Abstract:
The pyran ring system is widely present in the animal and plant kingdom and possesses diverse pharmacological activities. Pyran is a six-membered oxygen heterocycle with two double bonds. Out of the five carbon atoms of the pyran ring, four are sp2 carbons and one is sp3 hybridised. The pyran ring is not a true aromatic ring. In the nomenclature of this ring system, the position of the sp3 carbon is designated by adding H or, in other words, the use of the suffix H locates the positions of the double bonds. In structure I (Figure 1), the carbon at position 2 is sp3 and it is named as 2H-pyran. Similarly, in the case of the structure II, C-4 is the sp3 carbon and it is designated as 4H-pyran. Partially reduced pyrans such as III and IV are always named using the suffix 2H. The structure III is named as 3,4-dihydro-2H-pyran and, similarly, structure IV as 5,6-dihydro-2H-pyran. The same terminology is followed when the sp3 carbon is replaced by a carbonyl function in the structures V and VI. Thus, compounds V and VI are named as 2H-pyran-2-one and 4H-pyran-4-one as per the IUPAC convention. They are also named as 2-pyranone and 4-pyranone, respectively.
