Meso derivatives

The stereochemistry of the dimers produced from the dimaso-anthracenes is assumed to be trans, analogous to the head-to-tail dimers observed for the meso derivatives ... 

The vinyloxirane reaction was later extended to methylidene cyclohexene oxide and to related meso derivatives [53]. The effects of the diastereomeric ligands 42 and 43 (Fig. 8.5), derived from (S)-binaphthol and (S, S)- or (R, R)-feis-phenylethyl-amine respectively, were investigated. In the case of kinetic resolution of racemic methylidene cyclohexane epoxide 45 with Et2Zn, ligand 42 produced better yields, regioselectivity, and enantioselectivity than 43 (Scheme 8.27). 

Similar studies were carried out on the meso derivatives of the a, -dichloro- and dibromo-adipodinitriles (17) and (18) (see Scheme 21). These molecules have conformations similar to those of the corresponding esters and exhibit the same reactivity when exposed to ammonia, thus demonstrating for these systems a mechanism identical to that for the esters. 

The related sennosides, glycosides A, B, C and D, occur in amounts of 1.5-3% in the leaves of Egyptian senna shrubs that are cultivated primarily in India. Senna leaves were previously used by Arab physicians as a laxative drug and digestive stimulant. Heterobianthrones sennosides A (meso-derivative, 9-169) and B (trans-derivative, 9-170) are bianthrone glycosides derived from aloe-emodin and rhein, homobianthrones sennosides C meso-derivative) and D (tra s-derivative) are derived only from rhein. 

Addition of bromine has been the subject of a number of studies. When a neutral salt of maleic or fumaric acid (A = COO ) is brominated in water, the meso derivative is the predominant product. Bell and Pring also report that the meso product is obtained on bromination of the diethyl esters of fumaric and maleic acids. Terry and Eicheberger observed 78% meso product from disodium maleate, a cis addition product which is unexpected. The prevalent bromonium ion mechanism proposed by Roberts and Kim-ball could easily explain the trans addition as follows ... 

Little work has been devoted to the class Gastropoda, from which only a few derivatives have been isolated. Limatu-lone is a bicydic triterpene of the squalenoid type produced by the limpet Collisella limatula for protection against predation by crabs and fish (Albizati, Pawhk, and Faulkner, 1985 Pawlik, Albizati, and Faulkner, 1986). Despite the presence of four asymmetric carbon atoms, limatulone is optically inactive and it was determined by total synthesis that natural limatulone contains both the meso derivative and the racemic mixture (Mori, Takikawa, and Kido, 1993). 

Tetrahydro derivatives are formed when either quinoxaline or 6-chloroquinoxaline is reduced with lithium aluminum hydride in ethereal solution. Similar reduction of 2,3-dimethylquinoxaline gives the meso-(cts)-1,2,3,4-tetrahydro derivative. This is shown to be a stereospecific reduction since lithium aluminum hydride does not isomerize the dl-(trans)-compound. Low temperature, platinum catalyzed, hydrogenation of 2,3-dimethylquinoxaline in benzene also gives meso (cis) -l,2,3,4-tetrahydro-2,3-dimethylquinoxaline. ... 

Porphyrins are formally derived from the porphin (1) nucleus by substitution of some or all peripheral positions with various side chains. In the classical system of nomenclature T, introduced by H. Fischer,Sc the peripheral /5-pyrrolic positions are numbered front 1 to 8 and the methine positions (also named meso positions) between the pyrrole rings are designated a, //, y, and 5. The rings are lettered clockwise A, B, C, and D. The classical nomenclature was in the past more and more displaced by a nomenclature which numbers all the carbon... 

Nakajima reported the use of a chiral bipyridine N,N -dioxide 18 in the desym-metrization of acyclic meso epoxides (Figure 7.3). Although the enantioselectivity was not as high as in the method developed by Fu for meso-stilbene oxide (90% ee vs. 94% ee), it was higher for the same aliphatic epoxide (74% ee vs. 50% ee) [57]. Nakajima showed that mono-N-oxide derivatives 19 and 20 were much less effective than 18 in tenns of both yield and enantioselectivity, and accordingly proposed a unique mechanism for 18 involving a hexacoordinate silicon intermediate coordinated to both N-oxides of the catalyst. 

Picolinate and pyridine-2,6-carboxylate give stable complexes, with 4- and 6-coordination. Macrocycles like porphyrins afford silver(II) derivatives most remarkable is the reaction of the macrocycle meso-Me6[14]ane (Figure 4.13). 

