Excited racemization

Desilylation of 20, followed by oxidation of the secondary hydroxyl group furnishes racemic 1 in a yield of 81 %. Spectroscopic and bioassay characteristics of synthetic ( )-1 matched natural peripla-none B in every detail. The constitution and relative stereochemistry of the elusive American cockroach sex excitant periplanone B (1) has been secured in a very elegant way.5... 

Thus isomerization of the trans to cis ketone is stereospecific while isomerization of cis to trans results in about 28% racemization. Postulating different pathways for trans to cis and cis to trans isomerizations would appear at first hand to violate the principle of microscopic reversibility. However, what we are dealing with here are the excited state intermediates as reactants and these are not necessarily in equilibrium for the two isomers. For example, taking the analogy... 

Derivatives of FLEC are formed without any racemization of the sample and the derivatives of the D and L forms of the test molecules are eluted from reverse-phase columns in sequence. FLEC derivatives are fluorescent, having an excitation maximum at 260 nm and an emission maximum at 315 nm and are particularly useful in the analysis of amino acids. 

Trigonal ML3 metal complexes exist as optically active pairs. The complexes can show enantiomeric selective binding to DNA and in excited state quenching.<34) One of the optically active enantiomers of RuLj complexes binds more strongly to chiral DNA than does the other enantiomer. In luminescence quenching of racemic mixtures of rare earth complexes, resolved ML3 complexes stereoselectively quench one of the rare earth species over the other. 35-39 Such chiral recognition promises to be a useful fundamental and practical tool in spectroscopy and biochemistry. 

The confusion generated in the initial report185 on photoracemization of sulfoxides has recently been removed with the postulate that naphthalene singlet forms an excited complex with sulfoxides.186 Thus, despite the fact that the singlet state of 36 lies at 113 kcal, some 23 kcal above that of naphthalene, 36 quenches the fluorescence of this hydrocarbon with kes = 3.2 x 107Af -1 sec-1. From the dependence of the quantum yield of racemization on sulfoxide concentration (Eq. 34), a value of kes = 2.3 x 107Af-1 sec-1 was deduced. Since these values are the same within experimental error, it follows that the singlet state of naphthalene is responsible for photoracemization. 

The P- and M-cis-syn-conformers give photoreactions, whose rates are proportional to el when left-handed CPL is used. Therefore, a high optical yield requires a high g-factor 69 After the excitation cyclization occurs leading to a DHP, which is readily oxidized to helicene. The precursor remains racemic as racemization in the ground state, is easy. Racemization in the excited state, however, may be slow, compared to cyclization. According to Kagan 69) this mechanism explains the asymmetric synthesis well. 

The ortho substituents used were supposed to have no influcence on the absorption spectrum and to possess the same circular dichroism as the parent compound. The ortho substituted compound can only photocyclize in its exo-conformations, but equilibration by a rotation over the phenylethylene bond converts an M-exo into a P-endo form (or P-exo into M-endo) which cannot cyclize. Indeed, the optical yields of the 4-substituted [6]hclicenes from the ortho substituted (1+4) precursors are not lowered in comparison with the unsubstituted (2 + 3) precursor. On the other hand (4+1) precursors having a para substituent give again a low optical yield. The values found suggest that racemization in the excited state is slightly hindered, what may be due to steric hindrance. 

Modifying the parent phosphinooxazoline 38 to ligand 57, exciting improvement in the regio- and enantioselectivity has been realized (entries 1 and 2) [151,152]. Similar enantio- and regioselectivities obtained from the reaction of the racemic substrate 125 suggest a facile... 

The discovery that certain catalysts are capable of producing crystalline polymer from optically inactive (racemic) monosubstituted epoxides has caused a good deal of excitement in recent years because it seems virtually certain that such crystallinity must originate from an asymmetric synthesis of the polymer chain (24). 

The optically active 1,2-dioxetane of 2,4-adamantanedione (89) was synthesized. Thermal activation of 89 yielded chemiluminescence (Xmax = 420 nm characteristic of ketone fluorescence), pointing to intermediate 90 which is chiral only in its excited state due to the out-of-plane geometry of one of the two carbonyl groups. However, circular polarization of chemiluminescence measurement of 90 has not detected optical activity at the moment of emission. The authors have concluded that fast, relative to the lifetime of ketone singlet excited state, intramolecular n, it energy transfer caused racemization of 90196. 

Two HPLC methods for the determination of enantiomers of donepezil HC1 in rat plasma have been developed [36].The first method involves chiral separation of donepezil HC1 on an ovomucoid-bonded column, and native fluorescence detection of donepezil HC1 with excitation at 318 nm and emission at 390 nm. The fluorometric detection is without interference from background components and is about five times more sensitive than UV detection at 271 nm. The method was applied to monitoring the racemization of each enantiomer of donepezil HC1 in buffer solutions and in rat plasma. The second method involves separation of donepezil HC1 from background components of rat plasma on an a chiral column, collection of the donepezil HC1 fraction into a sample loop, concentration to a trap column, transfer of donepezil HC1 to a chiral column, resolution of the enantiomers of donepezil HC1 on the chiral column, and fluorometric detection of the enantiomers of donepezil HC1 with excitation at 318 nm and emission at 390 nm. The detection limits of donepezil HC1 and each enantiomer of donepezil HC1 were 1 ng/ml, respectively, with a 200 (A injection of deproteinized plasma samples. 

FIGURE 5 33 CpL (upp curve) total luminescence (lower curve) for a 0.05 M aqueous solution of Tb(III) with the ligand shown. CPL is observed from this racemic mixture following circularly polarized laser excitation at 488 nm 6... 

Arnold and colleagues have reported a series of chiral homoleptic yttrium and lanthanide fra(alkoxide) complexes [49, 50], These initiators (including complex 1) show high degrees of iso-selectivity and rapid rates, even at low temperatures. Thus, using the racemic mixture of the lanthanide initiator, stereoblock PLA was produced with a P, of 0.81 so far, this is the only known type of yttrium initiator able to exert such stereocontrol and a very exciting finding. Analysis of the stereoerrors indicates that an enantiomorphic site control mechanism is responsible for the iso-selectivity. 

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