Beckwith-Houk model

The transition-state model for these cyclizations (Scheme 34) differs fundamentally from the well-established Beckwith-Houk transition model for radical cyclizations [130,146-148]. Thus, while both models invoke chairlike transition states, without excluding the possibility of twist boatlike systems in some instances, the Beckwith-Houk model involves full conformational... 

Free-radical cyclizations using ethyl radicals generated by EtsB/air system or stannyl radicals systems provide a range of carbocyclic and heterocyclic hydroxylamines (equation 77). Stereoselectivity in these reactions is variable but can be semiquaUtatively predicted by Beckwith-Houk models . Depending on the substitution pattern of the emerging cyclic system, stereoselectivity can be very high, especially in fused polycyclic systems (equation... 

The two computational models differ on this point. The Beckwith model (ref. 6) indicates increased bending and torsional strain in the 6-endo TS. The Houk model (ref. 7) indicates increased bending strain only, and states this strain is due in part to the bending of the allylic carbon out of the alkene plane to accommodate a better angle of attack. 

In contrast, when 6-exo cyclizations are effected in the presence of an ot-alkoxy or a-silyloxy substituent, the stereocontrol can be excellent, as has been observed in a Si-tethered cyclization of 54a (Scheme 15) where a single diastereomeric product was obtained [134]. In this case, additional conformational constraints aided the cyclization, as became evident upon finding that the product configuration was inconsistent with the standard Beckwith-Houk chair-equatorial transition state model. For 54a there are two chairlike conformations (chair-equatorial and chair-axial) conceivably leading to cyclization the latter has the C-O and C=N bonds in an anti relationship to minimize dipole repulsion (Scheme 15). The... 

Beckwith pioneered the use of a Thiol-Oxygen-Co-Oxidation (TOCO) process for the transformation of 1,4-dienes and 1,3,6-trienes to 1,2-dioxolanes [90], As illustrated by the example in Scheme 51 [90a], this process involves phenylthio radical addition to the least substituted double bond, oxygen entrapment, peroxyl radical cyclization, oxygen entrapment and hydrogen atom transfer from the thiol. In accord with the Beckwith-Houk transition state model [91, 92], cyclization provides preferentially the c/s-3,5-disubstituted 1,2-dioxolanes. 

Figure 4.17 Chair-like (Beckwith—Houk) transition-state model.

Well-known "Beckwith transition states in which a maximum number of groups are in an equatorial orientation of a chair-like transition state provides a satisfactory rationalization for the formation of the products. A. L. J. Beckwith, C. J. Easton, T. Lawrence, and A. K. Serelis, Ausi. J. Chem. 36 545 (1983) see also D. C. Spellmeyer and K. N. Houk, A force field model for intramolecular radical additions, J. Org. Chem. 52 959 (1987) T. V. RajanBabu and T. Fukunaga, Stereochemical control in hex-5-enyl radical cyclizations Axial vs. equatorial 2-(but-3-enyl)cyclohexyl radicals, J. Am. Chem. Soc. Ill 296 (1989). 

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