Dithians Exchange

The synthesis starts simply with the coupling of the lithium derivative of 44 with 39 but takes a novel turn when a dithian exchange with pyruvic acid is used to remove the dithian from 45. The keto group in pyruvic acid is unstable, being next to another carbonyl group, while that in 39 is stable. Dithian exchange removes this instability by forming the pyruvic acid derivative 46. 

Sulfur transfer can also occur from sulfurated borohydride exchange resin <2001TL6741> the reaction proceeds in methanol at ambient temperature rapidly (<5min) and in high yield (91%). From l,4-di(acetylthio)butane, 1,2-dithiane can be obtained by a one-pot synthesis in excellent yield (98%) using BuzSn(OMe)2, FeCls, and a catalytic amount of CsF as a promoter in THF solution <1990TL3595>. 

Fluorination of thiophene and derivatives and of 1,4-dithiane [241] with KC0F4 gives a series of fluorinated derivatives. Chlorine-fluorine exchange in sulphides is also possible in some cases [242] (Figure 8.86). 

It has also been shown that the Sn/Li exchange at the 2-position of 2-stannyldithiane takes place within minutes at -78 C, whereas H/Li metallation does not occur at all at this temperature (Scheme 59, entry b). This reaction, when applied to 2-(u>-oxidoalkyl)-2-stannyl-l,3-dithiane, takes place within a few minutes and the intramolecular nucleophlic reaction, which immediately follows, is usually complete within 0.1 h at -78 C. The total yield of cyclization products by the tin route has proved to be twice as high as that by direct metallation (Scheme 68). ... 

Zincio-I,3-dithianes are valuable reagents. Based on reactions of organozincs 4-acetoxy-l,3-dioxanes serve as precursors of anti-l,3-diols." A synthesis of 1,1-diarylallenes involves Pd-catalyzed coupling of the organozinc species which are derived from 1-arylpropynes by lithiation and Li/Zn exchange with aryl halides. By an sp -sp bond formation process to introduce a dienyl substituent to C-2 of 2-cyclopentenones a facile synthesis of nakienone-A was developed. - ... 

Dithianes and gemdithioacetals could be alternatively oxidized indirectly by means of the redox catalysis method. The technique appeared to be particularly mild and mainly avoided inhibition and adsorption phenomena relative to the anode platinum interface. Thus aromatic hydrocarbons (e.g. 9,10-diphenylanthracene) [83] and judiciously substituted triphenylamines [84] afford quite stable cation radicals used homogeneously as oxidants. Their standard potential, E°x, will determine the rate of electron exchange with the concerned sulfur compound. The cleavage of a C—S bond in any dithiane can be regarded as fast enough to draw the redox catalysis process to the indirect oxidation. 

As a further test, we attempted to convert the 14-OH group to hydrogen in 10, to confirm that the 14-OH group participated in the abstraction of the 2 -hydrogen of dithiane 10. Compound 16 was synthesized from naltrexone methyl ether [22] and then subjected to the same acetal exchange reaction as above. Under the same conditions, acetal 18 was obtained instead of the objective ortho ester (Scheme 4), consistent with our hypothesis that the 14-OH group participated in the ortho ester formation. Subsequently, the obtained ortho ester 12 was readily hydrolyzed with acid to give ester 19 (Scheme 5). 

Scheme 4 Acetal exchange reaction of the 14-H compound 16. Reagents and conditions (a) n-BuLi, 1,3-dithiane, DME, -70°C, 49% (b) CuC12-2H20, CuO, CH(OMe)3, MeOH, 50°C, 63%...

The dithiane group can be converted into the dioxolane group by exchange with dry ethylene glycol-THF-mercuric chloride.11... 

Stork reagent) 1,3-dithiane cleavage, 328-329 Iodoalkanes. See Haloalkanes Iodoetherification, 272, 326 Iodolactones. See Halolactones Ion-exchange resins. See Benzene, ethenyl- ... 

CYCLIZATION n-Butyllithium. Hydrobromic acid. Hydrogen fluoride. Ion-exchange resins. 2-Lithio-2-trimethylsilyl-l,3-dithiane. Lithium diisopropylamide. Polyphos-phoric acid. Tetra-n-butylammonium iodide. Stannic chloride. Trifluoroacetic... 

Addition of an aUcyllithium to the aldehyde of 132, followed by cation exchange to the potassium alkoxide, led to 1,5-Brook rearrangement. The resulting potassium dithiane 133 was allylated, providing masked y-hydroxyketone 134 in good overall yield. 

The protonation of 1,3-dithian in FSO3H has been thoroughly studied. At slow S—H exchange, the monoprotonated species (151) appeared to be predominantly equatorial. The formation of a hydrogen-bonded complex between 1,3-dithian and FSO3H has been suggested in order to explain the observation that... 

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