Showing papers on "Pyran published in 1997"

Conical Intersection Mechanism for Photochemical Ring Opening in Benzospiropyran Compounds

Abstract: The photoinduced ring-opening process in spiropyran compounds has been modeled through CASSCF calculations on the lowest excited state (S1) of benzopyran. The documented S1 reaction coordinate is characterized by a ring-opening transition state connecting a cyclic intermediate to a much more stable acyclic structure. Remarkably this structure does not correspond to a real S1 intermediate but to a crossing point (i.e., a conical intersection) between the excited- and ground-state energy surfaces. At this crossing the excited-state system can undergo fully efficient decay to S0. Using the parent pyran molecule as a model, we have characterized two ground-state valleys which develop from the crossing point. The first valley leads to reactant (pyran) regeneration. The second valley leads to cZc-penta-2,4-dienal indicating that the primary ring-opened photoproduct is formed in an unstable conformation. The benzopyran/pyran reaction coordinates suggest that the first step in the benzospiropyran photochromic rea...

Palladium-Catalyzed Oxidative Cross-Coupling of 2-Phenylphenols with Alkenes

Abstract: 2-Phenylphenols react with alkenes such as acrylate esters in the presence of a palladium-copper catalyst system under air to give the corresponding 6-substituted 6H-dibenzo[b,d]pyran derivatives.

Synthesis and Photochromic Behaviour of Naphthopyrans, Pyranoquinolines, Pyranoquinazolines and Pyranoquinoxalines

Abstract: New chromenes annulated with different six-membered azaheterocycles were prepared, i.e., the 3H-pyra-no[3,2-f]quinolines 9/10 and 14, the 8H-pyrano[2,3-h]isoquinoline 11, the 8H-pyrano[3,2-f]quinazolme 12, the 8-H-pyrano[3,2-f]quinoxaline 13, and the 2H-pyrano[2,3-f]isoquinoline 15. The synthesis was achieved using conveniently substituted α,β-unsaturated aldehydes and organotitanium intermediates arising from azaheterocyclic phenols. Their photochromic behaviour (photocolouration yield, UV/VIS spectrum of photomerocyanines, rate constant of thermal bleaching) were studied besides those of corresponding naphthopyrans. The heterocycle effect and the role of substituents in the pyran moiety were investigated quantitatively through the study of the photochromic properties and the solvent effects. Diaryl-substituted azino-fused chromenes, especially isoquinoline derivatives, exhibit increased colourabilities and bathochromically shifted spectra for photomerocyanines which open up new prospects for photochromic applications.

Tungsten(II)−Carbene Complex Functions as a Dicationic Synthon: Efficient Constructions of Furan and Pyran Frameworks from Readily Available α,δ- and α,ε-Alkynols

Abstract: Treatment of tungsten−η1-α,δ- and −η1-α,e-alkynols 4−6 with RCHO/BF3·Et2O (R = alkyl, aryl) in cold diethyl ether effected cycloalkenation reaction, yielding tungsten−η1-furylidene and −η1-pyrylidene salts in excellent yields (>95%). The structures of these oxacarbeniums were elucidated through X-ray diffraction studies of the representative compounds 7 and 8 in addition to standard NMR and IR spectral data. In contrast with conventional metal carbeniums, these tungsten oxacarbeniums reacted with two molecules of nucleophiles such as H2O, NaBH3CN, and Grignard reagents, resulting in α,α-double addition reactions to afford furan and pyran derivatives in good yields. In the hydride case, unsymmetric α,α-double addition of η1-furylidenium salts was achieved via treatment with NaBH4/MeOH. Organocuprates also effected double alkylations of these salts but in a distinct 1,3-addition pathway. The reactions of these oxacarbeniums with CH2N2 were examined; the outcome depends on their vinyl substituents. When the ...

