Cyclotrimerization 5 6 5 ring-systems

A number of compounds with this ring system (163), including dispiro derivatives, are formed by the catalytic cooligomerization of ketazines or aldazines with butadiene (Scheme 186)249 reactions of this type are analogous to the catalytic cyclotrimerization of butadiene to cyclodecatriene.250... 

It is possible to carry out the [2+2+2] cyclotrimerization reaction in a regioselective manner by using a partially or completely intramolecular approach. Rhodium-catalyzed intramolecular cyclotrimerization of 1,6,11-triynes, which construct fused 5-6-5 ring-systems, has been studied extensively [33-36]. Cyclization of 1,6,11-triyne 47 catalyzed by RhCl(PPh3)3, gives the tricyclic benzene 48 in good yield (Eq. 14) [33a]. 

This rhodium-catalyzed intramolecular cyclotrimerization method has been utilized as the key step for the synthesis of the marine illudalane sesquiterpenoid, alcyopterosin E, 51, which has a fused 5-6-5 ring system as illustrated in Scheme 7.15 [37]. 

The final option available to a u-alkylpalladium intermediate from Heck alkylation occurs if another alkene or alkyne function is situated properly to participate in a further Heck-type carbopalladation (equation 161)318,319. In properly constructed systems, more than one further carbopalladation is feasible, and many examples of these cascade car-bopalladations have been reported. Several have been quite spectacular (equation 162)320. Fused, spirocyclic and bridged bicyclic ring systems have been prepared in this manner. The process may also create as many as five rings in one step, with five-,six- and three-membered rings321 being the most suitable for preparation (equation 163). Alternatively, the proper orientation of double and triple bonds allows cyclotrimerization to highly functionalized arenes or fulvenes (equation 164)322,279. 

Transition metal catalysed [2 + 2 + 2] cyclotrimerization of alkynes is a powerful tool to construct substituted aromatic compounds.[1] Recently, cyclotrimerization of alkynes in a water-ether biphasic system with the use of a water soluble rhodium catalyst has been reported.[2,3] Distribution of a hydrophobic substrate between the two phases keeps the concentration of substrate in the aqueous phase low. The biphasic system enables medium and large-sized ring systems to be synthesised on the basis of the concentration control. 

The ifaodium-catalyzed intramolecular cyclotrimerization of 1,6,11-triyne 419, forming 5-6-5 fiised-ring system 420, has been extensively studied (Scheme 2-41, eq. i).[220b,276] reaction has also been used as the key step in the synthesis of a marine illudalane sesquiterpenoid, alcyopterosin E (423) (Scheme 2-41, eq. 2)P as well as in die asymmetric synthesis of chiral diphosphine ligands 425 (Scheme 2-41, eq. 3). ... 

As illustrated in the early achievements from Reppe on the cyclotetramerization of acetylene and from Wilke on the cyclodimerization and cyclotrimerization of butadienes, nickel-catalyzed cycloadditions possess considerable potential in the construction of medium ring systems. Pioneering studies by Wender have demonstrated many practical implications that have advanced the... 

The alcyopterosins represent an interesting set of illudalanes that were isolated from deep-sea soft corals and whose members represent the first nitrate esters to be found in any natural product [10]. The rhodium-catalyzed intramolecular cyclotrimerization method has been utilized here to gain access to the 5-6-5 and 6-6-5, as well as the bicyclic ring systems of this set of marine illudalanes (Schemes 7.5 to 7.7). The synthesis of enantiopure triyne 21, as needed for alcyopterosin E (22), was straightforward, starting from readily available L-ascorbic acid (17) and isophorone (20), to provide the alkyne building blocks 18 and 19 in seven and six steps, respectively (Scheme 7.5) [11]. 

Rhodium-catalyzed [2+2+2] cyclotrimerization of alkynes in an aqueous-organic diphasic system was also employed in die formation of dihydrobenzo[c]furans as depict in the scheme below. Medium- and large-sized rings could be obtained by utilizing this approach <03JA7784>. 

Among the metallacycloalkenes with different ring size, metallacyclopro-penes and -pentadienes are to be mentioned because they are involved as intermediates in the cyclotrimerization of alkynes to benzene derivatives, and in this section attention is mainly directed to such species. Cotrimeriza-tions also are possible with alkyne-like systems. 

Cyclotrimerization of (CF3)2C2 also occurs in the reactions with Rh(I)-j8-keto-enolate rings and related systems ... 

Cyclization, Isomerization, and Coupling Reactions. Inter-(eq 42) and intramolecular (eq 43) cyclotrimerizations of alkynes are mediated by Wilkinson s catalyst. This is an extremely efQcient route to ring fused systems. Similarly, Diels-Alder-Uke [4 + 2] cyclization processes are promoted by RhCljPPhsjs dienophile components in these reactions need not be electron deficient, and they can be an alkene or alkyne (eqs 44 and 45) AUenes oligomerize in pathways determined by their substituents For instance, four molecules of aUene combine to give a spiro-cyclic system (eq 46), but tetraphenylallene isomerizes to give an indene (eq 47). ... 

Potential multidentate ligands have also been created by serendipity. Whereas the terpene derivative 28 had been prepared to build the AB rings of taxane 29 its treatment with the Grubbs catalyst 3 has led to its unexpected cyclotrimerization and the production in 70% of the multifunctional system of C3 symmetry 30 (Scheme 14) [44],... 

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