Component pyran ring

One-component syntheses, in which a pyran ring (type B,l) or a 1,5-dione structure (type B,2) is already present. 

B. One-Component Syntheses 1. From Compounds with Preformed Pyran Ring... 

Several synthetic compounds containing the naphthoquinone skeleton, such as 2-(4-cyclohexylcyclohexyl)-3-hydroxy- 1,4-naphthoquinone [169], are effective in the treatment of coccidial infection. Two structurally related antibiotics WS-5995A (36) and WS-5995B (37a), isolated from S. auranticolor sp. nov. near Tokyo, efficiently protect Eimeria tenella (a species of coccidia) infection. An inactive component WS-5995C (37b) can be readily converted to the active (36) by simple dehydration [ 170-172]. Compound (36), the structure of which resembles the aforementioned benz[a]anthraquinone antibiotics, is the first example of a 5//-benzo[d ]naphtho[ 2,3-6 ]pyran ring system found in nature. 

The cycloaddition reaction of o-thioquinones with acyclic 1,3-dienes is a finely tuned process that can involve either reagent behaving as the diene component. It has now been established that formation of a 2-spiro-linked thiopyran by a [2-1-4] reaction is kinetically controlled and that the [4-1-2] alternative leads to the thermodynamic product, a 1,4-benzoxathiin (Scheme 31). Cyclic dienes yield only the oxathiin <03T5523>. Incorporation of the thionocarbonyl diene unit into fiiran and pyran rings and reaction with carbohydrate glycols leads to tricyclic oxathianes, desulfurisation of which yields 2-deoxydisaccharides <03T4249>. 

Heterocyclic systems produced by annulation of a pyran ring to the biologically versatile thiazole nucleus can lead to an attractive scaffold for libraries of drug-like compounds. Yadav et al. reported a synthesis of these species by three-component coupling of glycine, acetic anhydride, and a 5-arylidenerhodanine 57 (Scheme... 

Pyran ring by 3-component reaction from / -dioxo compds., aldehydes... 

Note that each six-membered pyran ring contains five stereogenic centers, each marked (i ) or (S). In both polysaccharides, there are repeating glucose units, but the connecting ether units (marked in red and part of an acetal structure see Chapter 18, Section 18.6) have different absolute configurations. There is a vast difference in the characteristics of cellulose when compared to the amylose component of starch, in part due to stereochemistry. 

Bio active polyphenols are present in a variety of plants and are used as important components of human and animal diets [27]. Flavonoids are a broad class of low-molecular-weight secondary plant polyphenolics, which are benzo-y-pyrone derivatives consisting of phenolic and pyrane rings. Flavonoids are usually subdivided according to their substituents into fla-vanols, flavones, flavanones, chalcones, and anthocyanidines. 

Pyrano[2,3-b]pyiidines 26, 244 Pyran ring by 3-component reaction 26, 812 4H-Pyran -... 

A number of mefhods for fhe synthesis of fused pyrans were developed using multicomponent reactions in PEG. Thus, the annulation of a pyran ring using the three-component reaction of 1/5-cyclohexanediones, arylaldehydes, and malononitrile was found to proceed in the presence of barium(II) triflate xmder mild conditions in aqueous PEG 400 providing tetrahydrochromenes 105 (Scheme 58) [89]. The recyclability of the catalyst was studied using the... 

By connecting the alcohol and allylsilane moieties into a single molecule, the above Sakurai reaction becomes a two-component system (instead of three components as in the previous scheme). This enables the formation of the pyrane ring in a single step, as illustrated in studies directed... 

Over the years, many spiropyran structures have been prepared. The pyran component consists of benzopyran or naphthopyran and the heterocyclic part consists of indoline, benzothiazoline, benzoxazoline, benzoselen-azoline, phenanthridine, acridine, quinoline, benzopyran, naphthopyran, xanthene, benzodithiole, benzoxathiole, and saturated heterocyclic rings such as pyrolidine and thiazolidine. 

The non-aromatic nature of the pyran-2-one ring is evident in its behaviour as the diene component in Diels-Alder cycloadditions. With dimethyl acetylenedicarboxylate (DMAD, dimethyl but-2-ynedicarboxy-late), for example, it gives an adduct that spontaneously eliminates carbon dioxide to yield dimethyl phthalate (dimethyl benzene-1,2-dicarboxylate) (Scheme 4.7)... 

