Pyrans benzo

Table 2 Important ring syntheses for quinolines, benzo[b]pyrans, benzo[b]thiins, and their derivatives...

No tautomeric interconversions have been observed between 4//-pyrans (1, X = O) and 2//-pyrans (2, X = O), 4//-thiopyrans (1, X = S) and 2H-thiopyrans (2, X = S), or their benzo-fused derivatives. Under normal conditions, all these compounds exist as sole isomers that are initially formed, both in solution and in the solid state. The only tautomeric equilibrium reported was that between the two possible 2H isomers of the unsymmetrical sulfone 3 (77TL1149). 

By contrast, heating benzo-2//-pyran 6 with acetic acid leads to the formation of the 4H isomer 7 (Scheme 2) [77ACS(B)496]. 

Chemical Name 4-Hydroxy-a-(4-hydroxy-2-oxo-2H-1-benzopyran-3-yl)-2-oxo-2H-1-benzo-pyran-3-acetic acid ethyl ester... 

Starting from l.l-dichloro-7b-ethoxy-2-methyl-1,1 a,2,7-tetrahydrobenzo[/)]cyclopropa[prepared from the corresponding benzothiopyran by addition of dichlorocarbene, the three 1-benzothiepins 6a-c are formed upon treatment with strong bases, i.e. sodium methoxide or ethoxide in dimethyl sulfoxide.73 The optimal yield of each 1-benzo-thiepin compound depends on the molar equivalents of base, as follows from different ring-opening mechanisms. 

Bei der Reduktion von 4-Alkyl-(naphtho-[2,l-b]-pyryIium)-Salzen mit Natriumbora-nat werdenhauptsachlich 4H-(Naphtho-[2,l-b]-pyrane) gebildet1.2-Benzo-[c]-py-rylium-Salze werden durch Lithiumalanat zu lH-(Benzo-[c]-pyranen) reduziert2. 

The same substrate (451) and 4-phenylhydrazono-3,4-dihydro-2//-l-benzo-pyran-2,3-dione (454) [prepared from the dione (452) with benzenediazo-nium chloride] gave 3-(o-hydroxy-a-phenyUiydrazonobenzyl)-2(l//)-quinox-alinone (455) (EtOH-AcOH, reflux, 90 min 80%). ... 

For the preparation of the benzo[fc]cyclopropa d pyran 2-599, Ohta and coworkers have treated 3-ethoxycarbonylcoumarin (2-597) with dimethylsulfoxonium methylide (2-598), derived from the corresponding trimethylsulfoxonium iodide and NaH (Scheme 2.136) [307]. The isolated product was not 2-599, however, but an unexpected eye lope nia[fc]benzofuran derivative 2-600 probably formed through 2-599. 

The highly potent anti-HIV natural product daurichromenic acid (10-100) was synthesized by Jin and coworkers [36] using a microwave-assisted reaction of the phenol derivative 10-97 and the aldehyde 10-98 (Scheme 10.25). Normal heating gave the desired benzo[b]pyran 10-99 by a domino condensation/intramolecular SN2 -type cyclization reaction only in low yield. However, when the reaction mixture was irradiated twenty times in a microwave for 1-min intervals, 10-99 was obtained in 60% yield. This compound was then transformed into 10-100 by cleavage of the ester moiety. 

Compounds are generally classified according to their fully unsaturated parent compound (but see below). Thus substituted, partially saturated, and fully saturated derivatives of, for example, pyrrole are all indexed under pyrrole. Benzo and similar derivatives are included under the most unsaturated parent system (e.g., quinoline, thienofuran, etc.). For any given heterocyclic parent only one indicated hydrogen isomer appears in the text, typically the most stable or the lowest numbered form thus all instances of pyran, whether of the 2H- or 4H-form, are indexed under 2H-pyran. The charges and additional valences for any heterocyclic parent structure are not indicated. 

Pyran-4-one (56a) and its benzo derivative (chromone) show chemical properties in agreement with substantial jr-electron delocalization and consistent with a betaine structure 56b (Scheme 27). Experimental data have therefore generated numerous theoretical studies on the aromaticity of pyranones, which have been extensively reviewed.219 Earlier studies suggested that chemical shifts and coupling constants... 

