Stereochemistry and structure

Mass spectrometry of biologically active N-heterocycles 87MI4. 

Radical cations derived from N- and N,S-heterocycles, generation and structure investigation 90MI51. 

X-Ray fluorescence spectroscopy of N-arylthiosubstituted derivatives of saturated N-heterocycles 88MI21 90MI17. 

Srain effects on basicities of saturated N-heterocycles 89CRV1215. 

Amination of N-heterocycles (Chichibabin reaction) 87KGS1011 88AHC(44)1 90AHC(49)117. 

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Chemistry in three dimensions is known as stereochemistry At its most fundamental level stereochemistry deals with molecular structure at another level it is concerned with chemical reactivity Table 7 2 summarizes some basic definitions relating to molec ular structure and stereochemistry... 

The need for simple names to describe complex structures has been met in several ways, the most straightforward of which is to use a trivial name giving little or no structural information e.g. morphine, opuntiol). Such names are.often based on the Latin name of the species from which the compound was isolated e.g. opuntiol from Opuntia eliator). While this is acceptable for a newly isolated compound of unknown structure, it is less satisfactory once the structure is established. What is needed is some means of establishing the relationship of the compound to others in the same class, without going into too much detail with regard to structure and stereochemistry. This can be achieved by defining, for a particular group of structures, a parent structure. 

Predict the structure and stereochemistry of the products that would be obtained under the specified reaction conditions. Explain the basis of your prediction. 

Partial Synthesis of a-Amyrin Acetate Proof of the Structure and Stereochemistry... 

Other methods of identification include the customary preparation of derivatives, comparisons with authentic substances whenever possible, and periodate oxidation. Lately, the application of nuclear magnetic resonance spectroscopy has provided an elegant approach to the elucidation of structures and stereochemistry of various deoxy sugars (18). Microcell techniques can provide a spectrum on 5-6 mg. of sample. The practicing chemist is frequently confronted with the problem of having on hand a few milligrams of a product whose structure is unknown. It is especially in such instances that a full appreciation of the functions of mass spectrometry can be developed. 

A Dimethyl butynedioate undergoes a Diels-Alder reaction with (2 ,4 )-hexadiene. Show the structure and stereochemistry of the product. 

Structure and stereochemistry of the dibromides 2a-h were elucidated by various NMR methods including ID proton-proton NOE difference spectra and 2D... 

The aldol reaction is also important in the synthesis of more complex molecules and in these cases control of both regiochemistry and stereochemistry is required. In most cases, this is accomplished under conditions of kinetic control. In the sections that follow, we discuss how variations of the basic mechanism and selection of specific reagents and reaction conditions can be used to control product structure and stereochemistry. 

The stereochemical outcome is replacement of the C—B bond by a C—O bond with retention of configuration. In combination with stereospecific syn hydroboration, this allows the structure and stereochemistry of the alcohols to be predicted with confidence. The preference for hydroboration at the least-substituted carbon of a double bond results in the alcohol being formed with regiochemistry that is complementary to that observed by direct hydration or oxymercuration, that is, anti-Markovnikov. 

Bromination of 4-r-butylcyclohexene in methanol gives a 45 55 mixture of two compounds, each of composition CuH21BrO. Predict the structure and stereochemistry of these two products. How would you confirm your prediction ... 

On the basis of the mechanistic pattern for oxymercuration-demercuration, predict the structure and stereochemistry of the alcohol(s) to be expected by application of the reaction to each of the following substituted cyclohexenes. 

Carbocation intermediates are involved and the structure and stereochemistry of the product are determined by the factors that govern substituent migration in the carbocation. Clean, high-yield reactions can be expected only where structural or conformational factors promote a selective rearrangement. Boron trifluoride is frequently used... 

The photorearrangement of a dienone was noted<4) as early as 1830 in a study of the sesquiterpene a-santonin (1). However, the structure and stereochemistry of the various photoproducts were not conclusively established until 1965.(6) Upon irradiation in neutral media, a-santonin (1) undergoes rapid rearrangement to the cyclopropyl ketone, lumisantonin (2). However, if the irradiation is not terminated after a short period of time the lumisantonin itself rearranges into a linearly conjugated dienone (3). The dienone (3) can be isolated from the photolysis of either (1) or (2) in benzene or ether. In nucleophilic solvents (alcohol or water) the dienone (3) is also photo-chemically active and is further converted into an ester or an acid (photo-santonic acid) (4). 

Natural circular dichroism (optical activity). Although circular dichroism spectra are most difficult to interpret in terms of electronic structure and stereochemistry, they are so very sensitive to perturbations from the environment that they have provided useful ways of detecting changes in biopolymers and in complexes particularly those remote from the first co-ordination sphere of metal complexes, that are not readily apparent in the absorption spectrum (22). It is useful to distinguish between two origins of the rotational strength of absorption bands. 

Wu YC, Chen CH, Yang TH, et al. Cytotoxic aporphines from Artabotrys uncinatus and the structure and stereochemistry of artacinatine. Phytochemistry 1989 28 2191-2195. 

In order to determine the structure and stereochemistry of compounds 36 and 37, standard H- HCOSY, H- C shift correlation (HETCOR), DEPT, and H NOE NMR experiments were performed. The 13C NMR spectrum of compound 36 is based on DEPT, HETCOR, and long-range H- C shift correlation experiments, which allowed the correct assignment of most of the carbon signals <1997MI328>. 

The pyranofurooxazoline 109 can be prepared by a nitrene insertion reaction of the corresponding furan 110 upon treatment with ethyl azidoformate at — 50 °C under photolysis conditions. Compound 109 is moisture sensitive, and upon treatment with wet acidic THF was converted quantitatively to the more polar furanopyran 111. The structure and stereochemistry of 109 were proved unambiguously by X-ray diffraction, showing that the nitrene inserted anti to the bridgehead methyl group <1999JOC736> (Scheme 30). 

Buss, V., The Electronic Structure and Stereochemistry of Simple Carbonium Ions, 7, 253. 

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