Partially and fully saturated ring systems

Relatively few such heterocyclic systems have been studied by microwave spectroscopy some data are Included in Table 2. In 1,3-dioxolane the bent form is more stable than the 

Structure of Five-membered Rings with Two or More Heteroatoms 

Ring 1 Ring position 2 3 4 5 Examples of compounds studied 

C3N2 N N — — — Pyrazole substituted pyrazoles A - and A -pyrazolines pyrazolinones pyrazolidines pyrazolidinones 

C2N3 N N N — — l,3-Dimethyl-4-(l,2,3-triazolyl) sulfide 3-methyl-2-phenyl-l,2,3-triazol-l-ine-4-thione 

CN4 N N N N — 5-Amino-2-methyltetrazole S-aminotetrazofe monohydrate sodium tetrazolate monohydrate  

Relatively few such heterocyclic systems have been studied by microwave spectroscopy some data are included in Table 5. In 1,3-dioxolane the bent form is more stable than the twisted, and pseudorotation occurs. In 1,2,4-trioxocyclopentane the equilibrium conformation is twisted, and there is a barrier of 6.3 kJ mol 1 opposing pseudorotation (74PMH(6)53). 

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Chemical and physicochemical properties for saturated, partially saturated and fully conjugated ring systems are discussed under the appropriate section on structure, reactivity and synthesis. 

The reactivity and the synthesis of the fully unsaturated heterocycles compared to their partially or fully saturated members have been examined and developed in detail. The literature is mostly associated with ring synthesis, and a great deal of effort has been devoted over the last decade to develop various strategies and tactics for the preparation of these systems. Reactions in Section 11.04.9 are classified strictly according to the number of atoms in the... 

The Hantzsch-Widman system provides names for both fully saturated and fully unsaturated molecules. Partially saturated molecules are named by attaching appropriate hydro prefixes to the names of the fully saturated rings however the special suffixes applicable to four-and five-membered rings may also be used (this is not now recommended... 

Most of the work described so far deals with the synthesis of members of the many possible ring systems. Both saturated and partially saturated ring systems have been prepared, but most information comes from the fully conjugated ring systems. 

Isoxazolo[2,3-4]pyridines 44, isothiazolo[2,3-4]pyridines 46, and their fully saturated derivatives 45 and 47 (Scheme 16) were discussed in CHEC(1984) <1984CHEC(6)613> and CHEC-II(1996) <1996CHEC-II(8)249>. Very little information was available on the isothiazolo[2,3- ]pyridine ring system while most of the informations given on the oxygenated parent, isoxazolo[2,3- ]pyridines, concerned the fully saturated system. Careful examination of the literature clearly shows that the situation did not change much almost no references have been reported on isothiazolo[2,3- ]pyridines and most of the work done in the last decade concerns the synthesis and reactivity of hexahydro-isoxazolo[2,3- ] pyridines 45. Therefore, this chapter will briefly describe the new reactions of fully conjugated systems and will focus on the partially/completely saturated derivatives. 

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