REACTIVITY OF THREE-MEMBERED RINGS WITH TWO HETEROATOMS

REACTIVITY OF THREE-MEMBERED RINGS WITH TWO HETEROATOMS... 

The chemistry of seven-membered rings with three heteroatoms 1,2,4 has improved since CHEC(1984) was published. In the first volume, these compounds were included in Chapter 6.08 devoted to seven-membered rings with two or more heteroatoms and only few references on synthesis were reported. In CHEC-II(1996), a single chapter for these compounds mainly covered new synthetic methods, spectroscopy, and reactivity, but still there was a lack of activity reports. 

The ring bond order deviation from uniformity partially agreed with the order of reactivity computed on the basis of FMO energy gaps. The least aromatic was 1,2,5-oxadiazole, while 1,2,3-thiadiazole should be most aromatic (Table 36). The order of reactivity was oxadiazole, triazole, thiadiazole in all 1,2,3-, 1,2,5- and 1,3,4-series of the three heteroatom heterocycles. Except for 1,3,4-oxadiazole, the two other 1,3,4- five-membered heterocycles were predicted to be more reactive than their 1,2,3- isomers (Table 36). The prediction that 1,2,5-oxadiazole was the most reactive heterocycle as a diene for Diels-Alder reaction was unacceptable due to the fact that two C-N bonds should be formed in the course of the reaction, which usually requires an exceptionally high activation barrier. 

---