Protection/deoxygenation procedures

The 1,2-anhydro sugar (Brigl s anhydride) approach, developed by Danishefsky and coworkers (see Section V) also lends itself to the deoxygenation procedures of a- and (3-pyranosides having a single free OH group at C-2 and the remaining positions suitably protected. 

One approach started from partially protected rhamnosyl derivatives. Thus, from easily obtained 2,3-<3-isopropylidene-a-L-rhamnopyranoside, deoxygenation at C-4 by the Barton and McCombie procedure afforded the 4,6-dideoxy-a-L-/yxo-hexopyranoside derivative.234... 

In this chapter, methods for oxidation, reduction, and deoxygenation of carbohydrates are presented. In most cases, the reactions have been used on aldoses and their derivatives including glycosides, uronic acids, glycals, and other unsaturated monosaccharides. A number of reactions have also been applied to aldonolactones. The methods include both chemical and enzymatic procedures and some of these can be applied for regioselective transformation of unprotected or partially protected carbohydrates. 

Another well-adapted method for removing the oxygen functionality at Cl uses a radical reduction of a protected pyranosyl halide (O Scheme 28) [211]. Formally, this is not a deoxygenation since the hydroxy group at Cl has already been replaced by a halide. However, gly-cosyl halides are easily available from aldoses by a one-pot procedure [212] and combined with the radical reduction this two-step route gives easy access to a number of 1,5-anhydroalditols. 1,4-Anhydroalditols, on the other hand, are more difficult to obtain by this method since the corresponding furanosyl halides require more steps for their preparation. The rad-... 

The Ireland s)mthetic intermediate 176 was deoxygenated by the Barton procedure to provide 194 (O Scheme 22). Stepwise deprotection and protection of hydroxy groups in 194 afforded 195. De-O-tritylation and Swem oxidation of 195 gave the C10-C15 segment 192. As shown in Scheme 22, this aldehyde was transformed into the C10-C20 segment 197. 

The rosemary subcritical water extractions were carried out at 60 and 100 °C for 25 min. The procedure was as follows 2.0 g of sample were placed in 11 mL extraction cells prior to each extraction a heat-up time of S min was completed. The extracts obtained were inunediately protected from light and stored under refrigeration until dried. For this purpose, a Freeze Dryer (Unitop 400 SL, Virtis, Gardiner, NY, USA) was used. After freeze-drying, 10 mg of each extract were freshly dissolved in 1 ml of water and immediately injected into CE-MS without further purification. Water was deoxygenated by purging with He for 15 min prior its use as extraction solvent. 

Alternate procedures were also developed using Barton s reagent. In the first of these, coupling of the xanthate 3.1.4.3 obtained from 10-deacetylbaccatin III with protected side chain, followed by free radical deoxygenation and deprotection, gave 10-deacetoxy taxol (3.1.4.4) or 10-dehydroxydocetaxel (3.1.4.5), depending on the side-chain used (83). 

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