Search results for "Intramolecular force"

Intra- and intermolecular forces dependent main chain conformations of esters of α,β-dehydroamino acids

2013

Abstract Esters of dehydroamino acids occur in nature. To investigate their conformational properties, the low-temperature structures of Ac-ΔAla-OMe, Ac-ΔVal-OMe, Z-(Z)-ΔAbu-OMe, and Z-(Z)-ΔAbu-NHMe were studied by single-crystal X-ray diffraction. The ΔAla ester prefers the fully extended conformation C5. Both the ΔVal and (Z)-ΔAbu esters assume the conformation β, whereas the amide analogue of the latter prefers the conformation α. For the conformations found, DFT calculations using B3LYP/6-311++G(d,p) with the SCRF-PCM and M062X/6-311++G(d,p) with the SCRF-SMD method were applied to mimicking chloroform and water environment. The tendency of the ΔVal and (Z)-ΔAbu esters towards the confo…

New reactions and step economy: the total synthesis of (±)-salsolene oxide based on the type II transition metal-catalyzed intramolecular [4+4] cyclo…

2006

Studies on the viability of the type II nickel-catalyzed intramolecular [4+4] cycloaddition of bis-dienes show that it is influenced by both diene substitution and geometry. Both E- and Z-isomers of 19 and 20 react, albeit at markedly different rates, to afford cycloadducts, whereas only the Z-isomer of 9 (and not the E-isomer) reacts to give 8 and 25. Chemoselective elaboration of 8 to (±)-salsolene oxide (7) was used to confirm the cycloadduct structure while establishing a step economical route to the natural product.

Diastereoselective synthesis of spongian diterpenes. Total synthesis of the furanoditerpene (−)-spongia-13(16),14-diene

1999

Abstract An effective diastereoselective synthesis of the marine-sponge metabolite (−)-spongia-13(16),14-diene 1 is achieved starting from S-(+)-carvone via a homochiral phenanthrenone as the key intermediate for the construction of the furan ring system. S-(+)-Carvone was transformed into the phenanthrenone 2a in six steps (53% overall yield), using an intramolecular Diels-Alder reaction as the key step. Conversion of the enone function in 2a into an epoxyaldehyde function followed by cyclisation and aromatisation in acid conditions completed the construction of ring D.

Migration of bidentate anionic ligands in binuclear compounds derived from 1,2-bis(imino)propyl-palladium(II) complexes

1983

Abstract The complexes [Pd(AB){C(NR)CMeNR}(PPh3)] (I; AB = 2,4-pentanedionate, N-methylsalicylaldiminate, pyrrole-2-N-methylaldiminate; R = p-C6H4OMe) react with ‘MCl2’ (‘MCl2’ = CoCl2, NiCl2, CuCl2, ZnCl2, [PdCl2(NCMe)2], K[PtCl3(CH2CH2)]) to give either the binuclear adducts [(PPh3)(AB)Pd{C(NR)CMeNR}MCl2] (II) or the zwitterionic isomers [(PPh3)Cl2Pd{C( NR)CMeNR}M (AB)] (III) depending on the nature of the metal M and of the anionic ligand AB. The formation of products III is accounted for in terms of an intramolecular migration of ligands between the two metal centers Pd and M of the initially formed MCl2 adducts II.

Toward an Understanding of the Molecular Mechanism of the Reaction between 1-Methylpyrrole and Dimethyl Acetylenedicarboxylate. An ab Initio Study

1998

The molecular mechanism for the reaction between 1-methylpyrrole and dimethyl acetylenedicarboxylate (DMAD) has been studied using ab initio methods. Two alternative reaction pathways have been considered, both of which correspond to stepwise processes with initial, rate-determining formation of a common zwitterionic intermediate. This intermediate is formed by nucleophilic attack of the pyrrole ring to the carbon−carbon triple bond of DMAD. Closure of this intermediate (pathway A) affords a [4 + 2] cycloadduct, whereas intramolecular proton transfer (pathway B) affords a Michael adduct. The much larger potential energy barrier of the second step in pathway B relative to pathway A is respon…

Elongated push–pull diphenylpolyenes for nonlinear optics: molecular engineering of quadratic and cubic optical nonlinearities via tuning of intramol…

1999

Abstract Push–pull polyenes are of particular interest for nonlinear optics (NLO) as well as model compounds for long-distance intramolecular charge transfer (ICT). In order to tune the ICT phenomenon and control the linear and nonlinear optical properties, we have synthesized and investigated several series of soluble push–pull diphenylpolyenes of increasing length and having various donor (D) and acceptor (A) end groups. Their linear and NLO properties have been studied by performing electro-optical absorption measurements (EOAM) and third-harmonic generation (THG) experiments in solution. Each push–pull molecule exhibits an intense ICT absorption band in the visible characterized by an i…

Intramolecular Charge Transfer as Revealed by Results from the Measurement of Ground and Excited State Dipole Moments

1992

Intramolecular charge transfer and dual fluorescence of 4-(dimethylamino)benzonitrile: ultrafast branching followed by a two-fold decay mechanism.

2011

International audience; In this contribution we present new experimental and theoretical results for the intramolecular charge transfer (ICT) reaction underlying the dual fluorescence of 4-(dimethylamino) benzonitrile ( DMABN), which indicate that the fully twisted ICT (TICT) state is responsible for the time-resolved transient absorption spectrum while a distinct partially twisted ICT (pTICT) structure is suggested for the fluorescent ICT state.

Highly convergent total synthesis of (+)-anaferine and (−)-dihydrocuscohygrine

2019

A unified and highly convergent total synthesis of anaferine and dihydrocuscohygrine alkaloids has been devised, taking advantage of the dual role of N-sulfinyl amines as nucleophilic nitrogen sources and chiral auxiliaries. A bidirectional cross metathesis reaction followed by a double intramolecular aza-Michael reaction led us to create the whole skeleton of the natural products in a very simple manner.

Ligand-Based Charge-Transfer Luminescence in Ionic Cyclometalated Iridium(III) Complexes Bearing a Pyrene-Functionalized Bipyridine Ligand: A Joint T…

2012

Two new heteroleptic iridium(III) complexes [Ir(ppy)(2)(pyr(2)bpy)][PF(6)] ([1a][PF(6)]) and [Ir(dfppy)(2)(pyr(2)bpy)][PF(6)] ([2a][PF(6)]), where Hppy = 2-phenylpyridine, Hdfppy = 2-(3,5-difluorophenyl)pyridine, and pyr(2)bpy = 5,5'-bis(pyren-1-yl)-2,2'-bipyridine, have been synthesized and fully characterized. The single-crystal structures of pyr(2)bpy and the complexes 4{[1a][PF(6)]}·2CH(2)Cl(2)·9H(2)O and [2a][PF(6)]·0.25CH(2)Cl(2)·H(2)O have been determined. The effect of the pyrene substituents on the electronic properties is investigated through a comprehensive photophysical and theoretical study on the two complexes in comparison to reference complexes without substituents on the an…