Open chain

The four kelohexoses are fructose, sorbose, allulose and tagatose. See glucose for example of isomerization between open chain and cyclic structures in a typical hexose molecule. 

The largest series is that of the alkanes or paraffins, which are open chain molecules with saturated bonds, and have the general formula C H2n+2-... 

Open chain hydrocarbons which are undersaturated, i.e. having at least one carbon-carbon double bond are part of the olefin series, and have the ending -ene . Those with one carbon-carbon double bond are called mono-olefins or alkenes, for example ethylene CH2 = CH2. 

We will describe two cases open and closed chains of sites. For an open chain of sites, the energy of a spin configuration is... 

Both systems give the same results in the thennodynamic limit. We discuss the solution for the open chain at zero field and the closed chain for the more general case of H 0. 

In a similar way Table II summarizes how the phase changes upon interconversion among the isomers. Inspection of the two tables shows that for any loop containing three of the possible isomers (open chain and cyclobutene ones), the phase either does not change, or changes twice. Thus, there cannot be a conical intersection inside any of these loops in other words, photochemical transformations between these species only cannot occur via a conical intersection, regardless of the nature of the excited state. 

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 ... 

Division I. Acyclic compounds Acyclic stem nuclei). atoms are joined in open chains only. 

Analysis The cycHc ether can clearly be made from the open-chain diol ... 

If an open-chain organic molecule contains an electron acceptor and an electron donor site, two carbon atoms may be combined intramolecularly. This corresponds to the synthesis of a monocyclic compound. 

Cyclopentene-l-carboxaldehydes are obtained from cyclohexene precursors by the sequence cyclohexene - cyclohexane-1,2-diol -> open-chain dialdehyde - cyclopentane aldol. The main advantage of this ring contraction procedure is, that the regio-and stereoselectivity of the Diels-Alder synthesis of cyclohexene derivatives can be transferred to cyclopentane synthesis (G. Stork, 1953 G. BUchi, 1968). 

Open-chain 1,5-polyenes (e.g. squalene) and some oxygenated derivatives are the biochemical precursors of cyclic terpenoids (e.g. steroids, carotenoids). The enzymic cyclization of squalene 2,3-oxide, which has one chiral carbon atom, to produce lanosterol introduces seven chiral centres in one totally stereoselective reaction. As a result, organic chemists have tried to ascertain, whether squalene or related olefinic systems could be induced to undergo similar stereoselective cyclizations in the absence of enzymes (W.S. Johnson, 1968, 1976). 

Heterocyde syntheses are often possible from difunctional open-chain precursors, including olefins as 1,2-difunctional reagents, and an appropiate nucleophile or electrophile containing one or more hetero atoms. The choice of the open-chain precursor is usually dictated by the longest carbon chain within the heterocyde to be synthesized. 

The longest carbon chain within a heterocycle indicates possible open-chain precursors. We use this chain as a basis to classify heterocycles as 1,2- to 1,6-difunctional systems. 

Regioselectivity becomes important, if unsymmetric difunctional nitrogen components are used. In such cases two different reactions of the nitrogen nucleophile with the open-chain educt may be possible, one of which must be faster than the other. Hydrazone formation, for example, occurs more readily than hydrazinoLysis of an ester. In the second example, on the other hand, the amide is formed very rapidly from the acyl chloride, and only one cyclization product is observed. 

Open-chain carbon skeletons (including cyclic acetals, lactones, lactams, cyclic anhydrides, etc.)... 

Difunctional, Trifunctional, and OligofuuGtianal Open-Chain Reagents... 

Open-chain difunctional compounds containing no other elements than... 

Table 7. Open-chain difunctional sulfur compounds.

Table 9. Open-chain tiifunctional nitrogen, halogen and sulfur compounds.

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