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Unsaturated pyrans, synthesis

There are some examples of pyran synthesis by Method 2, which involves the reactions of unsaturated ketones with methylene-active nitriles. In a typical case of Method 2, the Michael reaction of a,j5-unsaturated carbonyl compounds 26 (77TL1835, 78JHC57,... [Pg.193]

H-pyran synthesis from, 3, 758 pyridine derivatives from, 2, 523 unsaturated... [Pg.711]

Substituted unsaturated pyrans prepared by RCM using 19 as catalyst can be immediately submitted to zirconium-catalysed kinetic resolution of the racemic product at 70 °C. This provides a new route to medicinally important agents containing 6-membered cyclic ethers. A one-pot synthesis can give 63% conversion with >99% enantiomeric purity229. [Pg.1525]

Unsaturated pyran 6 (Scheme 12.3) has been utilized (via 8) in the synthesis of the lactone fragment of tipranavir, a molecule discovered to possess potent activity against the HIV-protease enzyme [6]. Synthesis of enantiomericaUy enriched 7 (er = 95.5/4.5), a chiral product that would otherwise be difficult to access in the nonracemic form, is therefore achieved through a catalytic reaction of an easily available achiral substrate. Enantioselective preparation of 7 by an RCM process that involves the use of an achiral catalyst demands an effective protocol for preparation of the requisite O-substituted quaternary carbon stereogenic center. [Pg.345]

The intramolecular Michael addition11 of a nucleophilic oxygen to an a,/ -unsaturated ester constitutes an attractive alternative strategy for the synthesis of the pyran nucleus, a strategy that could conceivably be applied to the brevetoxin problem (see Scheme 2). For example, treatment of hydroxy a,/ -unsaturated ester 9 with sodium hydride furnishes an alkoxide ion that induces ring formation by attacking the electrophilic //-carbon of the unsaturated ester moiety. This base-induced intramolecular Michael addition reaction is a reversible process, and it ultimately affords the thermodynamically most stable product 10 (92% yield). [Pg.734]

Synthesis of highly functionalized 3,4-dihydro-2H-pyrans by high-pressure Lewis-acid-catalyzed cycloaddition of enol ethers and a,/i-unsaturated aldehydes [83]... [Pg.242]

Dihydro-2H-pyran-2-ones (e. g., 4-195) are valuable intermediates in the synthesis of several natural products [67]. Hattori, Miyano and coworkers [68] have recently shown that these compounds can be easily obtained in high yield by a Pd2+-catalyzed [2+2] cycloaddition of ct, 3-unsaturated aldehydes 4-192 with ketene 4-193, followed by an allylic rearrangement of the intermediate 4-194 (Scheme 4.42). In this reaction the Pd2+-compound acts as a mild Lewis acid. a,(3-unsaturated ketones can also be used, but the yields are below 20%. [Pg.307]

Both natural and non-natural compounds with a 2ff,5ff-pyrano[4,3-fc]pyran-5-one skeleton are of interest in medicinal chemistry. Several natural products, such as the pyripyropenes, incorporate this bicyclic ring system. The group of Beifuss has described an efficient microwave-promoted domino synthesis of the 2ff,5H-pyr-ano[4,3-fo]pyran-5-one skeleton by condensation of a,/3-unsaturated aldehydes with 4-hydroxy-6-methyl-2]-f-pyran-2-one (Scheme 6.244) [428]. It is assumed that in the presence of an amino acid catalyst a Knoevenagel condensation occurs first, which is then followed by a 6jr-electron electrocyclization to the pyran ring. While the conventional thermal protocol required a reaction time of up to 25 h (refluxing ethyl... [Pg.259]

Inverse type hetero-Diels-Alder reactions between p-acyloxy-a-phenylthio substituted a, p-unsaturated cabonyl compounds as 1-oxa-1,3-dienes, enol ethers, a-alkoxy acrylates, and styrenes, respectively, as hetero-dienophiles result in an efficient one step synthesis of highly functionalized 3,4-dihydro-2H-pyrans (hex-4-enopyranosides). These compounds are diastereospecifically transformed into deoxy and amino-deoxy sugars such as the antibiotic ramulosin, in pyridines having a variety of electron donating substituents, in the important 3-deoxy-2-gly-culosonates, in precursors for macrolide synthesis, and in C.-aryl-glucopyranosides. [Pg.182]

