Cyclohexyl backbones

Subsequently, these catalysts were evaluated in the enantioselective desymmetri-sation of achiral trienes, and three distinct trends in catalyst selectivity were found. Firstly, catalysts 56a-b with two phenyl moieties on the backbone of the A -heterocycle exhibited higher enantioselectivity than those with a fused cyclohexyl group as the backbone 55a-b. Secondly, mono-ort/io-substituted aryl side chains induced greater enantioselectivity than symmetrical mesityl wing tips. Thirdly, changing the halide ligands from Cl to I" increased the enantioselectivity. As a result, catalyst 56b turned out to be the most effective. For example, 56b in the presence of Nal was able to promote the desymmetrisation of 57 to give chiral dihydrofuran 58 in up to 82% conversion and 90% ee (Scheme 3.3). 

The proton decoupled carbon 13 NMR spectra for three poly( cyclohexylmethyl-co-isopropylmethyl) copolymers are shown in Figure 4. The backbone methyl group is observed as occurring between -4 and -1 ppm and consists of multiple resonances which are due to polymer microstructure. Multiple resonances are also observed for the methyl and tertiary carbon of the isopropyl group and for the methine carbon of the cyclohexyl group. Microstruc-tural assignments for these resonances remain to be made. It has also been found that increasing the bulky character of the substituent yielded broader resonance peaks in the carbon-13 NMR spectra. 

Jacobsen and coworkers discovered that chiral salicylimidato transition metal complexes activate epoxides in a stereoselective manner. The published mechanism indicates that one Cr° (salen)-N3 with (/ ,/ )-cyclohexyl backbone acts as Lewis acid and coordinates to the oxygen of PO, while a second catalyst molecule transfers the azide to the activated epoxide and thus opens the ring. The coplanar arrangement of the two chromium complexes prefers one enantiomer of PO and so induces stereochemical information [99,100, 121-129]. (cf. also Sect. 8.3) (Fig. 42). 

The use of amino alcohol derivatives with a cyclohexyl backbone was reported recently [30b]. 

The structure of the methyl methacrylate-co-N-cyclohexyl maleimide copolymers is shown in Fig. 33. The maleimide cycle is a rigid structure whose C - C bond contributes to the chain backbone. 

Various types of sorbents used for SPE can be grouped (Table 2.6) according to the primary mechanism by which the sorbent and the analyte interact [32,72]. Reversed-phase bonded silica sorbents having alkyl groups such as octadecyl (Qg, C18), octyl (C8, C8), or ethyl (C2, C2) covalently bonded to the silica gel backbone or cyclohexyl (CH) or phenyl groups and sorbents composed of polymeric resins such as polystyrene-divinylbenzene... 

The influence of the cyclohexane ring on the glass transition temperature and sub-Tg transitions has been studied by dynamic mechanical methods [10,16,42]. These comparative studies have focused on the influence of the cyclohexyl substituent on observed thermal transitions. A systematic study of cycloalkyl substituents on alternating carbons of a polyethylene backbone was performed to study the influence of these substituents on the dynamic mechanical spectra of the polymers [42], These workers also prepared materials in which the cycloalkyl substituents were spaced away from the polymer backbone by successively longer methylene chains, testing the influence of ring proximity... 

Figure 27. Side and top views of ca. one turn of the 14-helix extracted from the crystal structure of Gellman s trans-ACffC hexamer.239 A single S(14) H-bond circuit in the peptide backbone has been rendered as thick cylinders. Additionally, one residue has been rendered as a space filling model to provide a frame of reference between the two views and to more clearly show the spatial relationship between cyclohexyl groups.

Related to this work are also the investigations carried out on peptide nucleic acids (PNAs) which have recently been reviewed by Nielsen.39 PNAs are a new class of flexible oligomeric DNA analogues in which the base pairs are carried by a peptide backbone. When this backbone consists of cyclohexyl-derived amino acids, the molecules adopt a more rigid conformation and are able to also form a doublehelical structure.40... 

Figure 34 Synthesis of chiral salen indium complexes with cyclohexyl backbones...

Table 1). This may be due to the increased steric bulk of the BINAP backbone relative to the cyclohexyl backbone in complex 2, which may also be responsible for the low activity of the system. Interestingly, the same trend is not observed with the analogous A1 complexes (K" -SalBinap)Al(0 Pr) and (0NN0)Al(0 Pr), which have polymerization rates within the same order of magnitude and produce highly isotactic PLA.> i... 

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