3- Hydroxy-2-pyrone, Diels-Alder reactions

Development of base-catalyzed Diels-Alder reaction of 3-hydroxy-2-pyrone and its application to synthesis of bioactive compounds 99YGK84. 

A careful assessment of the constitution of compound 10 led to the development of a rather efficient strategy featuring the Diels-Alder reaction (see Scheme 3). Although the unassisted intermole-cular reaction between 3-hydroxy-2-pyrone (16)23 and a,/ -unsatu-rated ester 17 is unacceptable in terms of both regioselectivity and chemical yield, compounds 16 and 17 combine smoothly in refluxing benzene and in the presence of phenylboronic acid to give fused bicyclic lactone 12 (61% yield) after workup with 2,2-... 

Base-catalyzed Diels-Alder reactions are rare (Section 1.4). A recent example is the reaction of 3-hydroxy-2-pyrone (145) with chiral N-acryloyl oxazolidones 146 that uses cinchona alkaloid as an optically active base catalyst [97] (Table 4.25). Only endo adducts were obtained with the more reactive dienophile 146 (R = H), the best diastereoselectivity and yields being obtained with an i-Pr0H/H20 ratio of 95 5. The reaction of 146 (R = Me) is very slow, and a good adduct yield was only obtained when the reaction was carried out in bulky alcohols such as t-amyl alcohol and t-butanol. 

Table 4.25 Diels-Alder reactions of 3-hydroxy-2-pyrone (145) catalyzed by cinchona alkaloids...

Protic solvents such as i-PrOH and t-BuOH favor the diastereoselectivity of the reaction of 3-hydroxy-2-pyrone with acrylates [49b]. Further examples of proton-promoted Diels-Alder reactions are reported in Section 4.8. 

Okamura H., Iwagawa M. and Nakatani M. Development of Base Catalyzed Diels-Alder Reaction of 3-Hydroxy-2-Pyrone and Application to Synthesis of Biologically Active Compounds Org. Chem. Japan 1999 57 84... 

Okamura and coworkers151 studied the base catalyzed Diels-Alder reactions between 3-hydroxy-2-pyrone (224) and chiral l,3-oxazolidin-2-one based acrylate derivatives. Catalysis of the reaction between 224 and 225 by triethylamine gave fair to good de values, somewhat dependent on the solvent system used (equation 63, Table 7). Addition of 5% of water to the solvent isopropanol, for example, increased the de of the endo adduct 226 substantially. When the amount of water was increased, however, the triethylamine catalyzed reaction became less endo and diastereofacially selective, a small amount of exo 227 being obtained. Replacing triethylamine by the chiral base cinchonidine also improved the de, but now independently of the solvent system used. 

The intermediate J4 was prepared by an "intramolecular Narasaka type Diels-Alder reaction" of dienophile 10 and 3-hydroxy-2-pyrone (J L) in the presence of phenylboronic acid [7] ... 

Okamura and Nakatani [65] revealed that the cycloaddition of 3-hydroxy-2-py-rone 107 with electron deficient dienophiles such as simple a,p-unsaturated aldehydes form the endo adduct under base catalysis. The reaction proceeds under NEtj, but demonstrates superior selectivity with Cinchona alkaloids. More recently, Deng et al. [66], through use of modified Cinchona alkaloids, expanded the dienophile pool in the Diels-Alder reaction of 3-hydroxy-2-pyrone 107 with a,p-unsaturated ketones. The mechanistic insight reveals that the bifunctional Cinchona alkaloid catalyst, via multiple hydrogen bonding, raises the HOMO of the 2-pyrone while lowering the LUMO of the dienophile with simultaneous stereocontrol over the substrates (Scheme 22). 

The Deng group identified QN-derived thiourea 121 and QD -derived thiourea 124 to be also efficient promoters of enantio- and diastereoselective Diels-Alder reactions between the 2-pyrone diene 3-hydroxypyran-2-one and the dienophiles fumaronitrile, maleonitrile as well as acrylonitrile, while various C9-hydroxy acylated and alkylated (dihydro)cupreines and (dihydro)cupreidines failed for the same reactions under identical conditions (e.g., 97% yield, 15% ee, 64 36 endoxxo) [289], Catalysts 121 and 124 (5mol% loading), however, produced the corresponding Diels-Alder adducts 1-3 with synthetically useful enantioselectivities (85-... 

