Rotation, sign

Optical rotations sign employed to assign configuration ec by chiral H NMR. b No second cycle was undertaken. c Second cycle was undertaken using (17 )-15. 

For some carbonyl compounds, a reverse in optical rotation sign has been observed when the ORD spectra were measured in solvents of different polarity. For example, (5)-5-hydroxy-l,7-diphenyl-3-heptanone is dextrorotatory in trichloromethane (molar optical rotation [interaction between the y9-ketol moiety and the solvent is responsible for the change and reversal in optical rotation with increasing solvent polarity. 

Target/fu notion Resolving agent (number of recrystallization steps) Yield of crystalline diastereomeric salt (%) Configuration or optical rotation sign of resolved enantiomer ee or optical purity (op) (%) Reference... 

The high rotational strengths of molecules like 3 and 34-50 are remarkable. The helicity of the skeleton may account mainly for this. Molecules exhibiting the same rotational sign show the same helicity too, e.g. the enantiomer rotating counter clockwise is correlated to minus (M) helicity... 

It should be noted first that the fact that only enantiomers of tlie same optical rotation sign are found in proteins and nucleic acids - the homo part of homochirality - may be viewed as a necessity, most probably a consequence of natural selection. Protein molecules containing both d- and L-amino acids, nucleic acid constructed with the two enantiomers of the pentoses, would lack some of their key biological properties. Life as we know it could not exist with heterochiral proteins or nucleic acids. The alleged mystery thus concerns the chirality part, the actual nature of the enantiomers used l- rather than D-amino acids, and d- rather than L-pentoses. 

The absolute configuration of quebrachamine (XVI) already related to that of aspidospermine (XV see Volume VIII) has been firmly established by oxidative cyclization (mercuric acetate in acetic acid or potassium permanganate in dimethylformamide) of both enantiomers to 1,2-dehydroaspidospermidine (XVII) of the opposite rotational sign, followed by reduction to aspidospermidine (XII 19h, 58). The similarity in the IR- and NMR-spectra as well as the ORD-curves of (-H )-XII and... 

No Specific rotation is a physical property and the direction of rotation (sign) is part of that property. It must be measured in a polarimeter and cannot be estimated or guessed at. 

Bromofluoroketene ethyl trimethylsilyl acetal [Br(F)C=COEt(OSiMe3), fi -mix-ture] undergoes enantioselective aldol reactions with aldehydes in the presence of Masamune s catalyst. The enantioselectivity is markedly temperature dependent, with examples of high ee at —78 and —20°C, but of opposite rotation sign. 

The axial ratio of the ellipse is p, and the sign of p gives the sign of rotation (+ sign corresponds to the left-handed rotation, and - sign for the right-handed rotation). From Equations (4.31), (4.34), and (4.41), we have... 

Finally, following Mead and Truhlar [10], it may be seen that an interchange of A and B is equivalent to a sign reversal of <() followed by a rotation perpendicular to the AB bond, under the latter of which Aab) is invariant and Fab) changes sign. The net effect is therefore to induce the tiansitions... 

Conservation of orbital symmetry is a general principle that requires orbitals of the same phase (sign) to match up in a chemical reaction. For example, if terminal orbitals are to combine with one another in a cyclixation reaction as in pattern. A, they must rotate in the same dii ection (conrotatory ovei lap). but if they combine according to pattern H. they must rotate in opposite directions (disrotatory). In each case, rotation takes place so that overlap is between lobes of the it orbitals that are of the same sign. 

The small capital letter prefix refers to configuration, related to n glyceraldehyde, and not to the direction of optical rotation. The sign of optical rotation is expressed as (+) and (—) or as d and I or by the words dextro and Uuvo. Th is we have n.( —)-fructose and ,-(+).arabinose. 

It IS convenient to distinguish between enantiomeis by prefixing the sign of iota tion to the name of the substance Foi example we lefei to one of the enantiomeis of 2 butanol as (+) 2 butanol and the othei as (—) 2 butanol Optically pure (+) 2 butanol has a specific rotation ol of +13 5° optically pure (—) 2 butanol has an exactly oppo site specific rotation ol of —13 5°... 

The spatial ariangement of substituents at a chiiality centei is its absolute conflgura tion Neithei the sign noi the magnitude of rotation by itself can tell us the absolute con figuration of a substance Thus one of the following stiuctuies is (+) 2 butanol and the othei IS (—) 2 butanol but without additional mfoimation we can t tell which is which... 

Although the alkyl halide and alcohol given in this ex ample have opposite config rations when they have opposite signs of rotation it cannot be assumed that this will be true for all alkyl halide/alcohol pairs... 

Techniques for determining the absolute configuration of chiral molecules were not developed until the 1950s and so it was not possible for Eischer and his contemporaries to relate the sign of rotation of any substance to its absolute configuration A system evolved based on the arbitrary assumption later shown to be correct that the enantiomers... 

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