A versatile approach to the synthesis of (+)-mannostatin A analogues

The Photoreaction of Pyrylium Cation with Water: A DFT Study

Abstract:

The photochemical reaction of pyrylium cation with water to give a cyclopentene derivative has been studied at the DFT/B3LYP/6-31G+(d,p) level of theory. The calculation is in agreement with an electrocyclic disrotatory reaction from the S1 singlet excited state, giving a cyclopentene epoxide cation that can react with very low activation energy (2 kJ mol-1) with water to give the corresponding adduct. CASSCF calculations allowed to identify a conical intersection in the S1 state, giving the cyclopentene epoxide cation.

On the photochemical reaction of pyridinium salts with nucleophiles

DFT calculations on the photochemical reaction of 1-n-N-butylpyridinium salt in water with hydroxide anion is in agreement with a singlet state process where the S₂ state at λ = 253 nm can be converted into a Dewar isomer (2-butyl-2-azabicyclo[2.2.0]hexa-2,5-dien-2-yl cation). The Dewar isomer can react with hydroxide anion giving the product, 6-n-butyl-6-azabicyclo[3.1.0]hex-3-en-2-ol.

(1R,4S,5S)-5-((3-Hydroxypropyl)amino)-4-((1-methyl-1H-tetrazol-5-yl)thio)cyclopent-2-en-1-ol

Using environmentally friendly conditions, the nucleophilic ring-opening reaction of 6-azabicyclo[3.1.0]hex-3-en-2-ol with 1-methyl-1H-tetrazole-5-thiol provided a novel thiol-incorporated aminocyclopentitol, (1R,4S,5S)-5-((3-hydroxypropyl)amino)-4-((1-methyl-1H-tetrazol-5-yl)thio)cyclopent-2-en-1-ol, in excellent yield (95%). The newly synthesized compound was analyzed and characterized via 1H, 13C-NMR, HSQC, and mass spectral data.

Pyridinium salts: from synthesis to reactivity and applications

This review highlights the pyridinium salts in terms of their natural occurrence, synthesis, reactivity, biological properties, and diverse applications.

The synthetic potential of pyridinium salt photochemistry

The discovery in the 1970s by Kaplan, Wilzbach and Pavlik that pyridinium salts undergo a unique cyclization reaction to produce bicyclic-aziridines was virtually unrecognized for nearly three decades. It was only comparatively recently that the process was explored in more detail and its synthetic potential exploited. In this Perspective, photocyclization reactions of pyridinium salts will be discussed, starting with the initial discovery, covering related processes of pyrylium salts, and extending to applications to the synthesis of natural and non-natural products of biomedical interest.

Photochemical transformations of pyridinium salts: mechanistic studies and applications in synthesis

The discovery, understanding and synthetic exploitation of the photochemical transformation of pyridinium salts are described. The investigations surrounding the remarkable transformation of pyridinium salts into a host of structurally complex motifs have helped extend the comprehension of aromatic and heteroaromatic photochemistry. The synthetic community has, in recent years, recognised the potential inherent in these compounds and has since exploited the irradiation of variously substituted pyridinium salts as key steps in the preparation of advanced intermediates in numerous synthetic programmes.

Photocyclization Reactions of Cyclohexa- and Cyclopenta-Fused Pyridinium Salts. Factors Governing Regioselectivity

The results of studies described in this report show that irradiation of 1,2-cyclopenta-fused pyridinium perchlorate in aqueous base promotes a remarkably regioselective photocyclization reaction that results in exclusive formation of a single tricyclic allylic alcohol. Moreover, transformation of this photoproduct to a spirocyclic amido diester followed by enzymatic desymmetrization produces an enantiomerically pure monoalcohol. This chemistry comprises a highly concise sequence for the preparation of what should become a useful synthon in synthetic organic chemistry.