Chromanones,4 In the presence of this amine 4-mcthyl-6-hydroxy-2-pyrone (1) undergoes decarboxylative dimerization to form a coumarochromanone (2) in 42 yield. The pyrone group in 2 undergoes decarboxylative Diels Alder reactions (8, 32G 9, 175 176) with acetylenes to form 4-chromanoncs (3). 

Both 9-epiamino derivatives of 9-deoxyquinine and 9-deoxyquinidine promote exo-selective Diels-Alder reaction of 3-hydroxy-2-pyrones with enones. Actually, great latitude exists for tuning the reaction by partial structural changes of the catalysts. ... 

DIELS-ALDER REACTIONS Benzocyclopropene. 1,2-BisC3-tosyIethoxycarbonyl)-diazene. Cyclobutadiene iron tricarbonyl. 1,2-Dicyanocyclobutene. Diethyl ketomalonate. 1,3,4,6-Heptatetraene. 2-Hydroxy-5-oxo-5,6-dihydro-2/f-pyrone. 3-Hydroxy-2-pyrone. Isopropylidene isopropylidenemalonate. Lithium tetra-methylpiperidide. 4-Methoxy-5-acetoxymethyl-o-benzoquinone. 4-Methoxy-5-methyl-o-benzoquinone. frans-l-Methoxy-3-trimethyl-silyloxy-l,3-butadiene. Per-fluorotetramethylcyclopentadiene. 4-Phenyl-l, 2,4-triazoline-3,5-dione. Potassium f-butoxide. 

A full report has been published on the preparation of fused valerolactones from cyclopentadiene anions by condensations with epoxides, carboxylation, and finally cyclization (Scheme 26). Diels-Alder reactions of 3-hydroxy-2-pyrones usually lead to benzene derivatives by decarboxylation of the intermediate lactone. It has now been shown that these lactones can be isolated in good yields by suppression of the CO2 loss by carrying out the cycloadditions under a high pressure of carbon dioxide. 

In order to prepare the cyclohexenaldehyde 8, 3-hydroxy-2-pyrone 14 and ethyl 4-hydroxy-2-methyl-2-butenoate 15 are subjected to a Diels-Alder reaction in the presence of phenylboronic acid which arranges both reactants to the mixed boro-nate ester 19 as a template to enable a more efficient intramolecular Diels-Alder reaction with optimal control of the regiochemical course of the reaction. Refluxing in benzene affords the tricyclic boronate 20 as primary product. This liberates the intermediate cycloadduct 21 upon transesterification with 2,2-dimethylpropane-l,3-diol which, on its part, relaxes to the lactone 22. Excessive i-butyldimethyl-silyltriflate (TBSTf) in dichloromethane with 2,6-lutidine and 4-7V,A-dimethyl-aminopyridine (DMAP) as acylation catalysts protects both OH goups so that the primary alcohol 23 is obtained by subsequent reduction with lithiumaluminum-hydride in ether. 

The synthetic scope of the Diels-Alder reaction continues to be widened as a result of the studies of Corey s group. A brief note this year describes the use of 3-hydroxy-2-pyrone (195) as a vinylketene equivalent for the synthesis of dihydrophenols and cyclohexenones. These compounds are the result of the loss of CO2 from the initially formed adducts in a reverse [4 + 2]reaction, but in the reaction of (195) with maleic anhydride the adduct (196) is stable. 

Development of base-catalyzed Diels-Alder reaction of 3-hydroxy-2-pyrone and its application to synthesis of bioactive comporrrrds 99YGK84. Fluorination of O- and N-heterocycles with molecular fluorine in the synthesis of fluorine-containing bioactive compounds 98YGK107. 

Pyrones, which are the ring-oxygen equivalents of pyridones, are simply a- and y-hydroxy-pyrylium salts from which an 0-proton has been removed. There is little to recommend that 2- and 4-pyrones be viewed as aromatic they are perhaps best seen as cyclic unsaturated lactones and cyclic p-oxy-a,P-unsaturated-ketones, respectively, for example 2-pyrones are hydrolysed by alkali, just like simpler esters (lactones). It is instructive that, whereas the pyrones are converted into pyridones by reaction with amines or ammonia, the reverse is not the case - pyridones are not transformed into pyrones by water or hydroxide. Some electrophilic C-substitutions are known for pyrones and benzopyrenes, the oxygen guiding the electrophile ortho or para, however there is a tendency for electrophihe addition to the C-C double bond of the heterocyclic ring, again reflecting their non-aromatic nature. Easy Diels-Alder additions to 2-pyrones are further evidence for diene, rather than aromatic, character. 

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