Five-and Six-Membered Ring Forms

This is an abridged version of the document Conformational Nomenclature for Five- and Six-membered Ring Forms of Monosaccharides and their Derivatives. Recommendations 1980 [3],... 

IUPAC-IUB Joint Commission on Biochemical Nomenclature (JCBN), Conformational nomenclature for five- and six-membered ring forms of monosaccharides and their derivatives (Recommendations 1980), Eur.J.Biochem. Ill, 295-298 (1980) Arch. Biochem. Biophys.,207,469-472(1981) PureAppl. Chem.,S3,1901-1905(1981) ref.2,pp. 158-161. 

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

Conformations of Five- and Six-Membered Ring Forms 191 Box4-D Antifreeze and Ice-Nucleation Proteins... 

This means that three-membered rings can easily be made but tend to decompose under the conditions of their formation, that four-membered rings are difficult to make, that it is often difficult to stop five- and six-membered rings forming and that seven-membered rings can usually be formed when needed. 

On the other hand, the other direction of addition (yd-addition) produces a monosubstituted alkylidene that is sterically comparable to the initiating alkylidene, making this the desired direction of addition. Formation of a six-membered ring in a reaction involving a more reactive terminal alkylidene would transform 43b into 44b. The NMR spectrum of poly-6 reveals two carbonyl carbon resonances. Therefore it was speculated that poly-6 consist of a random distribution of five- and six-membered rings formed through what is nominally tail-to-tail and head-to-head cyclopolymerization of the two acetylenic bonds in the same monomer (Scheme 1). Other uncertainties in catalyst systems of the type are the rate at which alkylidene rotamers interconvert, the extent to which the reactivities of the two alkylidene rotamers differ, and the degree of selective formation of one rotamer when a triple bond reacts with a Mo=C bond. ... 

Conformational nomenclature recommendations for five- and six-membered ring forms of monosaccharides and their derivatives are contained in a publication from the lUPAC-IUB Joint Commission on biochemical nomenclature. ... 

Depending on the positioning of the bridge linking the ene donor and acceptor, three orientations are possible for the intramolecular ene reaction. These have been described by Oppolzer (1) as Types I, II, and III. Snider (5) has observed, in addition. Type IV [3]. Type I reactions are by far the most studied, especially for five- and six-membered ring forming reactions. It is conceivable, especially as activated ene systems are developed, that Types II-IV will become more important. 

In an intramolecular aldol condensation of a diketone many products are conceivable, since four different ends can be made. Five- and six-membered rings, however, wUl be formed preferentially. Kinetic or thermodynamic control or different acid-base catalysts may also induce selectivity. In the Lewis acid-catalyzed aldol condensation given below, the more substituted enol is formed preferentially (E.J. Corey, 1963 B, 1965B). 

The silyl enol ethers 209 and 212 are considered to be sources of carbanions. and their transmetallation with Pd(OAc)2 forms the Pd enolate 210. or o.w-tt-allylpalladium, which undergoes the intramolecular alkene insertion and. 1-elimination to give 3-methylcyclopentenone (211) and a bicyclic system 213[199], Five- and six-membered rings can be prepared by this reaction[200]. Use of benzoquinone makes the reaction catalytic. The reaction has been used for syntheses of skeletons of natural products, such as the phyllocladine intermediate 214[201], capnellene[202], the stemodin intermediate 215[203] and hir-sutene [204]. 

Cycloadditions Forming Five- and Six-Membered Rings by W. R. Dolbier... 

Radicals of the type 9, generated in other ways, also undergo these cyclizations. Both five- and six-membered rings can be formed in these reactions (see p. 985). 

Hexose monosaccharides can form both five- and six-membered rings. In most cases, the six-membered ring structure is more stable, but fructose is an important example of a hexose that is more stable as a five-membered ring. The structure ofy6-fhictose is shown in Figure 13-17. Notice that there are — CH2 OH fragments bonded to two positions of this five-membered ring. Examples and explore the structures of monosaccharides in more detail. 

The intramolecular version of ester condensation is called the Dieckmann condensation.217 It is an important method for the formation of five- and six-membered rings and has occasionally been used for formation of larger rings. As ester condensation is reversible, product structure is governed by thermodynamic control, and in situations where more than one product can be formed, the product is derived from the most stable enolate. An example of this effect is the cyclization of the diester 25.218 Only 27 is formed, because 26 cannot be converted to a stable enolate. If 26, synthesized by another method, is subjected to the conditions of the cyclization, it is isomerized to 27 by the reversible condensation mechanism. 

Similarly, phenols with unsaturated side chains can form five- and six-membered rings. In these systems the quaternary carbon imposes the (3, y-elimination. As with the above... 

The addition of a vinyl radical to a double bond is usually favorable thermodynamically because a more stable alkyl radical is formed. The vinyl radical can be generated by dehalogenation of vinyl bromides or iodides. An early study provided examples of both five-and six-membered rings being formed.329 The six-membered ring is favored when a branching substituent is introduced. 

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