Stannylenes synthesis

Another synthesis, described in detail in Scheme 12, was devised specifically for the introduction of deuterium at both ends of l-deoxy-D-t/ireo-pentulose.21,22 Stannylene methodology was used twice, first for glycol splitting with phenyliodonium diacetate, under strictly neutral conditions (necessary to preserve the benzylidene acetal), and secondly to convert the sequence -CHOH-CD3 to CO-CD3 by brominolysis. The final, labeled pentulose was l-deo y-D-threo-(1-2H3, 5-2H)pentulose. 

Scheme 42 Synthesis of a stannylene by desulfurization of the corresponding tetrasul-fide...

The synthesis of doubly bonded tin compounds by the coupling of stannylenes, however useful, is limited by the need for a stable stannylene and often a second divalent species (for example, a carbene or isonitrile). The simplest example of this reaction is the formation of tetrakis[bis(tri-methylsilyl)methyl]distannene from two molecules of the corresponding stannylene,83 with which it is in equilibrium in solution as evidenced by NMR spectroscopy.91... 

Ligand transfer from tin(IV) to tin(II) compounds with appropriate ligands such as chlorine (d) has been used infrequently in the synthesis of stannylenes. The only reactions reported so far have been performed with oxygen as substituents at tin (see also Sect. 6.4.3). 

The synthesis of asymmetrically substituted stannylenes is most efficiently achieved by ligand exchange reactions between two stannylenes, SnXj and SnY2 (see Chapter 4). For example, the stannylenes [(Me3Si)2NJClSn and (C5H5)ClSn can be synthesized according to Eqs. (36) and (37) 86,93 94). 

Decomposition with the elimination of arsenic or phosphorus ylide and formation of silylene (germylene, stannylene) Me2E14, which represents the process inverse to the synthesis of these betaines. 

Hodosi and Kovac reported the highly efficient P-mannosylation, which makes use of mannose-derived 1,2-O-stannylene acetal 15 in combination with an aglycon-derived triflate (Scheme 7.13).58 A remarkable feature of this method is its extreme simplicity. Even totally unprotected mannose can be used as precursor of 15. Undoubtedly, this method can be seen as one of the most facile methods for the stereoselective synthesis of P-mannosides. 

Soon after the isolation of 136, Tokitoh et described the synthesis of the first kinetically stabilized diarylstannylene stable in solution, that is, Tbt(Tip)Sn (169), by treatment of TbtLi with stannous chloride followed by addition of TipLi (Scheme 14.74). Under an inert atmosphere, stannylene 169 was found to be quite stable even at 60 °C in solution, and it showed a deep purple color (A,max =561 nm) in hexane. The Sn NMR spectrum of 169 showed only one signal at 2208 ppm, the chemical shift of which is characteristic of a divalent organotin compound as in the case of a monomeric dialkylstannylene (136). The bandwidth and the chemical shift of 169 were almost unchanged between —30 and 60 °C, indicating the absence of a monomer-dimer equilibrium. 

Tokitoh and co-workers further succeeded in the synthesis of overcrowded diarylstannylenes, Tbt(Tcp)Sn (170 Tcp = 2,4,6-tricyclohexylphenyl) and Tbt (Tpp)Sn [171 Tpp = 2, 4,6-tris(l-ethylpropyl)phenyl], by the exhaustive desulfurization of the corresponding tetrathiastannolanes with a trivalent phosphine reagent (Scheme 14.74). Since only a few convenient precursors have been available for the generation of stannylenes, this new method should provide us with a useful synthetic route for a variety of overcrowded stannylenes. The successful synthesis of a series of Tbt-substituted diarylstannylenes enabled the systematic comparison of their electronic absorptions with those of the previously reported overcrowded diarylstannylenes, which led to the elucidation of the substituent effect on the n—transition of stannylenes. 

The progress in the chemistry of plumbylenes is far behind those of its lighter congeners (silylenes, germylenes, and stannylenes). Most of the recent reports on plumbylenes are describe their synthesis and structural analysis there have been very few descriptions of their reactivities. 

Stannylenes are oxidized to keto-alcohols by dropwise addition of bromine in di-chloromethane. The reaction proceeds at room temperature at the speed of a titration. The attention of chemists who want to check their reaction by infrared (IR) examination in situ is drawn to the fact that the ketone may be chelated to tin in these conditions (vqj 1685 cm-1). The reaction is regiospecific, giving only one of the two possible keto-alcohols. It has been used in total synthesis for instance, in the synthesis of (+)-spectinomycin (described under Sec. QI.G) [12,13]. The replacement of bromine by IV-bromosuccinimide as oxidant has been reported [14]. 

The generation of hydroxy ketones by the brominolysis of stannylenes has been used several times in total synthesis. Experiment G [see Eq. (15)] describes a key step in the total synthesis of the antibiotic (+)-spectinomycin [12,13]. It is remarkable that the two oxygen atoms bound to the tin atom originate from hydroxyl groups, which are port of different functions, a hemiketal and a secondary alcohol. The oxidation is selective for one alcoholic function ont of three. The same product was obtained by W-bromosuccinimide oxidation of the tributylstarmyl ether. 

Direct synthesis of complexes of structure type B from germylenes, stannylenes and plumbylenes is possible in several cases. Many complexes of the very sterically hindered metallenes M[CH-(SiMe3)2]2 (M = Ge, Sn, Pb) have been obtained (Scheme 1).2 27... 

The ability of allyltin halides to extend their coordination sphere allowed the preparation of chiral hypervalent complexes with diamine ligands, which have been efficient in the asymmetric synthesis of homoallylic alcohols with up to 82% ee100. Similarly, a chiral hypervalent allyltin was prepared from a low valent tin (II) catecholate, chiral dialkyl tartrate and ally lie halide101. The allylation of aldehydes and activated ketones proceeded with high enantiomeric excess. Allyltins prepared from Lappert s stannylene and allylic halides were shown to be efficient as well, although the Lewis acid character of the tin atom is much less marked in that case102,103. 

Intramolecular Williamson reaction.1 The final steps in a total synthesis of octosyl acid A (1) from ribose involve a cyclization of 2 to 3 via a 2, 3 -stannylene... 

Acid-catalysed alkylation of an alcohol with O-alkyl trichloroacetimidate prepared from allyl alcohol and trichloroacetonitrile is readily accomplished Scheme 4.233]440 as previously discussed for the preparation of benzyl and tert-butyl ethers.311 However, these conditions are not compatible with many of the protecting groups employed in oligosaccharide synthesis. For such cases, two methods for 0-allylation under essentially neutral conditions have been devised. The first method takes advantage of the mild conditions and regioselectivity of stannylene alkylations (see section 4.3.3). The method is illustrated by the selective O-allylation of o-lactal, which began with stannylene formation on an 0.8 mole scale [Scheme 4.234].441... 

---