Cyclization hydrogen bonding

Gust, Moore, Moore and coworkers covalent cartenoid-porphyrin-quinone molecular triads 55-60 contain a cyclized hydrogen bond within the quinone acceptor framework [143], The naphthaquinone moiety of 55 is fused to a norbomene system whose bridgehead position bears a carboxylic acid, which can hydrogen bond to an adjacent quinone. Photoinduced electron transfer from the porphyrin to the quinone leads to a marked p/fg increase of the latter, resulting in a fast proton transfer ( pt 10 s ) to form the semiquinone. Back electron transfer from the semiquinone is attenuated as a consequence of the proton-stabilized charge-separated species. This leads to a two-fold increase in the quantum yield of the charge-separated state of 55, as compared to those of the reference triads 56 and 57 (see Volume III, Part 2, Chapter 2). 

During electrochemical fluorination retention of important functional groups or atoms in molecules is essential. Acyl fluorides and chlorides, but not carboxylic acids and anhydrides (which decarboxylate), survive perfluorination to the perfluorinated acid fluorides, albeit with some cyclization in longer chain (>C4) species [73]. Electrochemical fluorination of acetyl fluoride produces perfluoro-acetyl fluoride in 36-45% yields [85]. Electrochemical fluorination of octanoyl chloride results in perfluorinated cyclic ethers as well as perfluorinated octanoyl fluonde. Cyclization decreases as initial substrate concentration increases and has been linked to hydrogen-bonded onium polycations [73]. Cyclization is a common phenomenon involving longer (>C4) and branched chains. a-Alkyl-substituted carboxylic acid chlorides, fluorides, and methyl esters produce both the perfluorinated cyclic five- and six-membered ring ethers as well as the perfluorinated acid... 

It is interesting to note that 4-aminopent-3-en-2-one (85), which is held in a cisoid arrangement by hydrogen bonding, gives the product 86, which is stable in anhydrous solvent, but which cyclizes under the influence of water to give methyl 2-methyl-5-oxo-4-(2-oxopropylidene)-2-pyrroline-3-carboxylate (87). 

The 8-hydroxy-7-aminoquinoline-5-sulfonic acid was used to clarify the role of intramolecular hydrogen bonding on the cyclization rate of hydroxy Schiff bases in the preparation of 2-aryloxazolo[4,5-/i]quinoline-5-sulfonic acids 13. Irradiation... 

Apparently, cyclization involves the formation of open-chain intermediates 342, 343, further closing up to imidazolidines 344 and oxazolidines 345 which eliminate the secondary amine, thus leading to imidazolines 346 and oxazolines 347. The latter exist in the solution exclusively in the enolic forms 348, 349 which are stabilized by conjugation and intramolecular hydrogen bonds. 

Cyclization of the diazo compounds 1 a or 1 b, obtained from 2,4,6-trimethylpyrylium tetra-fluoroborate and ethyl diazoacetate or dimethyl diazomethanephosphonate, respectively, thus gives 1//-1,2-diazepines 2, which are stabilized by hydrogen bonding.71... 

In another study, the carrier protein was replaced by an enzyme compatible solid-phase resin (PEGA), and enzyme-catalyzed cyclization was used to probe substrate specificity. This study demonstrated also that oxo-esters are tolerated as substrates for TE domains, and then-preparation in library format served as an excellent tool for substrate specificity studies, as well as for preparation of cyclized peptides. Figure 13.11 shows how the TycA TE showed selectivity for only residues 1 and 9 (colored in red), and changes at all other residues were tolerated [42]. Hydrogen bonding interactions are shown in green. Several compounds made from this series were shown to demonstrate improved therapeutic indices (with respect to hemolysis) while retaining antimicrobial activity. 

Treatment of the phosphoric ester 224 with triethylamine breaks the intramolecular hydrogen bonding in the solution structure and promotes cyclization to give spirophosphorane 43 (Equation 44) <2003PS2117>. 

Investigations of hydrogen bonding of several thioamides have been carried out by calculations and spectroscopy. Characterization, hydrolysis and cyclization of thioamides have been discussed by using the results of calculations and spectroscopy. Comparison of amides and thioamides has been investigated.81 86 Rotation barriers for a series of amides and thioamides have been calculated.87 93... 

Since Pedersen s original work on the use of cations to template the formation of crown ethers [18-20], a large number of different templating agents for macro-cyclization reactions have been reported. While the initial work concentrated on the use of metal cations, further developments demonstrated that species with hydrogen bonding donor or acceptor properties could be equally useful to template the synthesis of macrocyclic molecules. 

The present volume contains 13 chapters written by experts from 11 countries, and treats topics that were not covered, or that are complementary to topics covered in Volume 1. They include chapters on mass spectra and NMR, two chapters on photochemistry complementing an earlier chapter on synthetic application of the photochemistry of dienes and polyenes. Two chapters deal with intermolecular cyclization and with cycloadditions, and complement a chapter in Volume 1 on intramolecular cyclization, while the chapter on reactions of dienes in water and hydrogen-bonding environments deals partially with cycloaddition in unusual media and complements the earlier chapter on reactions under pressure. The chapters on nucleophiliic and electrophilic additions complements the earlier chapter on radical addition. The chapter on reduction complements the earlier ones on oxidation. Chapters on organometallic complexes, synthetic applications and rearrangement of dienes and polyenes are additional topics discussed. 

Figure 11 Proposed mechanism of cyclization of dehydrated NisA by NisC. The cyclization reaction shown results in the formation of the B-ring of nisin. The possible stabilization through a /3-turn-like structure via hydrogen bonding between the amide NH of Cys and the carbonyl of Dha/Dhb is shown and may explain the high stereoselectivity observed in nonenzymatic cyclizations involving four amino acids as discussed in the text. Reprinted with permission from B. Li W. A. van der Donk, J. Biol. Chem. 2007, 282, 21169-21175.

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