Meso diastereoisomers

Look out for meso diastereoisomers in compounds that have a degree of symmetry In their overall structure. Inositol, one of whose dlastereomers is an important growth factor, certainly possesses some meso diastereoisomers. 

H-nmr spectroscopy readily enables the differentiation of chiral from achiral (meso) diastereoisomers of 3,5- and 2,6-dimethyl-4-phenylpiperidines because in the meso forms the two methyls have identical environments and give rise to the same resonance, while chiral isomers involve an axial-equatorial pair that results in two separate methyl signals (see 39). The configurations of both the chiral 2,6- and 3,5-dimethyl reversed ester analogs of pethidine have been confirmed by X-ray crystallography/27 41)... 

Truxillic acid exists as five meso diastereoisomers. The substituents, as in 85, can be either up (U) or down (D) (see Mislow17) ... 

The fluorescence decay curves of l c and IqC monitored at 377 nm and 500 nm can be described by the same decay laws used for the respective meso diastereoisomers. 

I The results obtained for the racemic diastereoisomers lj,c and indicate that even when the rate of conformational change between the different chain conformations is much faster than the rate of excimer formation, resulting in fluorescence decays similar to those of the meso diastereoisomers it is possible to extract some information on the conformational distribution of the chain from the obtained data. 

Various aliphatic and aromatic aldehydes can be used and either enantiomer of the metric diastereoisomer 76 can be made depending on the enantiomer of IpcdlCl used. Two allylic transfers occur, each with same sense of asymmetric induction and this is easier to see if 75 is redrawn and the intermediate 77 also redrawn before reaction. The ratio of the C2 to the meso diastereoisomers is usually greater than 90 10 and the ees uniformly >95%. 

Hence, provided that the contribution of monomer fluorescence can be neglected at the excimer wavelength, deviations from unity of the ratio (- 22/ 2 indicate that the excimer is in fact preformed in the ground state. This has recently been shown to be the case for the meso diastereoisomer of 2,4-di(2-pyrenyl)pentane, based on a combination of time-correlated single-photon counting and NMR measurements (20). 

The mass spectral evidence, as well as this chemical evidence, indicates that quadrigemine A has a symmetrical structure, but the n.m.r. evidence indicates that it is probably a mixture of diastereoisomers. On the basis of the optical rotation evidence derived from quadrigemine A, the degradation product (253), and the dimeric alkaloids (247), (248), and calycanthidine, it has been suggested that quadrigemine A is a mixture of the (RRSR) diastereoisomer illustrated (251), together with one or both of the meso diastereoisomers (RRSS) and (RSRS). 

Considerable interest arises in electrochemical reactions in which a chemisorbed radical may be formed from an initially optically active center. In certain cases, chirality of the radical produced may be preserved if chemisorption occurs and leads to stereochemically different products from those which may arise in the corresponding homogeneous reaction. Examples which have been considered are products arising from reduction of certain ketones and in decarboxylation. For example, the stereochemistry at the C center in a radical RR R"C derived from an acid RR R"C COOH may be preserved if the radical is chemisorbed. Also, the radio of dl to meso diastereoisomers in the case of dimeric products may differ according to whether the products are formed heterogeneously or homogeneously. Steric effects can also influence the reduction of cis and trans reactants or the formation of cis and trans products in appropriate cases. 

There are 10 possible diastereoisomers for the Ni(m so-l,7-CTH) isomer four are meso and six are racemic— idealized structures m-1.7-1 through 10, Fig. 2. Structures m-1,7-1 and 2, and m-1,7-3 and 4 are of the basic forms II and III (Fig. 1), respectively, and are meso-diastereoisomers of C, symmetry. Structures m-1,7-1 and 3 have the methyl groups attached to the asymmetric carbons in equatorial positions, while in isomers m-1,7-2 and 4 these methyl groups are in axial positions. 

Table 2 summarizes all of the diastereoisomers, their symmetries, and asymmetric designations for the Ni(l,4-CTH) system. All of the meso-diastereoisomers have symmetry and have the methyl groups attached to the asymmetric carbons either both axially or both equatorially orientated. 

Ensch and Hesse adopted a different strategy to constmct the 1 -lactam in their racemic synthesis of homaline (1) and its meso diastereoisomer R,S)-epi-homaline (20), namely the Sb(OEt)3-mediated lactamization of diamine 24, itself derived firom commercially available (RS)-3-amino-3-phenylpropanoic acid 5. N-Tosylation of (RS)-3-amino-3-phenylpropanoic acid 5 followed by esterification with ethanoHc HCl gave 21 in 96% yield. N-Alkylation of 21... 

By analogy to this racemic synthesis of (l .S,l -homaline (1) and its meso diastereoisomer (J, -epi-homahne (20), Ensch and Hesse s enantiospecific synthesis of (—)-(J J )-hopromine (2) reHed on the Sb(OEt)3-mediated lactamization of diamines 34 and 35 to form the desired -lactams 36 and 37, followed by stepwise alkylation and subsequent manipulation to give (—)-(R,R)-hopromine 2)7 Starting from L-aspartic acid 29, N-tosylation... 

N-meso diastereoisomers and one N-racemic diastereoisomer can occur. Carbon-13 NMR measurements suggest that the complex is a mixture of the possible stereoisomers. Base hydrolysis of [Co([15]aneN5)Cl] is quite rapid with fcoH = 2.45 x 10" M s at 30 C and / = 0.1 M, a result which can be rationalized in terms of the mcr-N3 donor set. " Acid aquation with feaq = 1.66 X lO"" s at 50°C is, as expected, quite slow. 

Depending on the crystallization conditions, solvated or unsolvated structures result and the configurations at vanadium varies (Fig. 64). In all structures, vanadium-vanadium and sulfur-sulfur distances are 2.9 and 2.0 A, respectively, and the structural unit has been compared to that of the mineral patronite (VS4), which consists of linear chains of vanadium(IV) ions bridged by S ligands (172). Molecules generally possess a center of inversion and thus are the meso diastereoisomers (172, 696), the exception is [V2(p-S2)2(S2CN—i-Bu2)4], which adopts the rac form, possibly in order to ease the steric crowding caused by the bulky isopropyl substituents (695). 

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