Triynes rhodium-catalyzed intramolecular

Extensively developed by Ojima and co-workers, SiCaT and carbonylative silylcarbocyclization (CO-SiCaC) represent a rapid entry into polycyclic molecules of interest.271 For instance, the rhodium-catalyzed intramolecular SiCaT of triyne 441 afforded tricyclic compound 442 in high yield, accompanied by a small amount of cycloadduct 443 (Scheme 111).270... 

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]. 

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]. 

In a study of rhodium-catalyzed [2 + 2 + 2] cycloadditions of alkynes <88JCS(P1)1357>, the intramolecular [2 + 2 + 2] cycloaddition of4,9-dioxadodeca-l,6,12-triyne catalyzed by Wilkinson s catalyst [(PPh3)3RhCl] over a prolonged period of time gave the dihydropyrano- and tetrahydrofuro-fused 6-6-5 tricyclic benzene derivative (67) in moderate yield (Equation (38)). It should be noted that an analogous bis-tetrahydrofuro-fused 5-6-5 tricyclic compound could be prepared (74%) under similar conditions, but after only 3 h at room temperature. 

Tanaka et al. overcame this limitation by designing the enantioselective completely intramolecular double [2- -2-1-2] cycloaddition. The reaction of diphenylphosphinoyl-substituted hexayne 94, prepared from triyne 93 in two steps, in the presence of the cationic rhodium(I)/tol-BINAP catalyst furnished C2-symmetric axially chiral biaryl bisphosphine oxide 95 in moderate yield with high enantioselectivity (Scheme 9.32) [25]. Subsequent recrystallization andreduction of bisphosphine oxide 95 furnished the corresponding enantiopure bisphosphine 96, which could be used as an effective chiral ligand for rhodium-catalyzed asymmetric hydrogenation and cycloaddition [25]. 

The reaction of [2+2+2] cycloaddition of acetylenes to form benzene has been known since the mid-nineteenth century. The first transition metal (nickel) complex used as an intermediate in the [2+2+2] cycloaddition reaction of alkynes was published by Reppe [1]. Pioneering work by Yamazaki considered the use of cobalt complexes to initiate the trimer-ization of diphenylacetylene to produce hexasubstituted benzenes [54]. Vollhardt used cobalt complexes to catalyze the reactions of [2+2+2] cycloaddition for obtaining natural products [55]. Since then, a variety of transition complexes of 8-10 elements like rhodium, nickel, and palladium have been found to be efficient catalysts for this reaction. However, enantioselective cycloaddition is restricted to a few examples. Mori has published data on the use of a chiral nickel catalyst for the intermolecular reaction of triynes with acetylene leading to the generation of an asymmetric carbon atom [56]. Star has published data on a chiral cobalt complex catalyzing the intramolecular cycloaddition of triynes to generate a product with helical chirality [57]. 

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