Pyridinium Salt Photochemistry in a Concise Route for Synthesis of the Trehazolin Aminocyclitol, Trehazolamine

[reaction: see text] A strategy for the concise synthesis of trehazolamine, the aminocyclitol core of the potent trehalase inhibitor trehazolin, has been developed. The methodology takes advantage of photocyclization reaction of 1-methoxyethoxymethyl-3-pivaloxymethylpyridinium perchlorate to generate a bicyclic-aziridine intermediate, which is transformed under aziridine ring opening conditions to the key intermediate, 3,5-diacetoxy-3-pivaloxymethyl-4-(N-acetylamino)cyclopentene. In addition, the strategy is used in an enantio-divergent sequence for preparation of the natural (+)-trehazolamine and its unnatural (-)-enantiomer. In this route, the chiral auxiliary containing 1-(tetracetyl-alpha-D-glucosyl)-3-pivaloxymethylpyridinium perchlorate undergoes photocyclization to generate separable, diastereomeric bicyclic-aziridines, which are then independently transformed to enantiomeric 3,5-diacetoxy-3-pivaloxymethyl-4-(N-acetylamino)cyclopentenes.

Stereoselective Synthesis of Polyhydroxylated Indolizidines Based on Pyridinium Salt Photochemistry and Ring Rearrangement Metathesis

Ruthenium-catalyzed ring rearrangement metathesis (RRM) reactions of stereochemically diverse, differentially protected 4-N-allylacetamidocyclopenten-3,5-diols, prepared by using pyridinium salt photochemistry, have been explored as part of a program to develop novel routes for the synthesis of polyhydroxylated indolizidines. The RRM reactions, which produce selectively protected 1-acetyl-2-allyl-3-hydroxy-1,2,3,6-tetrahydropyridines, were found to take in high yields and with high levels of regioselectivity. The significance of RRM reactions of 4-N-allylacetamidocyclopenten-3,5-diols in the context of polyhydroxylated indolizidine synthesis is demonstrated by an application to the concise preparation of the potent glycosidase inhibitor, (-)-swainsonine.

A novel approach to the synthesis of amino-sugars. Routes to selectively protected 3-amino-3-deoxy-aldopentoses based on pyridinium salt photochemistry

A new approach for the synthesis of selectively blocked 3-amino-3-deoxyaldopentoses is presented. The strategy is based on employment of a pyridinium salt photocyclization-aziridine ring-opening sequence to prepare stereochemically defined, enantiomerically enriched aminocyclopentendiol derivatives. Ring-opening reactions transform these substances into terminally differentiated aminopolyols, which serve as precursors to the target amino-aldopentoses. The utility of this strategy is demonstrated by its application to the syntheses of protected derivatives of D- and L-3-amino-3-deoxyxylose, L-3-amino-3-deoxyarabinose, and a late-stage intermediate in a potential route to N-acetylneuraminic acid.

The photohydration of N-alkylpyridinium salts: theory and experiment

Bicyclic aziridines formed by the irradiation of pyridinium salts in basic solution have recently been recognized to have great synthetic potential. We have undertaken a joint computational and experimental investigation of the mechanism of this photoreaction. We have computationally determined the structures and relative energies of the relevant stationary points on the lowest potential energy surface (PES) of the pyridinium and methylpyridinium ions. Two important intermediates are shown to be bound minima on the ground-state PES: azoniabenzvalene and a 6-aza[3.1.0]bicyclic ion with an exo-oriented substituent (analogous to prefulvene). We advance a mechanism which involves initial formation of this exo-bicyclic ion, followed by nitrogen migration around the ring via the azoniabenzvalene intermediate. Thus, the barrier separating the two intermediates is the factor that determines the degree of scrambling observed in the photoproducts when the carbon atoms are labeled with deuterium or substituted with additional methyl groups. For N-methylpyridinium, the exo-methyl bicyclic ion was computed to be approximately 1 kcalmol(-1) lower in energy than N-methyl-azoniabenzvalene. The transition state was computed to lie several kcal mol(-1) above the exo-methyl bicyclic ion (+8.4kcalmol(-1), 6-31G* RHF; +3.7kcalmol(-1), 6-31G* B3LYP), but still well below the energy available from the 254 nm excitation of the N-methylpyridinium ion. The computed relative energies correspond splendidly with several experimental findings which include the preference for exo products, the results of deuterium labeling, and the impact of additional substituent methyl groups on the product distribution.