bases

Substituted 5-(3,5-dimethyl-1H-pyrazol-1-yl)-1,3-oxazole-4-carbonitrile differently react with nitrogen bases having different numbers of labile hydrogen atoms. Treatment of the title compounds with secondary amines or morpholine results in nucleophilic replacement of the pyrazolyl substituent at C⁵, the ozaxole ring remaining unchanged. Their reactions with primary amines are accompanied by cleavage of the oxazole ring with formation of the corresponding enamino nitriles. Hydrazine hydrate acts in a similar way, but enehydrazino nitriles thus formed undergo fast cyclization to give new 4,5-diaminopyrazole derivatives. The latter can be converted into substituted pyrazolo[1,5-a]pyrimidines whose structure has been proved by X-ray analysis.     

Solid-phase synthesis of 1,2,3,4-tetrahydroisoquinoline derivatives employing support-bound tyrosine esters in the Pictet-Spengler reaction

The solid-phase synthesis of 1,2,3,4-tetrahydroisoquinoline-3-carboxamides employing carboxyl-supported, o-alkylated tyrosine esters in a Pictet-Spengler reaction is described. Esterification of [4-(hydroxyphenyl)thiomethyl]polystyrene (Marshall resin) with ethers of N-BOC-L-tyrosine using diisopropylcarbodiimide (DIC) and 4-dimethylaminopyridine (4-DMAP) afforded the solid-supported ester derivatives. Removal of the BOC group with trifluoroacetic acid (TFA) afforded the carboxyl-supported tyrosine ester, which was then treated with paraformaldehyde and TFA to afford the desired solid-supported counterpart. Acylation of the secondary amine with arylsulfonyl chlorides followed by reaction with amines resulted in the formation of the desired 2-arylsulfonyl-7-alkoxy-1,2,3,4-tetrahydroisoquinoline-3-carboxamides. Alternatively, the support-bound tetrahydroisoquinoline-3-carboxylate derivatives could be treated with an aldehyde and a reducing agent to give the corresponding support-bound tertiary amine. Exposure of these resin-bound products to amines afforded the corresponding 2-alkyl-7-alkoxy-1,2,3,4-tetrahydroisoquinoline-3-carboxamides after cleavage from the resin. Alternative routes to the desired chemotypes, as well optimization of the conditions for the Pictet-Spengler reaction and the conditions for the acylation and reductive amination of the support-bound secondary amines, are also described.