Paquette studies

Gleiter and Paquette studied the selectivity in the reactions of isodicyclopentadiene by molecular orbital calculation by the STO-3G level of theory using simple model compounds 81 and 82 (Scheme 33) [42], They found remarkable tilting of Tt orbitals due to the mixing of o and orbitals. The orbitals tilt their terminal p orbital inward on the top face. 

In the course of an unsuccessful search for the tris-homotropylium cation, Ohkata and Paquette studied several biscyclopropylcarbinyl systems around a seven-membered ring. Some of their relative rate data for 3,5-dinitrobenzoate solvolyses in 80 %... 

At elevated temperatures, COTs undergo carbon scrambling as revealed by substituted materials. In the definitive paper on this topic, Paquette studied the interconversions of the dimethyl-COTs in a flow system at 350-450°C with 1-3 s contact time. The most relevant conversions are those of the 1,2-material exclusively to the 1,4-isomer, kinetically, and of the 1,5-isomer exclusively to the 1,4-isomer kinetically. The 1,4- and the 1,3-isomers rearrange with little regio-specificity (Scheme 9.13). 

Another major contributor to this research area has been the ButenschSn laboratories, whose focus has been on the double addition of alkenyllithium reagents to benzocyclobutenedione chromium complex jg4 220,221 j ej-eas the addition of alkenyllithium reagents to the squarate esters used in the Paquette studies favor the tra 5-addition pathway, addition to 184 gives exclusively sy -addition products, enabling them to undergo facile dianionic oxy-Cope rearrangements. In a similar fashion, the diketones produced are well situated to undergo intramolecular aldol additions. 

The next seven references are cited not because of the experimental procedures described but because they indicate diversification in the types of enamines prepared and studied. Both Paquette (25) and Kasper 26) have condensed 2,5-methylene-l,2,5,6-tetrahydrobenzaldehyde (5-nor-bornene-2-carboxyaldehyde) (2) with several cyclic and open-chain aliphatic secondary amines. Kasper studied the ratio of endo to exo aldehyde formed upon hydrolysis of these enamines and the dihydro enamines. Paquette investigated the addition of sulfene to the enamines. -Fluoro-... 

An example of the use of NMR spectroscopy to ascertain with reasonable certainty the stereochemistry of a series of enamines has been provided by Paquette (25). Based on a study of the NMR spectra of the endo- and exo-5-norbornene-2-carboxaldehydes (168), the enamine mixtures were estimated to contain 80 to 90 % of the transoid form (170). 

For a systematic study, see Paquette LA, Basu K, Eppich JC, Hofferberth JE (2002) Helv Chim Acta 85 3033... 

In the second part of this work we studied the bromination of benzobarrelene at room temperature and at 150°C. Some dipolar addition reactions to the benzobarrelene have been reported by Paquette and al (ref. 7). Surprisingly, there is no report in the literature on bromination of benzobarrelene. However, Barkash and al. (ref. 8) has reported bromination of tetrafluorobenzobarrelene 2 and isolated from complex reaction mixture only compounds 8 and 9 (Scheme 2). 

Treatment of a-halosulfones 357 bearing a -hydrogens with base gives olefins by extruding SO2 from the episulfone intermediates 359. The reaction was found by Ramberg and Backlund in 1940 and named after them, and it has been very useful for the preparation of tailor-made olefins. The reaction has been investigated in detail " and utilized widely for olefin syntheses. An excellent review on this reaction by Paquett covers the literature up to 1975, so only recent studies are dealt with in this section. The generally accepted mechanistic path is shown below. 

The effect of substituents and ring size on this photochemical rearrangement was studied by Paquette and Meehan.(43) Irradiation of the following ketones resulted in steady state mixtures of starting ketone and photoketone whose ratios depended upon structure ... 

Our success in super-stabilization of cation 6 led us to the preparation of a higher homologue, that is, cyclooctatetraene (COT), fully annelated with BCO units 9 (9). As compared with a large number of studies on its radical anion or dianions, the studies on the cationic species of COT have been quite limited. There have been only one study by Olah and Paquette on the substituted COT dication (70), which is a typical 6n Hiickel aromatic system, and few sporadic studies on radical cations, which involve indirect spectral observations, such as electronic spectra in Freon matrix at low temperature (77,72) and constant-flow ESR study (13). 

The simplest of all of the homoaromatics is the homocyclopropenium cation [2], This species was first invoked as an unusually stable intermediate by Applequist and Roberts (1956) to explain the reactions of methylene cyclobutenes with electrophiles. Since that time a wealth of studies has provided data supporting the special stability of the monohomocyclo-propenium cation (Story and Clark, 1972 Paquette, 1978). The parent cation and several simple derivatives have been examined by a variety of techniques including the convincing, direct H and 13C NMR observation... 

There is, no doubt, a decrease in the homoaromatic stabilization when conjugation is interrupted in two (bishomoaromatic) or more places (Paquette et al., 1977a Paquette, 1978). In spite of this, several bishomoaromatic cations have been well characterized. The simplest bishomoaromatic the 4-cyclopentyl cation [29] is, as yet, unknown (see Olah et al., 1972, and references cited therein). The cation [29] was not detected by NMR studies, nor was it invoked as an intermediate in the solvolysis of appropriately substituted cyclopentenes. 