Aiming at the pyranose form of sugars, normal type hetero-Diels-Alder reactions were extensively used for the synthesis of functionally substituted dihydropyran and tetrahydropyran systems (5-10) (see routes A - D in the general Scheme 1) which are also important targets in the "Chiron approach" to natural product syntheses (2.) Hetero-Diels-Alder reactions with inverse electron demand such as a, p-unsaturated carbonyl compounds (l-oxa-1,3-dienes) as heterodienes and enol ethers as hetero-dienophiles, are an attractive route for the synthesis of 3,4-dihydro-2H-pyrans (11). [Pg.183]

For the synthesis of nonannulated 2-amino-4H-pyrans, acyclic carbonyl compounds 24 are used in Methods 1,2, and 3. Generally, readily accessible ketones 24, as well as unsaturated nitriles 25 contain electron-withdrawing substituents (R, X). Consequently, densely functionalized pyrans 22 are formed. UNs, widely used for aminopyran preparation, include 30-34... [Pg.185]

Moreover, unsaturated 3,5-diarylidenepiperidin-4-ones can be substituted for diarylidenecyclohexanones 130 (07BML6459). Analogously, a series of tertralones 132 were employed in the synthesis of tricyclic pyrans 133 (79M115) (Scheme 47). [Pg.208]

In contrast, ethyl 3-oxo-4-(triphenylphosphoranylidene)butyrate behaves as a C,0-bis(nucleophile) when reacted with unsaturated 5(4//)-oxazolones 584 with a leaving group at the exocyclic p-carbon. In this case, initial Michael reaction generates 585 that eliminates HX to produce a resonance stabihzed ylide 586. Cyclization of 586 with ring opening leads to the interesting ylide intermediate 587 used for the synthesis of 2//-pyran-2-ones (Scheme 7.185). [Pg.252]

Dihydrofuran reacts with /3,7-unsaturated a-keto esters with copper or zinc complex catalysts to generate furo[2,3-/ ]pyran derivatives in good yields with high stereoselectivity. The synthesis proceeds via an inverse electron demand hetero-Diels-Alder reaction <2000CC459>. [Pg.301]

By analogy with the formation of dihydropyrans from unsaturated carbonyl compounds and alkenes (see Section 2.24.2.7.l(i)), the synthesis of 4//-pyrans from the [4 + 23-cycloaddition of unsaturated carbonyl compounds and alkynes would seem to offer some potential. Such a reaction has indeed proved of value, but examples are largely restricted to the use of ynamines as the dienophile (76BSF987). [Pg.760]

The reaction between vinyl ethers and unsaturated carbonyl compounds, which provides a powerful synthetic route to dihydropyrans, has been adapted to the synthesis of pyran-2-ones (72CC863). 2-Chloro-1,1 -dimethoxyethylene, which is a protected form of chloroketene, undergoes cycloaddition with a number of enones to give the cis or trans isomers of 3-chloro-3,4-dihydro-2,2-dimethoxypyrans (338) and (339) or a mixture of both. Although... [Pg.796]

The thermal ring opening of the pyrazoline (351) occurs with elimination of nitrogen and the formation of a keto ester. The latter cyclizes with loss of ethanol to the pyran-2-one (352 Scheme 110) (02CB782). Since pyrazolines may be obtained from a 1,3-dipolar cycloaddition of diazoacetic ester and an unsaturated ketone, this route is in effect a further example of a type (i) synthesis (Scheme 85). [Pg.798]


See other pages where Unsaturated pyrans, synthesis is mentioned: [Pg.711]    [Pg.176]    [Pg.222]    [Pg.220]    [Pg.413]    [Pg.513]    [Pg.575]    [Pg.604]    [Pg.622]    [Pg.183]    [Pg.755]    [Pg.759]    [Pg.22]    [Pg.185]    [Pg.119]    [Pg.48]    [Pg.60]    [Pg.110]    [Pg.513]    [Pg.575]    [Pg.604]    [Pg.622]    [Pg.366]   
See also in sourсe #XX -- [ Pg.1525 ]




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Pyrans, unsaturated

Synthesis unsaturated

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