277-Pyran intermediates

Finally, a novel synthetic route involves formation of the pyridine ring from a fused pyran intermediate, e.g. (139) - (140) (70CB1250, 80JOC1918, 73JCS(P1)823). If a pyrylium salt is used, a quaternary pyridopyrimidinium salt such as (141) is formed 77KGS14S4). 

Dehydrohalogenation reactions have been involved in several routes for the total synthesis, involving pyran intermediates, of... 

In principle, pyrylium salts can be transformed to pyrans by the addiiion of an appropriate reagent to the 2-, 4-, or 6-position of the heterocycle. Although most transformations seem to be nucleophilic additions (because of the electrophilic character of the pyrylium substrates) some reactions, such as hydrogenation or one-electron reductions, are of a radical nature. Many nucleophilic additions to pyrylium salts proceeding via unstable pyran intermediates seem to be of general synthetic interest, especially in the fields of aromatic and other conjugated systems, l97 200 but are of little interest in pyran chemistry. 

The addition of ammonia or primary and secondary amines to pyrylium salts is usually accompanied by a rapid ring opening without detection of the expected 2//-pyran intermediates. The exceptional behavior of the salt 160a was observed in its reaction with Af./V-dimethylaniline, affording 85% of 4H-pyran 210, whereas analogous addition products from toluene and methoxy-benzene were too unstable to be isolated.214... 

Z)-(3-Silyloxyacrylonitriles 95 undergo cycloaddition reactions with a,P-unsaturated ketones to form a dihydro-pyran intermediate, which eliminates trimethylsilanol to furnish 3-cyano-4//-pyrans (Scheme 31) <1997S628>. 

Enynals 104 can undergo a palladium-catalyzed cyclization to form the pyrylium intermediate 105, which upon deprotonation affords the triene 106. In the presence of DM AD, a [2+2]-cycloaddition then occurs to form the tricyclic 4//-pyran intermediate 107, which undergoes expansion of the cyclobutene ring to afford the bicyclic 4//-pyrans 108 (Scheme 34) <2004S1409>. 

The use of pyran intermediates for the synthesis of these ring systems has presented few examples. The [4 + 2] cycloaddition of 5-ethenyl-3,4-dihydro-2//-pyrans with DEAD yields compounds (500) <86CB3204>. Reaction of the esters (501) with hydrazine hydrate gave the pyranodiazines (502) <86MI 716-01). 

Kim, S, Salomon, R G, Total synthesis of halicondrins highly stereoselective construction of a homochiral pentasubstituted H-ring pyran intermediate from a-D-glucose, Tetrahedron Lett., 30, 6279-6282, 1989. 

A retro-synthetic analysis of 1 as shown in Fig. 3 indicates that 3,6-dihydro-2H-pyran derivatives could serve as key intermediates in the synthesis of 1 and that hetero Diels-Alder reactions could be used to make these 3,6-dihydro-2H-pyran intermediate [3, 4]. 

This type of isomerization still seems to be of limited importance. For example, a mixture of unseparable valence-bond isomers 75 and 76 was obtained by thermolysis of an appropriate cyclobutene precursor (92TL883). Another 2//-pyran intermediate postulated in the Z-E interconversion of stereoisomeric retinals could be neither isolated nor identified (92JOC1110). 

A selective method for benzylic metalation of o-, m-, and p-substituted toluenes has been reported using BuLi/f-BuOKVTMP and rationalized by the ability of the mixed metal amide base to facilitate an anion migration from the kinetic (o-aryl) to the benzylic metalation site. Reaction of 2-(l-phenylethylidene)malononitrile and 2,2,2-trifluoro-1 -phenylethanone gives (2 ,4 )-2-cyano-6,6,6-trifluoro-3,5-diphenyl-hexa-2,4-dienamide via a vinylogous aldol reaction followed by rearrangement of the 2/f-pyran intermediate (Scheme 52). °°... 

When aldehydes were employed in place of CO2 in the cyclization reactions, the formation of dienones was observed. These are presumed to be formed by the electrocyclic ring opening of the proposed pyran intermediate (Figure 13.33) [78]. 

In 2007, Hoveyda and Gillingham disclosed the total synthesis of (+)-baconipyrone C (143), which represents the first, and a rare example of, application of a Ru-catalyzed alkene ROM/RCM to complex natural product synthesis (Fig. 38) [90]. By treatment of oxabicycle (144) with 2 mol% chiral Ru carbene catalyst, the key highly functionahzed pyran intermediate (145) was prepared in 63% yield and 88% ee. Notably, the chiral catalyst was generated in situ by subjection of... 

Intermediates. 3,4-Dihydro-2H-pyran [110-87-2] is prepared by a ring-expanding dehydration of tetrahydrofurfuryl alcohol. It is used as a protecting agent for hydroxyl compounds and as an intermediate. 2-Methylfuran is a chemical intermediate for 5-methylfurfural [620-02-0] (151) and... 

Despite the increasing information on the photochemistry of 2,4-dienones and other unsaturated ketones, as well as on the ring-chain valence isomerism of halogen-substituted pyran and dihydi opyran systems,the data are still very scarce. The intermediate formation of pyrans valence-isomeric with unsaturated carbonyl compounds in the pyridine syntheses based on reactions of ammonia with aldehydes or ketones, advocated by various authors (cf. Section II,B,2,f), is still rather speculative. (See also Section II,B,2,e for the valence isomerism of 5-chloro-2,4-dienones with pyrylium chlorides.)... 

Recently, the cyclopenta[c]pyran (122) was obtained as a secondary product along with 1,2,3-tribenzoylcyclopropane from phenacyl-trimethylammonium hydroxide a complicated sequence was proposed involving a Michael addition to 1,2-dibenzoylethylene intermediately formed. 

Dihydropyrones 302 are used as intermediates in the syntheses of tetrahydro-pyrane. There are two approaches (i, ii) to their preparation [74JA7807 84AG991 85JA1246 86JA7060 86MI1 87AG(E)15]. 

Transformations of 5,6-dihydro-2//-pyrans 8 into the corresponding 3,4-dihydro-2//derivatives 9 under basic conditions (88KGS291 98TL2025), as well as the reverse conversion under irradiation [97H(46)451], have been explained by the intermediate formation of electron-delocalized heterocyclic systems, giving rise to two possible isomers. 

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