Molecular rearrangements Wittig reaction

Several alternative transition structures for the [2.3]-Wittig rearrangements (Z)-(48) —> threo- 49), and ( )-(50) — erythro-(51), have been explored by ab initio molecular orbital calculations at the 6-31G level in an attempt to explain why the observed stereo selection is opposite to that for alkenes which do not bear a 1-carboxylic group.49 It has been concluded that coordination of lithium cation to two oxygen atoms and the C(4) carbon plays a significant role in reactions of (48) and (50), thereby making it easier to break the 0(2)—C(3) bond. 

Both intermolecular and intramolecular additions of carbon radicals to alkenes and alkynes continue to be a widely investigated method for carbon-carbon bond formation and has been the subject of a number of review articles. In particular, the inter- and intra-molecular additions of vinyl, heteroatomic and metal-centred radicals to alkynes have been reported and also the factors which influence the addition reactions of carbon radicals to unsaturated carbon-carbon bonds. The stereochemical outcome of such additions continues to attract interest. The generation and use of alkoxy radicals in both asymmetric cyclizations and skeletal rearrangements has been reviewed and the use of fi ee radical reactions in the stereoselective synthesis of a-amino acid derivatives has appeared in two reports." The stereochemical features and synthetic potential of the [1,2]-Wittig rearrangement has also been reviewed. In addition, a review of some recent applications of free radical chain reactions in organic and polymer synthesis has appeared. The effect of solvent upon the reactions of neutral fi ee radicals has also recently been reviewed. ... 

The enantioselective Wittig and epoxide rearrangements have been widely explored in the last few years. They are of interest since such processes can, in a single step, induce asymmetry and molecular complexity that would be difficult to generate otherwise. Advances in substrate scope, reaction efficiency and catalytic process constitute significant challenges for the future. 

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