Antiperiplanar arrangement

A different kind of closed five-membered transition state has been postulated in 5-e.vo-trig radical13 and metallo-ene14 cyclizations. In both cases simple diastereoselection is effected via similar transition states (36 and 42) in which the exocyclic appendages adopt antiperiplanar arrangements with respect to the endocyclic C-C bonds (C-2/3 and C-3/4). Thus, the appendages are in the cis orientation on the newly formed rings 37/38 and 43/44. 

The reason for this stereospecificity can be explained using orbital diagrams (Following fig.). In the transition state of this reaction, the C-H and C-Br a bonds are in the process of breaking. As they do so, the sp3 hybridised orbitals which were used for these c bonds are changing into p orbitals that begins to interact with each other to form the eventual n bond. For all this to happen in the one transition state, an antiperiplanar arrangement is essential. 

The stereodescriptor erythro indicates that the chlorine atom and the hydroxy group lie on the same side of the main chain in a Fischer projection formula. Since it does not define an absolute configuration, two enantiomers must be considered. The Fischer projection formula, which represents an eclipsed conformation, is simplest first converted into a sawhorse projection (also in its eclipsed conformation) and then one side of the molecule rotated until both the reference groups, the chlorine atom and the hydroxy group, adopt an antiperiplanar arrangement. The required Newman projection formula can then be derived from these formulae. 

Arylsulfenyl trifluoroacetates have been generated in situ and used for the hydroxysulfenylation of al-kenes. These reagents are prepared from diaryl disulfides and LTA in trifluoroacetic acid. Yields for the addition are generally good (42-95%). The hydroxysulfenate intermediate can be oxidatively cleaved by LTA (provided the groups can achieve an antiperiplanar arrangement) to form an a-aldehyde-acetoxy sulfide in good yield (40 88% Scheme 3). 7... 

The stereoelectronic requirements proposed for the migration step are an antiperiplanar arrangement of the C— Rm bond and the breaking O—O bond. It has been further suggested that one of the hydroxy nonbonding electron pairs must be antiperiplanar to the migrating carbon atom, as in (3). ... 

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