Chemical synthesis chiral substituents

A significant recent development has been the synthesis of PAn s in which chiral substituents have been covalently attached to the aniline rings. Chiral ethers were bound to the aniline monomer before its chemical oxidation by persulfate,139 producing chiral PAn s possessing strong optical activity as evidenced by their CD spectra. Previous routes to chiral PAn s had employed chiral dopant anions to induce chirality into PAn chains. 

Although these definitions are vague, it appears that partial synthesis is restricted to those modifications which do not change the basic chemical structure, whereas ex-chiral-pool synthesis generally leads to a new type of compound. For instance, the introduction of new substituents into a steroid nucleus is partial synthesis , whereas the conversion of D-glucose into the pheromone e.vo-brevicomin is ex-chiral-pool synthesis . 

Ugi has coined the term stereorelating synthesis for the sequence lithiation/reac-tion with electrophiles [62,118], and used this technique as a method for the chemical correlation of the structure and for the determination of the enantiomeric purity of many 1,2-disubstituted ferrocene derivatives obtained either by resolution or by asymmetric synthesis (for a compilation, see [118]). It is important to note that all stereochemical features discussed above for central chiral compounds, such as retentive nucleophilic substitution, remain valid when more substituents are present at the ferrocene ring and the conversion of functional groups in planar chiral ferrocenes can be achieved by the same methods as described. 

For the acetylenic sulfoxide, because of its configurationally stable pyramidal stereogenic sulfur atom (a lone electron pair, an oxygen and two different carbon substituents), it can exist in chiral forms. Therefore, in chiral acetylenic sulfoxide, the sulfoxide moiety not only serves as a chemical activator of the acetylene unit, it can also induce stereochemical control at the adjacent carbon centers to achieve enantioselective synthesis. In this article, we shall discuss the preparation of these a, /J-unsaturated synthons and their applications in Diels-Alder reactions, heterocycle and alkaloid syntheses. 

Chiral a,P-unsaturated sulfoxides 1.136 (Y = Tol, R = R CH=CH) also have been used in asymmetric synthesis. These compounds are prepared either by treatment of 1.137 with vinylic organometallic reagents, or from saturated precursors by classical chemical transformations [102, 173, 476, 484-487], Michael additions to these electrophiles are interesting only if R = CF3 [161], Organometallic additions or [4+2] cycloadditions require the introduction of a second electron-withdrawing substituent [73, 102], and acyclic 1.138 and cyclic 1.139 gem-di substituted sulfoxides have seen many interesting applications [101, 102,... 

A substituent-controlled "mono-Clatsen" rearrangement was also employed as a key reaction in the construction of chiral intermediates for the synthesis of pseudomonic acids from diacetyl-L-arabinal418. The acetate groups of the starting material are chemically differentiated by virtue of the accelerated rate of the first Claisen rearrangement (ascribed to a vinylogous anomeric effect). 

Asymmetric and enantioselective olefination reactions continue to be of interest. Wadsworth-Emmons reactions of 4-substituted cyclohexanones with the phosphonate (147), which carries a chiral benzopyrano-isoxazolidine substituent, proceed with diastereomeric excesses of 80-90% and hence provide another example of such an approach to enantiomerically pure, axially dissymmetric cyclohexylidene derivatives. A further example of trapping of in situ generated ketenes by Wadsworth-Emmons reactions to give allene carboxylates has been reported and the reaction has been extended to enantioselective synthesis by use of the optically active phosphonates (148) (Scheme 14). Moderate to good chemical yields and e.e. values up to 84% were obtained depending on the nature of (148) and the reactions conditions. 

---