Hydrogenation, catalytic chromone

Table 5 Formation of Chromans by the Catalytic Hydrogenation of Chromones...

A structurally unusual 3-blocker that uses a second molecule of itself as the substituent on nitrogen is included here in spite of the ubiquity of this class of compounds. Exhaustive hydrogenation of the chromone (13-1) leads to a reduction of both the double bond and the carbonyl group, as in the case of (11-2). The car-boxyhc acid is then reduced to an aldehyde (13-2) by means of diisobutylaluminum hydride. Reaction of that intermediate with the ylide from trimethylsulfonium iodide gives the oxirane (13-3) via the addition-displacement process discussed earlier (see Chapters 3 and 8). Treatment of an excess of that epoxide with benzylamine leads to the addition of two equivalents of that compound with each basic nitrogen (13-4). The product is then debenzylated by catalytic reduction over palladium to afford nebivolol (13-5) [16]. The presence of four chiral centers in the product predicts the existence of 16 chiral pairs. 

Both coumarin and chromone are converted by diborane then alkaline hydrogen peroxide into 3-hydroxy-chroman." " Catalytic reduction of conmarin or chromone satnrates the C-C double bond." For both systems, hydride reagents can of course react either at carbonyl carbon or at the conjugate position and mixtures therefore tend to be prodnced. Zinc amalgam in acidic solntion converts benzopyrones in 4-unsubstituted benzopyrylinm salts." ... 

Catalytic hydrogenation of a chromone-2-carboxylic ester in the presence of palladium-charcoal under an initial pressure of 10 atmospheres gave a small amount of the corresponding chromanone but no evidence was found of reduction of the pyrone carbonyl group [24]. When a similar ester (82) was reduced at atmospheric pressure in the presence of Raney nickel [36], both the pyrone double bond and the two carbonyl groups present were reduced to give the chromanol (83). 

The products of catalytic reduction of chromone-2-carboxylic acids or esters depend partly on the catalyst used, the amount of hydrogen present and the conditions of reaction. Ethyl chromone-2-carboxylate is reduced (in 86 per cent yield) to the corresponding chromanone in the presence of Raney nickel and hydrogen at a pressure of 50 Ib/in [184]. The 5-hydroxy analogue is similarly reduced at 80°C and 30 Ib/in in 44 per cent yield [8]. A nitro group on the benzene ring is more readily hydrogenated than the pyrone double bond (10 atmospheres, platinum-charcoal or palladium-charcoal catalyst) [24,40]. 

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