Asymmetric spontaneous

Lipids in model systems are often found in asymmetric clusters (see Figure 9.8). Such behavior is referred to as a phase separation, which arises either spontaneously or as the result of some extraneous influence. Phase separations can be induced in model membranes by divalent cations, which interact with negatively charged moieties on the surface of the bilayer. For example, Ca induces phase separations in membranes formed from phosphatidylserine (PS)... 

Asymmetric fission is observed in the spontaneous decomposition of sCf1Ji(15M and other very heavy nuclei. We may ask when the transition to symmetric fission would begin. The next elongated core, in the series represented in Figs. 11 and 12, would contain 31 spherons, and the transition to it should occur for 28 spherons in the core of the undistorted nucleus, that is, at N = 163 (calculated with use of Eq. 1). We conclude that lftf,Lw,(i,20 and adjacent nuclei should show both asymmetric and symmetric fission. 

Fig. 60 Schematic illustration for formation of cylindrical morphology in a blend of slightly asymmetric lower molecular weight PS-b-PI (/3-chain) with large symmetric PS-fc-PI (a-chain). a Molecule of /S-chain with non-zero spontaneous curvature, b Cylindrical morphology formed by neat /3 chains shown in a. Here mean curvature of cylinder (solid line) is larger than spontaneous curvature of /3-chain (dashed lines). c Cylindrical morphology formed by binary blend of /3-chains shown in a and large symmetric copolymers (a-chain). In this case, mean curvature of cylinder closely fits to spontaneous curvature of /3-chain. From [180]. Copyright 2001 American Chemical Society...

Isoquinolinium 369 and [2,7]naphthyridin-2-ium 371 salts have also been used for the preparation of 2,3,8,8a-tetrahydro-57/-oxazolo[3,2-tf]pyridine derivatives (Scheme 98) addition of Grignard reagents to 369 is followed by a spontaneous cyclization to 370 <1998JOC1767> while an asymmetric version of the Bradsher cycloaddition between 371 and chiral enol ether 372 gives 373 in good yield and selectivities <1996TL7019>. 

These schemes have been frequently suggested [105-107] as possible mechanisms to achieve the chirally pure starting point for prebiotic molecular evolution toward our present homochiral biopolymers. Demonstrably successftd amplification mechanisms are the spontaneous resolution of enantiomeric mixtures under race-mizing conditions, [509 lattice-controlled solid-state asymmetric reactions, [108] and other autocatalytic processes. [103, 104] Other experimentally successful mechanisms that have been proposed for chirality amplification are those involving kinetic resolutions [109] enantioselective occlusions of enantiomers on opposite crystal faces, [110] and lyotropic liquid crystals. [Ill] These systems are interesting in themselves but are not of direct prebiotic relevance because of their limited scope and the specialized experimental conditions needed for their implementation. 

Further variations on the epoxyketone intermediate theme have been reported. In the first (Scheme 9A) [78], limonene oxide was prepared by Sharpless asymmetric epoxidation of commercial (S)-(-)- perillyl alcohol 65 followed by conversion of the alcohol 66 to the crystalline mesylate, recrystallization to remove stereoisomeric impurities, and reduction with LiAlH4 to give (-)-limonene oxide 59. This was converted to the key epoxyketone 60 by phase transfer catalyzed permanganate oxidation. Control of the trisubstituted alkene stereochemistry was achieved by reaction of the ketone with the anion from (4-methyl-3-pentenyl)diphenylphosphine oxide, yielding the isolable erythro adduct 67, and the trisubstituted E-alkene 52a from spontaneous elimination by the threo adduct. Treatment of the erythro adduct with NaH in DMF resulted... 

The asymmetric Diels-Alder cycloadditions of enantiopure (5)-5-(/ -tolylsulfinyl)-1,4-benzoquinones with Dane s diene under thermal and Lewis acid conditions produce tetracyclic quinones after spontaneous elimination of the sulfinyl group.The Diels-Alder reaction of barrelene with o-benzoquinone produces tetracyclo[6.2.2.2 .0 ]tetradeca-9,ll,13-triene-4,5-dione. Under kinetic control, the Diels-Alder cycloaddition of 2,3-dicyano-p-benzoquinone (98) with cyclopentadiene in MeOH produces the single cycloadduct (99) (Scheme 38). ... 

Absolute asymmetric synthesis refers to the situation in which an asymmetric induction occurs in the absence of an externally imposed source of chirality [5]. Such reactions are invariably carried out in the crystalline state, where the asymmetric influence governing the enantioselectivity derives from the spontaneous crystallization of an achiral compound in a chiral space group. This phenomenon, which is analogous to the spontaneous crystallization of racemates as... 

In addition to bimolecular [2-1-2] photocycloaddition reactions similar to the one discussed above [11], absolute asymmetric syntheses employing spontaneously grown chiral crystals have been studied for a number of unimolecular photorearrangement reactions as well. The first report of such an investigation dealt with the photoisomerization of dibenzobarrelene diester 6 to the corresponding dibenzosemibullvalene compound 7 [12], an example of the well-known di-7i-methane (Zimmerman) photorearrangement. Achiral diester 6 crystallizes spontaneously from cyclohexane in the now familiar chiral space... 

We describe highly enantioselective asymmetric autocatalysis with amplification of chirality and asymmetric autocatalysis initiated by chiral triggers. Asymmetric autocatalysis correlates between the origin of chirality and the homochirality of organic compounds. We also describe spontaneous absolute asymmetric synthesis in combination with asymmetric autocatalysis. 

Spontaneous absolute asymmetric synthesis, that is the statistical formation of enantioenriched compounds from achiral reagents without the intervention of any chiral auxiliary, has been proposed as one of the origins of chirality. Without using... 

Spontaneous absolute asymmetric synthesis is described in the formation of enantiomerically enriched pyrimidyl alkanol from the reaction of pyrimidine-5-car-baldehyde and /-Pr2Zn without adding chiral substance in combination with asymmetric autocatalysis. The approximate stochastic distribution of the absolute conhgurations of the product pyrimidyl alkanol strongly suggests that the reaction is a spontaneous absolute asymmetric synthesis. 

Crystallization and reactivity in two-dimensional (2D) and 3D crystals provide a simple route for mirror-symmetry breaking. Of particular importance are the processes of the self assembly of non-chiral molecules or a racemate that undergo fast racemization prior to crystallization, into a single crystal or small number of enantiomorphous crystals of the same handedness. Such spontaneous asymmetric transformation processes are particularly efficient in systems where the nucleation of the crystals is a slow event in comparison to the sequential step of crystal growth (Havinga, 1954 Penzien and Schmidt, 1969 Kirstein et al, 2000 Ribo et al 2001 Lauceri et al, 2002 De Feyter et al, 2001). The chiral crystals of quartz, which are composed from non-chiral Si02 molecules is an exemplary system that displays such phenomenon. 

Chiral rhenium complexes, such as 1 and 4, are isoelectronic to the a-alkoxy vinyl-iron complexes discussed above and they exhibit analogous chemistry in many respects. Like the iron complexes, they are prepared as the Z-isomer and are readily alkylated by primary iodoalkanes and (bromomethyl)benzene with efficient 1,3-asymmetric induction97. Subsequent, spontaneous loss of halomethane produces the elaborated rhenium-acyl complexes. Two examples of the stereocontrolled preparation of diastereomeric rhenium-acyl complexes via this methodology are illustrated. 

E. Stochastic Production of S- and -Enantiomers without Adding a Chiral Source by Asymmetric Autocatalysis Spontaneous Absolute... 

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