Pauson-Khand reaction 866 Subject

Apart from studies by Pauson and coworkers, this reaction was not widely investigated until the seminal report by Schore in 1981 which detailed the first intramolecular example of a Pauson-Khand reaction [10]. Following this report, activity in this area flourished, and the frequent apphcation of this methodology in the total synthesis of natural and unnatural products demonstrated the versatility of this reaction. A detailed discussion of these efforts is beyond the scope of this article, but a number of excellent reviews on the subject have been published [11]. 

The diastereocontrolled cyclization of chiral enynes has been extensively studied [11], and its application with optically active enynes represents a major focus in the efforts to synthesize optically active cyclopentenones via the Pauson-Khand reaction. For the purposes of this review, the discussion of this subject... 

Another approach to the stoichiometric enantioselective Pauson-Khand reaction involves the use of chiral auxiliaries. Extensive investigations on this subject have been carried out by Periods, Moyano, Greene, and coworkers. Initial reports detailed the use of frans-2-phenylcyclohexanol as a chiral auxiliary for the... 

Numerous applications of the latter methods to stereoselective20 25- 94 105 and asymmet-ric26-33,io6-r is organic ancj natural product synthesis have been described however, those methods which use a stoichiometric amount of the metal are not the subject of this section. Few examples of transition metal catalyzed cyclopentenone and cyclopentadienone syntheses starting from alkenes and alkynes have been reported 34. It should be noted, however, that considerable efforts (and the first positive results) towards the catalytic use of zirconium13 and cobalt (in the Pauson-Khand reaction)35 36- U6l 117 have been reported. 

Access to bicyclic enones from 1,6-enynes by the Pauson-Khand method is rendered enantioselective by installing a chiral t-butylsulfinyl group at C-1. Cyclization of a-benzylidene-aroylacetamides to furnish 3-arylindanones is subject to 1,5-asymmetric induction when the amide moiety is derived from a bulky 4-substitulted oxazolidin-2-one. Chiral ligands for the Pauson-Khand reaction have also been studied. Phosphine-borane (111) derived from (-l-)-pulegone is an example. 

Not surprisingly, the cobalt-complexed ether products obtained by the Nicholas reaction can be subjected directly to the Pauson-Khand sequence (Scheme 7).244... 

Buchwald and coworkers studied Cp2Ti complexes and developed a titanium(II)-catalyzed Pauson-Khand-type reaction (Fig. 14) [133]. When a number of ortho-allylacetophenones 42 was subjected to 5-20 mol% of Cp2Ti(PMe3)2 in the... 

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 >... 

---