Chemoselective hydroxyl group transformation: an elusive target

Abstract

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.

Figure 1

Chemoselective transformation of the hydroxyl group. Four categories of conversions are described: direct conjugation reactions, oxidation, activation by generation of a leaving group and transformation to an alternative chemical handle.

Figure 2

Selective O- or N-arylation can be achieved by use of the appropriate catalyst. Copper-mediated catalysis favors formation of the O-arylation products while utilization of palladium yields the N-arylation products.

Figure 3

Oxidation of an N-terminal Ser or Thr residue of a protein or peptide. An electrophilic site is specifically generated at the N-terminus of the substrate enabling selective bioconjugation at this position. The requirement for the β-amino alcohol motif precludes the use of this transformation is a wide variety of substrates.

Figure 4

Tosylation of the hydroxyl moiety in the presence of a carboxylic acid functional group. Application of a strong base deprotonates both groups, but the corresponding ion pair is weaker for the hydroxyl moiety and can be exchanged with a tosyl group.

Figure 5

Hydroxyl group-targeted method to generate natural product analogs for protein target identification. A. A rhodium catalyzed reaction between an alcohol and a diazoester-functionalized coupling partner results in incorporation of an alkyne moiety. B. FK506 was used as a model natural product to demonstrate the utility of the devised strategy. Following functionalization of FK506 with an alkyne handle, this probe was incubated with RKO cells to facilitate the identification of the binding partner(s) of the natural product probe. Probe design dramatically altered the results of these experiments.

Figure 6

Chemoselective enrichment of hydroxyl-containing molecules through the formation of a silyl ether bond. Hydroxyl group-containing compounds are then retained on the solid-support resin while molecules deplete of this functional group are rinsed away. Upon traceless cleavage of the enriched molecules they can be immediately subjected to functional and structural characterization.

Figure 7

Tyrosine residues have been targeted utilizing several different strategies, reaction with the electron-rich aromatic ring at the position ortho to the hydroxyl or direct targeting of the phenol hydroxyl group. A. Coupling to an electron deficient diazonium salt. B. A three component Mannich-type reaction yields an o-substituted tyrosine moiety. C. An ene-type reaction is performed between tyrosine and a cyclic diazocarboxamide yielding a highly stable product. D. Metal-mediated coupling facilitates reaction at the phenolic site. Palladium promotes addition of allylic substrates. E. Cerium promotes a one-electron oxidative coupling to tyrosine.

Figure 8

Activity-based protein profiling can be used to target enzymes that utilize hydroxyl group-containing active site nucleophiles. A. Serine hydrolases can be targeted by several compound classes. Fluorophosphonate-based probes are promiscuous within the protein family. Compounds with attenuated reactivity such as carbamates and ureas enable selective examination of one or a small number of family members. B. The global nature of FP labeling can be utilized in the development of an assay to identify highly specific serine hydrolase inhibitors. Vehicle treated samples will display labeling of all FP-susceptible proteins. Addition of potential inhibitors will prevent susceptible enzymes from reacting with the FP probe, leading to a loss of a band or bands during gel-based analysis. Selective inhibitors will cause the loss of only one protein band (i.e., inhibitor 1) while promiscuous inhibitors will promote the loss of many (i.e., inhibitor 2).