Photohydration of pyridinium ions

Kaplan, Louis; Pavlik, James W.; Wilzbach, K. E.

doi:10.1021/ja00764a089

Cited by 76 publications

(50 citation statements)

References 2 publications

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“…The relative stereochemistry of the products has been assigned from NOE difference data, the most obvious differences being shown for compound 9h. 12 This last point appears to conflict with the results published in earlier papers, [6][7][8][9] where one might expect the solvent (SolvH) to attack from the opposite side to the newly formed aziridine. We attribute this unexpected observation to different stereoelectronic effects controlling the direction of attack.…”

Section: Methodsmentioning

confidence: 96%

“…11 The pyridine species were quaternised by heating in neat alkyl bromide or iodide inside a sealed tube and, after filtration, the pyridinium halides 13 were converted to the tetrafluoroborate salts 14 using silver carbonate and fluoroboric acid. Chloride 6,8,9 and perchlorate 7 salts have previously been used for photosolvolysis reactions, although we opted to use tetrafluoroborate salts; they were easy to make from the halide salt (which tended to undergo photoinduced charge transfer reactions) and were less likely to detonate than perchlorate salts.…”

Section: Methodsmentioning

confidence: 99%

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New Methods of Preparing Cyclopentenone Ketals: The Photosolvolysis of 3-Alkoxypyridinium Tetrafluoroborates

Penkett¹

1999

Synlett

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A new base catalysed method of ketal formation is reported, such that substituted cyclopentenone ketals are prepared from 3-alkoxypyridinium tetrafluoroborate salts.The photochemical properties of aromatic compounds are a source of amazement to many organic chemists. In particular the metaphotocyclo addition 1 has often been described as the reaction that leads to the greatest increase in molecular complexity. This light induced reaction between an aromatic ring and an olefin was concurrently discovered in 1966 by Wilzbach and Kaplan 2 and also by Bryce-Smith, Gilbert and Orger. 3 The metaphotocycloaddition has been very elegantly utilised by Wender for the synthesis of a number of key natural products, such as (±)-a-cedrene 4 and (±)-modhephene (4) 5 (Scheme 1). Scheme 1As yet there have been no reports of heteroaromatic compounds being involved with similar reaction sequences, although Wilzbach, Kaplan and Pavlik 6 reported an unusual photohydration reaction of pyridinium ions in 1972. These workers described how the irradiation of an alkaline solution of N-methylpyridinium chloride 5 led to the formation of a bicyclic aziridinyl alcohol 7. It was proposed that the reaction proceeded via an azabenzvalene 6 cation that was subsequently captured by water from the least hindered face. Scheme 2A renaissance of interest in this reaction has led to some important discoveries by Mariano 7 and Burger. 8,9 Mariano reported the results of photosolvolysis of various pyridinium salts and subsequent solvolytic aziridine ring opening of the products. 7 Burger then reported how this reaction could be used for b-lactam synthesis 8 and, more recently, has shown how polyhydroxylated aminocyclopentanes 9 can be made. We were intrigued to know what effect an electron donating group at the 3 position on the pyridine ring would have on the photosolvolysis of pyridinium salts. We reasoned that the incipient cation would be very stable and should be readily captured by the solvent to form a geminal bisalkoxy species. To test our hypothesis various 3-alkoxypyridinium salts 8 were irradiated in different solvents using a 400 W medium pressure mercury vapour lamp with a pyrex filter. Analysis of the different products showed that a series of cyclopentenone ketal derivatives 9 had been made (Scheme 3). Scheme 3The pyridinium salts were made according to standard literature procedures, starting from either 3-hydroxypyridine or 3-hydroxy-6-methylpyridine 10. The hydroxyl group was initially protected as an ether 11 using an alkyl halide. 10 In order to introduce larger alkyl groups at the 6 position of the pyridine ring, the 6-methylpyridine derivative was metallated using LDA and quenched with an appropriate alkyl halide. 11 The pyridine species were quaternised by heating in neat alkyl bromide or iodide inside a sealed tube and, after filtration, the pyridinium halides 13 were converted to the tetrafluoroborate salts 14 using silver carbonate and fluoroboric acid. Chloride 6,8,9 and perchlorate 7 salts have previously been used for photos...

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Section: Methodsmentioning

confidence: 96%

Section: Methodsmentioning

confidence: 99%

New Methods of Preparing Cyclopentenone Ketals: The Photosolvolysis of 3-Alkoxypyridinium Tetrafluoroborates

Penkett¹

1999

Synlett

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“…4 In the early investigation by Wilzbach and his coworkers, it was observed that irradiation of a basic aqueous solution of N-methylpyridinium chloride 1 results in stereocontrolled formation of the bicyclic allylic alcohol 3 (Scheme 1). The authors proposed that this process takes place by excited state electrocyclization (a photo-Nazarov cyclization) to produce the intermediate bicyclic allylic cation 2, which then undergoes addition of hydroxide to the least sterically hindered exo-face.…”

Section: Introductionmentioning

confidence: 99%

Observations Made in Exploring a Pyridinium Salt Photochemical Approach to the Synthesis of (+)-Lactacystin

2008

Bulletin of the Korean Chemical Society

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The key step in a strategy for the synthesis of (+)-lactacystin involving photocyclization reaction of a cyclopenta-fused pyridinium salt has been probed by using a model substrate. Observations made in this effort led to the discovery of a highly unusual cascade process that leads to stereoselective formation of an interesting tricyclic carbamate. The results of this study are presented and discussed in the context of a (+)-lactacystin synthetic approach.

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“…[10,11] We investigated the mechanism of the ring contraction of 3 to aminocyclopentadiene 8 with DFT calculations.T he mechanism in Pathway A( Scheme 5), analogous to that proposed by Fusi and Adamo,i sc onsistent with the experimental observation that an electron-withdrawing R 5 substituent is necessary to obtain 8 in ag ood yield. [10,11] We investigated the mechanism of the ring contraction of 3 to aminocyclopentadiene 8 with DFT calculations.T he mechanism in Pathway A( Scheme 5), analogous to that proposed by Fusi and Adamo,i sc onsistent with the experimental observation that an electron-withdrawing R 5 substituent is necessary to obtain 8 in ag ood yield.…”

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confidence: 99%

Formation of Aminocyclopentadienes from Silyldihydropyridines: Ring Contractions Driven by Anion Stabilization

et al. 2018

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Highly functionalized aminocyclopentadienes can be formed through the rearrangement of anions generated from readily prepared 6-silyl-1,2-dihydropyridines by desilylation with fluoride.T he scope and generality of the reaction was defined, and diverse transformations were performed on the highly functionalized products.Amechanism and driving force for the rearrangement were identified from experiments and DFT calculations. Scheme 1. Reactionso fsilyl-substituted DHPs 3.

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Photohydration of pyridinium ions

Cited by 76 publications

References 2 publications

New Methods of Preparing Cyclopentenone Ketals: The Photosolvolysis of 3-Alkoxypyridinium Tetrafluoroborates

New Methods of Preparing Cyclopentenone Ketals: The Photosolvolysis of 3-Alkoxypyridinium Tetrafluoroborates

Observations Made in Exploring a Pyridinium Salt Photochemical Approach to the Synthesis of (+)-Lactacystin

Formation of Aminocyclopentadienes from Silyldihydropyridines: Ring Contractions Driven by Anion Stabilization

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