Formation of Small Rings

Only a few papers on the formation of compounds with small rings have been published. One example is the [2 + 2]-cycloaddition of electron-rich enamines to Schiff bases under high pressure (1.4 GPa) (87JOC365). The reaction leads to substituted azetidines (1). Four-membered ring heterocycles, thietane derivatives (4), are formed by interaction of sulfene (2) with enamines (3) (86CB257 93JOC3429). 

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Intramolecular general acid catalysis in reactions of salicylic acid derivatives 196 Why are EM s for general acid-base catalysed reactions so low 198 EM and the nature of the transition state 200 The formation of small rings 205... 

The gem-dialkyl effect as a function of ring size relative EM s for the formation of small rings... 

It is instructive to consider why the presence of relatively bulky N-substituents results in the formation of small rings. Of the reactions of phosphorus trichloride with amines, that with t-butylamine has been widely studied (Scheme 6) Intermediates (17) and (18) have been isolated, but not (79), which evidently undergoes a facile cyclisation step to give (20). Two effects can be identified, which are expected to favour the formation of small rings. Firstly, an n.m.r. study of (18)... 

Now we need to explain why this is. The explanation comes in two parts, one of which may be more important titan the other, depending on the ring being formed. The first part is more applicable to the formation of small rings, such as the first example we gave you. 

These can be used for the formation of small rings thus Bradley and Durst have used an intramoleculai alkenyltin/aryl bromide coupling to form methylenebcnzocyclobutanones in medium to good yields [43J, and Baldwin and co-workers coupled acid chloride and alkenyltin functionalities to form five- and six-membered a-methylene lactones (Scheme 4-13) [44]. 

To minimize dimerization products, intramolecular condensations have to be carried out under carefully controlled conditions and in high dilution. The formation of small rings obeys the Baldwin rules (equation 45). ... 

Fused-ring bicyclic compounds have been formed by transannular attack of an amino group at phosphorus either to displace a phosphorus substituent, for example, formation of the salt (53), or by valence expansion to produce phosphorane (68). In the latter case the five-coordinate metalloid atom needs to be stabilized by the presence of electronegative substituents and the formation of small rings—preferably, five-membered rings. 

Bulky substituents lead to the formation of small-ring compounds. For example, LiF elimination from l-lithium-amido-3-fluoro-l,3-disiloxanes leads to four-membered (SiNSiO)-ring systems (Eq. 13). 

Special features of curing of epoxy resins derived from N,N-diglycidyl-amines and diffusion control of curing due to increase in Tg are discussed. The special features of amine curing of diglycidylamines include extensive formation of small rings and dependence of reactivities of epoxy groups. 

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