Heats of hydrogenation. IX. Cyclic acetylenes and some miscellaneous olefins

Strain-Promoted 1,3-Dipolar Cycloaddition of Cycloalkynes and Organic Azides

“…Interestingly, cyclooctyne was identified as the smallest isolable cycloalkyne, although its acetylene bond angle of 163°still significantly deviates from linear. The experimentally determined ring strain of cyclooctyne is *18 kcal/mol [58], compared to 12.1 kcal/mol for saturated cyclooctane [6]. Not surprisingly, ringstrain is accountable for the intrinsically low stability of medium-sized cyclic alkynes, with calculated ring strains of 25 kcal/mol and above [6], leading to fast degradation and/or polymerization of seven-membered and smaller cycloalkynes, and thwarting their isolation in pure form.…”

Visualizing Metabolically Labeled Glycoconjugates of Living Cells by Copper‐Free and Fast Huisgen Cycloadditions

“…[17] The cellular toxicity of the Cu I catalyst has precluded applications wherein cells must remain viable, [18] and hence there is a great need for the development of Cu I -free [3+2] cycloadditions. [19][20][21] In this respect, alkynes can be activated by ring strain, and, for example, constraining an alkyne within an eight-membered ring creates 18 kcal mol À1 of strain, much of which is released in the transition state upon [3+2] cyclcoaddition with an azide. [19,20] As a result, cyclooctynes such as 1 react with azides at room temperature without the need for a catalyst (Figure 1).…”

“…[19][20][21] In this respect, alkynes can be activated by ring strain, and, for example, constraining an alkyne within an eight-membered ring creates 18 kcal mol À1 of strain, much of which is released in the transition state upon [3+2] cyclcoaddition with an azide. [19,20] As a result, cyclooctynes such as 1 react with azides at room temperature without the need for a catalyst (Figure 1). The strain-promoted cycloaddition has been used to label biomolecules without observable cytotoxicitiy.…”

Beckmann rearrangement of cyclotriveratrylene (CTV) oxime: tandem Beckmann-electrophilic aromatic addition

“…The cationic intermediate of the Beckmann rearrangement is represented as a nitrilium cation in structure 6, since strain on the linear triple bond should be minimal in the 10-membered ring. 19 Cationic intermediate 6 is then attacked by the electron-rich dimethoxyphenyl ring in an electrophilic aromatic substitution process forming two new 6-membered rings and the new quaternary center C6. Intermediate 7 might be expected to undergo demethylation of O1, but deprotonation occurs instead forming the conjugated triene 8.…”