1.4- Diynes 1,4-dienes

The palladium-catalyzed cyclization reaction was used in the syntheses of several natural products such as siccanin [86], streptazolin [87], and ceratopi-canol (through a diyne, diene cascade) [80]. The production of the streptazolin precursor 149 through reductive cyclization of 150 is illustrative of the complexity that the reaction can provide (Eq. 29) [87]. 

A wide variety of main group heteroatom-containing ring systems are formed by the reaction of appropriate reagents with metallacycles derived from cycloaddition of Cp2Zr with alkynes, diynes, dienes, enynes or alkynenitriles. This is an exceptionally general process of considerable potential, especially for the synthesis of less-common types of heterocycles (Schemes IS and 16). ... 

The latest detectable compound had a retention time of 4.183 min. Its spectrum can be assigned either to a triene-triyne, i.e. R-(CH=CH)3-(C=C)3-R or to a diene-diyne-diene, i.e. R (C11=CII)2 (C=C)2 (CH= CH)2-R. In both cases a Cji chromophoric chain. In all the latest cases the spectral assignments are based on the Bohlmann tables [1,25] and Perkampus atlas [24] on ene-yne reference compound spectra. 

Cappel, D., Tiilhnann, S., Rrapp, A., Frenking, G. (2005). Direct Estimate of the Conjugative and Hyperconjugative Stabilization in Diynes, Dienes, and Related Compounds. Angewandte Chemie International Edition, 44, 3617-3620. 

Hydroboration of terminal diyne gives organoboron polymer having branched structure due to the further hydroboration reaction toward the initially formed vinylborane structures. That is, terminal diynes are taken to have a potential as a multifunctional monomer. When the polymerization between 1,7-octadiene and thexylborane was carried out in the presence of a small amount of 1,7-octadiyne, the molecular weights of organoboron polymers were found to be increased when the ratio of diyne/diene was increased [5]. 

The organoborate intermediates can also be generated from alkenylboronic esters and alkyllithium or Grignard reagents, or from ttialkylboranes and alkenyllithium compounds. Conjugated symmetrical and unsymmetrical diynes (289—291), stereochemically pure 1,3-dienes (292,293), and 1,3-enynes (294) including functionali2ed systems can be prepared (289,295). 

Diene, Enyne, and Diyne Metathesis in Natural Product Synthesis... 

Depending on the types of unsaturated functional units involved in the metathesis process, the reactions can be classified into three major categories diene, enyne, and diyne metathesis (Figs. 1-3). Another mode of classification... 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

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