Cycloadditions salts

Breslow supported this suggestion by demonstrating that the cycloaddition can be further accelerated by adding anti cliaotropic salts such as lithium chloride, whereas chaotropic salts such as guanidium chloride led to a retardation " "" ". On the basis of these experiments Breslow excluded all other possible explanations for the special effect of water on the Diels-Alder reaction " . 

Aminomethylindoles are particularly important synthetic intermediates. 3-Dimethyl-aminomethylindole (gramine) (153) and especially its quaternary salts readily undergo displacement reactions with nucleophiles (Scheme 60). Indole-2,3-quinodimethanes, generated from 2-methylgramine as shown in Scheme 61, undergo intermolecular cycloaddition reactions with dienophiles to yield carbazole derivatives (82T2745). 

In the alternative approach.the 1,3-dipolar system can be constructed in several ways. Treatment of a-chloroacylhydrazones of diaryl ketones and certain aralkyl and dialkyl ketones (382) with NaH in anhydrous THF gives l-(disubstituted methylene)-3-oxo-l,2-diazetidinium inner salts (383). Reaction of (383) with DMAD in methylene chloride gave (384), a 2 1 adduct with loss of CO. Double bond migration in (384) occurred on heating to give (385). The intermediate in the cycloaddition was found to be (386), which on heating lost CO to form a new ylide system which in turn underwent reaction with more DMAD <81JA7743). 

Oxadiazolium salts cycloaddition reaction, 6, 435 ring cleavage, 6, 434 synthesis, 6, 440... 

Pyrrolo[3,4-d]pyridazinium salts, 2,3-dimethyl-synthesis, 4, 291 Pyrrolo[ 1,2-6]pyridazinones oxidation, 4, 298 Pyrrolo[2,3-6]pyridine, 1-acetyl-cycloaddition reactions, 4, 509 Pyrrolo[2,3-6]pyridine, 3-bromo-nitration, 4, 505... 

The calculated barrier to dissociation of the [S3N2] dication into [SN]" and [S2N] in the gas phase is 10.9 kcal mof . However, lattice-stabilization effects allow the isolation of [MEg] salts (M = As, Sb) of this six r-electron system in the solid state from the cycloaddition of [SN] and [S2N] cations in SO2 (Eq. 5.11)."° The S-S and S-N bond distances in the planar, monomeric dication are shorter than those in the... 

The vinylogous 3,5-hexadien-2-one (16) adds in a 1,4 cycloaddition with zl -dehydroquinolizidine (17) to form compound 18 (26). A similar 1,4-cycloaddition reaction takes place between pyrylium salts and the pyrrolidine or morpholine enamines of cycloalkanones (26a). 

The 2,7-naphthyridine system 53 (Scheme 8.4.18) was combined with 2,4-dinitrochlorobenzene and 2-amino glycerol for in situ reaction of the resulting Zincke salt. The resulting naphthyridinium 54 was trapped by Bradsher cycloaddition with (Z)-vinyl ether 55, providing tetracycle 56 (X-ray) upon internal addition of one of the diastereotopic hydroxymethyl groups to the resulting iminium. This approach was also extended to the use of chiral 2,7-naphthyridinium salts, prepared via the analogous Zincke process. ... 

Chiral dihydropyridines such as 103 were also accessible from Zincke-derived N-alkyl pyridinium salt 102 (Scheme 8.4.34). The dihydropyridine underwent cycloaddition with methylacrylate, providing chiral isoquinuclidine derivative 104 as the major diastereomeric product. ... 

Whereas in all previously mentioned inverse cycloaddition reactions [h]-fused pyrido annelated systems are formed, some reactions are described which lead to [c]-pyridine annelated bicyclic systems. 5-(Butynylthio)pyrimidines (R = Ph, NHCOCH3) give on heating at 180°C in nitrobenzene 5-R-2,3-dihydrothieno[2,3-c]pyridines (89T803). 5-Propynyloxymethylpyrimidines also readily undergo cycloaddition into l,3-dihydrofuro[3,4-c]pyridines (89T5151) (Scheme 39). Considerable rate enhancements were observed with quaternized pyrimidinium salts. Whereas... 

In 1997 the application of two different chiral ytterbium catalysts, 55 and 56 for the 1,3-dipolar cycloaddition reaction was reported almost simultaneously by two independent research groups [82, 83], In both works it was observed that the achiral Yb(OTf)3 and Sc(OTf)3 salts catalyze the 1,3-dipolar cycloaddition between nitrones 1 and alkenoyloxazolidinones 19 with endo selectivity. In the first study 20 mol% of the Yb(OTf)2-pyridine-bisoxazoline complex 55 was applied as the catalyst for reactions of a number of derivatives of 1 and 19. The reactions led to endo-selective 1,3-dipolar cycloadditions giving products with enantioselectivities of up to 73% ee (Scheme 6.38) [82]. In the other report Kobayashi et al. described a... 

The first report on metal-catalyzed asymmetric azomethine ylide cycloaddition reactions appeared some years before this topic was described for other 1,3-dipolar cycloaddition reactions [86]. However, since then the activity in this area has been very limited in spite of the fact that azomethine ylides are often stabilized by metal salts as shown in Scheme 6.40. 

---