Water soluble photochromic compounds, compositions and optical elements comprising the compounds

Abstract: Described are novel water soluble photochromic naphthopyran and benzopyran compounds having an alkylaminoalkylamino group on the naphtho- or benzo-portions and/or pyran portions thereof. These compounds may be represented by one of the following graphic formulae: ##STR1## wherein R1 is an alkylaminoalkylamino group. Articles such as ophthalmic lenses or other plastic transparencies and coating compositions such as paints and inks that incorporate the novel water soluble compounds are also described.

The Synthesis, Structure, and Anticancer Activity of cis- and trans-4‘,7-Dihydroxyisoflavan-4-ols

Abstract: cis-4‘,7-Dihydroxyisoflavan-4-ol (4) and trans-4‘,7-dihydroxyisoflavan-4-ol (5), two proposed metabolites of daidzein (4‘,7-dihydroxyisoflavone), have been synthesized and fully characterized for the first time. The vicinal coupling constants of the pyran ring protons are compatible with a half-chair conformation. The cis isomer is anancomeric while the trans isomer consists of a 68:32 mixture of two ring inversion conformers. Molecular mechanical calculations are in agreement with the half-chair conformation of the pyran ring and suggest that the cis isomer is biased because of an unfavorable gauche interaction of the equatorial hydroxyl and the axial phenyl group. The isoflavanols 4 and 5 are comparable to genistein (4‘,5,7-trihydroxyisoflavone) in antitumor activity against human prostate cancer cells.

Activated Nitriles in Heterocyclic Synthesis: Synthesis ofPyrano[2,3-d]pyrimidine andPyrano[3,2-e][1,2,4]triazolo[1,5-c]pyrimidineDerivatives

Abstract: Several newnaphtho[2′,1′:5,6]pyrano[2,3-d ]pyrimidineshave been synthesized via hydrazinolysis of a2-ethoxymethylideneaminonaphtho[1,2-b ]pyran;polysubstitutedpyrano[3,2-e ][1,2,4]triazolo[1,5-c ]pyrimidineshave also been prepared.

Enantioselective synthesis of the lactone moiety of HMG-CoA reductase inhibitor: stereoselective synthesis of (+)-(4R,6R)-4-hydroxy-6-(2-phenylethyl)-tetrahydro-2H-pyran-2-one

Abstract: The enantioselective synthesis of the lactone moiety of (+)-compactin and (+)-mevinolin was established starting from a meso -3,5-dihydroxycyclohexanone derivative by employing an enantioselective deprotonation strategy

Hetero-Diels-Alder reaction of 3-aryl-2-benzoyl-2-propenenitriles with enol ethers. Synthesis of 2-alkoxy-3,4-dihydro-2H-pyran-5-carbonitriles

Abstract: The hetero-Diels-Alder reaction of 3-aryl-2-benzoyl-2-propenenitriles1a–d with enol ethers2a–c yieldscis/trans diastereoisomers of 2-alkoxy-4,6-diaryl-3,4-dihydro-2H-pyran-5-carbonitriles3 and4 in 79–98% yield. The similar reaction of1a–c with cyclic enol ether5 affords diastereoisomeric cycloadducts6 and7 withcis annulated pyran rings. Reaction of3 with sulfuric acid leads to 2-hydroxy-3,4-dihydro-2H-pyran-5-carbonitriles8 and9.

