Hydroxylation tributyltin

A similar approach with [l- C]tributyltin acetate showed that carbon hydroxylation occurred at the a-, as well as at the /8-, carbon atoms, followed by Sn-C bond-cleavage, to afford dibutyltin derivatives. 

Quebrachitol was converted into iL-c/j/roinositol (105). Exhaustive O-isopropylidenation of 105 with 2,2-dimethoxypropane, selective removal of the 3,4-0-protective group, and preferential 3-0-benzylation gave compound 106. Oxidation of 106 with dimethyl sulfoxide-oxalyl chloride provided the inosose 107. Wittig reaction of 107 with methyl(triphenyl)phos-phonium bromide and butyllithium, and subsequent hydroboration and oxidation furnished compound 108. A series of reactions, namely, protection of the primary hydroxyl group, 0-debenzylation, formation of A-methyl dithiocarbonate, deoxygenation with tributyltin hydride, and removal of the protective groups, converted 108 into 7. 

The 8-methyl-8,14-cycloberbine 364, derived from the protoberberine 324 via the betaine 363, was reduced with sodium borohydride or lithium aluminum tri-tert-butoxyhydride to give a diastereoisomeric mixture of cis-and trans-alcohols (7.8 1 or 1 7.8, respectively) (Scheme 64).t)n exposure to formaldehyde the mixture underwent N-hydroxymethylation and subsequent intramolecular substitution on the aziridine ring to give the oxazolidine 365. Removal of the hydroxyl group in 365 was accomplished by chlorination followed by hydrogenolysis with tributyltin hydride. Reductive opening of the oxazolidine 366 with sodium cyanoborohydride afforded ( )-raddeanamine (360), which has already been converted to ochotensimine (282) by dehydration. 

The deuterium KIE of 2.5 0.1, observed at 297 K in the cleavage of tributyltin allylsulphinate with [2H]TFA in toluene435 (equation 227) has been interpreted as suggesting a rigid compact and reactant-like early TS, and as confirming the concerted retro-ene mechanism of thermal desulphination of allylsulphinic acid (equation 228). The mechanism of hydroxyl radical-induced decarboxylation of 2-(alkylthio)ethanoic acid... 

Generating tin hydride in situ by using a catalytic amount of tributyltin chloride and sodium cyanoborohydride [24] did not solve the problem. Although this method inherently produces the desired low concentration of n-BujSnH to promote cyclization over direct reduction of the starting material and reduces drastically the amount of tin residue, a displacement reaction between solvent and the benzylic hydroxyl group takes place. Eventually, the best method was the use of high dilution conditions at approximately 70 °C in benzene followed by the KF workup. 

Cyclic thiocarbonates offer another class of substrates for radical deoxygenation (Scheme 3.9b). In particular, thiocarbonates formed from a diol derived from a primary and secondary hydroxyl are of particular interest, since they can be deoxygenated regioselectively with tributyltin hydride and AIBN.53 In these cases, the secondary position is deoxygenated owing to the higher stability of secondary over primary radicals. As expected, radical reduction of thiocarbonates derived from two secondary hydroxyls leads to a mixture of deoxygenated isomers.52b 53... 

Intramolecular substitutions offer a convenient and stereoselective method for the introduction of amino functionalities. A strategy for the preparation of vicinal m-hydroxy amino moieties entails the halocy-clisation of allylic trichloroacetimidates.77 Conversion of the hydroxyl of unsaturated sugar derivatives into a trichloroacetimidate, followed by Ar-bromosucciniinide (NBS) or JV-iodosuccinimide (NIS) mediated intramolecular cyclisation, gives bromo- and iodo-oxazoline derivatives (Scheme 3.13a). The oxazoline can be hydrolysed with mild acid to unmask the amino functionality, and the halogen can be removed by treatment with tributyltin hydride. 

As with peracids, hydroxylation is not the only useful oxidation possible. Dihydroxyarenes are readily converted to quinones using hydrogen peroxide in combination with a wide range of metal species 486 An interesting example is the use of supported chromium oxide-bis(tributyltin) oxide catalyst in the manufacture of vitamin Ki (Figure 3.115).478... 

Radical chemistry has been widely exploited for the modification of carbohydrates. In particular, tributyltin hydride-promoted deoxygenation provides a convenient method for the effective removal of hydroxyl groups without intervention of other functionalities. Stereoselective carbon-carbon bond formation at the anomeric center has also revealed its apphcability for the preparation of carbon analogues of 0-glycosides. The majority of this work has been reviewed in earlier publications and hence will not be covered in this account. Instead a selection of newer appHcations is provided here, which have been categorized according to the type of radical reaction carried out. 

Another modification made off the C-2 position of 1 is the trichloroacetimidate derivate (104), which was prepared by treating 57 with trichloroacetonitrile and DBU in C1CH2CH2C1 at 0 °C to yield product 104 in a 37% yield (Scheme 16) [39], Efforts were also made to remove the C-2 hydroxyl of 57 entirely. Hydroxyl 57 was treated with 1,1-thiocarbonyldiimidazole and catalytic DMAP in CH2C12 to afford imidazole 105 in 64% yield [50], Imidazole 105 was then refluxed with 2,2 -azobis (2-methylpropionitriIe) (AIBN) and tributyltin hydride in toluene to bestow C-2 dehydroxylated 57 in a C-8 epimeric mixture of C-8 a-H and p-H in 13 and 27% yield (106 and 107), respectively [50], Additionally, dilactone 108 was obtained by treating 57 with a mixture of chromium trioxide and pyridine in CH2C12 in a 37% yield [50],... 

Diol 164 was treated with l,l -thiocarbonyldiimidazole to produce the cyclic thiocarbonate (168) product in 67% yield over two steps from salvinorin A (Scheme 26) [49]. Unfortunately, these conditions were found to cause epimerization at C-8, the products of which were inseparable on silica gel. However, this meant that initial separation of the C-8 epimers of the c/s-diol starting material was not necessary. Radical reduction (tributyltin hydride, AIBN, toluene, 80 °C) of the epimeric thiocarbonate mixture afforded the separable C-l deoxy products in 47% combined yield. Subsequent acetylation of the C-2 hydroxyl under standard conditions (Ac20, pyridine) gave 1-deoxysalvinorin A (169) in 82% yield. 

Reductive radical elimination (Barton deoxygenation) of the 3, 4 -xanthate derivative of neamine (70) has been used to prepare the key intermediate 71 (compounds 70, 71) for the synthesis of the 3, 4 -dideoxyneamine analog gentamine Cia [52]. In this synthesis, a precursor possessing free hydroxyl groups at positions 3 and 4 was treated with a mixture of carbon disulfide, aqueous sodium hydroxide, and methyl iodide in DMSO to afford compound 70. Subsequent reduction of 70 with tributyltin hydride furnished the olefin 71, which affer cafalyfic hydrogenation and deprofecfion was converted to gentamine Cia. 

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