742 research outputs found
Enantioselective Total Synthesis of (+)-Cassiol
An enantioselective total synthesis of (+)-cassiol is reported. The complex derived from Pd-2(pmdba)(3) and enantiopure t-BuPHOX ligand catalyzes enantioconvergent decarboxylative alkylation to generate the quaternary carbon stereocenter at an early stage. The overall synthetic strategy involves a convergent late-stage coupling of two fragments. The synthesis features a longest linear sequence of eight steps
Efficiency of the Wang-Landau algorithm: a simple test case
We analyze the efficiency of the Wang-Landau algorithm to sample a multimodal
distribution on a prototypical simple test case. We show that the exit time
from a metastable state is much smaller for the Wang Landau dynamics than for
the original standard Metropolis-Hastings algorithm, in some asymptotic regime.
Our results are confirmed by numerical experiments on a more realistic test
case
Enantioselective Tsuji Allylations
The family of allylation reactions developed by Tsuji in the 1980s are capable of generating tertiary and quaternary carbon stereocenters from several synthetic precursors. Despite the utility of these transformations, they have seen little use in the synthesis of natural products. Recently, the power of these reactions was significantly enhanced by the development of enantioselective versions of these transformations. Applications of these methods to the enantioselective syntheses of natural products and pharmaceutical compounds highlight the importance of these developments
Pd-catalyzed enantioselective aerobic oxidation of secondary alcohols: Applications to the total synthesis of alkaloids
Enantioselective syntheses of the alkaloids (-)-aurantioclavine, (+)-amurensinine, (-)-lobeline, and (-)- and (+)-sedamine are described. The syntheses demonstrate the effectiveness of the Pd-catalyzed asymmetric oxidation of secondary alcohols in diverse contexts and the ability of this methodology to set the absolute configuration of multiple stereocenters in a single operation. The utility of an aryne C-C insertion reaction in accessing complex polycyclic frameworks is also described
Optimal importance sampling for overdamped Langevin dynamics
Calculating averages with respect to multimodal probability distributions is
often necessary in applications. Markov chain Monte Carlo (MCMC) methods to
this end, which are based on time averages along a realization of a Markov
process ergodic with respect to the target probability distribution, are
usually plagued by a large variance due to the metastability of the process. In
this work, we mathematically analyze an importance sampling approach for MCMC
methods that rely on the overdamped Langevin dynamics. Specifically, we study
an estimator based on an ergodic average along a realization of an overdamped
Langevin process for a modified potential. The estimator we consider
incorporates a reweighting term in order to rectify the bias that would
otherwise be introduced by this modification of the potential. We obtain an
explicit expression in dimension 1 for the biasing potential that minimizes the
asymptotic variance of the estimator for a given observable, and propose a
general numerical approach for approximating the optimal potential in the
multi-dimensional setting. We also investigate an alternative approach where,
instead of the asymptotic variance for a given observable, a weighted average
of the asymptotic variances corresponding to a class of observables is
minimized. Finally, we demonstrate the capabilities of the proposed method by
means of numerical experiments
Preparation of (S)-2-Allyl-2-methylcyclohexanone (Cyclohexanone, 2-methyl-2-(2-propen-1-yl)-, (2S)-)
[No abstract
High-Throughput Screening of the Asymmetric Decarboxylative Alkylation Reaction of Enolate-Stabilized Enol Carbonates
The use of high-throughput screening allowed for the optimization of reaction conditions for the palladium-catalyzed asymmetric decarboxylative alkylation reaction of enolate-stabilized enol carbonates. Changing to a nonpolar reaction solvent and to an electron-deficient PHOX derivative as ligand from our standard reaction conditions improved the enantioselectivity for the alkylation of a ketal-protected,1,3-diketone-derived enol carbonate from 28% ee to 84% ee. Similar improvements in enantioselectivity were seen for a β-keto ester derived and an α-phenyl cyclohexanone-derived enol carbonate
A reduced model for shock and detonation waves. II. The reactive case
We present a mesoscopic model for reactive shock waves, which extends a
previous model proposed in [G. Stoltz, Europhys. Lett. 76 (2006), 849]. A
complex molecule (or a group of molecules) is replaced by a single
mesoparticle, evolving according to some Dissipative Particle Dynamics.
Chemical reactions can be handled in a mean way by considering an additional
variable per particle describing a rate of reaction. The evolution of this rate
is governed by the kinetics of a reversible exothermic reaction. Numerical
results give profiles in qualitative agreement with all-atom studies
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