![]() 6 Based on this consideration and our previous work in cycloaddition reactions, 12 we considered that stable dienecarbamates 12a,d could be well adapted to our goal. 11 In this context, we envisioned that the use of electron-rich diene partners for the DA reaction might be a solution to improve efficiency of this cycloaddition. ![]() However, despite their potential usefulness, the azo-DA reactions have rarely been utilized as click reactions because of the requirement of catalysts or heating. 8 Several catalytic enantioselective cycloadditions using chiral Lewis or Brønsted acid catalysts have been also reported.8, 9 In addition, the easy synthesis of these azodicarbonyl compounds incorporating biological molecules makes them valuable reagents for the bioconjugation. Typical examples of such reactions include: (1) cycloaddition reactions (1,3-dipolar cycloadditions and hetero-Diels-Alder reactions), (2) nucleophilic ring-opening reactions of strained heterocyclic electrophiles (such as epoxides, aziridines and, aziridinium ions), (3) carbonyl chemistry (formation of imines, oxime ethers, hydrazones and amides), (4) additions to carbon-carbon multiple bonds (epoxidation, aziridination, Michael addition), (5) thiol-ene additions and (6) azide-phosphine reaction (Staudinger ligation).1, 1(a), 1(b), 1(c), 1(d), 1(e), 2, 2(a), 2(b), 2(c), 3, 3(a), 3(b), 3(c), 3(d), 3(e), 4, 4(a), 4(b), 5, 5(a), 5(b), 5(c), 5(d), 5(e), 6, 7 Despite the significant advance made in this field over the past decade, there is still a demand for the development of novel click reactions, especially ones requiring no catalyst to complement the existing arsenal.įor many years, stable azodicarboxylates have been recognized as highly reactive dienophiles in the azo-Diels–Alder (azo-DA) reaction (Scheme 1a) and have been broadly used for the synthesis of key intermediates for the preparation of natural and pharmaceutical products (Fig. The starting materials must be readily available, and the reaction should have broad functional group tolerance and scope. ![]() Click chemistry has become a powerful chemical tool in various research areas such as organic chemistry, 1 drug discovery, 2 bioconjugation, 3 radiochemistry, 4 and chemical biology.2(a), 5 As defined by Sharpless, 6 a click reaction proceeds rapidly under mild conditions to produce building blocks with high selectivity, specificity, and yield, without byproducts.
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