Cyclic form

The second application of the CFTI approach described here involves calculations of the free energy differences between conformers of the linear form of the opioid pentapeptide DPDPE in aqueous solution [9, 10]. DPDPE (Tyr-D-Pen-Gly-Phe-D-Pen, where D-Pen is the D isomer of /3,/3-dimethylcysteine) and other opioids are an interesting class of biologically active peptides which exhibit a strong correlation between conformation and affinity and selectivity for different receptors. The cyclic form of DPDPE contains a disulfide bond constraint, and is a highly specific S opioid [llj. Our simulations provide information on the cost of pre-organizing the linear peptide from its stable solution structure to a cyclic-like precursor for disulfide bond formation. Such... 

The two /3-turn structures, pc and Pe are the most stable among those considered. This is in accord with the unconstrained nanosecond simulations of linear DPDPE, which converged to these conformers [14]. Because the cyclic form is relatively rigid, it is assumed that the conformation it adopts in solution is the biologically active one, responsible for its high affinity and specificity towards the 5 opioid receptor. The relatively low population of the cyclic-like structure for the linear peptide thus agrees qualitatively with the... 

It is probable that many of the reactions of glucose in solution are due to the small amount of the open chain aldehyde present. If this reacts in a normal manner with a reagent, the equilibrium is disturbed, most of the cyclic form passes into (III) and ultimately the reaction proceeds to completion.-Fructose may be similarly formulated ... 

The 2-isoxazolin-5-ol system is of interest in that many reports indicate that in solution an equilibrium exists between the ring and open-chain forms. The direction of the equilibrium depends on the bulk and/or number of substituents, with increasing either of the above favoring the cyclic form (Scheme 133) (76T1369, 74BSF725). 

By combining M, D, T, and Q units, silicones can be made in a variety of structures, which include linear, branched, hyper-branched and cyclic forms. 

By use of potassium phthalimide we can isolate the intermediate. a-Oxo-S-aminovaleric (9, = 1) and a-oxo-e-aminocaproic acids (9, = 2) readily yield 2l -pyrroline-2-carboxylic acid (10, =1) and. d -piperideine-2-carboxylic acids (10, n = 2), respectively (7-10). The equilibria of the acids with their cyclic forms was observed in water solutions (11,12). 

Five- (70, = 1) and six-membered (70, n = 2) enamines substituted in position 2 generally exist in the cyclic form. Lukes and co-workers observed that partial ring-opening occurs with the pyrroline 157) or piperideine 163,164) derivatives by atmospheric moisture. This leads to the formation... 

By contrast, derivatives formed by the action of certain nucleophilic reagents (e.g, H2O2, alcohols, mercaptans, CN, SOsH, Grig-nard compounds) can only exist in a cyclic form. The most that can occur here is an equilibrium between the nondissociated and the dissociated form (for example, pseudocyanide and true cyanide),-if the newly formed C—O, C—S, or —C bond is sufificiently polar. 

The reaction products formed by other nucleophilic reagents (e.g., amino compounds or carbanions derived from carbonyl or nitaro compounds) can be considered as ring chain prototropic systems. The newly formed C—or C—bond in the cyclic form of these compounds is more or less polar (evidence for this is the sensitivity to... 

Ultraviolet and infrared spectroscopic investigations and also chemical behavior show unambiguously that the compounds which result from the last-rnentioned type of nucleophilic reagent and cotarnine possess the cyclic form. " Examples of these are cotar-nine anil (20a), cotarnine oxime (20b), cotarnine phenylhydra-zone (20c), anhydrocotarnine carbamide (20d), hydrocotarnyl-acetic acid (20e), anhydrocotarnine acetone (20f), and also the compound (21) obtained from two molecules of cotarnine and one molecule of acetone by the elimination of two molecules of water. The cyclic form had ben demonstrated earlier for anhydrocotarnine-nitromethane (20g) and anhydrocotarnine-acetophenone (20h). ... 

It is concluded that in the formation of the other pseudo salts and in the mutual interconversions of pseudo salts, it is similarly the cyclic form which reacts. 

Ring-chain tautomerism was observed in a series of l,2,3,6-tetrahydro-l,2, 4-triazine 4-oxides 5 in nonpolar solvents (e.g., CCI4) by NMR spectroscopy. Depending on the nature of substituents R and R, the ratios of the cyclic form of 1,2,4-triazine 5a to the open-chain form of hydrazone 5b were found to be up to 45 55 (77ZOR2617). 

Thomson -IOW Click Organic interactive to learn to draw cyclic forms of simple monosaccharides. 

If the carbonyl and the hydroxyl group are in the same molecule, an intramolecular nucleophilic addition can take place, leading to the formation of a cyclic hemiacetal. Five- and six-membered cyclic hemiacetals are relatively strain-free and particularly stable, and many carbohydrates therefore exist in an equilibrium between open-chain and cyclic forms. Glucose, for instance, exists in aqueous solution primarily in the six-membered, pyranose form resulting from intramolecular nucleophilic addition of the -OH group at C5 to the Cl carbonyl group (Figure 25.4). The name pyranose is derived from pyran, the name of the unsaturated six-membered cyclic ether. 

Carb-O. Historical development of carbohydrate nomenclature 0.1. Early approaches 0.2. The contribution of Emil Fischer 0.3. Cyclic forms 0.4. Nomenclature commissions 2-Carb-l. Definitions and conventions... 

Anomeric configuration in cyclic forms of higher aldoses 2-Carb-9. Dialdases... 

The present Recommendations deal with the acyclic and cyclic forms of monosaccharides and their simple derivatives, as well as with the nomenclature of oligosaccharides and polysaccharides. They are additional to the Definitive Rules for the Nomenclature of Organic Chemistry [13,14] and are intended to govern those aspects of the nomenclature of carbohydrates not covered by those rules. 

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