Jason Eames

Catalytic Allylic Oxidation of Alkenes Using an Asymmetric Kharasch–Sosnovsky Reaction

Kharasch and Sosnovsky reported the allylic oxidation of alkenes to give racemic allylic benzoates. This could be achieved efficiently using a tert-butyl perester as the oxidant, in the presence of a copper or cobalt salt. The use of C(2)-symmetric bis(oxazoline) ligands in the presence of copper(I) triflate with cyclic olefinic substrates gave the first synthetically useful asymmetric variant. The enantioselective control was good (up to 84 % ee) although yields were variable. In all cases the facial preference of the newly formed C-O bond was the same giving an S configuration at the allylic stereocenter. Lower stereocontrol was observed for large-ring alkenes and substantially reduced enantioselectivities were found with open-chain alkenes. This reaction has been further screened using a variety bis(oxazoline) and proline-derived ligands, which give a direct correlation between the chirality of the ligand and the enantioselectivity obtained. Individual substrates were found to be extremely sensitive to both the ligand structure and copper salt used as well as the presence of additives such as zinc, hydrazine, and molecular sieves.

Recent advances into the enantioselective protonation of prostereogenic enol derivatives

Abstract We discuss the recent developments into the enantioselective protonation of substituted enol derivatives and comment on the effect that reaction parameters (such as solvent, temperature and salt additive) have on the facial preference.

Polymeric Scavenger Reagents in Organic Synthesis

Recent advances in polymeric supported reagents have led to the development of a new synthetic technique. The use of polymeric supports in solid phase synthesis has led to the design and synthesis of ionic and covalent polymeric scavengers. In this Microreview, we discuss the concept of this strategy and comment on the efficiency and versatility of this new purification procedure.

AN EFFICIENT PROTOCOL FOR A SHARPLESS STYLE RACEMIC DIHYDROXYLATION

Abstract Dihydroxylation with solid OsCl 3 to provide the catalytic oxidant, K 3 Fe(CN) 6 as stoichiometric oxidant, quinuclidine as the accelerating ligand with added K 2 CO 3 and methanesulfonamide in a two-phase system (water and t-butanol) gives excellent yields of racemic syn diols from various alkenes (stilbenes, sulfides and phosphine oxides).

Recent Developments in Enantioselective Deprotonation Mediated by Sub-Stoichiometric Quantities of Chiral Bases

Recent developments in the enantioselective deprotonation of prostereogenic substrates by use of substoichiometric quantities of chiral bases are discussed. The effect that reaction parameters (such as solvent, temperature and additive) have on the stereoselectivity are outlined.

Rearrangements of phenylthio substituted 1,n-diols with toluene-p-sulfonic acid and with toluene-p-sulfonyl chloride

Abstract Rearrangement of a series of 1,n diols (n= 2 to 12), one OH group having an adjacent PhS group, under the two sets of conditions in the title, gives single compounds in high yield drawn from four possible classes of product. The effects of chain length help in our understanding of the mechanism of the rearrangements.

Kinetic and thermodynamic control in the stereospecific synthesis of cyclic ethers via phenylsulfanyl (PhS) migration

Abstract The factors controlling the stereospecific cyclisation of 1,3-diols with PhS migration to give THFs rather than oxetanes are reassessed in the light of evidence that these reactions are under thermodynamic control. Rearrangement of cyclic sulfites gives the nearest approach to kinetic control.

Katalytische Oxidation von Alkenen in Allylstellung über eine asymmetrische Kharasch-Sosnovsky-Reaktion

Kharasch und Sosnovsky beschrieben die Oxidation von Alkenen in Allylstellung, die mit einem tert-Butylpersaureester als Oxidationsmittel in Gegenwart eines Kupfer- oder Cobaltsalzes effizient verlauft und zu racemischen Allylbenzoaten fuhrt. Der Einsatz von C2-symmetrischen Bis(oxazolin)-Liganden in Gegenwart von Kupfer(I)-triflat ermoglichte bei cyclischen Olefinen als Substraten die erste praparativ nutzbare asymmetrische Variante dieser Reaktion. Die Ausbeuten waren recht unterschiedlich, doch die Kontrolle der Enantioselektivitat erwies sich als gut (bis zu 84 % ee). Die Seitenselektivitat bei der Bildung der neuen C-O-Bindung war in allen Fallen gleich (S-konfiguriertes Stereozentrum in Allylstellung). Geringere Stereokontrolle beobachtete man bei cyclischen Alkenen im Fall von grosen Ringen, und eine merklich geringere Enantioselektivitat fand man bei offenkettigen Alkenen. Man hat diese Reaktion mit einer Vielfalt von Bis(oxazolin)-Liganden und Liganden auf Prolinbasis weiter untersucht und dabei eine Korrelation der Chiralitat des Liganden und der Enantioselektivitat ermittelt. Einzelne Substrate erwiesen sich als auserst empfindlich sowohl gegenuber dem Kupfersalz und der Struktur des Liganden wie auch gegenuber Zusatzen wie Zink, Hydrazin und Molekularsieben.

Investigations on the Efficiency of Regioselective C-Deuteration of Endocyclic Enolates

Regioselective C-deuteration of a series of Endocyclic enolates (derived from cyclic aryl ketones) was efficiently achieved by quenching the corresponding “base-free” enolate in the presence of a suitable deuterium source. We discuss the structural nature of the deuterium donor and comment on the use of additives within this deuteration step.

Scope and limitation of the [1,4] SPh shift in the synthesis of allylic alcohols

Abstract Rearrangement of 4-PhS-1,3-diols with TsCl in pyridine gives allylic alcohols. We discuss the scope and limitation of this reaction using structural variations, which help to elucidate the mechanism of the reaction. All reactions proceed in high yield and give synthetically useful products.