Synthetic studies on bafilomycin A1: stereoselective synthesis of the enantiopure C1–C11 fragment

A common and versatile synthetic route to (-) and (+) pentenomycin I, (+) halopentenomycin I and dehydropentenomycin

A versatile and stereoselective total synthesis of (+) and (-) pentenomycin I, (+) halopentenomycins I and dehydropentenomycin from a common chiral polyhydroxylated cyclopentene through oxidation and protection/deprotection has been described. Stereoselective hydroxymethylation, stereoselective Grignard reaction and ring closing metathesis are the key features of our approach.

Cyclic α-Alkoxyphosphonium Salts from (2-(Diphenylphosphino)phenyl)methanol and Aldehydes and Their Application in Synthesis of Vinyl Ethers and Ketones via Wittig Olefination

Cyclic α-alkoxyphosphonium salts have been synthesized from (2-(diphenylphosphino)phenyl)methanol and aldehydes in 36-89% yields. These phosphonium salts are bench-stable solids and undergo Wittig olefination with aldehydes under basic conditions (K2CO3 or t-BuOK) to form benzylic vinyl ethers, which are readily hydrolyzed to 1,2-disubstituted ethanones under acidic conditions. The formation mechanism of these phosphonium salts via hemiacetal is also proposed.

Effects of phosphorus substituents on reactions of α-alkoxyphosphonium salts with nucleophiles

The effects of phosphorus substituents on the reactivity of α-alkoxyphosphonium salts with nucleophiles has been explored. Reactions of α-alkoxyphosphonium salts, prepared from various acetals and tris(o-tolyl)phosphine, with a variety of nucleophiles proceeded efficiently. These processes represent the first examples of high-yielding nucleophilic substitution reactions of α-alkoxyphosphonium salts. The reactivity of these salts was determined by a balance between steric and electronic factors, respectively, represented by cone angles θ and CO stretching frequencies ν (steric and electronic parameters, respectively). In addition, a novel reaction of α-alkoxyphosphonium salts derived from Ph(3)P with Grignard reagents was observed to take place in the presence of O(2) to afford alcohols in good yields. A radical mechanism is proposed for this process that has gained support from isotope-labeling and radical-inhibition experiments.

Total syntheses of (+)-vigulariol and (-)-sclerophytin A

The total syntheses of (+)-vigulariol and (-)-sclerophytin A are reported in 15 steps and 16 steps, respectively, from a known compound. The flexible, readily scalable synthetic strategy allows for rapid construction of a critical tricyclic intermediate and is demonstrated via the synthesis of these two marine natural products. A key reaction in this synthetic protocol is a combination Wittig/intramolecular Diels-Alder cycloaddition.

Enantiomeric impurities in chiral synthons, catalysts, and auxiliaries. Part 3

The enantiomeric excess of chiral reagents used in asymmetric syntheses directly affects the reaction selectivity and product purity. In this work, 84 of the more recently available chiral compounds were evaluated to determine their actual enantiomeric composition. These compounds are widely used in asymmetric syntheses as chiral synthons, catalysts, and auxiliaries. These include chiral alcohols, amines, amino alcohols, amides, carboxylic acids, epoxides, esters, ketones, and oxolanes among other classes of compounds. All enantiomeric test results were categorized within five purity levels (i.e. <0.01%, 0.01% to 0.1%, 0.1% to 1%, 1% to 10%, and >10%). The majority of the reagents tested were determined to have enantiomeric impurities over 0.01%, and two of them were found to contain enantiomeric impurities exceeding the 10% level. The most effective enantioselective analysis method was a GC approach using a Chiraldex GTA chiral stationary phase (CSP). This method worked exceedingly well with chiral amines and alcohols.