Monocyclic five-membered heterocycles

Oxidative procedures have been utilized for the synthesis of both monocyclic five-membered heterocycles and their ring-fused analogs, although the ease of synthesis of the precursors for the latter ring closures results in wider application of this procedure. A variety of oxidizing agents have been used and the conversion of the benzylidene hydrazidines (221) into the 4-arylamino-l,2,4-triazole (222) was effected with mercury(II) oxide (77BCJ953). 

The biological activity of azoles and their derivatives (indoles, purines, etc.) and their abundance as structural motif in natural products made them a prime target and test ground in the development of catalytic transformations. This chapter is mainly dedicated to the reactions of monocyclic five membered heterocycles and indole. The chemistry of other condensed systems of importance, such as purines, is discussed in Chapter 8. 

The syntheses of monocyclic five-membered heterocycles containing three oxygen or sulfur atoms can be classified into three groups, according to the number of building blocks required to form the ring in a one-step process ... 

Table 10 Ring Syntheses of Monocyclic Five-membered Heterocycles Containing Three Oxygen or Sulfur Atoms from Non-heterocyclic Compounds... 

Table 11 Formation of Monocyclic Five-membered Heterocycles containing three Oxygen or Sulfur Atoms by...

Elementoles, that is, monoelemental monocyclic five-membered heterocycles (arsole, stibole, and bismole), are also known in the literature as elementacyclopentadienes. 

Table 2.3 Ultraviolet spectra of monocyclic five-membered heterocycles ...

The parent compounds of the monocyclic five-membered heterocycles with one heteroatom are aromatic. When considering the three most important systems only, it appears that the aromaticity increases as follows furan < pyrrole < thiophene (< benzene). This sequence also applies to the respective benzo[Z ] condensed systems. 

S.2 Monocyclic Five-Membered Heterocycles 237 5 mol % [RuCl2(p-cymene)]2... 

---