Heterocyclic compounds three-membered ring opening

Durr et al. have described some novel complex cinnoline derivatives which show photochromic properties. Albini et al. have provided some new evidence on the mechanism for the photoisomerization of heterocyclic N-oxides. Simple pyridine N-oxides are exceptional. Thus irradiation of pyridine N -oxide in aqueous base affords the ring-opened product (23). Aoyama et al. have described the unprecedented photocyclization of the amide (24) to the lactam (25). The first examples of C2a+2Tr] photoreactions of a three-membered ring and an azo-compound have been described by Hunig and Schmitt. Nicolaou et al. have prepared the first stable example of a 1,2-dithiethane the procedure involves (2i +2Tr) photodimerization of C=S groups. 

The ease of ring formation helped us to make five-membered cyclic acetals from ketones and a seven-membered heterocyclic compound (see Chapter 6). In this chapter we have seen how three-membered epoxides are formed. Ring formation is generally preferred to bimolecular reactions forming open chain compounds providing that the ring is three-, five-, six-, or seven-membered. Four-membered ring are a special case discussed in Chapter 29 where all these points are developed more fully. 

A typical reaction of three-membered heterocycles is nucleophilic ring-opening resulting in the formation of 1,2-disubstituted aliphatic compounds. 

In four-membered heterocycles, the ring strain is less than in the corresponding three-membered compounds and is approximately equal to that found in cyclobutane. Nevertheless, ring-opening reactions forming acyclic products predominate. At the same time, analogy with the reactivity of the corresponding aliphatic compounds (ethers, thioethers, secondary and tertiary amines, imines) becomes more evident. 

With this large group of heterocycles, ring strain is of little or no importance. Ring-opening reactions are, therefore, rarer than in three- and four-membered heterocycles. The crucial consideration is rather whether a compound can be regarded as a heteroarene or whether it has to be classified as a heterocycloalkane or heterocycloalkene (see p 2). Various aromaticity criteria apply to heteroarenes, and as a consequence, different opinions have been expressed on this matter [1]. As will be shown by means of examples of the various systems, the nature and number of heteroatoms are the critical factors. The parent compound of the five-membered heterocycles with one oxygen atom is furan. 

In contrast, the reaction of zirconocene imide (91) with propylene oxide produced acyclic compound (95) exclusively (Equation 42) [47]. In this case, hydrogen transfer from the methyl group to the imide nitrogen proceeded in the intermediate (94) along with the ring opening of the epoxide. The complex (91) is reactive enough not only toward epoxides but also the other three-membered heterocycles such as aziridines and thiiranes. 

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