Enynes/dienynes substitution

In turn, the propensity of 1 to respond to steric hindrance can be used to control the site of initiation of an RCM reaction in a polyene substrate (Scheme 9) [20]. Thus, dienyne 25 reacts with the catalyst regioselectively at the least substituted site the evolving ruthenium carbene 26 undergoes a subsequent enyne metathesis leading to a new carbene 27, which is finally trapped by the disubsti-tuted olefin to afford the bicyclo[4.4.0]decadiene product 28. By simply reversing the substitution pattern of the double bonds, the complementary bicyclo [5.3.0] compound 32 is formed exclusively, because the cyclization cascade is then triggered at the other end of the substrate. Note that in both examples tri-substituted olefins are obtained by means of a ruthenium based metathesis catalyst [20] ... 

Certain substituted, 1,6-enynes and related dienynes undergo palladium-catalyzed cycliza-tion to give polycyclic structures containing a three-membered ring via cascade carbopal-ladation (equation 156)343 34S. Transformation of 66 into 67 is representative346. 

Suitable a, tw-enynes can be cyclized when allowed to react with catalytic quantities of [RhCl(PPh3)3]. Alken-2-ynes do not react and the yield is greatly diminished in the case of alk-2-enyne substrates. However, substitution of the alkene bond does not appear to inhibit the dicyclization of dienynes or the cyclization of alkynenones. The dienyne s reaction, shown in equation (21), is catalyzed more effectively by rhodium(I) complexes containing tertiary phosphite ligands. Under similar conditions, trienes can also be cyclized. The cyclopropyl ring can serve as an alkene bond precursor in cyclizations (equation 22). ... 

Similarly, enynes substituted at the alkyne with an aryl group led to products resulting from a formal intramolecular [4 + 2] cycloaddition occurring at an unusual low temperature (equation 64). On the other hand, substrates with = H or = Me, R = H gave cyclobutenes with Au(i)25,34 qj. catalysts. Dienynes react with cationic Au(I) catalysts leading to products of formal [4 + 2] cycloaddition as well (equation 65). Somewhat related cyclizations of allenes with alkynes and diynes have been described. 

Enyne metathesis RCM can be performed in tandem with RCM using appropriately spaced diene-yne substrates. Many examples of this reaction featuring various degrees of complexity have been reported. Representative examples of tandem enyne metathesis-RCM are depicted in Scheme 30 and include (i) formation of the fused bicyclic compound 257 from dienyne 255 for securinine total synthesis/ (ii) synthesis of cyclic ethers (e.g., 261 and 262) from diene-alkynes (e.g, 258) and control of the product distribution through alkene substitution pattern,and (iii) double tandem RCM-enyne metatheses (conversion of 263 into 266) of appropriate polyene-polyyne systems. 

Conjugated enynes were the major products obtained from palladium-catalysed reaction of vinyl bromides such as (32) with acetylenes (Scheme 53). Again the yields of simple enyne acetates are variable, with starting material and other products such as dienynes being isolated. The reaction demonstrates that when a substrate has a choice of undergoing either vinylic coupling or substitution (via a -allyl complex) the former process appears to be favoured. 

Reactions with conjugated enynes as dienes in Diels-Alder reactions yield cyclohexadiene or benzene products on reaction with alkene or alkyne dienophiles, respectively. These reactions proceed via a stepwise mechanism to avoid formation of a cyclic allene and are referred to as dehydro-Diels-Alder reactions. In 2008, Barluenga and Aguilar demonstrated that gold catalysts promote intermolecular hetero-dehydro-Diels-Alder reactions between dienynes and nitriles. Dienyne 99 combines with phenyinitrile (100) to afford substituted pyridine 101. ... 

Inter- and intramolecular additions of alkenes and dienes to propargylic alcohols catalyzed by thiolate-bridged diruthenium complexes have been described. The processes, a kind of allenylidene-ene reaction, generate 1,5-enynes and dienynes by reaction of propargylic alcohols with 2-arylpropenes [196] and 1,3-conjugated dienes [197], respectively. The intramolecular version of this reac-ti(Mi has been developed to give diastereo- [196, 198] or enantioselective syn-substituted chromanes (Scheme 58) [199]. Recently, the results of DPT calculations indicated that nucleophilic attack of the olefinic Jt-electrons on a carbocationic... 

Remarkably, a change of solvent from DCM to tetrahydrofuran (THF) resulted in an unprecedented switch of reaction regioselectivity, leading to the formation of 2,3-substituted styrenes 8. A variety of 4-aryl- or heteroaryl-substituted enynes was shown to react with good efficiency (Table 14.2, entries 1 to 8). Notably, in the case of bromo-substituted enyne, a clean reaction without protodebromination or formation of crosscoupling products occurred (entry 4). For a dienyne substrate, having both terminal and endocyclic double bonds, the former double bond reacted regioselectively, while the double bond of the cyclohexene subunit remained untouched (entry 9). 

---