Cycloaddition large rings formation

A very large number of these systems with ring junction heteroatoms exists, and this number is constantly increasing. Only illustrative examples of the preparation of such systems can be given here. The synthetic methods for the formation of this type of heterocycle can be usefully classified as follows (i) various cyclocondensations between the corresponding heterocyclic derivatives and bifunctional units, (ii) intramolecular cyclizations of electrophilic, nucleophilic or (still rare) radical type, (iii) cycloadditions, (iv) intramolecular oxidative coupling, (v) intramolecular insertions, (vi) cyclization of open-chained predecessors, (vii) various reactions (quite often unusual) which are specific for each type of system. Examples given below illustrate all these cases. 

The mode of dimerization of silaacrylates 389 depends on the steric bulk of the substituents195,196. Large substituents R prevent formation of dimers 390 and lead to intramolecular [2 + 2] cycloaddition to give l-oxa-2-silacyclobut-3-enes 391 (equation 112). Smaller substituents R allow head-to-tail dimerization to the eight-membered ring compound 390. 

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