Synthesis based on cyclohexadienes .24. A new total synthesis of pupukean-2-one and a facile entry to copa and ylanga type sesquiterpene skeletons

A novel tandem 5-exo-trig allyl and 3-exo-trig radical cyclisation and rearrangement to copa and ylanga type sesquiterpene skeletons from easily prepared cyclohexadienes is reported, A new total synthesis of pupukean-2-one 8, which belongs to a novel class of sesquiterpenes, involving a 5-exo-trig allyl radical cyclisation as the key step is also reported.

The nature of charge ordering in rare earth manganates and its strong dependence on the size of the A-site cations

Charge ordering in rare earth manganates of the type Ln(0.5)A(0.5)MnO(3) (Ln = rare earth, A = alkaline earth) is highly sensitive to the average radius of the A-site cations, [r(A)]. Tn the small [r(A)] regime (e.g., Y0.5Ca0.5MnO3), charge ordering occurs in the paramagnetic state, the transformation to an antiferromagnetic state occurring at still lower temperatures. At moderate [r(A)] values (e.g., Nd0.5Sr0.5MnO3), a ferromagnetic metallic state transforms to a charge-ordered antiferromagnetic state with cooling. These two distinct types of charge ordering and associated properties are explained in terms of the variation of the exchange couplings J(FM) and J(AFM) with [r(A)] and the invariance of the single-ion Jahn-Teller energy with [r(A)]. A qualitative temperature-[r(A)] phase diagram, consistent with the experimental observations, has been constructed to describe the properties of the manganates in the different [r(A)] regimes. (C) 1997 Academic Press.

Enantiodivergent total synthesis of microcarpalide from L-tartaric acid

Stereoselective approach for the synthesis of both enantiomers of bio-active decanolactone microcarpalide is described from L-tartaric acid. The synthesis of the key intermediates en route to the natural product is achieved from L-tartaric acid involving the elaboration of gamma-hydroxy amide derived from tartaric acid and ring opening of an epoxide derived from tartaric acid. (C) 2011 Elsevier Ltd. All rights reserved.

Halogen Bonding in 2,5-Dichloro-1,4-benzoquinone: Insights from Experimental and Theoretical Charge Density Analysis

Experimental charge density distribution in 2, 5-dichloro-1, 4-benzoquinone has been carried out using high resolution X-ray diffraction data at 90 K to quantitatively evaluate the nature of C-Cl center dot center dot center dot O=C halogen bond in molecular crystals. Additionally, the halogen bond is studied from geometrical point of view and the same has been visualized using Hirshfeld surface analysis. The obtained results from experimental charge density analysis are compared with periodic quantum calculations using B3LYP 6-31G(d,p) level of theory. The topological values at bond critical point, three-dimensional static deformation density features and electrostatic potential isosurfaces unequivocally establish the attractive nature of C-Cl center dot center dot center dot O=C halogen bond in crystalline lattice.

Stereoselective total synthesis of (+/-)-tochuinyl acetate and (+/-)-dihydrotochuinyl acetates

Details of the first total synthesis of the marine natural product dihydrotochuinyl acetate is described. Cyclopentenone annulation of p-methylacetophenone via a Claisen rearrangement-Wacker oxidation based sequence generated the cyclopentenone 3, a known precursor for the sesquiterpenes cuparene, laurene, alpha-cuparenone and beta-cuparenones. Conversion of the ketone moiety into a carboxylate followed by stereoselective alkylation and reduction transformed the cyclopentenone 3 into the primary alcohol 19. Birch reduction of the alcohol 19 followed by acetylation furnished (+/-)-dihydrotochuinyl acetate, whereas direct acetylation of 19 furnished (+/-)-tochuinyl acetate. (C) 1998 Elsevier Science Ltd. All rights reserved.

Modulation of lifetimes and diastereomeric discrimination in triplet-excited substituted butane-1,4-diones through intramolecular charge-transfer quenching

Triplet lifetimes have been determined for the diastereomers of a broad set of butane-l,4-dione derivatives (1-3). A remarkable dependence of lifetimes on conformational preferences is revealed in that the lifetimes are shorter for the meso diastereomers of 1-3 than those for the racemic ones. The intramolecular beta-phenyl quenching is promoted in the case of meso diastereomers by virtue of the gauche relationship between the excited carbonyl group and the beta-aryl ring, while a distal arrangement in the lowest energy conformation (H-anti) in racemic diastereomers prevents such a deactivation. The involvement of charge transfer in the intramolecular beta-phenyl quenching is suggested by the correlation of the triplet lifetimes of the meso diastereomers of compounds 2 with the nature of the substituent on the beta-phenyl rings. In the case of racemic diastereomers, beta-methoxy substitution on the beta-phenyl ring (2-OCH3, 3-OCH3) also led to a decrease of the triplet lifetimes when compared to those of the nonsubstituted compounds (2-H, 3-H). This shortening is accounted for by the deactivation of a small proportion of the excited molecules through beta-phenyl quenching. In addition to the above factors, the lifetimes in the case of meso diastereomers can further be controlled by increasing the energy spacing between the T-1 and T-2 states, since beta-phenyl quenching occurs from the latter for compounds 2 and 3. Through a rational conformational control, a surprisingly long triplet lifetime (300 ns) has been measured for the first time for a purely n,pi* triplet-excited beta-phenylpropiophenone dimer (1-rac).

