2-Alkoxy-2//-pyrans

On the other hand, variable yields of fluorinated 2-alkoxy-2//-pyrans 64 were found108,109 in mixtures formed in the reaction of perfluoroalkenes with dialkyl malonates. 

The chemoselective desilylation of one of the two different silyi enoi ethers in 10 to give the monosilyl enol ether II is realized by the Pd-catalyzed reaction of Bu3SnF. The chemoselectivity is controlled by steric congestion and the relative amount of the reagent[7,8]. An interesting transformation of the 6-alkoxy-2,3-dihydro-6//-pyran-3-one 12 into the cyclopentenone derivative 13 proceeds smoothly with catalysis by Pd(OAc)2 (10 mol%)[9]. 

H-Pyran, 2-alkoxy-4-methyl-2,3-dihydro-conformation, 3, 630 4H-Pyran, 2-amino-IR spectra, 3, 593 synthesis, 3, 758 4H-Pyran, 4-benzylidene-synthesis, 3, 762 4H-Pyran, 2,3-dihydro-halogenation, 3, 723 hydroboration, 3, 723 oxepines from, 3, 725 oxidation, 3, 724 reactions, with acids, 3, 723 with carbenes, 3, 725 4H-Pyran, 5,6-dihydro-synthesis, 2, 91 4H-Pyran, 2,6-diphenyl-hydrogenation, 3, 777 4H-Pyran, 6-ethyl-3-vinyl-2,3-dihydro-reactions, with acids, 3, 723 4H-Pyran, 2-methoxy-synthesis, 3, 762 4H-Pyran, 2,4,4,6-tetramethyl-IR spectra, 3, 593 4H-Pyran, 2,4,6-triphenyl-IR spectra, 3, 593... 

H-Pyran-6-carboxylic acid, rrans-2-alkoxy-5,6-dihydro- H NMR,3, 578 Pyrancarboxylic acids, oxo-decarboxylation, 2, 52 Pyran-5-carboxylic acids synthesis, 3, 792... 

Pyran-2-one, 3-acetyl-4-hydroxy-6-methyl-synthesis, 3, 792 Pyran-2-one, 4-alkoxy- H NMR,3, 581 Pyran-2-one, 6-alkoxy-synthesis, 3, 791... 

Pyran-3-one, 6-acetoxy-2,6-dihydro-Diels-Alder reaction, 3, 731 dimerization, 3, 722 Pyran-3-one, 6-alkoxy-synthesis, 3, 815... 

Pyran-3-one, 2-methyltetrahydro-reduction, 3, 729 Pyran-3-one, 6-methyltetrahydro-mass spectra, 3, 616 Pyran-3-one, 5-phenyl-synthesis, 3, 843-844 6H-Pyran-3-one, 6-ethoxy-2-methyl epoxidation, 3, 725 Pyran-4-one, 2-alkoxy- H NMR, 3, 581 Pyran-4-one, 2-amino-reactions, 2, 55 synthesis, 3, 814... 

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. 

H-Naphtho[2,l-fe]pyrans (1.19) with an amino or alkoxy residues in the 6-position (1.30) show particularly high colourability. The 1-amino- and l-alkoxy-3-hydroxynaphthalenes (1.28 and 129) required for the synthesis of these compounds are prepared from 2-naphthol as illustrated in Figure 1.9. ... 

Figure 1.9 Synthetic route to 6-amino and 6-alkoxy-3,3-diaryl-3H-naphtho[2,l-b]pyrans. (Reproduced with permission of Kluwer Academic/Plenum Publishers.)...

Bismuth(lll) chloride catalyzes the intramolecular hetero-Diels-Alder reaction of aldimines, generated in situ from aromatic amines and the A -allyl derivative of o-aminobenzaldehyde, in acetonitrile at reflux to generate [l,6]naphthyridine derivatives <2002TL1573>. The hetero-Diels-Alder reaction of 3-aryl-2-benzoyl-2-propeneni-triles and enol ethers yields 2-alkoxy, 6-diaryl-3,4-dihydro-2//-pyran-5-carbonitriles (Scheme 29) <1997M1157>. 

Dihydro-2J/-pyran (168) has not yet found any application in sugar synthesis. However, its 2-alkoxy derivatives (such as 193 and 196) are, as will be shown in this Section, useful substrates in the total synthesis of pentoses, hexoses, and many other sugars. In fact, a general method of carbohydrate synthesis is based on derivatives of 2-alkoxy-5,6-dihydro-2/f-pyran. Individual stages of the synthesis are described in Sections III, 2, a-d. 

Hydrogenation of the triple bond in 195 to a cis double-bond, followed by acid-catalyzed cyclization of the alkene, leads to 6-substi-tuted 2-alkoxy-5,6-dihydro-2//-pyrans (196). This method was used by H. Newman86 for the preparation of 2-ethoxy-5,6-dihydro-6-methyl-2H-pyran (196, R1 = Et, R = Me). 

