Tributyltin stereoselectivity

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Silyl ethers serve as preeursors of nucleophiles and liberate a nucleophilic alkoxide by desilylation with a chloride anion generated from CCI4 under the reaction conditions described before[124]. Rapid intramolecular stereoselective reaction of an alcohol with a vinyloxirane has been observed in dichloro-methane when an alkoxide is generated by desilylation of the silyl ether 340 with TBAF. The cis- and tru/u-pyranopyran systems 341 and 342 can be prepared selectively from the trans- and c/.y-epoxides 340, respectively. The reaction is applicable to the preparation of 1,2-diol systems[209]. The method is useful for the enantioselective synthesis of the AB ring fragment of gambier-toxin[210]. Similarly, tributyltin alkoxides as nucleophiles are used for the preparation of allyl alkyl ethers[211]. 

Although in the recent years the stereochemical control of aldol condensations has reached a level of efficiency which allows enantioselective syntheses of very complex compounds containing many asymmetric centres, the situation is still far from what one would consider "ideal". In the first place, the requirement of a substituent at the a-position of the enolate in order to achieve good stereoselection is a limitation which, however, can be overcome by using temporary bulky groups (such as alkylthio ethers, for instance). On the other hand, the ( )-enolates, which are necessary for the preparation of 2,3-anti aldols, are not so easily prepared as the (Z)-enolates and furthermore, they do not show selectivities as good as in the case of the (Z)-enolates. Finally, although elements other than boron -such as zirconium [30] and titanium [31]- have been also used succesfully much work remains to be done in the area of catalysis. In this context, the work of Mukaiyama and Kobayashi [32a,b,c] on asymmetric aldol reactions of silyl enol ethers with aldehydes promoted by tributyltin fluoride and a chiral diamine coordinated to tin(II) triflate... 

A high degree of stereoselectivity was achieved in reductive radical cyclizations with Coordination of the oxime function (e.g. 108) with samarium cation seems to play an important role, since the identical reaction with a tributyltin hydride/radical initiator system produces poor stereoselectivity (equation 79). ... 

Barton-McCombie deoxygenation is not always stereoselective the diastereo-meric ratios strongly depends on the nature of the protecting groups and of the ester moiety. However, in 2-C-trifluoromethyl-2-deoxyfuranose, the a compound is the major product of the reaction, due to steric hindrance of this a side. In 3-C-trifluoromethyl-3-deoxyfuranose, deoxygenation by tributyltin hydride yields only the a product, if it is performed with oxalate instead of thiocarbonate. Another possibility to obtain this selectivity is to perform the reaction with 1,2,5,6-di-O-isopropylidene-a-D-glucofuranose (Figure 6.34). ... 

R. Herranz, J. Castro-Picliel, and T. Garcia-Ldpez, Tributyltin cyanide, a novel reagent for the stereoselective preparation of 3-amino-2-hydroxy acids via cyanohydrin intermediates, Synthesis p. 703 (1989). 

D-glucal (10) and di-(tributyltin) oxide in dry benzene are refluxed using a Dean-Stark trap. Under these conditions stannyl complex 36 of the D-glucal is formed. Ogawa was the first to report on the stereoselective stannylation.12... 

Annelation.1 This reagent is used for conversion of 3-substituted aldehydes (or acetals) into methylenecyclohexanes. The conversion involves addition of the allylsilane group to the aldehyde or acetal to provide an adduct that undergoes radical cyclization via the allylic sulfide group. The phenylthio group is used to enhance 6-endo cyclization over the usual 5-exo cyclization. In addition it allows use of bis(tributyltin) as the initiator (14,173-174). Unfortunately the radical cyclization shows only slight stereoselectivity. 

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. 

In the second approach shown in Scheme 10, the vinyl radical was generated by addition of a tributyltin radical to the terminal acetylene group. In this case, the allyl radical formed in the first cyclization reaction might provide a large steric bias during the second ring closure and could thus impose a better stereoselectivity than in the first approach. The synthesis commenced from the same starting material 77 as in the synthesis of 78. The butenyl chain was attached first and the butynyl chain was introduced second to provide ketoester 83. 

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. 

The stereoselectivity observed in the cyclization of 4-unsaturated amides is dependent on the electrophilic halogen species. With /V-chlorosuccinimide the trans/cis ratio is reduced to 3 2, while with A -bromosuccinimide the fra .v-isomer is obtained in >98% purity. The structures of the iodolactones 4 were confirmed by reduction with tributyltin hydride to the trans- and rw-2.4-dimethy]-y-lactones. The high 1,3-trans selectivity strongly contrasts with the results obtained when 2-methyl-4-pentenoic acid is cyclized under thermodynamic conditions with iodine in acetonitrile. In this case a 92% yield and a 32 68 mixture of the f/ww/m-com pounds was obtained. 

---