As in the synthesis of other bipyridines, several routes to 4,4 -bipyridine have been devised where one of the pyridine rings is built up from simpler components. For example, a dimer of acrolein reacts with ammonia and methanol in the presence of boron phosphate catalyst at 350°C to give a mixture of products including 4,4 -bipyridine (3.4% yield), and in a reaction akin to ones referred to with other bipyridines, 4-vinylpyridine reacts with substituted oxazoles in the presence of acid to give substituted 4,4 -bipyridines. ° ° Condensation of isonicotinaldehyde with acetaldehyde and ammonia at high temperatures in the presence of a catalyst also affords some 4,4 -bipyridine, and related processes give similar results,whereas pyran derivatives can be converted to 4,4 -bipyridine (56% conversion), for example, by reaction with ammonia and air at 350°C with a nickel-alumina catalyst. Likewise, 2,6-diphenyl-4-(4-pyridyl)pyrylium salts afford 2,6-... 

A three-component one-pot coupling of 2,6-dimethylphenyl isocyanide, methyl 2,4-dioxopentanoate and alkynoic esters leads to the isatin-like compounds (or fused pyrans) 20 (Scheme 5). The reaction proceeds by initial formation of the zwitterionic intermediate 21, which is protonated by methyl 2,4-dioxopentanoate. Attack of the newly generated nucleophile then forms the intermediate 22. The pyrrolidinone ring is formed upon loss of methanol and electrocyclization then occurs to afford the desired 2//-pyrans (Scheme 5) <2005S1049>. 

In a one-pot three-component reaction, aromatic aldehydes, malononitrile and 1,3-dicarbonyl compounds react to form 2-amino-5-carboxy-4-aryl-47/-pyran-3-carbonitriles 87. The reaction proceeds by an initial Knoevenagel condensation of malononitrile with the aromatic aldehyde to afford the 2-benzylidenemalononitrile intermediate 88. Michael addition of the activated methylene group forms the 1,5-dicarbonyl equivalent 89, which upon ring closure affords 477-pyrans (Scheme 29) <2004SL871, 1999H(51)1101 >. 

Dimcthyl 2-(but-2-ynyl)-2-(5-oxopent-3-enyl)malonates 96 undergo ruthenium-catalyzed intramolecular cycli-zations to yield cyclohexyl fused 4//-pyrans 97, most likely via formation of and reductive elimination from the ruthenacycle intermediate 98 (Scheme 32). Likewise, internal alkynes tethered to an a, 3-unsaturated ketone via a three-component chain undergo mthenium-catalyzed cyclizations furnishing 4//-pyrans that are fused to five-membered rings 99 (Equation 50) <2000JA5877>. 

Clerodanes involve a (C61C6) group variously linked to furan (unsaturated C40), pyran (unsaturated C50), methyleneoxy and methylenedioxy rings. Clerodanes include bitter tastants and antifeedants as exemplified by the extremely bitter component columbin of the bitter tonic made from roots of Jateorhiza columba (columba root) (Menispermaceae). 

Pederamide (12) is an important intermediate in the projected total synthesis of pederin (13), a component of the vesicant of the beetle Paederus fuscipes. /rart5-2-Butene epoxide has been converted (in 16 steps) into pederamide. One of these involved the conversion of the tetrahydrofuran (14) into the pyran-2-one (15) in 96% yield.Synthetic studies on lasalocid A (16) have continued with the use of carbohydrates as substrates for the construction of both furanoid and pyranoid rings of the antibiotic. ... 

A review of the use of electrocyclic reactions in synthesis has been presented" and of asymmetric electrocyclic reactions that result in diastereo- or enantio-selectivity selection as a consequence of the influence of a chiral component, be it substrate or catalyst, on the electrocyclic bond-forming process." Computational studies of 0 electrocyclic ring-opening reactions of 2-pyrone and 6-fluoro-2-pyrone are pseudoper-icyclic 6-fluoro-2-pyranol are a borderline case and 2-pyranol, pyran, and 6-fluoro pyran reactions are pericyclic in character." ... 

Numerous other ethers containing pyran or furan rings are formed by the dehydration of aliphatic diols (e.g. linalool oxides, rose oxide or nerol oxide) and are components of many essential oils. For example, the furanoid 2R,5R)- E)- and (2J ,5S)-... 

Reduced Pyrans.— This widely occurring ring system attracts a great deal of synthetic attention, and the Diels-Alder reaction is particularly suited to the preparation of compounds in this class bearing a variety of substituents. The use of arylmethylenemalonaldehydes can lead to a variety of heterocycles since each such compound can behave as a 4tc or a 27t component. Olefins react to... 

Glycerol was found to be a very efficient solvent for another catalyst-free three-component reaction furnishing fused pyrans 105, 106, and 161-164 (Scheme 97) [141]. The 2-aminopyran ring was annulated to a variety of scaffolds including 1,3-cyclohexanones, a- and 3-naphthols, 4-hydroxy-6-methyl-2-pyrone, and barbituric acid using the reaction with aromatic aldehydes and methylene active nitriles. It was reported that glycerol recovered from the model reaction (105, Ar = Ph, R = CN, = Me) was reused 20 times in this reaction without... 

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