Ring closure torquoselective

Benzocyclobutenone and derivatives have been shown to react with diazomethylene anions to give 2,3-benzodiazepines under very mild conditions. A mechanism has been proposed which involves four-electron electrocyclic ring opening of the initial alkoxide, with important acceleration by the alkoxide anion. Torquoselectivity to set up an eight-electron electrocyclic ring closure to the benzodiazepine enolate has been attributed to the strong preference of the alkoxide anion for outward rotation. This mechanism has been supported by isolation of one of the /S-diazo alcohols, and its conversion to a benzodiazepine on LDA treatment.84... 

Photoirradiation at 300nm of iV-alkoxycarbonyl-l,2-dihydropyridines results in ring closure and the formation of 2-azabicyclo[2.2.0]hex-5-enes. Substituents are tolerated at the 2-, 3-, and 4-positions however, the yields are significantly lower than those for unsubstituted dihydropyridines. Irradiation of 2-substituted-l,2-dihydropyridines 112 proceeds via a torquoselective process to give only the endo-product 113 (Equation 5) <2001JOC1805, 2000T9227>. 

Finally, the preferred direction of disrotatory ring closure has been addressed just as in the electrocyclic ring opening of cyclobutene. In the six-electron case, tt acceptor substituents control the direction of rotation in the same sense as in cyclobutene but the torquoselectivity effect is attenuated. Further, steric effects tend to play a bigger role, and both effects were attributed to the disrotatory nature of the transition state. 

A stereoselective electrocyclization developed for the synthesis of reserpine results from a stereocentre six atoms away from the newly fornoing chiral centre that is responsible for the diastereoselectivity of the ring closure. The presence of allylic strain in the disfavoured transition state results in the torquoselective ring closure (Scheme 38). 

Computational studies of oxazolidinone-directed Nazarov cyclization show that the chiral oxazolidinone auxiliaries provide control over the torquoselectivity of 4n electrocyclic ring closure and the regioselectivity of subsequent deprotonation (Scheme 177). °... 

The formation of j8-lactams from aliphatic acid chlorides and imines has been reviewed. The reaction is assumed to proceed through in situ formation of a ketene, followed by interaction with the imine to form a zwitterionic intermediate, which undergoes an electrocyclic conrotatory ring closure to give the jS-lactam ring (Scheme 8). The stereoselectivity of the reaction is sensitive to both the electronic and steric effects of the substituents on the ( )- or (Z)-imine, and the former manifest themselves in the electronic torquoselectivity that occurs during the conrotating cyclization of a zwitterionic intermediate [shown for ( )-imine in Scheme 8]. ... 

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