Pyranones benzo

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... 

REDUCED PYRANS AND PYRANONES AND THEIR BENZO DERIVATIVES ... 

Ketones derived from pyrans are called pyranones (also commonly pyrones), and the parent compounds are pyran-2-one 17 and pyran-4-one 18. Trivial names are used for the related benzo analogs coumarin 19, isocoumarin 20, dihydrocoumarin 21, chromone 22, xanthone 23, and chromanone 24. 

Cleavage of the hetero ring of dibenzofuran by Li followed by the addition of ketones offers a useful route to 6,6-disubstituted dibenzo[/>,<7]pyrans 13 <06JOC8291> and a photochemical rearrangement of naphtho[2,3-b]benzofuranones was used to prepare intensely fluorescent indeno[ 1,2-/>]benzo[4,5-e]pyranones 14 <060BC3406>. 

Thiopyranone is the thio homologue of pyranone, a core constituent of many namral products. In their own right, thiochromenones, benzo annelated thiopyranones that are related to the class of flavones, are as well potent drug candidates and... 

An important role play carbonyl-bearing heterocycles, e.g. 2H- and 4//-pyranones, pyridones, diazinones and the benzo systems derived thereof, as well as hydrogenated heteroarenes like tetrahydropyrane, chromane, piperidine, morpholine and piperazine. 

The base-catalysed reaction of 6-aryl-3-carbomcthoxy-4-methylthio-2//-pyran-2-one 39 with tetrahydrothiopyran-4-onc affords the isothiochroman 40. Similarly, reaction with thiochroman-4-one gives a benzo[c]thiochromene. The carbanion derived from the thiopytanone attacks the pyranone at the electrophilic 6-position with concomitant ring cleavage and loss of CO2. Cyclisation involves nucleophilic attack at the 4-carbonyl function (Scheme 44) <01JOC5333>. 

As noted in Section 2.2, when an epoxycyclopentenone 7 was formed thermally, it largely isomerized into the isomeric pyranone 8 (Scheme 3 2000 TL9189). This is a general process and FVP of the benzo-, naphtho-, and benzofuro-cyclopentene epoxides 347, 349, and 351 leads to the isocou-marins 348, and analogs 350 and 352, the last of which is related to the natural product coriandrin (Scheme 68 2000TL3677). 

---