Attempts were made in order to obtain adducts 198 in enantiomeric form by cyclo-addition of 1-alkoxy-l,3-butadienes to optically active esters of glyoxylic acid96 the enantiomeric purities of the adducts were, however, poor.96 Optically active butyl 2-alkoxy-5,6-dihydro-2H-pyran-6-carboxylates (198, R1 = Bu) were obtained when the R group in 197 was a carbohydrate moiety. The diastereoisomers resulting from cyclo-addition were separated by chromatography (see Section VII and Ref. 350). 

Alkoxy-5,6-dihydro-2/7-pyrans are readily hydrolyzed8889 in dilute, aqueous acids to fnms-a./J-unsaturated aldehydes (206). Hydrolysis in a neutral medium leads to an equilibrium mixture of 5,6-dihy-dro-2ff-pyran-2-ol (207) and the corresponding cis-aldebyde 208. (For syntheses of stereoisomerie deoxy-DL-aldoses from aldehyde 79, see Section II.) The thermal isomerization of 193 has been investigated.1 "... 

Epoxidation of 2-alkoxy-5,6-dihydro-2H-pyran (193) with peroxy acids leads to both possible epoxides, with the (i-erythro stereoisomer preponderating.65... 

The majority of the total syntheses of desosamine described in the literature have been based on the same reaction-sequence, namely, 2-alkoxy-5,6-dihydro-6-methyl-2H-pyran (247) was epoxidized and the epoxide (248) was then opened with aqueous dimethylamine to afford a mixture of the dimethylamino sugars 249. Interestingly, despite the simple reaction-pathway, all syntheses performed between 1962 and 1969 met with only limited success. 

New possibilities in hetero-Diels-Alder condensation have been opened by the introduction of highly active l-methoxy-3-trimethylsilyloxy-, 4-benzoyIoxy-l-methoxy-3-trimethylsilyloxy-, and 2-acetoxy-l-alkoxy-3-trimethylsilyloxy-l,3-butadienes ( Danishefsky dienes, 5). These compounds readily react under atmospheric pressure, in the presence of Lewis acids, with normal aldehydes (e.g., acetaldehyde, benzaldehyde, furfural) to furnish 2,3-disubstituted or 2,3,5-trisubstituted derivatives of 2,3-dihydro-4H-pyran-4-one 7 capable of readily functionalizing to sugars (Scheme 5) [26]. This approach... 

Hetero-Diels-Alder reaction with inverted electron demand between a, 3-unsatu-rated carbonyl compounds (1-oxa-l,3-butadienes 11 Scheme 6) and enol ethers provides an access to 6-alkoxy-3,4-dihydro-2/f-pyrans 12 [31,32]. These heterocycles are also useful... 

A peroxy acid mediated oxidative rearrangement of 2-alkoxy-3,4-dihydro-2//- pyrans affords 5-alkoxytetrahydrofuran-2-carbaldehydes (79JCS(Pi)847>. This reaction pathway was used in developing a method for the synthesis of optically active monoalkylfurans. (S)-2-Ethoxy-5-s-butyl-3,4-dihydro-2//-pyran (319), obtained through a cycloaddition reaction of (S)-2-s-butylacrolein to ethyl vinyl ether, was converted to (S)-2-s-butyl-5-ethoxytetrahydrofuran-2-carbaldehyde (320) (Scheme 85). 

Absorption at 240-250 nm by 4//-pyrans has been noted (61CB1784), although this has been disputed and in general 4//-pyrans are characterized by a weak shoulder at ca. 225 nm (69JOC3169). Introduction of an ethoxycarbonyl or acetyl group at C-3 causes a shift of the maximum to 270 and 284 nm, respectively, which are further shifted to 285 and 296 nm by a second of these substituents. These maxima are considerably different from those of 0-alkoxy a,/3-unsaturated esters and ketones indicative of additional conjugation with the second double bond of the pyran. 

A combination of NMR and dipole moment measurements has proved useful for 2-alkoxy-tetrahydropyrans (69T3365, 76JA6477), 2-chlorotetrahydropyran <69JCS(B)855>, 2-amino-tetrahydropyran (82JCS(P2)249) and 2-dialkylamino-3-(2-tetrahydropyranyl)tetrahydro-pyrans (81RRC253). 

An anomeric effect is observed in 3,4-dihydro-2H- pyrans. For example, a 2-alkoxy group preferentially occupies an axial position (71DOKd96)367>. Indeed, a study of the NMR spectra of some 2-alkoxy-3,4-dihydro-2//-pyrans and their 4-methyl derivatives established that the anomeric effect was more important in the unsaturated heterocycles than in the corresponding tetrahydropyrans (72BSF1077). The axial preference of an alkoxy group is even more accentuated when the double bond is associated with a fused benzenoid ring, as in the 2-alkoxychromans. It is also of interest to note that the role of the polarity of the solvent on the conformational equilibrium is less important than for the saturated analogues. 

A study of the coupling constants between the hydrogen atoms at C-2 and C-3 in trans- 2-methoxy-4-methyl-3,4-dihydro-2//-pyran established that the conformer in which the alkoxy group is axial (187) predominates in the equilibrium mixture. However, the cis isomer exists preferentially as (188) despite the anomeric effect, since there is no 1,3-diaxial interaction between the 2-methoxy and 4-methyl groups (72BSF1077,78JOC667). 

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