Silicon connection, temporary

X. Zhen, P. Agback, J. Plavec, A. Sandstrom, and J. Chattopadhyaya, New stereocontrolled synthesis of isomeric ( -branched [1 D-nucleosides by intramolecular free-radical cyclization— opening reactions based on temporary silicon connection, Tetrahedron 48 349 (1992). 

S. J. Danishefsky, Route from glycals to mannose p-glycosides, J. Org. Chem. 59 1892 (1994) G. Stork and G. Kim, Stereocontrolled synthesis of disaccharides via the temporary silicon connection, J. Am. Chem. Soc. 114 1081 (1992) P. J. Garegg, Saccharides of biological importance Challenges and opportunities for otganic synthesis, Acc. Chem. Res. 25 575(1992). 

G. Stork, H. S. Suh, and G. Kim, The temporary silicon connection method in the control ol regio- and stereochemistry. Application to radical-mediated reactions. The stereospecific synthesis of C-glycosides, J. Am Chem. Soc. 7/3 7054 (1991). 

The temporary silicon connection , pioneered by Stork,67a and based on the protocol of temporarily bringing two reaction partners together by means of an eventually removable silicon atom, often as silyl ethers, has been surveyed676. This concept, i.e. that of converting an intermolecular reaction into its intramolecular equivalent, has proven a very valuable synthetic strategy for various types of reactions. 

The temporary silicon connection method introduced by Stork is a useful protocol for the preparation of silaoxacycles. The C-Si bond in the silaoxacycles is readily convened to C-OH with retention of stereochemistry by oxidative procedures. Utimoto and co-workers have developed a novel method for the synthesis of triols using the temporary silicon connection method [95BCSJ625], Treatment of the silacycle 184 with tin hydride furnished a mixture of silacycles, which were converted to the triacetate 187 without isolation. The endo isomer 185 was formed as the major compound. 

The most representative examples of intramolecular free radical cyclization in carbohydrate chemistry are the syntheses of C-branched nucleosides derivatives. The key step in C-branched nucleoside preparation is the regio- and stereo-controlled formation of a new C-C bond at the branching point of the ribofuranose ring [58]. Among published reports, a temporary silicon connection is becoming a growing interest in the syntheses of C-branched nucleosides by intramolecular radical cyclization. 

Beignet J, Tierman J, Woo CH, Kariuki BM, Cox LR (2004) Stereoselective synthesis of allyl-C-mannosyl compounds use of a temporary silicon connection in intramolecular allylations strategies with allyl silanes. J Org Chem 69 6341-6356... 

Detailed n.m.r. studies have been reported on some bicyclic uridine analogues reported last year by Chattopadhyaya s group, formed by intramolecular addition of free radicals to allyl ethers (see Vol. 25, p. 258).i66 The same group have described the preparation of the thymine nucleoside 134 by means of intramolecular free radical trapping by an allyl group tethered by a temporary silicon connection (Scheme 15). Similar sequences were used to make analogous compounds with the three-carbon branch at C-2 in a down position (D-ribo-), and at C-3 in the up orientation, but an attempt to make the compound with the C-2 up stereochemistry led to a tricyclic product after oxidation in which the intermediate bicyclic radical had attacked C-6 of the thymine ring. 167... 

---