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Radical methods are of central importance in organic synthesis [ 11. These reactions are performed under mild and neutral conditions, which usually avoids competing ionic side reactions. Carbon-centered radicals are compatible with a range of functional groups (e.g. aliphatic alcohols, amines, ketones, esters) and also show high chemoselectivity under carefully controlled reaction conditions. Furthermore, reactions involving loss of stereochemistry at the non-radical center are not problematic, and hence radical methods are emerging as a powerful synthetic tool in the field of carbohydrate chemistry.In this article we provide a broad overview of the application of radical methods in carbohydrate chemistry, including typical examples classified by the type of bond formed. The factors controlling the stereoselectivity of inter-and intramolecular C-C bond formation are now well understood and have been exploited in the synthesis of C-glycosides [2]. Intramolecular C-C bond formation using carbohydratebased chiral templates also provides a powerful route to branched-chain sugars [ 3 ] and carbocycles [4]. Finally, we include synthetically useful processes involving key carbon-heteroatom and C-H bond formation. Intermolecular Carbon-Carbon Bond Formation Synthesis of C-GlycosidesA powerful strategy for the formation of C-glycosides is the intermolecular addition of an anomeric radical to n-systems [2]. Anomeric pyranosyl radicals are readily generated by a variety of standard methods and are nucleophilic in character because of interaction of the SOMO with the non-bonding electron pair of the adjacent ring oxygen. Anomeric radicals therefore undergo addition to n-systems when the high-lying SOMO can interact with a low-lying LUMO as in electron-deficient Radicals in Organic Synthesis Edited
Radical methods are of central importance in organic synthesis [ 11. These reactions are performed under mild and neutral conditions, which usually avoids competing ionic side reactions. Carbon-centered radicals are compatible with a range of functional groups (e.g. aliphatic alcohols, amines, ketones, esters) and also show high chemoselectivity under carefully controlled reaction conditions. Furthermore, reactions involving loss of stereochemistry at the non-radical center are not problematic, and hence radical methods are emerging as a powerful synthetic tool in the field of carbohydrate chemistry.In this article we provide a broad overview of the application of radical methods in carbohydrate chemistry, including typical examples classified by the type of bond formed. The factors controlling the stereoselectivity of inter-and intramolecular C-C bond formation are now well understood and have been exploited in the synthesis of C-glycosides [2]. Intramolecular C-C bond formation using carbohydratebased chiral templates also provides a powerful route to branched-chain sugars [ 3 ] and carbocycles [4]. Finally, we include synthetically useful processes involving key carbon-heteroatom and C-H bond formation. Intermolecular Carbon-Carbon Bond Formation Synthesis of C-GlycosidesA powerful strategy for the formation of C-glycosides is the intermolecular addition of an anomeric radical to n-systems [2]. Anomeric pyranosyl radicals are readily generated by a variety of standard methods and are nucleophilic in character because of interaction of the SOMO with the non-bonding electron pair of the adjacent ring oxygen. Anomeric radicals therefore undergo addition to n-systems when the high-lying SOMO can interact with a low-lying LUMO as in electron-deficient Radicals in Organic Synthesis Edited
