Current Organic Synthesis - Volume 2, Issue 1, 2005

Compounds of certain transition metals in which the metals are in high oxidation states are reagents for the epoxidation and dihydroxylation of steroids. In recent years a variety of oxidative synthetic methods have been reported using transition metal compounds, in stoichiometric or catalytic amounts. The preparation of sterols containing oxygenated functions in suitable positions of the steroid nucleus can be accomplished using various reagents such as methyltrioxorhenium-hydrogen peroxide system, ruthenium tetraoxide, osmium tetraoxide and potassium permanganate.

Polymer synthesis via the palladium-catalyzed allylic substitution reaction between allylic esters and malonic esters, namely the Tsuji-Trost reaction, is summarized in this review. Due to the versatility and potential of the Tsuji-Trost reaction, four polymerization patterns for various functionalized polymers have been reported.

This paper reviews the latest developments in the synthesis of swainsonine and its analogues since the first review in this area by Nemr which covered the literature up to 1999 (Nemr, A. E. Tetrahedron 2000, 56, 8579- 8629).

Chemoselective ligation reactions are highly efficient and specific covalent bond forming reactions capable of proceeding within a physiological environment. Chemoselective ligations offer exquisite specificity, similar to enzymatic reactions, but with the significant advantage of accessing a much broader range of coupling partners. Thus, even among a multitude of reactive functional groups, two ligation partners exclusively react with one another. While chemoselective ligation has been used in many applications, the focus of this review is the application of chemoselective ligation reactions toward carbohydrate-based structures. More specifically, recent applications are presented in areas such as immobilized carbohydrate arrays, cell-surface engineering, glycoproteins / peptides, and glycosylated natural products.

Palladium-catalyzed C-N bond-formation is rapidly becoming a staple in the organic synthesis repertoire, and modern developments in the catalysis field have led to highly effective methods for aryl amination. Normally, this process involves the reaction of an aryl halide or triflate with an amine that is mediated by an appropriate ligand-palladium complex. The versatility of this method is documented in the many reports of its application for the synthesis of previously unknown entities or those that are otherwise difficult to prepare. The application of C-N bond-formation to nucleosides is particularly interesting. Since the nucleobases adenine and guanine contain exocyclic amino groups, purine nucleosides themselves could serve as amine components in Pd-catalyzed C-N bond formation. Alternatively, easily obtained halo purine nucleosides can be subjected to Pd-catalyzed amination. Thus, Pd-catalyzed C-N bond formation raises the possibility of a novel approach to preparing N-functionalized purine ribo and 2'-deoxyribo nucleoside analogs. Many such modified nucleosides are important for a wide range of biological studies. This review therefore discusses the recent developments in the Pd-catalyzed synthesis of N-functionalized nucleosides. Whereas the initial reports dealt with the tests of the method, more recent studies pertain to the applications. These have yielded facile access to: (a) dimeric nucleosides, which are products of nitrous acid-mediated DNA cross-linking, (b) C-8 arylamino nucleosides, which are produced by metabolism of aryl amines, (c) C-6 and C- 2 adducts of polycyclic aromatic hydrocarbon epoxides, which are products of polycyclic aromatic hydrocarbon metabolism implicated in tumorigenesis by these compounds. Although Pd-catalyzed C-C bondformation has also received recent attention, this is not discussed in the present review.

The regioselective Baeyer-Villiger oxidation of a wide range of α,ααdisubstituted β-ketoesters has been developed to synthesize, in good yields, α,α-disubstituted α-acetoxy esters. The reactions were performed using m-chloroperbenzoic acid in the presence of triflic acid. The rearrangement was shown to occur with retention of configuration of the migrating group. The corresponding α,ααdisubstituted α-hydroxy acids were obtained in good yields, after hydrolysis.

Five-membered hypervalent iodine heterocycles derived from benziodoxole and benziodazole oxide have recently emerged as reagents of choice for various synthetically useful oxidative transformations. In particular, IBX (2-iodoxybenzoic acid) and DMP (Dess-Martin periodinane) are widely used for the selective oxidation of primary and secondary alcohols and for a variety of other important oxidations. IBX-amides and IBX-esters are a new class of pentavalent iodine reagents with a pseudo-benziodoxole structure and a reactivity pattern similar to IBX.