The molybdenum cofactors – perspective from crystal structure

Abstract: The field of molybdopterin-containing enzymes has experienced an infusion of excitement as a result of several reports of high-resolution X-ray crystallographic structures including the tungsten-containing aldehyde oxidoreductase from the hyperthermophile Pyrococcus furiosus and three different studies on a single enzyme, DMSO reductase. The tungstoprotein was the first of the reported structures and contained several crucial features [1]. The structure provided unequivocal proof for coordination of Mo to the dithiolene moiety of molybdopterin, a finding subsequently confirmed in all of the other structures. Thus the crystal structures have categorically ruled out alternative modes of binding of Mo to the pterin, as proposed by Fischer et al. [2]. Secondly, the structure of the W-containing aldehyde oxidoreductase revealed an unexpected feature of the W cofactor, i.e., chelation of W to the dithiolenes of two molybdopterin molecules. A third important finding of this study was the presence of a pyran ring resulting from the fusion of the 3b-hydroxyl of the side chain of the pterin to C-7 of the pterin ring, now known to be a universal property of the protein-bound cofactor. The 2 :1 ratio of pterin to metal is not unique to W-containing enzymes, since both Rhodobacter DMSO reductase and Escherichia coli formate dehydrogenase have been found to contain bis(MGD)-Mo [3]. So far, the 2 :1 pterin-to-Mo ratio has been found only in MGD-containing enzymes. The presence of the tricyclic pterin was unexpected, since three derivatives of molybdopterin, Form A, Form B, and dicarboxamidomethyl molybdopterin contain linear 6-alkyl side chains [4]. Two other derivatives, Form B and urothione, contain a thiophene ring rather than a pyran ring [4]. Since these derivatives are formed from molybdoenzvmes under mild conditions, the formation of the pyran ring must be a freely reversible process, with ring opening occurring as soon as the pterin is released from the protein. It is likely that the pyran adduct formation stabilizes the pterin ring against oxidative modifications that could disrupt the structure of the cofactor. The structures of Form B and urothione suggest that oxidation of the pterin ring could cause cis-trans isomerization of the dithiolene, leading to weakened binding of Mo and facilitating the formation of the thiophene ring. The crystal structures of aldehyde oxidoreductase from Desulfovibrio gigas [5], and DMSO reductase from Rhodobacter sphaeroides [6] as well as Rhodobacter capsulatus [7] have elicited considerable interest, the former because of its homology to the xanthine oxidase family of hydroxylases and the latter because of the considerable differences seen in Mo coordination in three different structural studies. The implications of these findings are discussed below.

An Expeditious Synthesis of 9,10-Dihydrophenanthrene by Condensation of 2H-Pyran-2-ones with α-Tetralone†‡

Abstract: 7-Substituted 1-aryl-3-hydroxy-4-methoxycarbonyl-9,10-dihydrophenanthrene (3) and its derivatives were synthesized from 6-aryl-3-methoxycarbonyl-4-methylsulfanyl-2H-pyran-2-ones (1) by carbanion induced ring transformation reactions; the salient feature of this procedure is to provide a one-pot synthesis of functionalized polycyclic aromatic hydrocarbons.

1-substituted 3-dimethylaminoprop-2-en-1-ones as building blocks in heterocyclic synthesis : routes to 6-aryl- and 6-heteroaryl-2h-pyran-2-ones and 6- and 4-arylpyridin-2(1h)-ones

Abstract: Several new 6-substituted-3-acylamino-2H-pyran-2-ones 6a–j have been prepared from the reaction of enaminones 4a–f with N-acyl- and N-benzoyl-glycines; the enaminones 4a–c react with malononitrile in ethanol solution and in the presence of a base to yield amides 11a–c which are converted into 6-aryl-1,2-dihydro-2-oxypyridine-3-carbonitriles 13a–c on reflux in acetic acid.

Semi-empirical AM1 and PM3 molecular orbital calculations on the mechanism of the hydrolysis of unsaturated lactones: substituted (E)-5,5′-diphenylbifuranylidenediones and 3,7-diphenylpyrano[4,3-c]pyran-1,5-diones

Abstract: Semi-empirical molecular orbital (AM1 and PM3) calculations including solvent effects by the SCRF model on the mechanism of the addition of nucleophiles to unsaturated five- and six-membered bislactones of the Pechmann dye type indicate a similar mechanism for both systems. The rate-determining step appears to be the addition of a second nucleophile to the enol of the ring-opened monolactone. The five-membered lactones are found to be more reactive than their six-membered analogues. Electron donor substituents (p-Me, p-MeO) increase and acceptor substituents (p-Cl, m-NO2) decrease the activation energy.