2.3- dihydro-4H-pyran

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... 

Hydroxy-6-methyl-5,6-dihydropyran-2-one (597) is brominated by NBS at C-3 (78JHC1153). More than one product is often obtained in this type of reaction, for example from 2,3-dihydro-4//-pyran (593), but in acetic acid this reaction gives 2-acetoxy-3-bromotetrahydropyran (599) (58JOC1128). 2,3-Dihydro-4H-pyran reacts normally with hydroboration reagents to give tetrahydropyran-3-ol in 80% yield (70JOC2282). 

Hot mineral acids open dihydropyran rings and recyclization to a benzene ring is feasible when a suitable substituent is present, as for example in 6-ethyl-3-vinyl-2,3-dihydro-4H-pyran (605) (74T1015). 

The failure of 2,3-dihydrofuran, tetrahydropyran, 2,3-dihydro-4H-pyran, and 1,4-dioxane to undergo the reaction demonstrates the unusual chemoselectivity achieved by selective molecular adsorption on the semiconductor surface. This same adsorption equilibrium presumably allows for the much more... 

Adedeji et al. (1993) used a direct thermal desorption technique (220°C) to analyse the volatiles from beans that might cause the thermal degradation and transformation of sugar into common volatile compounds such as 3,5-dimethyl-2,4(3H,5H)-furandione and 3,5-dihydroxy-6-methyl-2,3-dihydro-4H-pyran-4-one. This last compound was detected at a high concentration (3880 ppm) in Mexican vanilla, being the third most abundant compound after vanillin and 2-furfural, and far more abundant than vanillic acid, p-hydroxy-benzaldehyde or p-hydroxybenzoic acid. 

Isobutoxy-217, 2-butyl-,218 2-phenyl- (eq. 12.113),218 and 2-ethoxycarbonyl-219 substituted 2,3-dihydro-4H-pyrans were hydrogenated over Raney Ni at room temperature and elevated hydrogen pressures, although rather long reaction times were required for the 2-isobutoxy and 2-phenyl derivatives. 

C6H11CIO trans-2-chlorocyclohexanol 6628-80-4 415.92 35.868 2 8119 C6H11NO 2-aminomethyi-2,3-dihydro-4H-pyran 4781-76-4 474.25 41.416 2... 

Goodwin, T E, Crowder, C M, White, R B, Swanson, J S, Evans, F E, Meyer, W L, Stereoselective addition of organocopper reagents to a novel carhohydrate-derived 2,3-dihydro-4H-pyran-4-one, J. Org. Chem., 48, 376-380, 1983. 

Similarly prepared were 3-acetoxy-2,3-dihydro-4ff-pyran-4-one (2 Rj = H, Rj = OAc) [from l-methoxy-2-acetoxy-3-trimethylsilyloxy-l,3-butadiene (1 Rj = H, R2 = OAc) and paraformaldehyde in 67% yield] and 5-acetoxy-3-benzoyloxy-2,3-dihydro-4H-pyran-4-one (from l-benzoyloxy-2-t-butyldimethylsilyloxy-3-acetoxy-4-methoxy-l,3-butadiene and paraformaldehyde in 75% yield). 

Phosgene was noted not to have any action upon pinene [719], but it was found to add to the double bond of 2,3-dihydro-4H-pyran to give 2-chlorotetrahydropyran-3-carbonyl chloride. This product, when heated under reduced pressure, readily eliminates HCl [914] ... 

In contrast, phosgene reacts with the 6-membered oxygen heterocycle, 2,3-dihydro-4H-pyran, at the double bond, rather than by ring opening [914]. This reaction is described more fully in Section 10.1.2. 

When the reaction was conducted at room temperature under the catalysis of Yb[(-)BNP]3, the asymmetric induction was improved to 73% ee. The effect of the central metal ion of the chiral catalysts on the optical yield of the product, 2-phenyl-2,3-dihydro-4H-pyran-4-one, is shown in Fig. 2. The degree of enanti-oselection is highly sensitive to and dependent on the ionic radius of lanthanide ions [31]. 

Die leicht zuganglichen 2-Alkoxy-2,3-dihydro-4H-pyrane (vgl. Tab. 67, Beispiele 10, 11) lassensich mitguten Ausbeutenzu dengesattigten 2-Alkoxy-tetrahydropyranen hydrieren, sofern unter AusschluB von Wasser gearbeitet wird andernfalls konnen in ho-hen Ausbeuten Pentan-diole-( 1,5) entstehen13 14. 

CgH,202, Mr 140.18, liquid, [a] +236° (C2H5OH). Sexual attractant of the male butterfly Hepialus cali-fornicus. H. hecta produces the isomeric (R)-6-ethyl-2,3-dihydro-2-methyl-4H-pyran-4-one (CgH,202, Mr 140.18 a). Besides the homologous 2,6-diethyl-2,3-dihydro-4H-pyran-4-one (C9H14O2, Mr 154.21) the... 

Wu and his co-workers reported an experimental and theoretical study on the hydrogen-bond-promoted enantioselective hetero-Diels-Alder reaction (HAD) of Danishefsky s diene 105 with benzaldehyde 106, Scheme 3.37 [52], The reaction was achieved catalytically by a series of a,ct,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol (TADDOL) derivatives through hydrogen-bonding activation and afforded 2-phenyl-2, 3-dihydro-4H-pyran-4-one 108 in good enantioselectivity. 

Basavaiah developed a convenient, one-pot stereoselective synthesis of spiro-oxindoles via TiCU catalyzed coupling of 2-acetyl-6-methyl-2,3-dihydro-4H-pyran with various isatin derivatives [328]. The reaction involves a tandem construction of C-C and C-0 bonds and its catalytic asymmetric version remained to be developed (Scheme 14.145). 

The preparation of branched-chain 4-eno-pyran-2-uloses derived from levoglucosenone is mentioned in Chapter 14. Studies on the addition of methanol to 2-C- and S-C-nitro-a- and P-D-cryt/iro-hex-2-enopyranosides as well as the preparation of C-1 glycosides by the addition of organometallic agents to 2,3-dihydro-4H-pyran-4-ones can be found in Chapter 3. 

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