Stannyl Subject

When dienones such as 55 are subjected to the epoxidation conditions the electron-poorer C=C double bond is selectively epoxidized. The other C=C bond can be functionalized further, for example, it can be dihydroxylated, as shown in the synthesis of the lactone 56 (Scheme 10.11) [82]. Stannyl epoxides such as 57 (Scheme 10.11, see also Table 10.8, R1 = n-Bu3Sn) can be coupled with several electrophiles [72], reduction of chalcone epoxide 58 and ring opening with alkyl aluminum compounds provides access to, e.g., the diol 59 and to phenylpropionic acids (for example 60). Tertiary epoxy alcohols such as 61 can be obtained with excellent diastereoselectivity by addition of Grignard reagents to epoxy ketones [88, 89]. 

While germyl cations are not known in aqueous solutions, reports on the formation of stannyl cations appeared as early as 192324. Since then, numerous investigations25-27 established that ions of the type R3M(OH2)n+ (M = Sn, Pb) can be prepared in aqueous solutions. The early literature on this subject was reviewed in 1966 by Tobias28. Among earlier studies one should mention a series of publications by the group of Rabenstein29,30 who studied plumbyl cation complexes in aqueous solutions by H NMR. 

Michael addition of trialkylstannyllithium to cyclohexenone SAMP- or RAMP-hydrazone gives 3-stannyl derivatives with de values of 42-44%, while subsequent alkylation of the enolate formed affords trans-products with de values >96%. The hydrazones obtained are subjected to ozonolysis to give 3-stannylcyclohexanones trans-1 with ee values up to 96%17. 

Protic-acid-catalyzed Michael additions (59) are subject to most of the limitations of base-catalyzed Michael additions (regioselectivity and stereoselectivity of enol generation, polyaddition, etc.), and hence, the stereochemistry has been little studied (60). At low temperatures silyl and stannyl enol ethers,+ ketene acetals, and allyl species are unreactive to all but the most reactive activated olefins. However, it was discovered by Mukaiyama and co-workers that enol ethers and ketene acetals react with a,/f-unsaturated carbonyl compounds in the presence of certain Lewis acids (4,61,62). Sakurai, Hosomi, and co-workers found that allylsilanes behave similarly (5,63,64). 

Transmetallation can be employed in order to avoid the use of strongly basic conditions. One such variant is the [2,3]-Wittig-Still rearrangement wherein stannyl ethers can be converted to homoallylic alcohols. Several examples of this tranformation in the synthesis of amino acid components of bioactive polyoxins have been reported by Ghosh. In their synthesis of 5-0-carbomylpolyoxamic acid, a bioactive amino acid nucleoside, E and Z-allylic stannyl ethers, such as 45, derived from an isopropylidene L-threitol derivative, were subjected to the [2,3]-Wittig-Still rearrangement. 

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