Triple-bond protection

Russian investigators9 found that a silyl substituent on one carbon atom of a triple bond protects this unsaturated center from hydrogenation. Schmidt and Arens,10 seeking to selectively reduce the central triple bond of I without disturbance of the two terminal triple bonds or of the two double bonds present, made use of the... 

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). 

The synthesis of the key intermediate aldehyde 68 is outlined in Schemes 19-21. The two hydroxyls of butyne-l,4-diol (74, Scheme 19), a cheap intermediate in the industrial synthesis of THF, can be protected as 4-methoxybenzyl (PMB) ethers in 94% yield. The triple bond is then m-hydrostannylated with tri-n-butyl-tin hydride and a catalytic amount of Pd(PPh3)2Cl238 to give the vinylstannane 76 in 98 % yield. Note that the stereospecific nature of the m-hydrostannylation absolutely guarantees the correct relative stereochemistry of C-3 and C-4 in the natural product. The other partner for the Stille coupling, vinyl iodide 78, is prepared by... 

Terminal silylation of alkynes affords two main benefits the triple bond is protected against chemical attack, either for steric reasons or because the potentially acidic proton is masked, and, perhaps paradoxically, the bond is activated towards regioselective electrophilic attack under certain conditions. 

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. 

Diethynylallenes, i.e. derivatives of the parent system 8 in Scheme 5.1, are obtained when the propargylic substrate already contains an additional triple bond as shown in Scheme 5.14. Here the bispropargyl carbonate 107 - for example with R = n-hexyl - is coupled with tris(isopropyl)silylacetylene to provide the protected hydrocarbon 108 in excellent yield (94%) [39, 40]. 

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. 

In addition to the synthesis of stable derivatives of 27, I also looked into the possibility of stabilizing 23 (Section 2, Scheme 6) and 19. The molecules I designed for this course were 124 and 125, in which the aromatic rings were expected to contribute a certain degree of stability. Furthermore, the triple bonds in 124 and 125 should be kinetically protected by the protons as illustrated. Most importantly, the length of the C9-C1Q bonds in the phenanthnene moieties should be approximately 1.34 A, which was exactly equal to the length of a carbon-carbon double bond (1.34 A). A combination of all these factors should furnish stable 124 and 125, which retain all the basic structural characteristics of 23 and 19, respectively. 

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. 

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%). 

Because enol ethers are more susceptible than triple bonds to electrophilic attack, the addition of alcohols to enol ethers can also be catalyzed by acids.170 One utilization of this reaction involves the compound dihydropyran (28), which is often used to protect the OH... 

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. 

The number of triple bonds of the heavier nonmetals that are known is considerably smaller—perhaps a dozen It has already been noted above that protecting a triple bond stcricaliy is considerably more difficult than for the case ol a corresponding double bond. One very interesting aspect of the C=S bond is that, in contrast to the CssC in acetylenes, the triple bond does not ensure linearity at the carbon atom (Fig. 18.2). The reasons are not completely clear but may be related to the nonplararity of RjGe—GeR, and R-Sn=SnR> (see page 863). 

The carbonyl Co2(CO)6 forms stable 7i-complexes of alkynes 0/2 complexes). Four effects on alkyne reactivity are expected from this coordination (i) protection of the triple bond (ii) stabilization of the carbonium ion on the a-carbon (or propargylic cations (iii) syntheses of common and medium-size cycloalkynes and (iv) steric effects. 

As regards the protecting effect, the complex is stable to Lewis acids. Also, no addition of BH3 occurs. As Co2(CO)6 can not coordinate to alkene bonds, selective protection of the triple bond in enyne 137 is possible, and hydroboration or diimide reduction of the double bond can be carried out without attacking the protected alkyne bond to give 138 and 139 [32], Although diphenylacetylene cannot be subjected to smooth Friedcl Crafts reaction on benzene rings, facile /7-acylation of the protected diphenylacetylene 140 can be carried out to give 141 [33], The deprotection can be effected easily by oxidation of coordinated low-valent Co to Co(III), which has no ability to coordinate to alkynes, with CAN, Fe(III) salts, amine /V-oxidc or iodine. 

With the TES protected alcohol at C-19 and the free alcohol at C-15 the subsequent reactions could be performed. Hydroboration of the triple bond and aqueous work-up creates vinylboronic acid 10, which is the other coupling partner with 6. 

---