Stannyl ethers oxidation

An oxidative Prins cyclisation of the allylsilane-tethered a-stannyl ethers offers a route to tetrahydropyrans avoiding the need for catalysis by a Lewis acid (Scheme 17) . 

A bromide was introduced in the reaction instead of a fluoride in performing the anodic oxidation of a-stannyl ethers in dibromomethane solvent with tetrabutyl-ammonium perchlorate as the electrolyte (Scheme 19) [28]. The bromide ion was generated by the reduction of the solvent at the cathode of an undivided cell. 

Selectivity can be an overriding commodity in cases where reactivity is dictated by logic and accepted concepts. Such is the case with stannylene acetals of diols and trialkyl-stannyl ethers of alcohols. Enhanced nucleophilicity of oxygen attached to tin and well-documented stereoelectrorric effects associated with methine carbon atoms of trialkyhin ethers lead to remarkably selective reactions of (7-substitution and oxidation in polyhydroxy compounds. 

Selective O-Substitution and Oxidation Using Stannylene Acetals and Stannyl Ethers... 

S. David, Selective O-Substitution and Oxidation Using Stannylene Acetals and Stannyl Ethers, in Preparative Carbohydrate Chemistry, S. Hanessian (Ed.), Marcel Dekker, New York, 1996. 

David, S. Selective O-substitution and oxidation using stannylene acetals and stannyl ethers. In Preparative Carbohydrate Chemistry. Hanessian, S. (ed.) Marcel Dekker New York, 1997, pp. 69-84. 

Concomitant cyclisation and bromination are achieved during the anodic oxidation of unsaturated a-stannyl ethers in a tetrabutylammonium perchlorate dibromomethane electrolyte (94CC2361). 

Silyl enol ethers, 23, 77, 99-117,128 Silyl enolates, 77 Silyl peroxides, 57 Silyl triflate, 94 Silyl vinyl lithium, 11 (E)-l -Silylalk-1 -enes, 8 Silylalumimum, 8 Silylation, 94 reductive, 26 a-C-Silylation, 113 O-Silylation.99,100 / -SilyIketone, 54 non-cydic, 55 Silylmagnesium, 8 Silyloxydienes, 112 Sodium hexamethyldisilazide, 89 Sodium thiosulphate pentahydrate, 59 Stannylation, see Hydrostannylation Stannylethene, 11 (Z)-Stilbene, 70 (E)-Stilbene oxide, 70 /3-Styryltrimethylsilane, 141 Swern oxidation. 84,88... 

Further evidence for the above-mentioned mechanism of HOMO elevation by group 14 elements is provided by studies of thioethers. The decrease in oxidation potential of silyl ethers as compared to ethers is not realized in the case of a-silylthioethers whereas a-stannyl substituents in thioethers cause a considerable cathodic shift in oxidation potential. Moreover, the effect is geometry-dependent. Values for substituted cyclic dithianes 15 are summarized in Table 21. The difference between Si and Sn in this case is illustrative. The lone nonbonding pair in the 3p orbital of sulfur is much too low in energy compared to... 

The DFT study of the 3 + 2-cycloaddition between ketene and TV-silyl-, IV-germyl-, and TV-stannyl-imines shows that the TV-germylimine reaction is a two-step process the TV-stannylimine reaction is a competition between two- and three-step processes whereas the TV-silyl process follows a three-step process44 A new and convenient synthesis of functionalized furans and benzofurans based on 3 + 2-cycloaddition/oxidation has been reported. The cyclization of cyclic 1,3-bis-silyl enol ethers (48) with l-chloro-2,2-dimethoxyethane (49), via a dianion, produced 5,6-bicyclic 2-alkylidenetetrahydrofurans (50), which are readily oxidized with DDQ to 2,3-unsubstituted benzofurans (51) (Scheme 13)45 The Evans bis(oxazoline)-Cu(II) complex catalyses the asymmetric 1,3-dipolar cycloaddition of a -hydroxyenones with nitrones to produce isoxazolidines.46 The... 

