Direct Atom-Efficient Esterification between Carboxylic Acids and Alcohols Catalyzed by Amphoteric, Water-Tolerant TiO(acac)2

NHC-catalyzed C-O or C-N bond formation: efficient approaches to α,β-unsaturated esters and amides

Simple and efficient NHC-catalyzed transformations of bromoenal or α,β-dibromoenal into α,β-unsaturated esters or amides with high stereoselectivity through C-O or C-N bond formation have been demonstrated. The NHC-catalyzed processes occur under mild conditions. The ready availability of the starting materials, avoidance of external oxidants and the usefulness of the products all make the strategy attractive.

Titanium-catalyzed esterification and transesterification reactions facilitated using microwave heating

Background: Esters can be biologically active against a range of potential targets. In addition, esterification has been used successfully to facilitate the penetration of polar compounds into cells, where the ester group can then be removed by nonspecific cellular esterases unmasking drug molecules. While direct esterification of carboxylic acids with alcohols using acid catalysts can prove effective in many cases, there are instances when substrates are acid sensitive or where the use of strong acids leads to side reactions and product decomposition. To overcome this, other methods are needed for the preparation of esters in the laboratory. Results: A methodology for titanium-catalyzed esterification and transesterification using microwave heating as a tool is presented. The reactions are complete within 1 h of heating at 160°C. The substrate scope of the methodology has been investigated. It is possible to use aromatic, aliphatic and heteroaromatic acids in the esterification protocol. Acid-sensitive alcohols, including furfuryl alcohol, are also suitable substrates. For transesterification, the reaction is again amenable to microwave heating. Conclusion: A range of esters can be prepared using a methodology based around the application of microwave heating and the use of a titanium catalyst.

Selective Monoesterification of Malonic Acid Catalyzed by Boric Acid

Boric acid catalyzes the monoesterification of malonic acid, likely through a chelation mechanism that is not available to the monoester product. Under more forcing conditions, diesters form to some extent, but conditions can be optimized to favour the monoester product (56–80%). With the easily handled solid acid catalyst, these reactions can be run with excess alcohol as solvent or with stoichiometric amounts of alcohol in acetonitrile with moderate heating.

Highly Diastereoselective Thioglycosylation of Functionalized Peracetylated Glycosides Catalyzed by MoO2Cl2

Among 18 oxometallic species, MoO2Cl2 was found to be the most reactive in catalytic thioglycosylation of O-acetylated glycosides with functionalized thiols in CH2Cl2, leading cleanly to 1,2-trans-thioglycosides with exclusive diastereocontrol. The new catalytic protocol is applicable to a monoglycoside building block and beta-(1-->6)-S-linked-thiodisaccharide synthesis. [reaction: see text].

O-Isopropylidenation of carbohydrates catalyzed by vanadyl triflate

Vanadyl triflate has been identified as a mild and efficient catalyst for the chemoselective O-isopropylidenation of functionalized carbohydrates with acetone and acetone equivalents. The current protocol is compatible with a diverse array of protecting groups and the products can be readily isolated by simple aqueous wash.

Highly Enantioselective Aerobic Oxidation of α-Hydroxyphosphonates Catalyzed by Chiral Vanadyl(V) Methoxides Bearing N-Salicylidene-α-aminocarboxylates

An unprecedented vanadyl(V) methoxide complex 4 derived from 3,5-dibromo-N-salicylidene-l-tert-leucinate enables highly enantioselective aerobic oxidations of alpha-hydroxyphosphonates at ambient temperature with selectivity factors ranging from 3 to >99.

Chiral N-salicylidene vanadyl carboxylate-catalyzed enantioselective aerobic oxidation of alpha-hydroxy esters and amides

A series of chiral vanadyl carboxylates derived from N-salicylidene-L-alpha-amino acids and vanadyl sulfate has been developed. These configurationally well defined complexes were examined for the kinetic resolution of double- and mono-activated 2 degrees alcohols. The best chiral templates involve the combination of L-tert-leucine and 3,5-di-t-butyl-, 3,5-diphenyl-, or 3,4-dibromo-salicylaldehyde. The resulting vanadyl(V)-methoxide complexes after recrystallization from air-saturated methanol serve as highly enantioselective catalysts for asymmetric aerobic oxidation of alpha-hydroxyl-esters and amides with a diverse array of alpha-, O-, and N-substituents at ambient temperature in toluene. The asymmetric inductions of the oxidation process are in the range of 10 to >100 in terms of selectivity factors (k(rel)) in most instances. The previously undescribed aerobic oxidation protocol is also applicable to the kinetic resolution of C-13 taxol side chain with high selectivity factor (k(rel) = 35). X-ray crystallographic analysis of an adduct between a given vanadyl complex and N-benzyl-mandelamide allows for probing the stereochemical origin of the nearly exclusive asymmetric control in the oxidation process.