5-exo-trig-ring closure

Michael addition to give the azaheterocycles 7-39 through a favorable 6-exo-trig ring closure (Scheme 7.13). Even highly sensible substrates such as aryl-allyl ethers and amines could be transformed to the desired products in excellent yields. 

Treatment of epoxide (34) with Bu3SnH/AIBN in the presence of Mgl2 first forms iodohydrin synthon, which rapidly reacts with Bu3Sn to form a cyclohexanol derivative (35) via 6-exo-trig ring closure of (3-hydroxyl radical as shown in eq. 3.12 [50-54]. Since the epoxides can be obtained from alkenes with peroxides, this is an indirect radical cyclization method of alkenes. 

Scheme 8. Synthesis of substituted tetrahydropyrans 31 and 33 via 6-exo-trig ring closures [53, 56]...

A combination of AuCl3/AgOTf (5 mol% each) as the catalyst allowed the 6-exo-trig ring closure of ArX(CH2)2CH=CH2 (X = 0, NTf, CHj) in (CH2C1)2 at 80 "C, affording dihydrobenzopyrans, tetralins, and tetrahydroquinoUnes, respectively. The kinetic isotope effects, observed for the deuterated arene derivatives, are consistent with an electrophilic aromatic substitution. ... 

The first type of process is characterized by cyclization reactions, which are found in a plethora of examples and hence can be considered as the flagship of the different classes being discussed in this section. In spite of the fact that this reaction type distinguishes a broad scope of subsections, the 5-exo-trig ring closure can be regarded as the most frequent and productive one. Furthermore, 6-endo and 6-... 

Trialkyltin hydrides can serve as chain carriers in radical reactions. The low Sn-H bond energy (74 kcal/mol) facilitates easy homolytic cleavage by an initiator radical to produce tin radicals that are in a position Lo cleave a Cl-C bond with formation of a carbon radical. The resulting carbon radical cycli/es stereoselectively to pyrurt 6 in a b-exo-trig ring closure. ... 

Unfortunately, the scope of the process appears to be limited by the need to form a five-membered ring in the second ring closure event [64]. For example, reduction of 220 affords 221 and 222, each the result of a 5-exo-trig cyclization (Table 10). Apparently, the rate of 6-exo-trig cyclization of the putative... 

The formation of a cyclohexyloxyl radical via 6-exo-trig manner is promoted by the rigid bicyclic structure derived from the sugar benzylidene group (eq. 3.57a). Moreover, eq. 3.57b indicates that radical ring closure to a formyl group is preferable to that onto an olefinic group. 

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