Abundance of generic homoclinic tangencies in real-analytic families of diffeomorphisms

We consider one-parameter families of two-dimensional diffeomorphisms with homoclinic tangencies. Various authors considered the dynamical complexities due to such tangencies satisfying certain nondegeneracy conditions. In this paper we provide methods to actually verify, for real analytic families, that there are homoclinic tangencies which satisfy these (generic) nondegeneracy generic conditions.     

Fluorinated (hetero)cycles via ring-closing metathesis of fluoride- and trifluoromethyl-functionalized olefins

Ring-closing metathesis (RCM) has been shown to be a viable tool to incorporate fluoride and trifluoromethyl substituents in (hetero)cyclic ring systems. 2-Fluoroacrylamides were cyclized to the corresponding lactams, and trifluoromethyl-substituted olefins were cyclized to yield trifluoromethylated cyclopentenes, pyrrolines and a dihydrofuran derivative.     

Heteroclinic attractors: Time averages and moduli of topological conjugacy

In this paper we consider attracting heteroclinic cycles. We recall that these cycles usually have no S.B.R. measure. This is related with the fact that certain time averages do not converge. We obtain a topological interpretation of the asymptotic properties of these non-converging time averages. In terms of these asymptotic properties we obtain a complete set of moduli for the attracting heteroclinic cycles.     

A short and scalable route to orthogonally O-protected 2-deoxystreptamine

A seven-step synthesis of orthogonally O-protected 2-deoxy-streptamine has been developed from readily available neomycin, with an overall yield of 28%. Key chemical transformations include a chemoselective glycosidic bond hydrolysis and two regioselective protective group manipulations involving acetylation and deacetylation. The synthetic route is amenable to scale-up for the production of multigram quantities of enantiopure and orthogonally O-protected 2-deoxystreptamine, a versatile scaffold for the generation of libraries of RNA-targeting ligands.     

Computed CH Acidity of Biaryl Compounds and Their Deprotonative Metalation by Using a Mixed Lithium/Zinc‐TMP Base

With the aim of synthesizing biaryl compounds, several aromatic iodides were prepared by the deprotonative metalation of methoxybenzenes, 3‐substituted naphthalenes, isoquinoline, and methoxypyridines by using a mixed lithium/zinc‐TMP (TMP=2,2,6,6‐tetramethylpiperidino) base and subsequent iodolysis. The halides thus obtained, as well as commercial compounds, were cross‐coupled under palladium catalysis (e.g., Suzuki coupling with 2,4‐dimethoxy‐5‐pyrimidylboronic acid) to afford various representative biaryl compounds. Deprotometalation of the latter compounds was performed by using the lithium/zinc‐TMP base and evaluated by subsequent iodolysis. The outcome of these reactions has been discussed in light of the CH acidities of these substrates, as determined in THF solution by using the DFT B3LYP method. Except for in the presence of decidedly lower pK_a values, the regioselectivities of the deprotometalation reactions tend to be governed by nearby coordinating atoms rather than by site acidities. In particular, azine and diazine nitrogen atoms have been shown to be efficient in inducing the reactions with the lithium/zinc‐TMP base at adjacent sites (e.g., by using 1‐(2‐methoxyphenyl)isoquinoline, 4‐(2,5‐dimethoxyphenyl)‐3‐methoxypyridine, or 5‐(2,5‐dimethoxyphenyl)‐2,4‐dimethoxypyrimidine as the substrate), a behavior that has already been observed upon treatment with lithium amides under kinetic conditions. Finally, the iodinated biaryl derivatives were involved in palladium‐catalyzed reactions.     

Deprotonative metalation of aromatic compounds by using an amino-based lithium cuprate

Deprotonative cupration of aromatic compounds by using amino‐based lithium cuprates was optimized with 2,4‐dimethoxypyrimidine and 2‐methoxypyridine as the substrates and benzoyl chloride as the electrophile. [(tmp)₂CuLi] (+2 LiCl) (tmp=2,2,6,6‐tetramethylpiperidino) was identified as the best reagent and its use was extended to anisole, 1,4‐dimethoxybenzene, other substituted pyridines, furan, thiophene and derivatives, and N‐Boc‐indole (Boc=tert‐butyloxycarbonyl). Of the electrophiles employed to attempt the interception of the generated aryl cuprates, aroyl chlorides, iodomethane, and diphenyl disulfide efficiently reacted. In addition, different oxidative agents were identified to afford symmetrical biaryls. Finally, palladium‐catalyzed coupling with aryl halides was optimized and allowed the synthesis of different aryl derivatives in medium to good yields.     

Stereoselective Mannich Reactions in the Synthesis of Enantiopure Piperidine Alkaloids and Derivatives

Piperidine alkaloids are members of the alkaloid family that is characterized by the presence of a six-membered nitrogen-containing heterocycle. Piperidine alkaloids are found mainly in plants and often exhibit interesting biological and pharmacological activities. Despite the accumulation of these natural products in plants, relatively low quantities of alkaloids are produced in absolute terms and thus synthesis of alkaloids and derivatives thereof remains relevant to identify targets for drug discovery. Throughout the years, researchers have come up with a myriad of methods to synthesize piperidine derivatives. This review describes methods that employ stereoselective Mannich reactions to create the core of piperidine alkaloids. Asymmetric induction in the Mannich reaction has been achieved by a range of methods that have been divided into three conceptual approaches: (1) chiral pool-based (internal asymmetric induction), (2) chiral auxiliary-based (relayed asymmetric induction) and (3) asymmetric catalysis-based (external asymmetric induction). Of each approach, we describe the reaction mechanism and rationalize the stereochemical outcome of the Mannich products.