Direct asymmetric aldol-Tishchenko reaction of aliphatic ketones catalyzed by syn-aminoalcohol-Yb(III) complexes

The asymmetric direct aldol condensation of aldehydes with ethyl- and propylketones is catalyzed by syn-alpha-aminoalcohol-Yb(OTf)3 complexes, yielding the anti-1,3-diol monoesters with high diastereocontrol and good enantioselectivity. Three adjacent stereogenic centers are created in a simultaneous aldol condensation and Evans-Tishchenko reduction in an acyclic system.     

Asymmetric aldol-Tishchenko reaction catalyzed by Yb-complexes with basic amino acid-derived ligands

Simple amino acid-derived esters have been identified as promising chiral sources for the ytterbium-catalyzed aldol-Tishchenko reaction of aromatic aldehydes with aliphatic ketones. The 1,3-anti-diols with three stereogenic centers were isolated in excellent yields, complete anti-diastereocontrol and enantioselectivities of up to 50% ee.     

A note on matricial minimality

Let A be a complex n×n matrix, θ a matricial norm and r(A) the spectral radius of A. Then, it is known [2,7], r(A?r(θ(A)). In our present note we investigate when r(A?αI_n)=r(θ(A?αI_n))?α?C.     

Matricial logarithmic derivatives

If θ is a norm on Cⁿ, then the mapping $\text{A→}\text{lim}{}_{\mathrm{h\downarrow 0}}\text{6I+hA6}{}_{\mathrm{\theta }}\text{?1}\text{/h}$ from M_n(C) (=C^{n × n}) into R is called the logarithmic derivative induced by the vector norm θ. In this paper we generalize this concept to a mapping γ from M_n(C) into M_k(R), where k ? n. Denoting by α(B) the spectral abscissa of a square matrix B (the largest of the real parts of the eigenvalues), we show, in particular, that α(A) ?α(γ(A)). As a byproduct we obtain simple sufficient conditions for the stability of a matrix.     

Rate and product studies in the solvolyses of N,N-dimethylsulfamoyl and 2-propanesulfonyl chlorides

Contrary to earlier suggestions of an S(N)1 pathway for solvolyses of N,N-dimethylsulfamoyl chloride (1), an extended Grunwald-Winstein equation treatment of the specific rates of solvolysis in 32 solvents shows an appreciable sensitivity towards changes in both solvent nucleophilicity and solvent ionizing power. The actual values are very similar to those obtained in earlier studies of the solvolyses of sulfonyl and phosphoryl chlorides, solvolyses which are believed to proceed by an S(N)2 pathway. The observation of similar selectivities in aqueous-alcohol solvents further supports this assignment. In a recent report, an addition-elimination (association-dissociation) pathway was proposed for solvolyses of 2-propanesulfonyl chloride (2). A severe multicollinearity problem has been removed by the addition of several specific rates of solvolysis in fluoroalcohol-containing solvents. The new analyses using the extended Grunwald-Winstein equation lead to sensitivities similar to those for and the previously studied related compounds, and these solvolyses are also best described as following an S(N)2 pathway.     

Structure assignment, total synthesis, and antiviral evaluation of cycloviracin B1

The first total synthesis of the antivirally active glycolipid cycloviracin B(1) (1) is described. The approach is based on a two-directional synthesis strategy which constructs the C(2)()-symmetrical macrodiolide core of the target by an efficient template-directed macrodilactonization reaction promoted by 2-chloro-1,3-dimethylimidazolinium chloride 14 as the activating agent. Attachment of the lateral fatty acid chains to the lactide core thus formed features not only one of the most advanced ligand-controlled addition reactions of a functionalized dialkyl zinc reagent to a polyfunctional aldehyde, but also a highly demanding Julia-Kocienski olefination of a tetrazolyl sulfone bearing electrophilic and base-labile beta-hydroxy ester motifs. By virtue of the flexibility of this synthesis plan, it was possible to prepare a series of macrodiolide cores differing only in the absolute stereochemistry at the branching points as well as a host of model compounds for the fatty acid appendices of cycloviracin. Comparison of these derivatives with the natural product allowed us to establish the as yet unknown absolute stereochemistry of 6 chiral centers of 1 as (3R,19S,25R,3'R,17'S,23'R). Thereby, the (13)C NMR shifts of the anomeric position of the beta-glycosides residing at those positions turned out to be excellent probes for the absolute configuration of the attached aglycones. The concise set of data thus obtained also makes clear that the proposed structure of the fattiviracins, a seemingly closely related family of glycoconjugates, is not matched by the published data. Finally, the biological activity of synthetic 1 and some of the key intermediates obtained en route to this natural product was investigated, showing that the entire construct is necessary for appreciable and selective antiviral activity.     

A concise synthesis of the fully functional lactide core of cycloviracin B with implications for the structural assignment of related glycolipids

The absolute stereochemistry at the site of attachment of the fatty acid residues to the lactide core of the glycolipids cycloviracin B1 (1) and glucolipsin A (13) has been elucidated as (3R,3'R) by comparison of their 13C NMR data with those of the three possible, differently configured core structures 9, 12, and 14. Moreover, a careful analysis of this set of NMR data allows us to conclude that the structures previously proposed for a seemingly closely related class of antivirally active compounds, i.e., the fattiviracin family, need revision. The key step en route to the symmetrical dilactones 9 and 12 consists of a highly efficient cyclodimerization process which exploits the template effect exerted by potassium cations on the hydroxy acid cyclization precursor. The latter is obtained in excellent overall yield by a sequence involving ring-opening Claisen condensation of pentadecanolide to form the functionalized beta-ketoester 4, asymmetric hydrogenation catalyzed by [(BINAP)RuCl2]2.NEt3, and a beta-selective glycosylation reaction using trichloroacetimidate 6. The unsymmetrical dilactone 14, in contrast, is prepared by a stepwise approach based on a Yamaguchi lactonization as the means to close the macrocyclic ring.     

Catalytic asymmetric aldol reactions in aqueous media

Nature has perfected the stereospecific aldol reaction by using aldolase enzymes. While virtually all the biochemical aldol reactions use unmodified donor and acceptor carbonyls and take place under catalytic control in an aqueous environment, the chemical domain of the aldol addition has mostly relied on prior transformation of carbonyl substrates, and the whole process traditionally is carried out in anhydrous solvents. The area of aqua-asymmetric aldol reactions has received much attention recently in light of the perception both of its green chemistry advantages and its analogy to eon-perfected enzyme catalysis. Both chiral metal complexes and small chiral organic molecules have been recently reported to catalyze aldol reactions with relatively high chemical and stereochemical efficiency. This tutorial review describes recent developments in this area.