New rearrangements of 2-imino-2//-l-benzopyran-3-carboxamides under the action of anthranilic acid as an N-nucleophile have been revealed. Depending on the conditions 2-(2-oxo-2//-l-benzopyran-2-yl)-3//-quinazolin-4-ones or 2-oxo-2//-l-benzo-pyran-3-((V-2-carboxyphenyl)carboxamides were found to be the products. 

The chemistry described in Scheme 97 has been applied to the synthesis of 4-aryliso-chroman-3-acetic acids , interesting precursors of benzo[c]pyran antibiotics . 

Five-membered oxygen- and sulfur-containing heterocyclic ketones reveal notable reactivity (03RCB961, 03RCB1380). Benzo[fc]furan-3-one 204 with arylidenemalononitriles 30 gives dibenzo[b,d]pyrans 206 instead of expected pyran 205 as a result of Michael reaction and the exchange of methylene components (03RCB961) (Scheme 77). 

B. 1,3-Dimethyl-6H-benzo[b]naphtho[1,2-d]pyran-6-one (4). Under an argon atmosphere, a 250-mL, oven-dried, round-bottomed flask, equipped with a reflux condenser, is charged with freshly distilled N,N-dimethylacetamide (DMA, 130 mL), 3,5-dimethylphenyl 1-bromo-2-naphthoate (3, 3.24 g, 9.12 mmol), palladium(ll) acetate (205 mg, 0.913 mmol), triphenylphosphine (481 mg, 1.83 mmol), and sodium acetate (1.50 g, 18.3 mmol) (Notes 4 and 5). The orange suspension is degassed three times, placed in a preheated (130°C) oil bath (Note 5), and stirred at 130°C for 12 hr (Note 6). Removal of the solvent at 40°C (0.1 mbar, 0.075 mm) gives a black oily residue, which is Chromatographed (5 x 40 cm column, silica gel 0.063 - 0.2 mm, 170 g, 1 cm of charcoal at the top of the column eluent hexane / diethyl ether 5 1), to yield 2.00 g (80%) of the lactone 4 as a slightly yellow solid. Recrystallization from diethyl ether / hexane delivers 1.63 g (65%) of colorless or pale yellow crystals (Note 7). 

Dimethyl-6H-benzo[b]naphtho[ 1,2-d]pyran-6-one 6H-Benzo[b]naphtho[ 1,2-d]pyran 6-one, 1,3-dimethyl- (13) (138435-72-0)... 

If the triplet energy of the carbonyl compound is below that of the diene, triplet-triplet transfer will become inefficient and photocycloaddition may occur. For example, the triplet energy of 1,4-benzo-quinone is about 50 kcal mole - V41 and photocycloaddition to dienes can indeed occur.42 The products are spiro-pyrans 13 (not vinyl oxetanes), which may arise via the allylic radical intermediate 12. 

Pitts, J. N., Jr., D. M. Lokensgard, W. Harger, T. S. Fisher, V. Mejia, J. Schuler, G. M. Scorziell, and Y. A. Katzenstein, Mutagens in Diesel Exhaust Identification and Direct Activities of 6-Nitro-benzo[a]pyrene, 9-Nitroanthracene, f-Nitropyrene, and 5H-Phenanthro[4,5-bco ]pyran-5-one, Mutat. Res., 103, 24f-249 (1982a). 

Particularly important is the influence of the metal countercation e.g., cleavage of the achiral lactone l,3-dimethyl-6//-benzo[ ,]naphtho[1.2-d]pyran-6-one in toluene at — 40 CC, using different metal alkoxides, e.g., (+)-menthol as inexpensive chiral 0-nucleophiles, gives atropisomer ratios of 81 19 for lithium, 66 34 for sodium, and 63 37 for potassium. In other solvents (e.g., THF), this selectivity order may be reversed14. 

Dihydro-5,7-dihydroxy-4-oxo-4H-l-benzo- pyran-2-yl)-2,3-dihydroxyphenyl]-5,7-dihydroxy- 2-(4-hydroxyphenyl)-4//-l-benzopyran-4-one 2,3-Dihydro-5 -hydroxyamentoflavone... 

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