Alder-Ene type cycloisomerizations

A catalyst system consisting of Pd2(dba)3 and AcOH or PhC02H effectively promotes the Alder-ene-type cycloisomerization of 1,6-enyne 34 to give 1,4-diene 35 (phosphine ligand is sometimes necessary). A cationic Pd(II) hydride species is believed to be generated in these reactions, the anion of which affects catalyst activity and selectivity (eq 22). 

Fig. 10.6 Postulated catalytic cycle for the Pd-promoted Alder-ene type cycloisomerization of 1,6-enynes...

Intramolecular variants of the Alder-ene type couplings between alkynes and alkenes have been extensively explored by means of palladium catalysis [73]. Recently, such a cycloisomerization of enynes was also accomplished with ruthenium catalysis (Scheme 4.32) [74]. 

Palladium-Catalysed Cycloisomerizations 10.2.1 Alder-Ene Type Cyclizations... 

From this short overview it appears that the majority of the recent studies on enantioselective cycloisomerizations have been focused so far on asymmetric Alder-ene type cyclizations with Pd and Rh catalysts, since these reactions represent an economical access into synthetically usefiil cyclopentene and cyclohexene frameworks (Sects. 10.2.1 and 10.3.1). For these processes, efficient chiral catalysts have been afforded mainly by atropisomeric diphosphines, but also DuPHOS, Skewphos and phosphine-oxazolines can occasionally represent suitable auxiliaries. 

The Alder-ene cyclization of allylic silyl ethers represents a clever use of cycloisomerization chemistry, as the enol ether products can be easily unmasked to yield aldehydes. Palladium-catalyzed cycloisomerization of 1,6- and 1,7-enynes containing an allylic oxygen most often gives rise to 1,3-dienes (see Section 10.12.4.1). However, enynes of type 63 underwent facile Alder-ene cyclization to the corresponding five- or six-membered rings (Equation (40)) using both [CpRu(MeCN)3]PF6 41 and the Cp analog ([Cp Ru(MeCN)3]PF6, 64).53... 

Kummeter et al. [34] successfully employed this type of Alder-ene cycloisomerization as an entry to a sequentially Ir-catalyzed cycloisomerization-Murahashi reaction sequence in a one-pot manner, where the intermediate aldehyde arising from the Alder-ene tautomerization step was condensed with cyano acetic esters without addition of acids or bases, furnishing five-membered heterocycles 19 with super-Michael acceptors in the side chain (Scheme 12.10). 

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