The phosphine-promoted [3 + 2] cyclizations between γ-substituted allenoates and arylideneoxindoles have been applied to the stereoselective synthesis of spiro(cyclopentene)oxindoles with trisubstituted cyclopentene units. It has been demonstrated that PPh(3) operates a very efficient control of the relative stereochemistry of the three stereogenic centers of the final spiranic products. Focused experiments have been carried out then so as to access carbocyclic analogues of an important series of anticancer agents inhibiting MDM2-p53 interactions.
Triphenylphosphine promoted reactions between 3-arylideneoxindoles and δ-aryl-substituted penta-2,3-dienoates afford an unprecedented access to spirocyclic oxindoles with functionalized six-membered rings. In these new [4 + 2] cyclization processes, the allenoates operate as the four-carbon synthons, thanks to the involvement of the substituted δ-carbons. These reactions give excellent control of the relative stereochemistry of the three stereogenic centers. The stereochemistry of the final product has been ascertained by X-ray diffraction studies.
During the last few years, synthetic methods based on phosphine organocatalysis have experienced extremely fast developments. This short overview is intended to demonstrate that these methods can afford suitable tools for the constitution of either focused or DOS inspired libraries of small molecules for biological screening. It also reports selected examples where these organocatalytic methods have been used conveniently as key steps in the formal or total synthesis of complex natural products. Especially, synthetic applications of the inter-and intramolecular cyclizations of unsaturated substrates have been considered in this review.
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