Cyclopentanones from alkynes

This precedents prompted several groups to develop reaction protocols that lead exclusively to the reduced products (the reductive PKR), in order to use them in synthesis. Thus, Becker obtained diazabicyclooctanones like 208 from amines (207) as the only reaction product when they used DSAC conditions under an inert atmosphere. The nitrogen atmosphere was essential as in air they obtained mixtures of the cyclopentanones and the cyclopentenones. This group has used this methodology for the synthesis of azaadamantanes like 209 as part of the structure of certain antagonists (Scheme 59) [ 182-184], Addition of TFA to the reaction favors the formation of reduced products. A series of alkynes produced cyclopentanones as the major product when reacted with norbornene, Co2(CO)8 and TFA. The authors think TFA reacts with the cobalt complex 210, prior to the reductive elimination that gives the final product 211 (Scheme 60) [185]. 

On the other hand, lithium enolates derived from substituted endocyclic ketones have largely been exploited in the synthesis of steroids since the regioselectivity of their deprotonation can be controlled and high levels of 1,2- and 1,3-stereoselection occur9,418. The control is steric rather than electronic, with the attack directed to the less substituted ji-face of the enolate for conformationally rigid cyclopentanones, whereas stereoelectronic control becomes significant for the more flexible cyclohexanones. Finally, an asymmetric variant of the formation of a-branched ketones by hydration of camphor-derived alkynes followed by sequential alkylation with reactive alkyl halides of the resulting ketones was recently reported (Scheme 87)419. 

The Pauson-Khand reaction is a powerful tool for the synthesis of cyclopentanones, 246, from a>-alkenylacetylenes, 245, and carbon monoxide.176 Enyne cyclization has been catalyzed with nitriles using catalytic (77S-CsH5)2Ti(PMe3)2 95177-179 and other variants have since been discovered where the desired cyclopentenones can be directly prepared from the enyne and CO using (77S-CsHs)2Ti(CO)2 68 (Scheme 33) 176,180-184 Addition of PMe3 to the latter reaction mixture has proved to be beneficial. Stoichiometric reactions established that the initial step in the catalytic cycle is reductive coupling of the alkyne and the olefin to form the titanacycle. Carbon monoxide insertion followed by reductive elimination generates the observed product. 

Hydroacylation is especially interesting in its intramolecular version, converting unsaturated aldehydes to cyclopentanones. Numerous examples of transition metal catalyzed hydroacylations have been described, mostly with 4-alkenals of various substitution patterns. The reaction is used for the construction of starting materials in prostaglandin synthesis and the preparation of other cyclopentanoid systems. Rhodium catalysts, mainly of the Wilkinson type, are used. The steric course of hydroacylation is believed to occur in a m-addition manner. This was deduced from results of intermolecular alkyne hydroacylation56 (vide supra) and the intramolecular hydroacylation of deuterated E- and Z-isomers of 7,5-unsaturated aldehydes39-5 . 

A synthesis of dienediynes from 2-(hydroxymethylene)cyclopentanone involves the di-enol ditriflate [bis(trifluoromethanesulphonate)] 120, which reacts with an alkyne in the presence of a palladium catalyst to give 121, accompanied by only small amounts of the regioisomer 122. The major product on treatment with the same or different alkyne yields the dienediyne 123 2-(Alkyn-l-yl)-l-methylpyrrolidines 125 (R = Me, pentyl, MesSi or Ph) have been obtained from the thiolactam 124 by sequential alkylation, treatment with a lithium acetylide and reduction with LAH. The phenylthio group in a-(phenylthio)-lactams is displaced by an alkynyl group by the action of an alkyn-l-ylzinc chloride or of di(alkyn-l-yl)zinc compounds. Thus the j -lactam 126 gave 127 and the azaprostacyclin II 128 gave 129 1... 

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