Novel chemoselective tosylation of the alcoholic hydroxyl group of syn-alpha,beta-disubstituted beta-hydroxy carboxylic acids

Total synthesis of 6-deoxyerythronolide B via C-C bond-forming transfer hydrogenation

The 14-membered macrolide 6-deoxyerythronolide B is prepared in 14 steps (longest linear sequence) and 20 total steps. Two different methods for alcohol CH-crotylation via transfer hydrogenation are deployed for the first time in target-oriented synthesis. Enyne metathesis is used to form the 14-membered ring. The present approach represents the most concise construction of any erythronolide reported, to date.

Chemoselective hydroxyl group transformation: an elusive target

The selective reaction of one functional group in the presence of others is not a trivial task. A noteworthy amount of research has been dedicated to the chemoselective reaction of the hydroxyl moiety. This group is prevalent in many biologically important molecules including natural products and proteins. However, targeting the hydroxyl group is difficult for many reasons including its relatively low nucleophilicity in comparison to other ubiquitous functional groups such as amines and thiols. Additionally, many of the developed chemoselective reactions cannot be used in the presence of water. Despite these complications, chemoselective transformation of the hydroxyl moiety has been utilized in the synthesis of complex natural product derivatives, the reaction of tyrosine residues in proteins, the isolation of natural products and is the mechanism of action of myriad drugs. Here, methods for selective targeting of this group, as well as applications of several devised methods, are described.

Enatiomerically pure hydroxycarboxylic acids: current approaches and future perspectives

The growing awareness of the importance of chirality in conjunction with biological activity has led to an increasing demand for efficient methods for the industrial synthesis of enantiomerically pure compounds. Polyhydroxyalkanotes (PHAs) are a family of polyesters consisting of over 140 chiral R-hydroxycarboxylic acids (R-HAs), representing a promising source for obtaining chiral chemicals from renewable carbon sources. Although some R-HAs have been produced for some time and certain knowledge of the production processes has been gained, large-scale production has not yet been possible. In this article, through analysis of the current advances in production of these acids, we present guidelines for future developments in biotechnological processes for R-HA production.

Synthesis of (+/-)- and (-)-vibralactone and vibralactone C

Mander reductive alkylation of methyl 2-methoxybenzoate with prenyl bromide and hydrolysis of the enol ether afforded methyl 6-oxo-1-prenyl-2-cyclohexenecarboxylate. This was converted in five steps (reduction of the ketone, saponification, iodolactonization, ozonolysis, and intramolecular aldol reaction) to a spiro lactone cyclopentenal. An efficient first synthesis of (+/-)-vibralactone was completed by retro-iodolactonization with activated Zn, formation of the beta-lactone (vibralactone C), and reduction of the aldehyde. Except for the novel use of an iodolactone to protect both the prenyl double bond and carboxylic acid, no protecting groups were used. A similar sequence starting with asymmetric reductive alkylation of the (2S)-2-methoxymethoxymethylpyrrolidine amide of 2-methoxybenzoic acid with prenyl bromide afforded (-)-vibralactone confirming the absolute stereochemical assignment that was based on computational methods.

Synthesis of (+/-)-vibralactone

Reductive alkylation of methyl 2-methoxybenzoate with prenyl bromide and hydrolysis afforded methyl 6-oxo-1-prenyl-2-cyclohexenecarboxylate. Reduction of the ketone, hydrolysis, iodolactonization, ozonolysis, and intramolecular aldol reaction provided a spiro lactone cyclopentenal. Retro-iodolactonization with activated Zn, formation of the beta-lactone, and reduction of the aldehyde completed an efficient first synthesis of (+/-)-vibralactone. No protecting groups were used except for the novel use of an iodolactone to protect both the prenyl double bond and carboxylic acid.