Cyclopentyl ring

Conversely, processes which convert carbons to sfp- carbons are more favorable for five-membered than for six-membered rings. This can be illustrated by the data for acetolysis of cyclopentyl versus cyclohexyl tosylate. The former proceeds with an enthalpy of activation about 3kcal/mol less than the latter." A molecular mechanics analysis found that the difference was largely accounted for by the relief of torsional strain in the cyclopentyl case." Notice that there is an angle-strain effect which is operating in the opposite direction, since there will be some resistance to the expansion of the bond angle at the reaction center to 120° in the cyclopentyl ring. 

Nicolaou and coworkers reported efficient enantioselective syntheses of ( )-kinamycin C (3), ( )-kinamycin F (6), and ( )-kinamycin J (10) [39], Nicolaou s retrosyntheses of these targets are shown in Scheme 3.13. The authors envisioned that all three metabolites could be accessed from the common precursor 82. The a-hydroxyketone function of 82 was envisioned to arise from an intramolecular benzoin reaction of the ketoaldehyde 83. This key bond disconnection would serve to forge the cyclopentyl ring of the kinamycin skeleton. The ketoaldehyde 83 was deconstructed by an Ullmann coupling of the aryl bromide 84 and the a-iodoenone 85. The latter were anticipated to arise from the bromojuglone derivative 86 and the enantiomerically enriched enone 87, respectively. 

The enantioselectivities were found to be relatively independent of the solvent used. In 1998, Zhang reported a bisphosphinite based on a rigid bis-cyclopentyl ring system (BICPO, 95, 96) which induced 45.7 to 95% ee in the hydrogenation of a-dehydroamino acid derivatives [85]. 

One can attribute the selective formation of materials with the ester and allyl units trans to one another, to the preference for the allyl unit to occupy a pseudoequatorial rather than a pseudoaxial orientation in the product-determining transition state. Compare, for example, transition state formulation 68 with 69. This stereochemical outcome is fortunate, as later on in the sequence, it is necessary for the allyl unit (after functional group modification) to swing across the top face of the cyclopentyl ring system during the conversion of 62 to 63. Were the substituents cis to one another, this would not be possible. 

Scheme 21). Fusion of a cyclohexyl ring at C2-C3 66 (n = 2) also led to the formation of pyrazoles 68, whereas the fusion of a cyclopentyl ring at C2-C3 66 (n = 1) led to a complete change in periselectivity, the benzodiazepines 67 being the only observed products (Scheme 22) [75JCS(P1)102]. 

Fusion of a cyclohexyl ring at C1-C2 also produced only the pyrazole, but when a cyclopentyl ring was fused at C1-C2 the diazo compound 71 reacted via all three possible modes of reaction (Scheme 23). This was the first example of a 3-aryl-l-diazoalkane reacting by both 1,5- and 1,7-electrocycliza-tion. This effect can be attributed to the fact that in 71a the termini separation is slightly greater than in the acyclic case 64 but not as great as in 69. So, in contrast to 69, both 6m- and 8w-electrocyclizations are possible, and the carbene reactions are also competitive [79JCS(P1)1433]. 

It would appear that there is considerable potential in using fluorinated 5-hexenyl radical systems to synthesize specifically fluorine-substituted cyclopentyl ring systems. 

The Barton decarboxylation procedure was used in the total synthesis of (-)-verrucarol by K. Tadano et al. The initially formed thiohydroxamic ester was decarboxylated to leave a methylene radical on the cyclopentyl ring, which was then trapped by molecular oxygen. Reductive work-up in the presence of f-BuSH finally provided the hydroxylated product. ... 

Oxandrolone (31)is rapidly absorbed into the systemic circulation after an oral dose. It undergoes oxidation on the cyclopentyl ring, to give a diol (80) called 16j8-hydroxy-oxan-drolone(seeFig. 15.10). The major plasma metabolites are glucuronide conjugates of both (31)and (80)(91). 

Transannular amidomercuration of macrocyclic lactams leads to heterocyclic cyclohexyl mercury compounds, which contain an annulated cyclopentyl ring76. Reductive alkylation with 2-propenenitrile and reducing agents proceeds with stereoselective addition tram to the cyclopentyl ring. 

Bicyclic pentenes are used as trapping agents in addition reactions of 3-butynyl radicals81. Addition occurs exclusively trans to the annulated cyclopentyl ring, while the regioselectivity is low when polar substituents are not present. Under thermal conditions, the reaction finishes after iodine atom abstraction to yield the overall trans-addition product. With photochemical initiation and in the presence of hexabutylditin, further cyclization and iodine atom abstraction occurs to form exocyclic vinyl iodides. 

The cis-sclcctivc introduction of a cyclopentyl ring via a radical cyclization is the key step in the synthesis of ( )-silphinene. an angular fused triquinane48. Conversion of readily available 3,5.5-trimethylcyclohexenone to the radical precursor 3, followed by Barton deoxygenation16 generates the radical, which cyclizes stereoselectively to the tricyclopentanoid skeleton 4. 

The drug exerts its aetion very much similar to that of biperiden hydroehloride. However, it is found to be twice as potent as the biperiden salt perhaps due to the presenee of a more eompaet cyclopentyl ring in place of the biperiden ring. 

Introduction of a methyl group to the cyclopentyl ring accelerates the rearrangement of cyclopentylphenyl ketone 28d) whose limiting stage appears to be that of a 1,2-shift in an intermediate a-hydroxycarbonium ion, cf Ref ... 

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