Cram studies

Abd Elhafez, F. A., and D. J. Cram Studies in stereochemistry, XVII. The course of the solvol5rtic substitution-reactions in the 1,2-diphenyl-l-propyl system. J. Amer. chem. Soc. 73, 339 (1953). 

The process of inversion (racemization) without exchange is called isoinversion. This involves an ion-pair mechanism. Indeed, in the presence of a crown ether, to separate the ions, the percentage of racemate increases (307). In fact, the stereochemical course of many metal-alkoxide catalyzed reactions in nonpolar solvents can be drastically modified by addition of catalytic amounts of crown ethers to the medium. For this reason, ion pairs, in low dielectric media, play a remarkable role as intermediates in reactions in which the negative ion is a carbanion. For example. Cram studied the rate of exchange (ke) and racemization kj as a function of the substituents present with deuterium-labeled 9-methylfluorene (309). The most interesting case was kjk < 0.5, racemization without exchange. This happens for X = di-... 

In cases of more effective 7t-electron donor or tz-donor neighboring groups, as is the case in forming /3-phenylethyl (studied by Don Cram from UCLA Nobel Prize in chemistry, 1987) or /3-halogen bridged species, these have sufficient electrons to form 2e-2c bonds (with some intermediate delocalization). 

Addition of Grignard reagents to ketones and aldehydes was one of the reactions which led to the formulation of Cram s rule. Many ketones and aldehydes have subsequently been examined to determine the degree of stereoselectivity. Cram s rule is obeyed when no special complexing functional groups are present near the reaction site. One series of studies is summarized in Table 8.2. 

As in the previous categories in this section, there are numerous compounds which have been prepared based on a sugar subunit. Examples may be found in Refs. 7,35,42-45, 57, 82-85, 117—121,175,176,193 and 208. Much of the work in these references has been reported by Stoddart and his coworkers, who have pioneered this field. As with the compounds prepared by Cram, the goal was to prepare a chiral receptor for ammonium ions which could be utilized in enzyme model studies. 

Cram has published several studies indicating that complexation of ammonium salts was greatly enhanced by using cyclic polyethers over the corresponding acyclic ones. These molecules were analogs of the binaphthyl systems under study and were generally prepared by phenoxide substitution on the appropriate tosylate. The approach is illustrated below in Eq. (7.9). ... 

In his original paper,2 Cram disclosed an alternative model that rationalizes the preferred stereochemical course of nucleophilic additions to chiral carbonyl compounds containing an a heteroatom that is capable of forming a complex with the organometallic reagent. This model, known as the Cram cyclic or Cram chelate model, has been extensively studied by Cram9 and by others,410... 

One study showed that the Cram s mle product predominates with metal hydride reducing agents, but the other product with Bouveault-Blanc and dissolving metal reductions Yamamoto, Y. Matsuoka, K. Nemoto, H. J. Am. Chem. Soc., 1988, 110, 4475. 

The structures and conformational properties of a simple hemicarcerand, created earlier by Cram, see <96JA5590>, as well as the complexation and decomplexation with guest molecules have been computationally studied <96JA8056>. 

The p a values of two systems with two intra-annular COOH groups have been measured. In Cram s study (Bell et al., 1982), the macrocyclic diacid [36] and an open-chain monomeric analogue [37] had almost identical pK values (see Table 18). In contrast, Gennari s compound [38], which contains two intra-annular COOH groups and in addition two"ethyl ester groups, has a different acidity from that of the analogues [39] and [40], as shown in Table 19 (Gennari et al., 1992). But in both systems, the difference Ap a between pK i and p a2 was comparable (ApA a [36] = 1.7, Ap/La[38] = 1.5). The increase from p ai to P a2 may occur for two... 

The stereochemistry of addition of organometallic reagents to chiral carbonyl compounds parallels the behavior of the hydride reducing agents, as discussed in Section 5.3.2. Organometallic compounds were included in the early studies that established the preference for addition according to Cram s rule.118... 

Cram, D. Studies in Stereochemistry. I. The Stereospecific Wagner-Meerwein Rearrangement of the Isomers of 3-Phenyl-2-butanol. J. Amer. chem. Soc. 71, 3863 (1949). 

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Anionic units have not only been attached to macrocyclic polyethers via flexible arms but have also been incorporated into the cycle itself. Alberts and Cram (1976, 1977) have studied the ion-binding properties of crown ethers containing / -diketone units, such as [72] and [73]. 

Total resolution of [269] into its two enantiomers was achieved by liquid-liquid chromatography through complexation to L-valine adsorbed initially on diatomaceous earth (Timko et al., 1978). On the basis of comparative chromatographic studies, the separation factors (a) and the EDC values were correlated (Cram et al., 1975) (Table 59). 

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Reich and Cram 8 > studied the patterns of electrophilic substitution of the monosubstituted [2.2]paracyclophanes. It was at once clear that the directive influences of the substituents X (see below) could not be correlated with transannular resonance effects in the ground state 84>. The product pattern predicted on the basis of electrostatic ground-state models, such as the canonical structures 65 for electron-releasing and 66... 

Cram et al.ss> have studied the stereochemical course and solvolysis reactions of optically active l-tosyloxy[2.2]paracyclophane 91 (see scheme 91—95). Reactions 92-+91, 92- -93, 92- -94, and 92- -95 were all shown to proceed as expected, with complete retention of configuration. 

Recent studies on chiral sulfoximides, espcially in the laboratories of Cram, Johnson, Andersen, and Stirling, have been centered on the stereospecific interconversions between chiral sulfoxides, sulfimides, and sulfoximides, as well as on the construction of new stereochemical cycles. For the sake of brevity, only some selected results from these extensive studies are presented here to illustrate various stereospecific syntheses of chiral sulfoximides. 

The procedure most commonly used for the stereospecific preparation of optically active sulfoximides involves the reaction of optically active sulfoxides with arylsulfonyl azides in the presence of copper (98,118,131,178,179). This reaction occurs with retention of configuration at sulfur and with high stereospecificity. The stereospecific sulfoxide-sulfoximide conversion is a key reaction in the stereospecific sulfoxide-sulfimide-sulfoxiraide set of interconversions carried out by Cram and co-workers (98) and shown in Scheme 9. A similar cycle of interconversions studied independently by Andersen and co-workers (179) was used to determine the stereochemical course of the sulfoxide-sulfoximide transformation (see Scheme 10). 

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