Pyranosides, rearrangement

The rearrangement provides a stereocontrolled synthesis of the C-pyranoside 2, present in pseudomonic acids.2... 

B. Fraser-Reid, D. B. Dilshian, and R. Tsang, Striking contrasts in the stereoselectivities of spiro-Claisen rearrangements of pyranosides versus catbocycles, Tetrahedron Lett. 25 4579 (1984). 

Claisen rearrangements routes to multiply substituted pyranoside diquinanes, J. Org. Chem. 54 5357 (1989). 

V. Bolitt, C. Mioskowski, S. G. Lee, and J. R. Falck, Direct preparation of 2-deoxy-D-gluco-pyranosides from glucals without Ferrier rearrangement, J. Org. Chem., 55 (1990) 5812-5813. 

Intramolecular nitrene insertion of pyranoside azides is a further route to oxazepanes <95JCS(Pl )1747>. Thus thermolysis of the diazide (94) affords the tetrazolooxazepane (95) in high yield. When the rearrangement is carried out under ultra-violet irradiation the yield of (95) is reduced when it is also accompanied by small amounts of the isomer (96). 

In a stereocontrolled route to thromboxane B2, Corey and coworkers used the Eschenmoser rearrangement for the preparation of lactone (91 Scheme 14). The product of the 3,3-sigmatropic shift, amide (90), is directly iodolactonized, thus avoiding often troublesome amide hydrolysis conditions. Another application involving a carbohydrate derivative was demonstrated by Fraser-Reid and coworkers (Scheme 15). Reductive elimination of benzylidene (92), followed by in situ alkylation, Wittig reaction, DIBAL-H reduction and rearrangement, led to amide (94), which was transformed into the corresponding pyranoside diquinane by double radical cyclization. 

The /3-tetraacetate, by etherification of the glycosidic hydroxyl group with methyl iodide and silver oxide, furnished a convenient source for the preparation of methyl tetraacetyl-/3-D-altroside when mutarotation occurred before methylation, considerable methyl tetraacetyl-a-D-altro-side was isolated. The latter was obtained also by rearrangement of the /3-isomer with titanium tetrachloride. These methyl tetraacetylaltro-sides do not contain orthoester linkages, and may be assumed to possess normal pyranoside rings. From the values of - -66.0 and — 61.0°... 

SCHEME 8.34 Synthesis of a carba-pyranoside using a Ti(IV) induced rearrangement. 

SCHEME 8.37 Synthesis of carba-pyranosides using the Claisen rearrangement. 

Unsaturated pyranosides are easily converted into cyclohexane derivatives by a HgCl2-catalyzed rearrangement discovered by Ferrier in 1979 (The Ferrier-II rearrangement) [5]. 

IrDeoxyglycosides The reaction of DAST with a pyranoside having a free hydroxyl group at Q and substituted at Ci with a methoxy, acetoxy, phenylthio, or azido group results in a glycosyl fluoride with the original Q-substituent now at C2. The rearrangement involves inversion of stereochemistry at both centers. 

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