Beckwith rules

On the other hand, Beckwith rules [22] are useful guidelines for predicting the stereoselectivity in the cyclisation of substituted hexenyl radicals i) 1- or 3-substituted radicals give preferentially cij-disubstituted cyclopentane derivatives and ii) 2- or 4-substituted radicals give mainly rrans-disubstituted derivatives. These rules can be explained in terms of 1,3-diaxial interactions present in a chair-like transition state. Some examples which demonstrate such an effect are [19b] ... 

In synthetic work, amidyl radicals, prepared by BusSnH/AIBN-mediated homolysis of O-benzoylhydroxamic acid derivatives, have been shown to cyclize in a A-exo fashion to give -lactams (Scheme 10). In addition, radicals generated in this way have also been reported to undergo 5-exo cyclization to give mixtures of cis- and Iranx-pyrrolidinones with the steric nature of the Al-substituent having little effect on the stereochemical outcome (Scheme 11). The major products detected were those predicted by application of the Beckwith rule. ... 

The additions of thiyl radical to alkenes and alkynes can also occur intra-molecularly, thus leading to sulfur heterocycles. This subject has been explored by Surzur s group [32-33]. Based on steric and stereoelectronic factors, the Baldwin-Beckwith rules [34] predict the 5-exo ring closure as the favored pathway. Scheme 6 shows that the pent-4-enylthiyl radical, generated from the allyl sulfide, yields both five- and six-membered rings, in the ratio of 1 19. The predominance of the larger cycle, in contrast with the above-mentioned rules, is due to the reversibility of the reaction. The product distribution reflects a thermodynamic control. 

The cyclizations are stereoselective following the Beckwith rules and allow the elaboration of highly substituted cyclopentane derivatives. 

No single factor can be identified as determining the outcome of radical addition. Nonetheless, there is a requirement for a set of simple guidelines to allow qualitative prediction. This need was recognized by Tedder and Walton,2 17 Beckwith ei ah, Giese. and, most recently, Fischer and Radom.4 With the current state of knowledge, any such rules must be partly empirical and, therefore, it is to be expected that they may have to be revised from time to time as more results become available and further theoretical studies are carried out. 1 lowever, this does not diminish their usefulness. 

In addition to nucleophilic reactions, Baldwin s rules also apply to homo-lytic and cationic processes. Table 21 lists rate constants for ring closure of lower -alkenyl radicals (71), in which intramolecular addition to the double bond occurs in the exo-mode (Beckwith, 1981). It is unfortunate that EM-... 

For a detailed discussion of the rules for radical ring-closure, see A.L.J. Beckwith, C.J. Easton and A.K. Serelis, 1980, J. Chem. Soc., Chem. Commun., 482. For Baldwin s earlier rules for ring-closure, see J.E. Baldwin, 1976, J. Chem. Soc., Chem. Commun., 734. 

Generation of a radical in a molecule that contains a site of unsaturation presents an opportunity for (yclization. These types of radical cyclizations have been developed into very useful synthetic reactions. The synthetic utility of these cyclizations is enhanced by the abhity to predict the regiochemistry of cyclization by applying the Baldwin rules. The Baldwin rules cover the formation of three- to seven-membered rings by various reactions and are based on consideration of both kinetic and thermodynamic factors. (See Beckwith, A. L. J. Easton, D. J. Serelis, A. K. J. Chem. Soc., Chem. Commun. 1980, 482-483, and Baldwin, J. E. J. Chem. Soc., Chem. Commun. 1976, 734-736.)... 

The cyclization is highly stereoselective according to Beckwith s radical cyclization rules [73]. Thus, the allylic benzyl ether 41 (a 1 1 mixture of diastereoisomers) undergoes a... 

Fortunately, in recent times, considerable knowledge has been gained about both termination modes. Beckwith [29] reviewed the topic of regio-selectivity and stereo-selectivity in several types of radical reactions. He suggested that the disproportionation reaction (of cyclohexyl radicals) is under stereo-electronic control and is not based that much on thermochemical factors. Relatively simple rules may be applied to predict the outcome of such radical reactions. Reviews on this topic have been published by Gibian and Corely [21], Alfassi [24] and Moad and Solomon [22, 30]. In general, the relative... 

Beckwith, H. 2003. What Clients Love. Warner Business Books New York, NY. (Offers these additional communication suggestions relevant to projects use your office space to communicate to clients that they belong and you care about them occasionally erring is acceptable, but never let your communication suggest you don t care and apply the rule of three—people tend to be able to remember three ideas, topics, etc.)... 

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