Pauson-Khand reactions dienynes

Fenestranes are compounds of theoretical interest in which the central carbon atom undergoes severe planarization distortion. Reactions sequences involving double intramolecular Pauson-Khand reactions of ene-diynes, or intramolecular Pauson-Khand of dienynes followed by photochemical [2 + 2] cycloaddition, successfully lead to [5.5.5.5]- or [4.5.5.5]fenestrane, respectively [42], For instance, compound 37 was obtained from ene-diyne 36 in moderate yield as a single all-czs stereoisomer [43] (Scheme 18). 

Cyclization of enynes, dienynes, and diynes with aryl isocyanides. This combination of reagents (1 2) generates a Ni(0) catalyst (1) which is easier to handle than the air-sensitive bis(l,5-cyclooctadiene)nickel(0), Ni(COD)2. Reaction of 1,6-cnyncs (2) with an aryl isocyanidc in the presence of this Ni(0) catalyst combined with Bu3P (2 cquiv.) results in bicyclic iminocyclopentenes (3) which can be hydrolyzed to the corresponding ketone (4). The overall reaction is an alternative to the Pauson-Khand reaction. 

The Ir-tolBINAP catalyst also functions well in the desymmetrization of dienynes, where a highly enantioselective and diastereoselective Pauson-Khand-type reaction proceeded to give vinyl-substituted bicyclic cyclopentenones with two chiral centers (Scheme 11.19) [31]. 

Jeong and coworkers have executed a desymmetrization of a dienyne by asymmetric Pauson-Khand-type reaction. Intriguingly, the use of a Rh catalyst resulted in preferential formation of one diastereomer, while a switch to the analogous Ir system provided the other diastereomer in excellent selectivity [Eq. (10.62)] the system has been shown to be tolerant of oxygen in the linker as well as modest substitution on the alkyne (Ph) ... 

The scope of Pauson-Khand type reactions has been expanded by exploring the use of various carbon components. Dienyne 350 is a versatile substrate, which can be subjected to two cycloaddition pathways (Scheme 2-21). In 2003, Wender et al. reported the Rh-catalyzed PK-type reaction of350. Under unoptimized conditions, 350 underwent three competing cycloadditions, i.e., 1) an intramolecular [4+2] cycloaddition to afford product 351, 2) a new version of a [2+2+1] cycloaddition to afford product 352, and 3) an unprecedented [4+2+1] cycloaddition to afford product 353. After further refinement, the [2+2+1] product 352 was obtained in excellent yield.t >... 

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