Sugars photochemical deriv

Photochemical Balz-Schiemann reaction of diazonium fluoroborates continues to be the method of choice to prepare fluoro substituted imidazoles. Thus, 4(5)-fluoroimidazole 1565 is prepared from 4(5)-nitroimidazole 1559 in moderate yield by conversion of the nitro group to the diazonium salt followed by irradiation in aqueous tetrafluoroboric acid. Sugar-fluoroimidazole derivatives 1566 and 1567 were prepared using an iV-silyl derivative of 1565 (Scheme 404). 

There are several other methods for generation of (per)fluoroalkyl radicals in reactions with diazines. In particular, photochemical decomposition of perfluoroalkyl mercury derivatives was used for perfluoroalkylation of uracils 568 (Scheme 120) [297]. Whereas for the parent uracil the method gave satisfactory results (30-54 % yields), the procedure was unfruitful for the sugar-modified derivatives (6-11 % yields). 

Dimethylthiocarbamates are known to undergo photochemical cleavage, leading, in the case of monosaccharide derivatives, to deoxy sugars and to free alcohols (see Scheme 12). ... 

The photochemically induced radical addition of alcohols to enones has been described by Fraser-Reid [104-109]. Here again, the sense of addition depends on the steric effects of substituents, attack anti to the C-5 substituent being preferred [108,110]. Other uses of sugar-derived enones to trap radicals have been reported [111]. Enolone 77 gave interesting results in terms of selectivity [112]. In this instance, radical addition occurs with an equatorial selectivity, whereas cuprate addition occurs with an axial selectivity [9,62]. 

Unsaturated sugar 37 is used as the starting material for cyclopropanation by non-photochemical means to form cyclopropanopyranoside 38 and, by further degradation and cis-trans isomerization, chrysanthemic derivatives 39 (Scheme 19) [41]. 

Spirocyclic derivatives 118 of pyranoses were prepared in moderate yields by tethering the succinimide on C-6 of the protected monosaccharide skeleton [24c]. The photochemical behavior of 1,6-anhydro sugars 119 [78e], 121 [78f), and 123 [78e] that bear the succinimide moiety in 2-, 3-, and... 

Not only acids but also a-hydroxylactones (y- and S-) when submitted to the system DIB-iodine underwent oxidative decarboxylation, accompanied by -fragmentation. This reaction took place under non-photochemical conditions in various solvents, at 20-58°C. The substrates - mainly sugar derivatives - eventually yielded, after a mechanistically interesting route, j8-iodo or a,/ -unsaturated compounds. An example follows [71] ... 

DIB, BTI and perfluoro-DIB were suitable precursors the last was most effective under photochemical conditions, whereas DIB and BTI were used either thermally or photochemically for DIB, best results were obtained with added trifluoroacetic acid, photochemically. Not only alkyl but other more complex free radicals could also be introduced, notably acyl radicals. Products derived from acids bearing sugar moieties were obtained in low to moderate yield. In the case of acetonides complete retention of the initial configuration was observed. This high diastereoselectivity... 

A number of analogues has been prepared for photochemical studies. The photochemical reaction of 5-halouracil derivatives in DNA has been investigated for some time. Hydrogen abstraction of the sugar moiety from the 5 side by dU is largely conformation-dependent. Competitive CT and C2 p H abstractions are observed in B-DNA, predominant CT H abstraction occurs in DNA-RNA hybrids, and stereospecific C2 a-hydroxylation occurs efficiently in Z-DNA. In a more recent study, the photoreaction of 5-iodouracil con-... 

Functionalization of several pyranose sugar derivatives, e.g. 28, 30, 33, was achieved with a variant of this method in which 3-oxabicyclo[4.1.0]heptanyl radicals were generated, either photochemically from benzoate esters, or from bromides. These photochemical transformations left unprotected functional groups in the substrate intact, including bromomethyl, ester, and ether groups, and rather effectively demonstrated the tolerance of free-radical intermediates for such functionality. 

The preparation of simple deoxy-sugars by reductive radical methods such as deoxygenation of thiocarbonyl derivatives of alcohols is now considered a routine operation [135] and is therefore not discussed here. Photochemical chemical reduction of halides, for example the photolysis of primary iodides for the preparation of... 

Ethylene sulfite also reacts with chlorine gas under photochemical conditions to form 2-chloroethyl chlorosulfite CICH2CH2OSOCI in excellent yield [67JCS(C)314], Some anomalous results were reported in the reaction of ethylene sulfite to furnish fluoroethanol, but they could not be substantiated (83M11). Tewson attempted to carry out nucleophilic opening of a cyclic sulfite derived from a sugar with tetrabutylammonium fluoride, which led to the hydrolysis of cyclic sulfite to furnish the corresponding diol (83JOC3507). 

The 6-aldehyde derivatives of 4,5-unsaturated 4-deoxyhexopyranosides, which can be obtained by a photochemical route according to the method of Horton and coworkers, have proved to be useful starting-materials for the preparation of higher-carbon sugars. Horton and Liav have... 

Acetone-sensitized irradiation of (77) in acetonitrile solution affords the intramolecular adduct (78) in 36% yield. The cycloaddition is quite specific and affords the [5 S, 6 S, 7 S]-lactone. Acetone-sensitized irradiation of the thymine derivative (79) results in the formation of the cis,syn-dimQr (80). A patent has been filed dealing with the synthesis of (2 + 2)-adducts such as (81). A study of the conformational effects within the sugar moieties of (82) has been investigated. Systems such as (82) are photochemically reactive and undergo (2 + 2)-cycloaddition to afford the adduct (83) on irradiation at 254 nm. ... 

Polymer-bound protected sugar derivatives such as 43, which are soluble in organic solvents, have been synthesized using xanthate transfer technology. The synthesis of thioacetal 44 was achieved under photochemical conditions. 

Photochemical cross-pinacolization of acetalated pyranos-3-ulose derivatives with methanol has provided a route to C-hydroxymethyl sugar derivatives (Scheme 62), although significant amounts of 4,6-O-ethylidene-D-allono-1,5-lactone were... 

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