1,721,025 research outputs found
Asymmetric Synthesis of Methylphenidate and Quinolizidinones by Addition of Aldehydes to Piperidine-Based Conjugated N -Acyl-iminium Ions
Get PDFThe synergistic effect of Lewis acids and MacMillan organocatalyst allows the α-regioselective and enantioselective α-amidoalkylation of piperidine derivatives with enolizable aldehydes. The potentiality of the synthetic method is illustrated by a short asymmetric synthesis of the eutomer of the blockbuster drug Ritalin and by a concise formal synthesis of 1-alkyl-4-substituted quinolizidine alkaloids
Surface hopping investigation of benzophenone excited state dynamics
Get PDFWe present a simulation of the photodynamics of benzophenone for the first 20 ps after n -> pi* excitation, performed by trajectory surface hopping calculations with on-the-fly semiempirical determination of potential energy surfaces and electronic wavefunctions. Both the dynamic and spin-orbit couplings are taken into account, and time-resolved fluorescence emission is also simulated. The computed decay time of the S-1 state is in agreement with experimental observations. The direct S-1 -> T-1 intersystem crossing (ISC) accounts for about 2/3 of the S-1 decay rate. The remaining 1/3 goes through T-2 or higher triplets. The nonadiabatic transitions within the triplet manifold are much faster than ISC and keep the population of T-1 at about 3/4 of the total triplet population, and that of the other states (mainly T-2) at 1/4. Two internal coordinates are vibrationally active immediately after n -> pi* excitation: one is the C-O stretching and the other one is a combination of the conrotatory torsion of phenyl rings and of bending involving the carbonyl C atom. The period of the torsion-bending mode coincides with oscillations in the time-resolved photoelectron spectra of Spighi et al. and substantially confirms their assignment
Dynamics of acetone photodissociation: a surface hopping study
Get PDFWe present on the fly surface hopping simulations of the dynamics of photoexcited acetone in the n->pi* band, taking into account both the spin-orbit and the dynamic couplings and allowing for the C-C bond dissociation. The S0, S1, T1 and T2 states were considered and the propagation time was 50 ps. According to the simulation results, after excitation to S1 both Internal Conversion (IC) to S0 and InterSystem Crossing (ISC) to T1 or T2 take place at comparable rates; T2 plays an important role and the simultaneous treatment of the spin-orbit and dynamic couplings is shown to be mandatory to describe the photodynamics. We propose a mechanism that explains the observed fast and slow decay rates of the S1 state of acetone
SYNTHESIS OF GAMMA-HYDROXY KETONES BY LICLO4-CATALYZED ADDITION OF LITHIUM ENOLATES TO 1,2-EPOXIDES
No full textA simple, efficient, anti-stereoselective, highly regioselective method is described for the synthesis of gamma-hydroxy ketones by the direct opening of 1,2-epoxides with lithium enolates derived from simple ketones in anhydrous THF, in the presence of LiClO4
Improved stereoselective synthesis of both methyl alpha- and beta-glycosides corresponding to the amino sugar component of the E ring of calicheamicin (I)(gamma 1) and esperamicin A(1)
No full textThe monosaccharide component (alpha and beta-anomer) of the E Ring of calicheamicin (I)(gamma 1) and esperamicin A(1) has been synthetized by an efficient and improved stereoselective procedure starting from methyl 2-deoxy-alpha- and beta-D-ribopyranoside. The synthetic procedure makes use, in each case, of a cyclic sulphate and of the regioselective ring opening of an intermediate activated aziridine. (C) 1997 Elsevier Science Ltd
REGIOCHEMICAL CONTROL OF THE RING-OPENING OF AZIRIDINES BY MEANS OF CHELATING PROCESSES - SYNTHESIS AND RING-OPENING REACTIONS OF CIS- AND TRANS-AZIRIDINES DERIVED FROM 4-(BENZYLOXY)CYCLOHEXENE
