Intramolecular cation

PET reactions [2] can be considered as versatile methods for generating radical cations from electron-rich olefins and aromatic compounds [3], which then can undergo an intramolecular cationic cyclization. Niwa and coworkers [4] reported on a photochemical reaction of l,l-diphenyl-l, -alkadienes in the presence of phenanthrene (Phen) and 1,4-dicyanobenzene (DCNB) as sensitizer and electron acceptor to construct 5/6/6- and 6/6/6-fused ring systems with high stereoselectivity. 

However, the greatest amount of work in the area of intramolecular cation-radical coupling reactions involves annulation of less ionizable alkenes (or alkynes) with enol ethers or vinyl sulfides. Typically, these reactions are used to form six-159,163 or five-membered165 rings, usually stereospecifically (Schemes 67 and 68, respectively). As seen... 

With the larger Ms(calix) crown (3.127),65 two dynamic processes are observed. The intermolecular association/disassociation equilibrium in which the complex equilibrates with the uncomplexed ligand and free metal cation is slow on the NMR time scale. The complex also exhibits a faster intramolecular vibration of the metal cation from one binding site to another (Scheme 3.26). Compound 3.127 is also interesting because the mechanism of this fast, intramolecular cation shuttling process may have important implications on the analogous movement of metal cations through transmembrane ion channels (Section 2.2). 

With the cylindrical cryptands, each macrocycle may bind one cation so that both mono- and dinuclear cryptates may be formed. Although the 12-membered (N202) macrocycles of ligand 5 are too small to bind two cations within each of the macrocycles, variable temperature 13C-NMR measurements have revealed intramolecular cation exchange between identical sites at the top and bottom of this cryptand, for Ca2+, Sr2+, and Ba2+. Cation jump between the two sites is fast with respect to intermolecular cation exchange, modeling the elementary jump processes of cations between binding sites in membrane channels (91). 

Ketyls from dicarbonyl compounds in the thiophene series have also been examined. The Bologna group has studied the conformational equilibria in the anion-radical of thiophene-2,5-dicarbaldehyde. They have also investigated the intramolecular cation exchange process within ion-pairs of this ketyl, and of analogous ketyls from the isomeric thienothiophenes, and the consequence for this process of variation in the size of the cation and its complexation by macrocyclic polyethers. The anion-radicals of the isomeric l,2-dithienylethane-l,2-diones (thenils) were recorded several years ago. " ... 

On constant current electrolysis (9.4 mA 2 Fmol ), 171 underwent intramolecular cationic [5 + 2] cycloaddition to afford the /3-isomer 170 as a sole product (34%). In the case of the tetra-substituted phenol 173, it was converted to a mixture of two stereoisomers (172a and 172b) in 70% yield (relative ratio a//S = 3/1), as shown in Scheme 34 . Both of them were converted successfully to the target molecule 169 . 

In the case of 3,4-dimethoxyphenol (185) bearing an a,/3-unsaturated CO system, one-pot synthesis of the corresponding tricyclic compound (186) was performed in ca 80% yield by a combination of electro- and photochemical reactions, as shown in Scheme 38. Here, intramolecular cationic [5 + 2] cycloaddition does not take place, because of the... 

Phenols bearing an olefinic side chain have been known to be oxidized electrochem-ically and the resulting cyclohexa-2,5-dienones undergo intramolecular cationic [5 + 2]... 

A variety of disconnections are possible utilizing intramolecular cationic reactions. A few of the more useful are... 

Intramolecular cation-7t interaction in organic synthesis (particularly, S3mthesis of chiral dihydrop37ridines) 070BC2903. 

Enantiomerically pure 4-alkyl substituted derivatives of tryptophan required for the asymmetric syntheses of ergot alkaloids has been obtained [35]. The author used the method [36] to produce 4-alkyl substituted indoles and combined this organometallic reaction with an enantioselective enzymatic transformation. An efficient eight stage synthesis ofN-benzenesulphonyl-3-(3 -methoxyprop-2 -en-r-yl)-4-(r-hydroxy-2 -trimethylsilymethyl-prop-2 -en-r-yl)-indoles from 4-carbomethoxyindole has been described [37]. The use of these benzylic alcohols for intramolecular cation-olefine cycloadditions yielding either a tetracyclic or a tricyclic product was also demonstrated. 

Fig. 217. Possible mode of intramolecular, cation selective bridge in the kappa carrageenan molecule in aqueous solution at low" temperature ( ) ionic bonding and (------) electrostatic force...

Electrophilic additions to the C—C of the endo-diacid (506) and dialcohol (505) by such reagents as CI2, Bt2,12, PhSCl, and Hg(OAc)2 results in the formation of the intramolecular cation-trapping products (507 X = I, AcOHg, etc.). " " Isomer ratios... 

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