| Popis: |
Palladium is probably the most useful metal in organic syntheses. It has shown great utility in various reactions such as C-C, C-N, C-O bond formation under mild conditions. The presence of abundant amount of palladium-chemistry related literature in the form of books, reviews emphasizes the growing importance of these reagents. Nowadays organopalladium chemistry is being used in various fields such as new methodology development, natural product synthesis, synthesis of polymers. Regio- and stereoselectivity is another facet of Pd catalyzed methodologies which has been extensively utilized in the last decade to obtain enantiopure compounds. The main emphasis of this work is to utilize Pd catalyzed allylic alkylation to synthesize new heterocycles including furans, isoxazolines and new cyclopentane amino-acid analogs in an enantioselective manner. The stereochemical outcome of these reactions is influenced by desymmetrization catalyzed by hydrolytic enzymes namely lipases. Chapter 1 reviews the recent advances in the field of palladium catalyzed synthesis of bicyclic furan analogs and provides a mechanistic explanation for these processes. Chapter 2 describes synthesis of new optically pure isoxazoline-2-oxide and furan analogs using Pd(0) catalyzed intramolecular cyclizations. Starting from a meso-diol, optically pure compounds were prepared without utilizing chiral ligands at any stage of the synthesis. The stereochemical outcome of the product (>99 % ee) was influenced by desymmetrization catalyzed by Pseudomonas cepacia lipase and the stereoselective nature of the palladium catalyzed transformations. Chapter 3 describes Pd(0) catalyzed allylic alkylation of allylic esters using various 1°, 2° nitroalkanes. This reaction resulted in the formation of nitro substituted aldehydes and ketones via an isomerization-alkylation step. The effect of various solvents, catalyst-ligand systems and bases was also studied. The presence of versatile nitro group in these compounds which can be easily converted to a ketone, reduced to amine or transformed into carboxyl group, imines, hydroxylamines, makes them an attractive starting material for various other synthetic compounds. Chapter 4 describes the chemoenzymatic synthesis of L-carbafuranomycin and related cyclopentane amino acids analogs. The synthesis utilizes the hydrolytic enzymes to induce enantioselectivity in the whole process. Out of all the peptidomimetics and related compounds, unnatural amino acids such as bicyclic and carbocyclic amino acids are of valuable interest as they have provided new building blocks for large number of potential drug candidates. The work presented here provides a more general and efficient route to these class of unnatural amino acids. |
