Ring formation Closing reaction

Clipping. The clipping method relies on the synthesis of the rotaxane by formation of the macrocycle (host) in the presence of a dumbbell shaped molecule. The partial macrocycle undergoes a ring closing reaction around the dumbbell shaped molecule, thereby forming the rotaxane. 

Only a few radical reactions have been applied to the functionalization of 1,3-dioxanes or 1,3-dithianes bearing one exo- or mdo-Ao ih c bond. In all cases, ring formation was the goal. The common reagent system, BusSnH/AIBN, was used to achieve a stereoselective ring closing hydrostannylation of an alkenyl alkyne subunit (AIBN = 2,2 -azobisiso-butyronitrile Equation 36) <1998JOC9626>. 

Scheme 15.5 Illustration of the formation of the trans- and a.v-ring-closed isomers of 34 by electrochemical ring-closing reaction.

Several examples of ring-closing metathesis indicate the highly selective nature of this reaction.161,162,224-226 The application of various Mo and Ru carbene catalysts allow formation of five-, six-, and seven-membered mono- and bicyclic rings. Formation of substituted cyclopentenes, for example, was shown... 

The tandem cyclopropyl ring-opening/nitrilium ring-closing reaction in the presence of triflic acid results in the stereoselective formation of cyclic imines 161 (Scheme 5.67).699... 

It is noted that, while numerous type c (Section 13.01.6.3) and type d (Section 13.01.6.4) ring-constmction manifestations of the ruthenium-catalyzed ring-closing metathesis reaction for the azepine ring formation have been reported, the corresponding type b possibility, with an iV-vinyl component, has not as yet been reported. 

Alternatively, ring closure metathesis reaction is applied to the formation of cyclic polymers, although the main chain does not consist of hydrocarbon. Such a method was reported in the preparation of cyclic poly(THF) as shown in Fig. 33 [112]. Such ring-close metathesis was also applied for a preparation of catenane [113]. 

This change in mechanism has been attributed to the formation of a n chelate complex via coordination of the second olefinic bond present in the diene at the Ru atom, which then favours the intramolecular ring-closing reaction over intermolecular condensation polymerisation [1],... 

As discussed before, the strategy to synthesize catenanes is not necessarily restricted to olefin metathesis as the ring closing reaction. Any reaction which can be carried out under conditions where the hydrogen-bonded dimers exist should be appropriate. A first example was realized by the formation of anhydride linkages with dicyclohexylcarbodiimide as the reagent in benzene as the solvent [57], as schematically illustrated in Scheme 5.19. 

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