The advent of well-defined catalysts for olefin metathesis which combine high activity, durability, and excellent tolerance towards polar functional groups has revolutionized the field. The past decade has seen the rapid embrace of these reagents as tools for advanced organic and polymer chemistry and the success of this development is witnessed by a plethora of elegant applications to the synthesis of natural and nonnatural products. This review article provides an overview of these developments and intends to familiarize the reader with some very recent advances which hold the promise to expand the scope of the reaction even further. Moreover, the positive impact of metathesis on the fundamental logic of retrosynthetic planning is demonstrated by means of typical examples. Finally, it will be shown that metathesis is by no means restricted to alkenes as substrates, and some comments on metathesis reactions following unconventional mechanistic pathways will also be presented.

Olefin Metathesis and Beyond

Olefinmetathese und mehr

This review covers the progress made in the chemistry, synthesis and biology of carbasugars until May 2004. It includes the extensively studied carbapyranoses as well as the scarcely considered carbafuranoses. Specific topics discussed include the natural occurrence of carbasugars, biosynthesis and biological activity of carbasugars, conformational analysis of carbasugars, and the compilation of synthetic methods of carbafuranoses and carbapyranoses.

Synthesis and conformational and biological aspects of carbasugars.

Polysaccharide-Based Conjugates for Biomedical Applications

The C7N aminocyclitol family of natural products.

(+)-Valienamine (16) was prepared in seven steps and in an overall yield of 17% from commercially available 2,3,4,6-tetra-O-benzyl-D-glucopyranose. Stereoselective addition of vinylmagnesium bromide to the 1,3,4,5-tetra-O-benzyl-6,7-dideoxy-L-xylo-hept-6-en-2-ulose (2) gave diene 3 (86%). Ring-closing alkene metathesis of 3 in the presence of 0.15 equiv. of Grubb's catalyst 1 gave the cyclohexene 4 (58%), that was converted into (+)-valienamine (16) in three steps and in 47% yield. Similarly, ring-closing alkene metathesis of the D-mannose-derived diene 20 gave the cyclohexene 21 (89%).

Carbasaccharides via Ring‐Closing Alkene Metathesis. A Synthesis of (+)‐Valienamine from D‐Glucose

Chitosan (1) was prepared by basic hydrolysis of chitin of an average molecular weight of 70000 Da, 1H-NMR spectra indicating almost complete deacetylation. N-Phthaloylation of 1 yielded the known N-phthaloylchitosan (2), which was tritylated to provide 3a and methoxytritylated to 3b. Dephthaloylation of 3a with NH2NH2⋅H2O gave the 6-O-tritylated chitosan 4a. Similarly, 3b gave the 6-O-methoxytritylated 4b. CuSO4-Catalyzed diazo transfer to 4a yielded 95% of the azide 5a, and uncatalyzed diazo transfer to 4b gave 82% of azide 5b. Further treatment of 5a with CuSO4 produced 2-azido-2-deoxycellulose (7). Demethoxytritylation of 5b in HCOOH gave 2-azido-2-deoxy-3,6-di-O-formylcellulose (6), which was deformylated to 7. The 1,3-dipolar cycloaddition of 7 to a range of phenyl-, (phenyl)alkyl-, and alkyl-monosubstituted alkynes in DMSO in the presence of CuI gave the 1,2,3-triazoles 8–15 in high yields.

Francisco Sarabia

Papers

Carbasaccharides via Ring‐Closing Alkene Metathesis. A Synthesis of (+)‐Valienamine from D‐Glucose

2‐Azido‐2‐deoxycellulose: Synthesis and 1,3‐Dipolar Cycloaddition

Carbasaccharides via Ring-Closing Alkene Metathesis. A Synthesis of (+)-Valienamine from D-Glucose.