No full textThe regiochemical outcome of the ring opening of aziridines bearing a polar remote functionality was verified in a conformationally semirigid bicyclic system in which the polar functionality (OBn) is in an homoallylic relationship to the aziridine ring. The couples of diastereoisomeric unactivated cis-3 and trans-4 and activated aziridines cis-5 and -7 and trans-6 and -8 derived from 4-(benzyloxy)-cyclohexene were prepared, and some of their opening reactions were studied. The regioselectivity observed in the opening reactions of the cis derivatives turned out to depend largely on the opening (standard, strongly acidic, or metal-assisted) reaction conditions, thus providing a nice regioalternating process. The results obtained are rationalized by admitting the incursion of chelate bidentate intermediate structures in which the proton or the metal is actively involved
Regioselectivity and stereoselectivity of nucleophilic addition to glycal and imino-glycal vinyl epoxides and their carba analogs: A rationalization based on HSAB theory and MEP
No full textBesides the usual parameters for softness (Fukui indexes) and hardness (atomic charges), novel parameters, obtained from the molecular electrostatic potential on the van der Waals surface (ESP-indexes), were considered as stereochemical and possible hardness descriptors. The effect of oxirane ring activation by electrophiles was modeled by scanning the aforementioned parameters over the epoxide ring opening in the neutral substrates and in two corresponding protonated derivatives. C(1) is a softer center than C(3), but oxirane activation rapidly levels softness and shifts hardness from C(3) to C(1); with carbocycles, C(1) and C(3) hardness are leveled, and with heterocycles, C(1) becomes harder than C(3). ESP-indexes are suitable parameters to account for stereoselectivity and more reliable hardness descriptors than the atomic charges. An explanation for the prevailing syn-C(1)-selectivity observed with glycals and iminoglycals epoxides versus the anti-C(3)-selectivity observed with carba analogs is given. A rationalization of Ferrier rearrangement is also provided
MILD LIBF4-PROMOTED AMINOLYSIS OF OXETANES
No full textLiBF4 in acetonitrile efficiently catalyzes the aminolysis of trimethylene oxide and 2-octyl oxetane under mild conditions (r.t. or 80 degrees C) to give the corresponding gamma-amino alcohols in very good yields
Theoretical Conformational Study of Six-Membered Cyclic Allyl Epoxides
No full textAccurate “ab initio” calculations (MP2 method) were performed to outline the conformational profile of a number of six-membered cyclic allyl epoxides differing either in the nature of the cycle fragment (Y) bound to the unsaturation, or in the substitution at the endocyclic carbon bound to the epoxy ring and bridging the epoxy ring with the Y fragment. In particular, we calculated structures 4 (Y=CH2), 5 (Y=O), 6 (Y=NH), 7 (Y=S), 8 (Y=CF2), 9 (Y=NH2+), 10 (Y=CO), 11 (Y=BH) and 12 (Y=NCOOH), where the fragment of the endocyclic carbon bridging “Y” and the epoxy fragment is either non-substituted (4a – 12a) or bears a methyl side chain trans (4b–12b) or cis (4c–12c) to the epoxidic oxygen. Saturated analogs (Y=O and Y=CH2) were also computed to test the method and to evaluate the conformational profile in the absence of the unsaturation. Minimum energy conformations were found which differ in the relative position of the Y group and the epoxy oxygen, with respect to a plane containing the epoxy ring carbons and the adjacent saturated endocyclic carbon: they may be on the same side (conformer A) or on opposite sides (conformer B). Conformers A are generally more stable. The conjugation effect of Y with the double bond lowers the barrier between the two conformers to the extent that in a few cases only conformer A is associated with a minimum of energy. On the basis of the elongation of the allylic oxirane C–O bond, we postulated the order of reactivity of epoxides 4–12 in the oxirane ring-opening process, and a mechanism based on the more reactive conformer A. A comparison was also made between MP2 and DFT calculation methods
Trifluoromethansolfonati di Lantanidi (III). Nuovi efficaci catalizzatori per l'amminlisi di ossirani
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