A Novel Example for Optical Resolution of Racemic Ketones Originating from Batrachotoxin Synthesis

A Novel Example for Optical Resolution of Racemic Ketones Originating from Batrachotoxin Synthesi

Highly Diastereofacial <i>Anti</i>-Aldol Reaction: Practical Synthesis of Optically Active <i>anti</i>-2-Alkyl-3-Hydroxycarboxylic Acid Ester Units^†

A variety of esters derived from commercially available norephedrine were used in diastereoselective anti-aldol reactions. The aldol reaction of designed 2-(N-2-methylbenzyl-N-2,4,6-trimethylbenzyl)amino-1-phenylpropanol esters 4a−d with aldehydes furnished anti-2-alkyl-3-hydroxycarboxylic acid esters in excellent diastereomeric ratios (>98:2) when LDA−Cp2ZrCl2 (0.3 equiv) was used for enolization, followed by transmetalation into the zirconium enolate for aldolization. The novel auxiliary 3 for the anti-aldol reaction does not exhibit the ordinary basicity of tertiary amines; 3 can be extracted from acidic media with organic solvents. Its use is, therefore, very advantageous not only for extraction of the aldol products from the acidic water solutions, but also for recovering the chiral auxiliary 3 after the reductive cleavage. Treatment of aldol or 3-protected aldol products with DIBAL-H or LiAlH4 affords the versatile synthons, 2-alkyl-propane-1,3-diols or those 3-protected diols in >98% ee's together with 3 in nearly quantitative recovery

A Novel Example for Optical Resolution of Racemic Ketones Originating from Batrachotoxin Synthesis

A Novel Example for Optical Resolution of Racemic Ketones Originating from Batrachotoxin Synthesi

Highly Efficient <i>O</i>-Glycosylations with <i>p</i>-Tolyl Thioribosides and <i>p</i>-TolSOTf

A wide variety of p-tolyl thioriboside donors are examined for O-ribosylations of primary and secondary alcohols. p-Tolylsulfenyl trifluoromethanesulfonate (p-TolSOTf) is very effective in promoting O-ribosylations with p-tolyl thioriboside; all reactions are completed within 1−15 min to provide the desired products in good yield with reliable α/β selectivity. A wide range of functional groups are tolerated under these conditions. The described O-ribosylation conditions are very useful for the generation of ribosaminouridine library molecules in solution or on polymer support

New Chiral Derivatizing Agents: Convenient Determination of Absolute Configurations of Free Amino Acids by ¹H NMR

The chiral carbonate reagents 5 allow for the direct and unambiguous determination of the absolute configurations of a wide range of free amino acids using 1H NMR. By using a ∼3:1 mixture of (S)-5 and (R)-5, absolute configurations of the corresponding carbamates are determined by only analyzing the nitrogen protons

Fe/Cr- and Co/Cr-Mediated Catalytic Asymmetric 2-Haloallylations of Aldehydes

The first example to couple aldehydes and 3-bromo-2-halopropenes in a catalytic asymmetric manner is reported. The coupling reaction is effected by the use of a chiral sulfonamide−Cr complex (prepared in situ from 1d, CrBr3, Fe(III) or from Co(II), Et3N, and Mn), TMSCl, and 2,6-lutidine. The method reported here is operationally simple and scalable, furnishing 3-halohomoallylic alcohols with a synthetically useful level of enantiomeric excess

A New Oxyma Derivative for Nonracemizable Amide-Forming Reactions in Water

An Oxyma derivative, (2,2-dimethyl-1,3-dioxolan-4-yl)methyl 2-cyano-2-(hydroxyimino)acetate (2), displayed remarkable physicochemical properties as a peptide-coupling additive for peptide-forming reactions in water. Short peptides to oligopeptides could be synthesized by using 2, EDCI, and NaHCO3 in water without measurable racemization. Significantly, a simple basic and acidic aqueous workup procedure can remove all reagents utilized in the reactions to afford only coupling products in consistently excellent yields

Concise Synthesis of Capuramycin

A concise total synthesis of capuramycin (1), a promising preclinical TB drug lead, is achieved by high-yield formations of the cyanohydrin 5a and 4′′,5′′-glycal derivative 12. Capuramycin can be synthesized in eight steps from the uridine building block 5a with >30% overall yield. The synthetic intermediates reported here are useful for generation of analogs to improve pharmacokinetic properties of capuramycin

Polymer-Supported (2,6-Dichloro- 4-alkoxyphenyl)(2,4-dichlorophenyl)methanol: A New Linker for Solid-Phase Organic Synthesis

An acid and base stable hydroxytetrachlorodiphenylmethyl (HTPM) linker is developed for polymer-supported organic synthesis. The linkers reported here are utilized for loading carboxylic acids, amines, alcohols, and phenols, and are stable to Brønsted and Lewis acids, Brønsted bases, and a wide variety of nucleophiles. However, the HTPM linkers can conveniently be cleaved by the solvolytic displacement reactions with 20% TFA

Synthesis of Ureidomuraymycidine Derivatives for Structure– Activity Relationship Studies of Muraymycins

One of the key constituents of the muraymycins is the 6-membered cyclic guanidine, (2<i>S</i>,3<i>S</i>)-muraymycidine (or <i>epi</i>-capreomycidine). In order to diversify the structure of the oligopeptide moiety of the muraymycins for thorough structure–activity relationship studies, we have developed a highly stereoselective synthesis of ureidomuraymycidine derivatives with the lactone <b>4a</b>