The 1.3-dipolar cycloaddition reaction of azide and cyclooctyne was first discovered by Blomquist and Liu in 1953, and confirmed by Wittig and Krebs in 1961. Cyclooctynes are the smallest cycloalkyne structures that can exist stably. Larger cycloalkyne structures have better stability and lower ring strain, which means lower reactivity. There are also some smaller cycloalkynes that can be synthesized in-situ and isolated, but most are not stable. In 1970, Krebs’ used sulfur atoms to reduce ring strain and synthesized thiepin derivatives. Since 1970, cyclooctyne derivatives have attracted a lot of attention.
In 2004 Bertozzi first used cyclooctynes instead of terminal alkynes for ‘click’ reactions. At present, this reaction has been widely used in materials chemistry and biochemistry research. The ‘click’ reaction involving cyclooctyne is called “ring tension-driven alkyne-azide coupling reaction” (SPAAC). Compared with traditional The “copper-catalyzed alkyne-azide coupling reaction” (CuAAC) reaction has the following advantages:
1. This reaction does not require the participation of copper. Copper particles are somewhat toxic to cells. At the same time, copper ions may bind to the active site of the protein, reducing the binding of the protein to the API. Also due to these advantages, the reaction can be performed in vivo.
2. Fewer side reactions compared to terminal alkynes.
3. It is not sensitive to the reaction environment, and can react under many conditions such as acidity, alkalinity and water.
However, cyclooctyne compounds also have some disadvantages that need to be overcome, such as:
1. The traditional cyclooctyne reaction speed is very slow, and a high concentration is often required to promote the completion of the reaction;
2. Lack of 1,4/1,5 addition selectivity (both for SPAAC and CuAAC);
3. Poor water solubility.
Research has found that this can be improved by:
1. Reducing the electron density can increase the reaction speed (Fig1, DIFO);
2. Increase ring tension by increasing the number of Sp2 carbon atoms (Fig1, DIBO, DBCO, DARAC);
3. Adding nitrogen atoms can increase water solubility;
4. Increase ring tension (Fig. 1, BCN).
At present, some new cyclooctynes have been synthesized. Among them, the most widely studied and utilized are azadiphenylcyclooctynes (DIBAC/DBCO), which have excellent chemical activity and stability, and can The scalability has attracted extensive research in the materials and biological communities.
There are two common synthetic methods:
1. The method of Van Delft (Scheme 1, a), which has the advantage of high overall yield. No particularly dangerous and complex reactions are used;
2. Popik’s method (Scheme 1, b), the advantage of this method is that it has fewer steps, high atom economy, and is suitable for scale-up. Overall it is easier to operate.
At present, the synthesis method of Popik is the mainstream synthesis method.
Through the search, it is found that the 15 most reported molecules are as follows:
The current applications of SPAAC reactions mainly include:
1. Molecular Imaging
2. Targeted drug delivery
3. Macromolecular Derivatization
4. Hydrogels
Judging from the number of articles about SPAAC in recent years, this type of reaction and application has received more and more attention, especially in the field of targeted drugs and molecular imaging, compared with traditional CuAAC cycloaddition, SPAAC is incomparable Comparable advantages, but also many difficulties. First of all, the synthesis of this compound has always been a difficult problem. The two routes reported above both used the process of bromination of alkenes and then elimination to obtain alkynes. This scheme is not only difficult to purify, but also requires low temperature (-78 degrees). This severely hinders other applications of the molecule. If the above problems can be solved, the author believes that the molecule can not only play a role in the above aspects, but also can be widely used in ADC drugs, PROTACs, molecular glues, double antibodies and other drugs that require linker. In addition, the size of the molecule is too large, and it will be very limited for oral drugs or drugs that need to pass through the BBB. The development of a new generation of SPAAC substrates is also a very important direction.