Die Reduzierbarkeit meso-ionischer Verbindungen mit Hydriden ist ebenfalls struk-turabhangig. Sydnone werden z. B. durch Natriumboranat nicht angegriffen1 1,3-Thia-zon-(4)-Derivate reagieren dagegen unter Bildung von 4-Oxo-1,3-thiazolidinen2. 

The prefix D or L must be given when a derivative of a meso form has become asymmetric by substitution. It is also necessary to define the configurational prefixes as D or L in the case where there are more than four contiguous asymmetric carbon atoms. 

Compounds containing carboxyl groups on adjacent carbons (succinic acid derivatives) can be bisdecarboxylated with lead tetraacetate in the presence of O2 263 jjjg reaction is of wide scope. The elimination is stereoselective, but not stereospecific (both meso- and dl- 2,3-diphenylsuccinic acid gave trans- stilbene) a concerted mechanism is thus unlikely. The following mechanism is not inconsistent with the data ... 

To summarise in authentic tasks, we have established that stmcture-property relations can be described by a dynamic system of stmctures, properties and their interrelations. Within the limits of our study we have derived a generalised conceptual schema, which we expect to be useful to teach macro-micro problems in which stmcture-property relations can be explicitly used (Figs. 9.2, 9.3 and 9.4). The system of nested stmctures, systematically assigned to appropriate scales, and the properties of the different stmctural components reveal a conceptual schema necessary for macro-micro thinking. The system of relevant nested stmctures and the explicit relations between stmctures and properties form the backbone of macro-micro reasoning. Depending on the task, a number of different meso-levels are relevant and... 

Treatment of methylated diphenyl ethers with BuLi/TMEDA in cyclohexane/diethyl ether followed by addition of CuCl resulted in excellent yields of the corresponding dibenzofinans <96JCR(S)362, 96JCR(M)2016>. An efficient route to meso benzoxabicyclo[2.2. IJheptyl derivatives beginning fi om p eronal has been developed (Scheme 24). 

In the future, further studies should be addressed to improve the chemose-lectivity and diastereoselectivity of the reductive coupling process, especially searching for novel reagents and milder experimental conditions. As a matter of fact, a few novel reductive couphng procedures which showed improved efficiency and/or stereoselectivity have not been further apphed to optically active imines. For example, a new electrochemical procedure which makes use of the spatially addressable electrolysis platform with a stainless steel cathode and a sacrificial aluminum anode has been developed for imines derived from aromatic aldehydes, and the use of the N-benzhydryl substituent allowed 1,2-diamines to be obtained with good yields and dl-to-meso ratios... 

An early example of the use of a subcatalytic amoimt of sparteine for the activation of an organolithium nucleophile was reported by Lautens et al. in the carbometallation of a meso-unsaturated oxabicycle 25, with ring opening leading to the substituted cycloheptene derivative 26 (Scheme 4) [4]. Both yield and enantiomeric excess remained virtually unchanged when the ratio n-BuLi sparteine was lowered to 1 0.15. However, when a 3 mol% amount of the ligand 1 was used, a 20% decrease in enantioselectivity was observed. 

Fig. 3. Haptens and multivalent antigens prepared in this syudy. The viologen derivative, 4,4 -bipyridinium, l-(carboxypentyl)-l -methyl-dichloride (1), divalent antigen 2, and trivalent antigen 3. Anti-porphyrin antibodies were elicited for [5-(4-carboxyphenyl)-10,15,20-tris-(4-methylpyridyl)]porphine (3MPylC) or meso-tetrakis(4-carboxyphenyl)porphine (TCPP). meso-Tetrakis(4-methylpyridyl)porphine (TMPyP) was used to investigate the specificity of the antibody dendrimer for porphyrins...

The thermal solid-to-solid cyclization reaction of diallene derivatives also proceeds stereospecifically. Reaction of 1,6-diphenyl-1,6-di(p-tolyl)hexa-2,4-diyne-l,6-diol (113) with HBr gave meso- (114) and rac-3,4-dibromo-l,6-di-... 

For dihydrodiols derived from substituted benzenes, the key to their significance lies in the availability of two adjacent chiral centers with an established absolute stereochemistry. The dihydrodiol from benzene is, of conrse, the meso compound, although enantiomers produced by subsequent reaction with a chiral reagent are readily separated. There are useful reviews containing nnmerous applications (Carless 1992 Ribbons et al. 1989), many of which involve, in addition, the nse of di-flnoro-, di-chloro-, or di-bromobenzene-2,3-dihydrodiols. 

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