Anodic oxidation of homo allyltrimethylsilylmethyl ethers 238 or homo allyl trimethyl-stannyl methyl ethers in the presence of tetrabutylammonium tetrafluoroborate results in the formation of fluorine- containing tetrahydropyrans 239249(equation 131). The process involves formation of a resonance stabilized carbocation and its intramolecular cycliza-tion by the participation of a neighboring vinyl group, followed by attack of fluoride ion. This process is a convenient way to form the C—F bond involving electrochemical steps. 

First, the utility of organostannyl compounds for generating carbo-cations or carbon radicals is discussed. Oxidation of organostannyl compounds such as a-stannyl sulfides, amines, esters, and ethers with metallic oxidants or photochemical methods gives their cation radicals, from which carbocations and carbon radicals are generated by cleavage of the carbon-tin bond. These reactive intermediates are employed for carbon-carbon bond formation, particularly in intermo-lecular reactions. 

The reaction proceeds as follows a cation radical CR16 initially formed by one-electron oxidation fragments into 2-dithianyl cation C16 and a stannyl radical, and the cation C16 reacts with the silyl enol ether (Scheme 11). Formation of the stannyl radical was confirmed by trapping the stannyl radical with carbon tetrabromide to give tributyl-stannyl bromide. 

In the electrochemical oxidation, similar reaction was observed (Scheme 12). Cation radical CR26 generated by electrochemical oxidation of a-stannyl sulfides cleaves to give carbocation C26, which react with allyltrimethylsilane or the silyl enol ether of cyclohexanone to give the usual addition products. In this electrochemical reaction, stannyl derivatives also afforded the desired product 27 or 28 in better yield compared with the corresponding silyl derivatives. 

The low oxidation potentials of stannyl compounds gave us a clue to develop the selective cross-coupling reaction between the a-posi-tions of two carbonyl compounds. Generally, silyl enol ethers or ketene silyl acetals are employed for this purpose to prepare 1,4-di-... 

This vanadium method enables the cross-coupling only in combinations of silyl enol ethers having a large difference in reactivity toward radicals and in their reducing ability. To accomplish the crosscoupling reaction of two carbonyl compounds, we tried the reaction of silyl enol ethers and a-stannyl esters based on the following consideration. a-Stannyl esters (keto form) are known to be in equilibrium with the enol form such as stannyl enol ethers, but the equilibrium is mostly shifted toward the keto form. When a mixture of an a-stannyl ester such as 45 and a silyl enol ether is oxidized, it is very likely that the stannyl enol ether will be oxidized preferentially to the silyl enol ether. The cation radical of 45 apparently cleaves immediately giving an a-keto radical, which reacts with the silyl enol ether selectively because of the low concentration of the stannyl enol... 

The stemona alkaloid stemonamide (49) was synthesized starting from a-stannyl acetate 47 and 2-stannyl pyrrolidine 48. The oxidative coupling of stannyl acetate 47 with acetylenic silyl enol ether affords the functionalized C-7 unit which corresponds to the side arm of the pyrrolidine ring. Then, introduction of the C-7 unit to the pyrrolidine ring is performed by the oxidative generation of acyliminium ion. The carbon skeleton of stemonamide was thus constructed efficiently as shown in Scheme 19 by employing organotin compounds. ... 

The p-stannyl silylenol ether is a useful protection device for a, -enones, as the ethers are relatively unreacdve towards most nucleophiles and are reconverted to the enone on mild oxidation. This form of protection was developed and employed in the acquisition of a crucial disubsdtuted cyclohex-2-enone (7) required in the synthesis of ( )-periplanone-B, a sex pheromone of the American cockroach. Similarly, this sequence was successful in effecting (Z, ) to ( , ) isomerization of isoacorageimacrone (8) to acoragermacrone (9), when other methods e.g. photoisomeiization) failed (Scheme 10). ... 

---