Volger (1968) and by Paquette et al. (1968). The culmination of these extensive studies, summarized by Hogeveen and Kwant (1975), clearly establishes the non-classical, homoaromatic structure [33] for this fascinating cation. 

The parent trishomocyclopropenyl cation [47], first proposed by Winstein et al. (1959), has been invoked as an intermediate in various solvolysis studies, observed under stable ion conditions by NMR spectroscopy, and studied theoretically (see Story and Clark, 1972 Paquette, 1978). Similarly, the related ions [48]—[51] have all been advanced as trishomoaromatics (see Story and Clark, 1972 Paquette, 1978). 

Medium ring annelations across the 2,8-positions of semibullvalene have been extensively studied by the Paquette group (see Paquette and Burson, 1978, and references cited therein). These annelations result in the favouring of one tautomer or the other but do not lead to a homoaromatic species. The previously described elassovalenes [82] (page 298) may also be... 

The homoenolate conjugate addition has been used very recently by Paquette and Cheney [17] to synthesise the key diquinane intermediate 18 in their studies directed towards the total synthesis of trixikingolide (Scheme 5.13). 

The stereoselective or stereospecific formation of these compounds and their interaction with butyllithium was studied with the help of NMR. Paquette and Freeman first applied asymmetric induction to the synthesis of four-membered rings, especially with the sulfene-enamine 2 -F 2 cycloaddition. The in situ generation of sulfene 68 by dehydrochlorination with butyllithium of the sulfonyl chloride allowed the formation of cycloadduct 69 in 88 % yield. In a variation, the sulfene may be generated by base-induced... 

Paquette has used the chloroketal derivatives of thietane 1,1-dioxide as an especially well-suited model for the theoretical study of the thermally induced intramolecular six-electron ring contraction rearrangement, which is pointed out in Eq. (54) for the ketals of 2-bromocyclobutanone that give cyclopropylcarboxylates. Desulfurization of thietanes to cyclopropane did not prove too successful. ... 

By far the most thoroughly investigated azocines are the 2-methoxy derivatives prepared and studied by Paquette and coworkers (7lAG(E)ll). The synthesis involves addition of chlorosulfonyl isocyanate to a cyclohexadiene, conversion to the imidate and introduction of another double bond by allylic bromination and dehydrohalogenation. Valence isomerization then ensues and 2-methoxyazocine (98) or alkyl homologs are isolated from this sequence in multigram quantities as stable yellow oils. NMR data (see 98) clearly indicate the monocyclic azocine structure the spectra are invariant from -70 to 180 °C and indicate less than 2% of the bicyclic valence isomers. 

Synthetic studies towards the spongistatins have been reported by the following groups Anderson, Barrett, Boyes, Hoffmann, Ley, Mead, Nakata, Paquette, Roberts, Roush, Samadi and Vogel. 

Paquette, L. A. Poupart, M.-A. Studies directed toward the total synthesis of cerorubenic acid-III. 1. Expedient construction of the tetracyclic core by oxyanionic sigmatropy. J. Org. Chem. 1993, 58, 4245-4253. 

Parallel studies by Masamune et al. led nevertheless to the conclusion that the argento carbonium ion 1 proposed by Paquette et al. may not necessarily be involved in such Ag -catalyzed rearrangements.25 26... 

Homocubanes were studied or used in synthesis more extensively. Paquette et al.36,37 showed that the rates of homocubane isomerization to norsnoutane in benzene with silver perchlorate followed a second-order catalytic rate law (Scheme 3.22). Kinetic data suggested that a single site was involved in the silver interaction with homocubane. 

The addition of silyl radicals to conjugated dienes to form allylic-type radicals and their subsequent intramolecular addition to C=C double bonds has been investigated108 109. Paquette and Usui provided a convenient route to triquinane 101 by exposure of 100 to (TMS)3SiH and AIBN in refluxing toluene (equation 56). An unoptimized 51% yield was obtained108. The addition of (TMS)3SiH to substituted 1,5-dienes in the presence of CO has been studied by Ryu and coworkers110. An example is shown in equation 57. From... 

In a related observation, Furstner and co-workers reported that while both the FMC and the MC2 lead to the same E Z alkene ratio in the case of 18-RCM, the use of the same conditions with 19 leads to product enriched selectively in either of the two isomers <02MI657> depending on catalyst selection (Scheme 9). Furstner notes that this "illustrates the subtle influence of remote substituents on the stereochemical outcome of RCM in the macrocyclic series." Studies concerning the effects of chelation of the reactivity of ruthenium carbene complexes were reported by Furstner and co-workers <02OM331>. Paquette and co-workers observed that the use of either the GMC or the MC2 led to different outcomes in the macrocyclization RCM of 1,2-amino-alcohol-templated ene-dienes <02HCA3033, 02MI615> hinting that catalyst selection is also an important consideration in those processes. 

Due to the inherent dissymmetry of monosubstituted triquinacenes, dimerization of such compounds will generate a pair of coupling products (meso and dl) if the monomer is racemic. Only when bonding between optically pure triquinacenes of the same chirality results can it be guaranteed that the formation of meso dimer will be precluded. In their study of the pinacolic reduction of <2/-2,3-dihydrotriquinacen-2-one, Paquette and co-workers showed that approximately equal amounts of the dl diol 458 and meso isomer 459 resulted.405 When enantiomerically pure (+)-380 was used, 458 was the exclusive reductive coupling product because of enforced enantio-... 

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