So findet man in [Mo4(NO),(S2),0]'l l l a l und [ M O~( N O )~( S~)~( S )~]~-2[lb1, fur die nahezu gleiche Stochiometrie gilt, vier verschiedenartig koordinierte S:--Liganden. Sake von 1 und 2 lierjen sich unter wenig unterschiedlichen Bedingungen durch Reaktion von ( M~N O }~+ -K o m p l e x e n~~~ mit S$-bzw. H2S erhalten. Es ist uns jetzt gelungen, den neuen diamagnetischen Komplex [ M o~( N O )~( S~)~( S~) O H ]~-3 (Fig. la) in Kristallen der (NH&[Mo2(NO),(S2),(S5)OH] -2 H 2 0 3b zu isolieren und durch Elementaranalyse, ESCA-, UVNIS-, IR-''] und Raman-Spektroskopie sowie magnetische Messungen und vollstilndige Rbntgen-Strukt~ranalyse[~~ zu charakterisieren. In dem Zweikernkomplex 3 fungieren je eines der Anionen OH-, SZ-(,,dachfiirmig" koordiniert) und S:-als BrOckenliganden. Ein Komplex mit einem verbriickenden S:--Liganden war bisher nicht bekannt. Die pentagonalbipyramidale Koordination der Molybdiinatome, die auch in 1 und 2 vorkommt, wird durch einen ,,side-on" gebundenen Sg-und einen NO-Liganden vervollstiindigt. Die ilquatoriale Ebene der Bipyramide wird hierbei nur durch Schwefelatome gebildet. Strukturchemisch besonders interessant ist, darj die achtgliedrige heterocyclische Einheit (S5M02(S2)J die Gestalt des Cyclooctaschwefels hat (vgl. Fig. lb); das achte Sake K,.~(NH3,.51M02(NO)2(S*)3(S~)OHl.2H~0 3a und 2 s2Ringatom (S? wird hierbei im Schwerpunkt der S:--Gruppe angenommen. (Mit dieser Art der Betrachtung von S:--Liganden IieDe sich vielleicht eine Systematik der Strukturchemie von S$--Komplexen entwickeln!) Die Bildung von 3 erscheint dadurch begunstigt, darj zurn einen die pentagonal-bipyramidale Umgebung der Zentralatome, die for (MONO}' +-Komplexe hilufig beobachtet wird, nicht gestort und zum anderen eine Kronenform des (S5M02(S2)}-Heterocyclus wie in Ss erreicht wird. Wir erwarten, daB es moglich sein wird, weitere Metallkomplexe rnit ,,Einheiten" henustellen, die formal Derivaten des Cyclooctaschwefels entsprechen.Bemerkenswert ist, daD sich 3 erst nach llngerer Zeit bildet (im Gegensatz zu 1 und 2), wobei wahrscheinlich die Oxidation von S$zu S:-durch Luftsauerstoff eine wesentliche Rolle spielt (vgl. Arbeitsvorschrift['l). [3] Hauptabsorptionsbanden im IR-Spektrum (Anionen-Schwingungen von 3b; CsI-ReDling): ca. 3580 (v(0H)). 1560 s (v(NO)), 810 m (v(Mo(OH)Mo)), 608 m (v(MoN); S(NO)), 521 m (NSz)), 474 w/425 m (v(S5)), 341/315 m (v(MoS)) cm-'. [4] 3b kristallisiert monoklin in dcr Raumgruppe PZl/n, (1 = 11 14.8(4). b -1588.0(5),c-1206.7(4) pm,p=97.09(3)", 2-4; R-0.065 fllr 2587 un-abhPngige Reflexe (Syntex P21; M o~) . Die Einkristallstrukturanalyse am isostrukturellen 38 fllhrt praktisch zur gleichen Struktur des Anions 3. [5] Arbeitsvorschrift: Ein Gemisch aus 2.5 g (NH4)6M0702,.4H20, 2.1 g NH20H. HCI und 15 g KSCN b m . 15 g N H d C N (3. bzw. 3b) in 20 mL Wasser wird 1 h unter Riihren auf 80°C erwlrmt. Die Msung, die [MO(NO)(NH,O)(NCS)~]~-[2] enthllt. wird nach Zugabe von 55 mL Ammoniumpolysulfid-LBsung [ erhalten durch Einleitung von HZS (a. 1.5 h) in eine Suspension von 9 g Schwefel...
m.p. =58-59.5"C) (cyclooctacosane 3 : m.p. =47-48"C, cyclohexatetracontane 13 : m. p. = 8 1 -83 "C).In the I3C-NMR spectrum of l b (100.6 MHz, CDC13, 30°C) the signals at 6=29.65 and 29.61 appear for the 28and 46-membered ring, respectively. Compared to the two isolated macrocycles, the signals for the 28-membered ring and 46-membered ring are shifted to lower field by 0.52 ppm and 0.11 ppm, respectively. A similar shift has previously been observed for the 28-membered ring of another catenanel7I. These shifts to low field are most probably caused by van der Waals' interactions. This explanation is consistent with the smaller shift of the 46-membered ring, since in this case the interactions are distributed over more carbon atoms.The electron impact (EI) mass spectrum (70 eV, 240"C, direct inlet) displays the molecular ion of catenane l b as a low intensity peak (m/z 1036,0.7% of the base peak at m/z 57). The two macrocyclic components of l b are represented by peaks of relatively high intensity at m/z 644 (13%) and m/z 392 (18%), respectively. H-Transfers between the two macrocycles, as frequently observed in catenanes'", are indicated by the occurrence of ( M e -H) and ( M e -H2) peaks for both macrocycles. By analogy to ionmolecule reactions of small it must be assumed that these ions are formed by loss of H2 from intermediate, unstable, protonated macrocycles.The CI mass spectrum with methane as reactant corroborates the results of the EI spectrum.
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