1 -trimethylsilyl- 1,3-diynes

To a mixture of 30 g of zinc powder (Merck, analytical grade) and 30 ml of absolute ethanol is added 3.5 ml of 1.2-dibromoethane. The mixture is heated until an exothermic reaction (evolution of ethene and temporary reflux) starts. The activation is completed by heating the mixture for an additional 10 min under reflux. After cooling to about 50 C, the trimethylsilylated diyne (0.05 mol, see Chap. VI for silylation methods) is added in one portion. The introduction of N2 is started and the mixture is heated for 30 min under reflux. After cooling to room temperature, the work-up is carried out in a way similar to that in exp. 2 (no aqueous ammonia is nsed). Z-CjH- CI CHC CSiMe, b.p. 75 020 mmHg, njy(20 ) 1.4592, is obtained in a high yield. 

A modification of this method is known When a sodium telluride solution in ethanol at 30°C is treated with l,4-di(trimethylsilyl)buta-l,3-diyne for 3 h, the result is tellurophene in 84% yield (78CB3745). 

An important stage in the synthesis has been reached. It was anticipated that cleavage of the trimethylsilyl enol ether in 18 using the procedure of Binkley and Heathcock18 would regiospecifically furnish the thermodynamic (more substituted) cyclopentanone enolate, a nucleophilic species that could then be alkylated with iodo-diyne 17. To secure what is to become the trans CD ring junction of the steroid nucleus, the diastereoisomer in which the vinyl and methyl substituents have a cis relationship must be formed. In the... 

With the exocyclic alkylidene at C-13 properly in place, the elaboration of the l,5-diyn-3-ene moiety can now be addressed. Cleavage of both acetate and trimethylsilyl functions in 86 with basic methanol, followed by triethylsilylation of the newly formed tertiary hydroxyl group, efficiently affords alkyne 25 (86 % overall yield). This substance was regarded as a viable candidate for a Pd-catalyzed coupling reaction.12 Indeed, treatment of 25 with (Z)-chloroenyne 26 in the presence of a catalytic amount of Pd(PPh3)4 and Cu1 results in the formation of enediyne 24 in 91 % yield. 

The basic building block for the protected expanded [n]pericyclinones 89 [391 was obtained by simple acetalization of l,5-bis(trimethylsilyl)penta-l,4-diyne-... 

Trimethylstannyi)copper. This reagent is prepared as the complex 1 as shown. Reaction of l-trimethylsilyl-l,3-diynes (2) with 1 (excess) forms tram-bis-(trimethylstannyl)enynes 3. Methyllithium reacts selectively with the trimethyl-... 

When trimethylsilyl substituted diyne 607 was reacted with methyl vinyl ketone, the reaction proceeded with complete regioselectivity and without aromatization to afford 608 with 56% yield (equation 174). The regioselectivity observed was considered to result from a metallacyclopentene intermediate which was built up of the nickel atom, the double bond of methyl vinyl ketone and the less substituted triple bond of 607. 

Rothwell and colleagues352 studied the titanium mediated [2 + 2 + 2] cycloaddition of alkenes with monoynes and diynes. Among the reactions studied, the reaction between styrene (29) and diyne 609 in the presence of titanium catalyst 610 proved cleanest (equation 175). The reaction yielded 614 via a [2 + 2 + 2] cycloaddition followed by a titanium mediated suprafacial [1,5] H-shift involving 611-613. The cis relationship between the trimethylsilyl group and the phenyl group indicated that the initially formed titananorbornene 611 had an endo stereochemistry. 

In the majority of pentatetraenylidene complexes prepared or generated so far, the pentatetraenylidene ligand is derived from suitable C5 precursors. Usually penta-1,3-diynyl derivatives like the alcohol HC = C—C = C—CPh20H, its trimethylsilyl ether, or the 5,5,5-tris(dimethylamino)-substituted penta-l,3-diyne are employed. 

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. 

The attempts to apply both of the above-described reactions for the preparation of the parent heterocycle 22 (R1 = R2 = H) with the use of penta-l,4-diyn-3-one were only partially successful. The yields of 22 (R1 = R2 = H) were lower than 5%, the main products being unidentified oligomers (92MI3). The first synthesis of 22 (R1 = R2 = H) of preparative significance was when l,5-bis(trimethylsilyl)penta-l,4-diyn-3-one was the... 

The addition of telluride anion to l-(trimethylsilyl)penta-l, 4-diyn-3-ones apparently represents a general method for the preparation of 2-substituted l-telluracyclohexa-2,5-dien-4-ones 22 (92MI3). In actual fact, the reaction of l-(trimethylsilyl)-5-phenylpenta-l,4-diyn-3-one with telluride anion affords 22 (R1 = Ph, R2 = H) in 38% yield, whereas only 12% of this compound is attained in the reaction with l-phenylpenta-l,4-diyn-3-one (87JOC3662). 

Later Walton and his co-workers46 could show that 2 equivalents of 2-trimethyl-silyl-ethyne-magnesiumbromide couple with cyclooctatetraene dibromide to form l,12-bis(trimethylsilyl)dodeca-3,5,7,9-tetraene-l,l 1-diyne (47). 

Control of the regioselectivity in Co-catalysed cross-cyclization has been solved in an ingenious way utilizing 1,5-hexadiyne (116) as one component and bis(trimethyl-silyl)acetylene (118) as the other. Although bulky bis(trimethylsilyl)acetylene (118) itself cannot cyclotrimerize to hexasilylbenzene due to steric hindrance, it reacts with the cobaltacyclopentadiene 117 formed from the less bulky diyne 116 to produce the... 

Diynes 6, which consist of a trimethylsilyl alkyne tethered to a tertiary propargyl alcohol, undergo ruthenium catalyzed cycloisomerization in aqueous acetone to form the dienone intermediate 7. Concomitant electrocyclization affords 2-trimethylsilyl-2-//-pyrans in high yield (Scheme 3) <2004OL4235>. 

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