H-Pyran

As dienol ethers, 2H-pyrans show typical C=C stretching vibrations at 1600-1650 cm in the IR spectrum. In the H NMR spectmm, chemical shifts of olefinic protons, for example, 2 at 5 = 4.89/5.60 (3-H/4-H CCI4) are observed. 

2H-Pyrans show the chemical properties ofoxacyclohexadienes for instance, the O/C-2 bond can take part in thermal electrocyclic ring-opening with formation of dienones, for example, 4 5  

Moreover, highly substituted derivatives 3 of 2H-pyran are obtained by cycloisomerization of propargyl vinyl ethers 8 mediated by successive treatment with AgBp4 and l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) [7]  

The Ag salt catalyzes a primary (3,3)-sigmatropic propargyl Ciaisen rearrangement of 8 to the aUenic system 9 subsequently, DBU isomerizes 9 to a dienone 10, which undergoes spontaneous electrocyclization to the 2-monosuhstituted 2H-pyran-5-carboxylate 3 (cf. p. 299). 

2H-Pyrans can undergo (4+ 2)-cycloadditions with activated multiple bonds. For instance, 2,2,4,6-tetramethyl-2H-pyran (4) undergoes a regioselective Diels-Alder reaction with methyl propiolate to the bicycUc adduct 11, which yields methyl 2,4-dimethylbenzoate by thermal cycloreversion and acetone elimination  

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D-fMO MO-Nonitol, 2,6-anhydro-3,S,7-trideoxy-l-C-([hydroxy-(tetrahydro-2-methoxy-5,6-dimethyl-4-methylene-2 H-pyran-2-yl) acetyl] amino)-5,5-di methyl-1, S,9-tri-0-rriethyl-, 2R-[2( j(,2[S (5-q], 5P,6P] -... 

H- Pyran-4-one, 3,5-diacetyl-2,6-dimethyl-dipole moment, 3, 626 (37JCS1088) 4H-Pyran-4-one, 2-dichloromethyl- C NMR, 3, 588 (82S500)... 

H-Naphtho[2,3-h]pyran, 5,10-diacetoxy-2-methyl-oxidation, 3, 670 Naphtho[2,3-c]pyran synthesis, 3, 770... 

Naphtho[ 1,2-h]pyran-4-one, 3,6-diacetyl-2-methyl-deacetylation, 3, 713 Naphtho[ 1,2-h]pyran-4-one, 5,6-dihydro-synthesis, 3, 811... 

Naphtho[2,l -h]pyran-1 -one, 2,3-dihydro-dehydrogenation, 3, 724 halogenation, 3, 731 Naphthopyranones synthesis, 3, 805 Naphthopyran-2-ones, dihydrosynthesis, 3, 802 Naphtho[l, 2-h]pyran-2-ones synthesis, 3, 802 Naphtho[2,1 -h]pyran-3-ones synthesis, 3, 803, 856 Naphtho[2,1 -c]pyran-4-ones synthesis, 3, 831 Naphtho[2,3-c]pyran-l-ones synthesis, 3, 831 Naphthopyrans H NMR, 3, 580 IR spectra, 3, 594 synthesis, 3, 743, 748, 750, 763 UV spectra, 3, 598 Naphthopyrans, dihydrosynthesis, 3, 778, 783 Naphtho[l,2-h]pyran-4-thione, styryl-properties, 3, 708... 

Smith PW, Sollis SL, Howes PD, Cherry PC, Starkey ID, Cobley KN, Weston H, Scicinski J, Merritt A, Whittington A, Wyatt P, Taylor N, Green D, BetheU R, Madar S, Fenton RJ, Motley PJ, Pateman T, Beresford A (1998) Dihydropyrancarboxamides related to zanamivir a new series of inhibitors of influenza virus sialidases. 1. Discovery, synthesis, biological activity, and structure-activity relationships of 4-guanidino- and 4-amino H-pyran-6-carboxamides. J Med Chem 41 787-797... 

An Improved Preparation of 3-Bromo-2(H)-pyran-2-one An Ambiphilic Diene for Diels-Alder Cycloadditions. 

When chlorinated and dehalogenated in the presence of a base, dimethyl iV-acetylstizobolate, a derivative of the nonproteinogenic stizolobic acid, afforded a derivative of the pyrrolo[3,4-h]pyran system 73 (86LA1968). 

The influences on the absorption spectra and the other photochromic properties of compounds with substituents in the 3//-naphtho[2,l-h]pyran ring and on the 3,3 -aryl groups have been stndied in detail. Electron-donating gronps in one or both of the 3-phenyl gronps, especially in the p-position, show a marked bathochromic shift in the absorption maxima of the coloured state, whilst electron-withdrawing groups have the opposite effect (Table 1.4). Substitutions in the a-position have little effect on the absorption maxima but have a very marked effect on the rate of return back to the colourless state, presumably due to stabilisation of the open chain form (Table 1.4). 

H-Thieno[3,4-c]pyran 4H-Thieno[3,2-c]pyran 5H-Thieno[3,2-h]pyran [42376-93-2] [55209-33-9] [272-81-1]... 

Examples of photochemically induced im electrocyclizations in oxygen-containing systems are relatively rare. 2//-Phenanthro[9,10-h]-pyran-4-carboxamides (16), for example, have been obtained in this way by irradiation of dienones (17).17 Similarly, the a-pyran (18) is formed on irradiation of ( )-//-ionone (19)18 a triplet excited state is believed to be involved and the reaction proceeds via the Z-isomer (20). The reverse process involving ring opening is more common and can lead to a wide variety of photochemically derived products. Thus, the products of irradiation of 2,2-dimethylchromene... 

Thus, the potential role of DMHF as a flavor impact compound and as an intermediate to form additional flavor compounds in heat-abused citrus products should be carefully considered. 2,3-Dihydro-3,5-dihydroxy-6-methyl-4-H-pyran-4-one was also isolated from dehydrated orange juice (37) and is known as a novel nonenzymic browning reaction product. This pyranone has a high threshold value, over 200 ppm (38), and appears to possess negligible odor character. 

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