**4. α,β-Unsaturated acylazolium intermediate**

An important strategy of non-umpolung transformation reaction proceeds through the α,β-unsaturated acylazolium conjugation addition of various bisnucleophile, a wide variety of carbo cycles and heterocycles are synthesized. 1,2 addition followed by cyclization [4, 5, 60]. α,β-unsaturated acylazoliums can be generated from α,β-unsaturated aldehyde with external oxidants [61–66], ynals, 2-bromo enals [67–69], α,β-unsaturated esters [70] or acyl fluorides [71, 72], etc. (**Figure 19**).

The main application of acylazolium intermediate found in the biosynthesis of clavulanic acid (potent β-lactamase inhibitor) from conjugation addition of L-arginine to the α,β-unsaturated acylazolium, demonstrated by Merski and Townsend (**Figure 20**) [73, 74].

Lupton reported the Claisen type reaction of α,β-unsaturated enol esters to the corresponding acylazolium/enolate pair followed by rearranged to 2,3-dihydropyrozones (**Figure 21**) [71, 72].

**Figure 19.** *Generation of α,β-unsaturated acylazoliums.*

*N-Heterocyclic Carbene Mediated Organocatalysis Reactions DOI: http://dx.doi.org/10.5772/intechopen.100642*

#### **Figure 20.**

*Acylazolium intermediate found in the biosynthesis of clavulanic acid.*

**Figure 21.** *Claisen type reaction of α,β-unsaturated enol esters to 2,3-dihydropyronones.*

The generated electrophilic acylazolium intermediates involved in various annulation and cycloaddition reaction with bis-nucleophiles. α,β-unsaturated acylazolium with various cyclic and acyclic bis-nucleophiles for the synthesis of dihydropyranones and dihydropyridines in a formal [3 + 3] annulation reaction (**Figure 22**), demonstrated by Biju et al. [68].

Cycloaddition reaction of α-bromoenals with 1,3-diketones by using different achiral NHC catalyst obtained the product with the same absolute configuration but different stereodirecting substituents (**Figure 23**) [67].

#### **5. Conclusion**

N-heterocyclic carbenes have had a broad scope in the field of organic chemistry, often tolerating the construction of complex molecules from simple starting materials. The various modes of NHC catalysis such as the generation of Breslow intermediates, homoenolates, α,β-unsaturated acylazoliums, NHC enolates can be engaged in the synthesis of various heterocycles and carbocycles. The use of enantiomerically pure carbene catalyst can result in asymmetric synthesis of the

#### **Figure 23.**

*Cycloaddition reaction of α-bromoenals with 1,3-diketones.*

target molecules. This chapter has focused on the reactivity pathways which expand the variety reactions with suitable reaction companions beyond the old-fashioned aldehydes with enhanced catalytic conventions.

*N-Heterocyclic Carbene Mediated Organocatalysis Reactions DOI: http://dx.doi.org/10.5772/intechopen.100642*
