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Updated: Jun 25, 2026
Constructing Cyclic Peptides Using an On-Tether Sulfonium Center
Published on: September 28, 2022
Electron-deficient alkynes as cleavable reagents for the modification of cysteine-containing peptides in aqueous
Hoi-Yan Shiu1, Tak-Chung Chan, Chi-Ming Ho
1Department of Chemistry and Open Laboratory of Chemical Biology, Institute of Molecular Technology for Drug Discovery and Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong, China.
Abstract:
An efficient method has been developed for the chemoselective cysteine modification of unprotected peptides and proteins in aqueous media through the formation of a vinyl sulfide linkage by using electron-deficient alkynes, including alkynoic amides, esters and alkynones. The terminal alkynone-modified peptides could be converted back into the unmodified peptides (81% isolated yield) by adding thiols under mild conditions. The usefulness of this thiol-assisted cleavage of the vinyl sulfide linkage in peptides has been exemplified by the enrichment of a cysteine-containing peptide (71% recovery) from a mixture of cysteine-containing and non-cysteine-containing peptides.
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02:01Alkynes to Carboxylic Acids: Oxidative Cleavage
02:27Preparation of Alkynes: Alkylation Reaction
Alkylation of terminal alkynes with primary alkyl halides in the presence of a strong base like sodium amide is one of the common methods for the synthesis of longer carbon-chain alkynes. For example, treatment of 1-propyne with sodium amide followed by reaction with ethyl bromide yields 2-pentyne.
02:38Electrophilic Addition to Alkynes: Halogenation
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
02:42Acidity of 1-Alkynes
The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.
02:40Alkynes to Aldehydes and Ketones: Acid-Catalyzed Hydration
Analogous to alkenes, alkynes also undergo acid-catalyzed hydration. While the addition of water to an alkene gives an alcohol, hydration of alkynes produces different products such as aldehydes and ketones.
02:34Preparation of Alkynes: Dehydrohalogenation
Alkynes can be prepared by dehydrohalogenation of vicinal or geminal dihalides in the presence of a strong base like sodium amide in liquid ammonia. The reaction proceeds with the loss of two equivalents of hydrogen halide (HX) via two successive E2 elimination reactions.