1,6-Diynes ring closure

The reaction of Ba[P(SiMe3)2]2(THF)2 with diphenylbutadiyne in toluene for 12 days induces a m-addition of the diyne to the phosphide, followed by a 1,3-silyl group shift and ring closure. The dinuclear complex 132 is then isolated in good yield.283 Its complex structure contains Ba-C a bonds (2.881(5), 2.899(5) A), side-on Ba-alkyne (3.003(6), 3.363(6) A) and arene interactions, and Ba-phospholide bonds (Ba-P = 3.487(2) A) (Figure 65). 

This reaction has been used to prepare many a-methylene lactone derivatives.537 The above alkyne carbonylations were all catalyzed by phosphorus ligand-containing complexes. Some phosphorus-free catalysts for these reactions are also known, such as PdCl2 in presence of thiourea.538 This catalyst system also effects ring closure of 1,6-diynes (equation 131).539... 

Dodeca-5,7-diyne undergoes photoreduction at one triple bond on irradiation in pentane or an alcohol (equation 26) , as does the 1,4-diyne 8, although this second dialkyne also gives rise to a cyclopentadiene product which may be formed by initial ring closure of the diyne to give a cyclopentadienyl biradical (equation 27). 

Intramolecular bridging is also observed. For functionalized dienes (e.g., RCH=CHCOCH=CHR), ring closures occur with the formation of thiacyclohexanones, while the related bis alkenylketones result in the formation of the corresponding thiacyc-lohexadienones . Simple thiophene derivatives are formed by insertion of diynes into SCI2 . Similar chemistry is reported for Se halides, and tricyclo thia- and selena ethers are obtained from sequential addition of SCI2 and SeCb to cyclooctatetraene . 

TMS methylpyrazole (279) is formed by the reaction of l,4-bis(TMS)-buta-l,3-diyne (207) with anhydrous hydrazine in abs. ethanol154. The reaction is initiated by a nucleophilic attack of hydrazine at carbon-1 of 207, forming a hydrazone via rearrangement and, after partial desilylation and ring closure, 279 can be isolated (equation 126). 

Electrocycylic ring closure combined with Diels-Alder addition has been employed to derive all four rings of the tetracyclic ring system in the steroids by means of a one pot synthesis (ref.198). Thus, from the triflate of hex-4-ynol and the readily available hexa-1,5-diyne a triyne intermediate was obtained which with chromium or tungsten hexacarbonyl formed a triyne carbene complex. This with a 1,3-dialkoxybuta-1,3-diene in acetonitrile with carbon monoxide 0 atmosphere) initially at ambient temperature and then at 110°C over 24 hours afforded a 62% yield of the two products shown [(1) R = TBS (2) R = H, (1) (2) 5 2]. 

Two general methods have been used for the preparation of annulenes involving, respectively, ring-closure of diynes and photolytic ring--opening of polycyclic valence isomers of annulenes. 

Chlorotris(triphenylphosphine)rhodium(I) Ring closures with diynes... 

We found that methyl or ethyl substituted diynes give Acyclic Diyne Metathesis Condensation (ADIMET) which is similar to the Acyclic Diene Metathesis (ADMET) of dienes. The catalysts we used for ADIMET reactions are the Schrock type carbyne complex ( BuO)3W=C Bu or W2( BuO)6. For ADIMET reactions we used no or only small amonts of solvent. With higher amounts of solvents Ring Closure Metathesis (RCM) of diynes occured. 

The intramolecular alkyne coupling of diyne 11.281 was used in a synthesis of bryostatin 11.283 to close the macrocycle 11.282 (Scheme 11.94). This reaction was followed by a gold(l)-catalysed 6-endo ring closure of 11.282 and some routine transformations to complete the synthesis. As so often in macrocyclizations, a low concentration (0.002 M) was found to be important. 

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