Endo-lrig cyclizations

The benzene derivative 401 by the intermolecular insertion of acrylate[278], A formal [2 + 2+2] cycloaddition takes place by the reaction of 2-iodonitroben-zene with the 1,6-enyne 402. The neopentylpalladium intermediate 403 undergoes 6-endo-lrig cyclization on to the aromatic ring to give 404[279],... 

Fluoride anion promoted cyclizations of benzylsilane dihydroisoquinolinium salts have been employed to form five- and six-membered azacyclic rings (Scheme 45). For example, CsF in either protic (EtOH-H2O) or aprotic (MeCN) solvents has been used to form the isoquinoline alkaloid (l)-xylopinine (121) from (118). Yields for this conversion in the range 25-70% have been obtained. - The A-arylisoqui-nolinium salt (119) is reported to cyclize under aprotic conditions only (Bu"4NF in refluxing THF) to give (122) in 60% yield. A betaine intermediate (120) has been proposed for these transformations. The conversion of this intermediate (R = Bn n = 0) to (122), formally a 5-endo-lrig cyclization, can then be formulated as a six-electron electrocyclization. Iminium ion-benzylsilane cyclizations have been accomplished, sometimes with greater efficiency, photochemically (see Section 4.4.3). 

The mechanism was proposed to occur via a co-ordination of the chloride anion associated with the A -acyhminium cation, to the thiourea portion of the organocata-lyst 117. Thus the attack of the indole onto this electrophile is stereochemically controlled. There are two mechanistic possibihties in the ring forming stage of this process. Either a 5-enolexo-endo-lrig cyclization occurs followed by Plancher... 

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