Ring structure regioselectivity

C). Regioselectivity was observed in the cyclizations of the ketones 274 and 276, from which only products with angular aromatic ring structures, 275 (15%, m.p. 212°C) and 277 (16%), were isolated. The strong preference for formation of an angular aromatic system was clearly noted in the Elbs reaction of ketone 279, which produced 280 (m.p. 150°C) by cyclization with... 

From a synthetic point of view, the regioselectivity and stereoselectivity of the cyclization are of paramount importance. As discussed in Section 11.2.3.3 of Part A, the order of preference for cyclization of alkyl radicals is 5-exo > 6-endo 6-exo > 7-endo S-endo > 1-exo because of stereoelectronic preferences. For relatively rigid cyclic structures, proximity and alignment factors determined by the specific geometry of the ring system are of major importance. Theoretical analysis of radical addition indicates that the major interaction of the attacking radical is with the alkene LUMO.321 The preferred direction of attack is not perpendicular to the it system, but rather at an angle of about 110°. 

The NMR and infrared spectra of the derivatized model compounds are useful In establishing the structures and the D.F. of the modified polymers. Careful assignment of all peaks in the 13C-NMR spectra for each of structures 7-13 confirms the regioselectivity of the substitution on the oxyphenyl unit and inertness of the phenyl sulfone units. The chemical shifts of the key carbons for the analysis, those of the oxyphenyl rings, are summarized in Table I. 

The transition state was shown to have a four-centered nonplanar structure and the product showed a strong jS-agostic interaction.59 Molecular-mechanics (MM) calculations based on the structure of the transition state indicated that the regioselectivity is in good agreement with the steric energy of the transition state rather than the stability of the 7r-complex. The MM study also indicated that the substituents on the Cp rings determine the conformation of the polymer chain end, and the fixed polymer chain end conformation in turn determines the stereochemistry of olefin insertion at the transition state.59... 

Proton and 13C NMR spectral data of 33 derivatives of 64 have been tabulated and assigned <1996MRC409>. Several 3-oxo derivatives of 64 have been studied by proton and 13C NMR spectroscopy as part of a comprehensive investigation of their structural and spectroscopic properties <2003BCJ2361>. The regioselectivity of the formation of the six-membered ring in derivatives of 67 from 1,3-diketones has been established by proton NMR spectroscopy and nuclear Overhauser effects <1997CHE535>. 

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