Protective groups triple bonds

Russian chemists [228] found that trimethylsilyl groups protect adjacent triple bonds against hydrogenation with poisoned Pd-catalysts. A similar effect is shown in reductions of trimethylsilylated 1,3-diynes with (activated) zinc powder [226]. One disadvantage of the zinc method is that the zinc salts present in the reaction mixture can cause cleavage of the =C-Si bond (this was shown in a separate experiment in which a trimethylsilylated 1,3-diyne was heated with a solution of zinc bromide or chloride in ethanol [2]). It seems therefore important to keep reaction times of the reductions with zinc as short as possible and to activate the zinc powder with a limited amount of dibromoethane. 

Alkynes and Allenes.—Ethynyl steroids (239) readily form a stable hexacarbonyl-dicobalt complex (240) on treatment with octacarbonyldicobalt. The complex serves to protect the triple bond during reactions elsewhere (c.g. olefin reduction with di-imide, or hydroboronation). The ethynyl group is regenerated by treating the complex with iron(iii) nitrate in ethanol. ... 

The telomer obtained from the nitromethane 65 is a good building block for civetonedicarboxylic acid. The nitro group was converted into a ketone, and the terminal alkenes into carboxylic acids. The acyloin condensation of protected dimethyl dvetonedicarboxylate (141) afforded the 17-membered acyloin 142, which was modified to introduce a triple bond 143. Finally, the triple bond was reduced to give civetone (144)[120). 

This study demonstrates that the addition of the 2-diazopropane with the triple bond of propargyl alcohols is regioselective, and affords new antibacterial 3H-pyrazoles. The photochemical reaction of these 3H-pyrazoles selectively leads to a- and 6-hydroxy cyclopropenes. The overall transformation constitutes a simple straightforward route to substituted cyclopropenyl alcohols without initial protection of the propargyl alcohol hydroxyl group. 

With respect to the naked metal atoms, this is the largest metalloid duster that has ever been structurally determined by diffraction methods. The Ga2 unit in the center of the 64 naked Ga atoms is remarkable and unique in this entire field of chemistry [Figure 2.3-28(c)]. The Ga2 unit, which contains a bond that is almost as short (2.35 A) as the above-mentioned Ga-Ga triple bond (2.32 A) and resembles the Ga2 unit of a-Ga (2.45 A), is surrounded by a Ga32 shell in the form of a football with icosahedral caps [see d-Ga (Figure 2.3-17)]. The apex and base atoms of this Ga32 unit are naked and are oriented towards each other in the crystal in an unusual fashion (see below). The Ga2Ga32 unit is surrounded by a belt of 30 Ga atoms that are also naked . Finally the entire Ga framework is protected by 20 GaR groups. 

Both of the stereoisomeric DL-tetroses were obtained21 from 1,1-diethoxy-2-butyn-4-ol (18a). In two steps, involving acetylation of 18a and partial hydrogenation of the triple bond in derivative 18b, cis-4-acetoxy-l,l-diethoxy-2-butene (19) was prepared. ci.s-Hydroxylation of 19 with potassium permanganate, followed by acetylation, led to 20. Hydrolysis (basic, and then acidic) of the protecting groups yielded DL-erythrose (28%). 

Comparative hydrogenation experiments show that the addition of hydrogen to carbon-carbon double and triple bonds in rotaxane axles proceeds at a lower rate compared with the isolated axles [72], Functional groups of a linear molecule can thus be protected by rotaxanation. 

To obtain non-ionic siloxane surfactants, Wersig et al. [11] performed the hydrosilylation of a non-protected oUgoethoxylated but-2-yne dialcohol. Dioxane was applied as solvent, hexachloroplatinic acid as the reaction s catalyst at 100°C. The difference between the activation energies of dehydrocondensation and the triple bond hydrosilylation reactions can be increased by the use of an appropriate solvent, such as dioxane. The selectivity toward hydrosilylation has also been explained by the protective association of hydroxyl groups in coiled POE chains. Thus, in this case the hydrosilylation reaction has been carried out successfully without any protection. 

A novel substituent modification reaction involves the use of the Co2(CO)g moiety as an alkyne-protecting group. To induce carbon-carbon double bonds to undergo addition reactions selectively in the presence of carbon-carbon triple bonds, the triple bond can be rendered inert by reaction with Co2(CO)g. Addition reactions can then be carried out on the free olefinic group in the Co2(CO)g(RC2R ) complex. Finally, the carbonylcobalt group can be removed with ceric ammonium nitrate or ferric nitrate. An example (49) of the overall reaction sequence is given in Eq. (29). 

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