Rules for ring closure

Baldwin, J. E. Rules for Ring Closure J. Chem. Soc, Chem Commun 1976, 734-736. 

Baldwin, J. E. Thomas, R. C. Kruse, L. I. Silberman, L. Rules for ring-closure Ring formation by conjugate addition of oxygen nucleophiles. J. Org. Chem. 1977, 42, 3846-3852. 

The formation and the hydrolysis of acyclic and cyclic acetals have been studied in rather great detail [91]. Several reviews on this topic are available [92] and some comments have been made [13] concerning the carbohydrate series. We have shown in Schemes 1,2, and 3 that a common feature of this reaction seems to be the intermediacy of an oxocarbenium ion. However, the cyclization of such an intermediate has been questioned more recently [93] in the light of the Baldwin s rules for ring closure [94]. At least for the five-membered ring, an SN2-type displacement mechanism far the protonated form (B) of die hemiacetal (A) (favorable 5-exo-tet cyclization) has been proposed rather than the unfavorable 5-endo-trig cyclization of the oxocarbenium ion (C) (Scheme 5). Except when the formation of the enol ether (D) is structurally impossible, the intermediacy of such a compound remains feasible. 

J. E. Baldwin and M. J. Lusch, Rules for ring closure Application to intramolecular aldol condensations in polyketonic substrates, Tetrahedron 38 2939 (1982). 

Step 4 Intramolecular aldol condensation favors the five-membered ring over the seven-membered ring (see Baldwin s rules for ring closure). 

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. 

An astounding aspect of antibody catalysis is the ability of these progranmiable biocatalysts to preferentially form the less thermodynamically favored product (66, 71-74). The intramolecular cyclization reaction of frons-epoxyalcohol 18 is an archetypal example of antibody catalysis of a disfavored transformation, which preferentially forms the tetrahydrofuran 19 under nncatalyzed conditions due to the overwhelming stereoelec-tronic constraints predicted by Baldwin s rules for ring closure (Fig. 9) (75, 76). 

Baldwin JE. Rules for ring closure. J. Chem. Soc., Chem. Commun. 1976 (18) 734-736. 

Baldwin has formulated rules for ring closure in a systematic manner . For digonal systems these are (/) 3- and 4-exo-dig, disfavoured ( ) 5- to 1-exo-dig, favoured (i/7) 3 to 1-endo-dig, favoured. In his terminology the choice in equation (58) is between S-exo-dig di di 6-endo-dig. On the basis of his survey Baldwin concludes that enrfo-ring closures at digonal carbon predominate. For the possibilities we encountered most often, namely, 5- to 7-rlngs, we find that first-row nucleophilic sites, e.g. O, N, C, favour 5-exo-dig and 6-exo-dig closures. These will be illustrated here and in later sections. 

It should be noted that the radical cyclization proceeds to yield a five-membered ring and not a six-membered one. This is predicted by the Baldwin rules for ring-closure and is classified as a 5-exo-trig closure ". LiAlH4 has also been used to generate the cyclopropyl radical which also underwent a 5-exo-trig ring-closure. ... 

Cyclization Reactions-Baldwin s Rules for Ring Closure 231... 

CYCLIZATION REACTIONS-BALDWIN S RULES FOR RING CLOSURE... 

Baldwin rules for ring closure. Set of empirical rules, stereochemical in nature, predicting the relative facility of ring closure reactions. 

Table 8. Selection rules for ring closures and openings...

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