Cycloalkenes chirality

Molecules that are chiral as a result of barriers to conformational interconversion can be racemized if the enantiomeric conformers are interconverted. The rate of racemization will depend upon the conformational barrier. For example, -cyclooctene is chiral. E-Cycloalkenes can be racemized by a conformational process involving reorienting of the... 

Klunder A. J. H., Zhu J., Zwanenburg B. The Concept of Transient Chirality in the Stereoselective Synthesis of Fnnctionalized Cycloalkenes Applying the Retro-Diels-Alder Methodology Chem. Rev. (Washington, D. C.) 1999 99 1163-1190 Keywords stereochemistry, stereoselective synthesis, retro-Diels-Alder methodology... 

Kinetic resolution of chiral dienes has been realized by RCM with enantiomeri-cally pure molybdenum carbene complexes [888-890]. High enantiomeric excesses of recovered diene (> 99% ee) and cycloalkene (93% ee) could be obtained. 

Hoveyda et al. reported a novel method for synthesizing of chromene 71 by ROM-RCM of cycloalkene 70 bearing the phenyl ether at the 3-position [Eq. (6.48)]." ° The yield is improved when the reaction is carried out under ethylene gas. In the case of cyclopentene 70a (n = 0) or cyclohexene 70b (n = 1), the yield is poor because the starting cycloalkene is in a state of equihbrium with the product and a thermodynamic product should be formed under these reaction conditions. They obtained enantiomeric ally pure cycloheptene derivative (5)-70e using zirconium-catalyzed kinetic resolution of 70e developed by their group, and chromene 71c was synthesized as a chiral form via ROM-RCM using lb [Eq. (6.49)] ... 

Isomerizations may take place over some catalysts more than over others. Regio- as well as stereoisomers may be formed [13, 49. Such side reactions are undesirable in the reduction of pro-chiral alkenes, and especially in the hydrogenation of some cycloalkenes. 

Homochiral molecules readily available from inexpensive sources are useful synthetic building blocks and chiral auxiliaries. 1,4-01-0-benzyl-L-threitol has been used in construction of homochiral crown ethers that are useful as enzyme model systems. Topologically controlled diastereoselective delivery of the Simmons-Smith reagent for 2-cycloalken-l-one 1,4-d1-0-benzyl-L-threitol ketals was recently reported. ... 

The stereochemistry of ring-opening polymerizations has been studied for epoxides, episul-fides, lactones, cycloalkenes (Sec. 8-6a), and other cyclic monomers [Pasquon et al., 1989 Tsuruta and Kawakami, 1989]. Epoxides have been studied more than any other type of monomer. A chiral cyclic monomer such as propylene oxide is capable of yielding stereoregular polymers. Polymerization of either of the two pure enantiomers yields the isotactic polymer when the reaction proceeds in a regioselective manner with bond cleavage at bond 1. 

Face Selectivity of Nitrile Oxide Cycloadditions with Chiral Cycloalkenes and the Like... 

Table 9. a.x-Disubstituted Alkanoates and 4,4-Disubstituted 2-Cycloalken-l-ones by Alkylation of Eno-lates from Substituted Chiral Bicyclic Lactams, Followed by Cleavage1 3,5 7,13... 

The chiral nature of ( )-cycloalkenes, whose jump rope conformational interconversions are restricted because of non-bonding interaction across the ring, was first pointed out in 1952 when Blomquistpresented the enantiomeric figures of ( )-cyclononene in his paper describing the syntheses of cis- and trans-isomers of cyclononene. 

Chirality of ( )-cycloalkene has been customarily classified as planar in nature, and specified following Cahn, Ingold and Prelog s convention u,12). 

Although an unsuccessful optical resolution21 of ( )-cyclodecene suggested its optical instability, Robert s dynamic NMR studies 22) of the racemization process in deuterated ( )-cycloalkenes 13 succeeded in providing a t1/2 value of 104 sec (room temperature). This optical instability, found in the parent compound, could explain Westen s fruitless attempt to prepare the chiral ( )-cyclodecenone 16 from the (+)-methanesulfonate 15 23). 

As mentioned earlier, preparation of (Z),( )-l, 5-cyclooctadiene (31) in an optically active modification 2) first demonstrated the chiral nature of ( )-cycloalkenes. In this classical experiment, Cope and coworkers obtained ( + )-31 by the Hofmann elimination of the (-l-)-ammonium salt 30. They were also successful in obtaining (+)-31 by optical resolution of racemic 31 through complexing with a chiral Pt(II)... 

The chiral C2-conformer 42 of a ( )-cycloalkene can transform into the enantiomeric Cf-conformer 44 through a planar Cs-conformer 43, and this rope jump racemization can be prevented by anchoring one end of the unsaturated center onto the ring by means of an extra-bridge. This bridging creates a bicyclic anti-Bredt rule compound 45, revealing that all anti-Bredt rule compounds (45) with one double bond are necessarily asymmetric (C, symmetry) and have one asymmetric carbon atom. 

As was the case with chiral ( )-cycloalkenes, Cope and coworkers 62) were the first to prepare single-bridged allene in an optically active modification 63). 

Introduction of the allene structure into cycloalkanes such as in 1,2-cyclononadiene (727) provides another approach to chiral cycloalkenes of sufficient enantiomeric stability. Although 127 has to be classified as an axial chiral compound like other C2-allenes it is included in this survey because of its obvious relation to ( )-cyclooctene as also can be seen from chemical correlations vide infra). Racemic 127 was resolved either through diastereomeric platinum complexes 143) or by ring enlargement via the dibromocarbene adduct 128 of optically active (J3)-cyclooctene (see 4.2) with methyllithium 143) — a method already used for the preparation of racemic 127. The first method afforded a product of 44 % enantiomeric purity whereas 127 obtained from ( )-cyclooctene had a rotation [a]D of 170-175°. The chirality of 127 was established by correlation with (+)(S)-( )-cyclooctene which in a stereoselective reaction with dibromocarbene afforded (—)-dibromo-trans-bicyclo[6.1 0]nonane 128) 144). Its absolute stereochemistry was determined by the Thyvoet-method as (1R, 87 ) and served as a key intermediate for the correlation with 727 ring expansion induced... 

Copper-catalyzed allylic oxidation allows the functionalization of unactivated alkenes into chiral allylic carboxylates.1347 The use of oxazoline-containing ligands give good enantioselectivities, but the reaction is extremely slow.1348-1351 Chiral bipyridine complexes, in turn, are much more active and give products in good yields and enantioselectivities up to 70% when applied in benzoyloxylation of cycloalkenes with rm-butyl perbenzoate.1352,1353... 

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