Strain preventing

All the above-mentioned acyclic 1,3-diradicals are less stable than the a-bonded isomers. Therefore, in addition to using various substituents, other factors should be further considered in our design of persistent singlet 1,3-diradicals. In Sect. 5.2, ring structure is taken into account. Strain prevents the ring closure in the singlet state. Two linkers between the radical centers multiply the through-bond interactions. 

Reinecke W, DJ Jeenes, PA Williams, H-J Knackmuss (1982) TOL plasmid pWWO in constrncted halobenzoatedegrading Pseudomonas strains prevention of meta pathway. / SacterioZ 150 195-201. 

The higher-energy cis isomer has the shorter wavelength. Steric strain prevents full coplanarity of the cis phenyl groups, and the conjugative effect is attenuated. 

One of the most interesting developments in the stereochemistry of organic compounds in recent years has been the demonstration that trans-cyclooctene (but not the cis isomer) can be resolved into stable chiral isomers (enantiomers, Section 5-IB). In general, a Wa/w-cycloalkene would not be expected to be resolvable because of the possibility for formation of achiral conformations with a plane of symmetry. Any conformation with all of the carbons in a plane is such an achiral conformation (Figure 12-20a). However, when the chain connecting the ends of the double bond is short, as in trans-cyclooctene, steric hindrance and steric strain prevent easy formation of planar conformations, and both mirror-image forms (Figure 12-20b) are stable and thus resolvable. 

Further studies by this group ° revealed that the preferred exo cycloaddition mode that is observed experimentally is supported by molecular mechanics calculations. In cases in which ring strain prevents an intramolecular cycloaddition, the isomunchnone can nevertheless be intercepted by iV-phenylmaleimide in an intermolecular reaction. 

In this case, it is the conrotatory photochemical cyclization that is prevented by strain (it was tried—cyclooctadienyl anion is stable for at least a week at -78 °C in broad daylight) as the product would be a 5,5 traws-fused system. The same strain prevents thermal electrocyclic ring closure of cyclooctadienyl cations. 

The conjugate addition of nucleophiles to activated halogeno-olefins has been shown to result in a Darzens intermediate, which in the presence of a carbonyl group affords an epoxide. In cases where excessive strain prevents epoxide formation, formation of a cyclopropane ring ensues (Scheme 4). ... 

Carbonylation Cascades. The norbornene enamide shown underwent a palladium-catalyzed 5-g o-trig cyclization followed by carbonylation (1 atm) to give a spirocyclic product as a single di-astereoisomer (eq 84). In this case ring strain prevents the competing /3-hydride elimination pathway. Similar diastereoselective three-component cascade processes proceed smoothly in excellent yield (eq 85). ... 

Frossard, C.R, Steidler, L., and Eigenmann, RA. (2007) Oral administration of an IL-lO-secreting Laclococcus lactis strain prevents food-induced IgE sensitization. J Allergy Clin Immunol 119,952-959. 

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