Nonlinear electrical conduction and broad band noise in the charge-ordered rare earth manganate Nd0.5Ca0.5MnO3

Measurements of the dc transport properties and the low-frequency conductivity noise in films of charge-ordered Nd0.5Ca0.5MnO3 grown on Si substrate reveal the existence of a threshold field in the charge-ordered regime beyond which strong nonlinear conduction sets in along with a large broad band conductivity noise. Threshold-dependent conduction disappears as T --> T-CO, the charge-ordering temperature. This observation suggests that the charge-ordered state gets depinned at the onset of the nonlinear conduction. (C) 1999 American Institute of Physics. [S0003-6951(99)05247-X].

Photon-photon total cross-section

We study the total inelastic gamma gamma cross-section and discuss predictions from different models, with a special attention to their dependence on the input parameters. In particular we examine the results from a simple extension of the Regge Pomeron exchange model and those from the eikonalized mini-jet model. We then compare both of them with recent LEP data.

Rearrangement approaches to sesquiterpenes containing multiple contiguous quaternary carbon atoms. Total synthesis of (+/-)-myltayl-8(12)-ene and (+/-)-6-epijunicedranol

Details of the first total syntheses of the sesquiterpenes myltayl-8(12)-ene and 6-epijunicedran-8-ol are described. The aldehyde 13, obtained by Claisen rearrangement of cyclogeraniol, was transformed into the dienones 12 and 18. Boron trifluoride-diethyl ether mediated cyclization and rearrangement transformed the dienones 12 and 18 into the tricyclic ketones 16 and 17, efficiently creating three and four contiguous quaternary carbon atoms, respectively. Wittig methylenation of 16 furnished (+/-)-myltayl-8(12)-ene (11), whereas reduction of the ketone 17 furnished (+/-)-6-epijunicedranol (23).

Synthesis based on cyclohexadienes: Part 26 - Total synthesis of some naturally occurring phthalides from Alternaria species

Total synthesis of the naturally occurring phytotoxic phthalides, silvaticol 7, zinniol 5 and the phthalides 1 and 2, is reported from the substrate 16 derived from the Alder-Rickert reaction of 1-methoxy-2-methyl-3-trimethylslyloxycyclohexa-1,3-diene 15 with dimethyl acetylenedicarboxylate.

Structural instability of the charge ordered compound Nd0.5Sr0.5MnO3 under a magnetic field

We report an anomalous magnetostriction behavior of the charge ordered compound Nd0.5Sr0.5MnO3. We have found that the applied magnetic field not only gives rise to a large negative magnetoresistance but also produces a huge positive magnetovolume effect. This unusual effect is explained considering that the applied magnetic field induces a structural transition at which the volume drastically increases. This effect is also seen in the anisotropic magnetostriction which shows clear anomalies at the field induced transition.

Synthesis and characterization of novel charge transfer complexes formed by N,N%-bis(ferrocenylmethylidene)-p-phenylenediamine and N-(ferrocenylmethylidene)aniline

N,N'-Bis(ferrocenylmethylidene)-p-phenylenediamine 1 and N-(ferrocenylmethylidene) aniline 2 are readily synthesized by Schiff base condensation of appropriate units. Iodine (I-2), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetrachloro-1,4-benzoquinone (CA), tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) form charge transfer complexes with 1 and 2. IR spectroscopy suggests an increase in the amount of charge transferred from the ferrocenyl ring to the oxidant in the order, I-2 < CA < TCNQ < TCNE approximate to DDQ. EPR spectra of the oxidized binuclear complexes are indicative of localized species containing iron- and carbon-centered radicals. The Mossbauer spectrum of the iodine oxidized complex of 1 reveals the presence of both Fe(III) and Fe(II) centers. Variable temperature magnetic and Mossbauer studies show that the ratio of Fe(III)/Fe(II) centers varies as a function of temperature. The larger Fe(II)/Fe(III) ratio at lower temperatures is best explained by a retro charge transfer from the iodide to the iron(III) metal center. There is negligible solvent effect on the formation of the iodine oxidized charge transfer complex of 1. (C) 1999 Elsevier Science S.A. All rights reserved.

Theory of coexisting charge and spin-density waves in (TMTTF)(2)Br, (TMTSF)(2)PF6 and alpha-(BEDT-TTF)(2)MHg(SCN)(4)

We discuss a recently formulated microscopic theory of the unusual coexistence of spin density waves (SDWs) and charge density waves (CDWs) that has been seen in recent experiments on (TMTTF)2Br, (TMTSF)2PF6 and α-(BEDT-TTF)2MHg(SCN)4.

Theory of coexisting charge and spin-density waves in (TMTTF)(2)Br, (TMTSF)(2)PF6, and alpha-(BEDT-TTF)(2)MHg(SCN)(4)

Recent experiments indicate that the spin-density waves (SDWs) in (TMTTF)(2)Br, (TMTSF)(2)PF6, and alpha-(BEDT-TTF)(2)MHg(SCN)(4) are highly unconventional and coexist with charge-density waves (CDWs). We present a microscopic theory of this unusual CDW-SDW coexistence. A complete understanding requires the explicit inclusion of strong Coulomb interactions, lattice discreteness, the anisotropic two-dimensional nature of the lattice, and the correct hand filling within the starting Hamiltonian. [S0031-9007(99)08498-7].

Enantioselective total synthesis of macrosphelides A and E

Enantioselective synthesis of 16-membered trilactone macrolides, macrosphelide A and E from (S)-lactic acid is described. Key features of the synthesis include the utility of a hitherto unexplored beta-ketophosphonate derived from lactic acid and Yamaguchi lactonization leading to the title compounds. (C) 2011 Elsevier Ltd. All rights reserved.