Cyclisation reactions cyclopentanones

Recent studies on radical-induced cyclisation reactions have led to a simple, one step method for the preparation of hydroazulenes from appropriately substituted cyclopentanone precursors. Treatment of 1, for example, with triphenyltin hydride and AH3N in refluxing toluene gave 2 in 79% yield. 

The iodoamide 110 and iodoester 111 both cyclise to cyclopentanone, but to avoid further reaction in the case of the ester it was necessary to trap the tetrahedral intermediate with Me3SiCl. Six-membered rings formed from homologous starting materials only in very low yield.52... 

A related reductive cyclisation has been developed by Schafer et al. in which the cathodic cyclisation of A-(oxoalkyl)pyridinium salts led to indolizidine and quinolizidine derivatives <95AG(E)2007, 03EJO2919>. Electrolyses of the pyridinium salts were carried out in a divided beaker-t5q)e cell at a mercury pool cathode under constant current, using 1 M aqueous sulfuric acid as the electrolyte. In this way, cyclisation of cyclopentanone 129 to the isomeric quinolizidines 130 and 131 was achieved in high yield and with excellent diastereoselectivity (Scheme 38). The stereochemical course of the reaction with cyclohexanone 133 was not as well defined, with three of the four possible diastereoisomers being given in a ratio of 10 21 26 (for 134,135 and 136 respectively). 

Michael-cross-benzoin cyclisation reaction afforded the corresponding chiral polyfunctionalised cyclopentanones bearing three contiguous stereocentres in good yields, moderate to excellent diastereoselectivities of up to 98% de, and good to high enantioselectivities of up to 96% ee, as shown in Scheme 2.31. 

The intramolecular asymmetric Stetter reaction of aliphatic aldehydes is generally more difficult to achieve due to the presence of acidic a-protons. Rovis and co-workers have demonstrated that the NHC derived from pre-catalyst 130 promotes the intramolecular Stetter cyclisation with enoate and alkyhdene malonate Michael acceptors 133. Cyclopentanones are generally accessed in excellent yields and enantioselectivities, however cyclohexanones are obtained in significantly lower yields unless very electron-deficient Michael acceptors are employed... 

The product selectivity in the reaction of methylene cyclopropanes with alkynes is strongly substituent-dependent the reaction is catalysed by Ni(0)/tris(o-tolyl Iphosphite (eqn.47). The intramolecular hydroacylation of 4-pentenal is catalysed by low valent Co/PPh3 complexes and produces mainly cyclopentanone.Cyclopentenones and cyclohexenones are the products of the intramolecular carbonylation of vinyl iodides (eqn.48). A large variety of heterocycles are accessible the intramolecular Pd catalysed cyclisation of unsaturated a-haloamides. Treatment with base and PdfOAclj/PPhj converts 2,6-dibromo-l,6-dienes into cyclopentanes with exocyclic double... 

---