Cyclopent-2-enones complexes

Many methods have been reported for the enantioselective synthesis of the remaining PG building block, the (J )-4-hydroxy-cyclopent-2-enone. For example, the racemate can be kinetically resolved as shown in Scheme 7-28. (iS )-BINAP-Ru(II) dicarboxylate complex 93 is an excellent catalyst for the enantioselective kinetic resolution of the racemic hydroxy enone (an allylic alcohol). By controlling the reaction conditions, the C C double bond in one enantiomer, the (S )-isomer, will be prone to hydrogenation, leaving the slow reacting enantiomer intact and thus accomplishing the kinetic resolution.20... 

Reactions of 4-amino-l-metalla-l,3-dienes, generated from (1-alkynyl) carbene complexes 1 by addition of secondary amines (Scheme 32), with alkynes may lead to production of different compounds, depending on the substitution pattern as well as on the reaction conditions employed.3r Whereas successive addition of a secondary amine and one molecule of a terminal alkyne to a (l-alkynyl)carbene complex lj affords cyclopentadi-enes (Scheme 35), addition of two molecules of a terminal alkyne to complex lj yields cyclopenta[h]pyrans (Scheme 36). In an extension of this reaction sequence, (methylene)cyclopent-2-enones 103 are obtained from (1-alky-nyl)carbene chromium compounds 1 (R = rc-Pr, Ph, CH2CH2CH2OTBDS) by addition of secondary amines (NR) = NMe2, morpholino) and disubsti-tuted alkynes (Scheme 37).9e... 

In a more complex application, a 2-substituted cyclopent-2-enone moiety is markedly less reactive towards cycloaddition than the unsubstituted cyclopentenone present in the same molecule 19. Thus, the tetracyclic product 20 can be obtained exclusively (82% yield). 

There are several examples known where methylenecyclopropanes have been employed as cosubstrates in reactions of the Khand—Pauson type. Generally, this cobalt-mediated reaction is performed with hexacarbonyldicobalt complexes of alkynes, preferably strained alkenes such as norbornene or cyclobutene, and carbon monoxide to produce cyclopent-2-enones. Stoichiometric amounts of cobalt (in the form of the alkyne complexes) are normally needed. Methylenecyclopropane (1) can be employed as the strained alkene and reacts with a variety of alkynes to yield spiro-fused products. Silica gel or zeolites function as promoters of the reaction, giving rise to improved yields. ... 

A mixture of propylene oxide, (trimethylsilyl)manganese pentacarbonyl, and methyl acrylate in ether allowed to react for 24-96 h in a polypropylene syringe pressurized at 5 kbar intermediate siloxymanganese carbonyl complex (Y 82%), in acetonitrile irradiated (350 nm) at room temp, for 1-12 h, then hydrolyzed with water product (Y 53%). Acetylene derivs. furnished enones which were converted to cyclopent-2-enones. F.e. and stereoselectivity s. P. DeShong, D.R. Sidler, J. Org. Chem. 53, 4892-4 (1988). 

This type of reaction can also be catalyzed by Pd complexes. Thus, e. g. hepta-1,6-diene afforded 5-ethyl-2-methyl-cyclopent-2-enone and 2-ethyl-5-methyl-cyclopent-2-enone in 16% yield when reacted with CO at 200 °C and 1000 atm in the presence of a 2% solution of diiodo-bis-(tributylphos-phine) palladium (II) [818]. 

More recently, Jacobsen and coworkers have found that epoxides undergo a highly enantioselective ring-opening with TMS-azide when catalysed by (salen)Cr(III) complexes such as (5,5)-4. This asymmetric ring opening shows a second-order rate dependence on catalyst concentration . Applications of the process have included kinetic resolution of terminal epoxides , an efficient synthesis of (/ )-4-(trimethylsilyloxy)cyclopent-2-enone, the dynamic kinetic resolution (equation 11) of epichlorohydrin, an enantioselective route to carbocyclic nucleoside analogues and a formal synthesis (equation 12) of the protein kinase inhibitor 5. 

The coupling of allylic chlorides with trimethylsilylalkynes in the presence of [PdCl2(PhCN)a] also exhibits good regio- and stereo-selective control to give 4-trimethylsilyl-5-chloro-l,4( )-dienes. Furthermore, upon treatment with fluoride, these produce the corresponding 1,4-enynes. Dicyclopentadienylzirconium diene complexes are readily carbonylated, and the adducts, after hydrolysis with acid, afford cyclopent-3-enones, sometimes in synthetically useful yields. ... 

Numerous new examples of annelation reactions involving alkyne-hexacar-bonylcobalt complexes and norbornene derivatives have been reported it seems that the alkyne and alkene units are joined at the less hindered position of each, subsequent addition of the carbonyl group leading to the observed cyclopent-2-enone derivatives. A new synthesis of tetrasubstituted cyclopent-2-enones (39) by cyclocarbonylation using tetracarbonylnickel has been presented. 

Grubb s ruthenium carbene catalyst to afford medium-sized carbocycles (Equation (10))." Iron tricarbonyl systems containing diazoketones tethered to the terminal position of the acyclic coordinated diene have been utilized to effect ring formation, via inter- and intramolecular carbene reactions, affording new cyclohexa-2,4-dienone and cyclopent-2-enone tricarbonyl iron complexes. ... 

The enantioselective intermolecular reductive aldol reaction with enone instead of ester is also an important C-C bond formation reaction. The coupling reaction of cyclopent-2-enone and benzaldehyde with hydrosilane in the presence of 1 mol% of the phebox-Rh complex 5-Ph smoothly proceeded to give the corresponding anti-p-hydroxyketone 29 with the high ee value (Scheme 20) [25], On the basis of the absolute configuration of 29, the transition state of the reaction might involve the Re face attack of the Rh enolate to the Re face of the aldehyde captured on the rhodium atom leading to form 2S, l R-diastereomeric product. 

Aza-MBH reaction between acrylonitrile (30) and imines (29) has been achieved with 98% ee using chiral phebim/Pd(II) complexes (32) to form a-methylene- -aminonitriles (31). Aza-MBH reactions of ArCH=NTs with electronically and 0 sterically deactivated Michael acceptors can be achieved by the use of electron-rich phosphanes (PArg) and pyridines (33) as catalysts (Scheme 21) Nucleophilic and steric influences, respectively, are exerted by new multifunctional chiral phosphines and BINOL derivatives used to cocatalyse aza-MBH reactions of 5,5-disubstituted cyclopent-2-enone and RCH=NTs in THF, with 99% yield and 85% ee ... 

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