Furan ring ketones

The unsaturated tetraoxaquaterene (accompanied by linear condensation products) was first synthesized in 18.5% yield by the acid-catalyzed condensation of furan with acetone in the absence of added lithium salts. Other ketones also condensed with furan to give analogous products in 6-12% yield.A corresponding macrocycle was also prepared in 9% yield from pyrrole and cyclohexanone. The macrocyclic ether products have also been obtained by condensation of short linear condensation products having 2, 3, or 4 furan rings with a carbonyl compound. ... 

The second group of reactions is called vicinal difunctionalization. They embrace the C2 and C3 positions of the furan ring simultaneously. Thus, complex 3 (X = O, R = R = R = H) reacts with benzaldehyde dimethyl acetal to give 4H-furanium cation (the product of electrophile addition at C4), which experiences further attack by the methoxide group with formation of the acetal 8 (950M2861). This reaction is possible in the presence of the Lewis acid (BF3—OEt2). Reaction with methyl vinyl ketone in methanol, when run in identical conditions. 

Reaction of the potassium salt of salicylaldehyde with chlo-roacetone affords first the corresponding phenolic ether aldol cyclization of the aldehyde with the ketonic side chain affords the benzofuran (1). Reduction of the carbonyl group by means of the Wolf-Kischner reaction affords 2-ethyl-benzofuran. Friedel-Crafts acylation with anisoyl chloride proceeds on the remaining unsubstituted position on the furan ring (2). The methyl ether is then cleaved by means of pyridine hydrochloride (3). lodina-tion of the phenol is accomplished by means of an alkaline solution of iodine and potassium iodide. There is thus obtained benziodarone (4)... 

The formation of 2 furane rings was achieved in one transformation by Ma and co-workers. Allenoic acids and allenyl ketons were reacted in the presence of a palladium catalyst to give the unsymmetrical bifuryl product, arising from the cyclization of both allene derivatives mediated by the same palladium centre followed by their coupling (3.73.) 91... 

PCC cleaves the furan ring, giving a conjugated endione. The unreacted alcohol attacks one of the ketones, yielding a cyclic hemiacetal. 

Cyclization reactions by 1,3-dehalogenation or dehydrohalogenation d> have provided additional routes to cyclopropanones. The application of the former reaction to the formation of 3-ring ketones was suggested by the isolation of the cycloadduct 37 from the reaction of a, -dibromo-benzyl ketone with sodium iodide in the presence of the trapping agent furan.39> More recently, Giusti n> has synthesized several cyclopropanone... 

Methylfurolabdane containing a 2,3-disubstituted furan substructure, isolated from the cuticular wax of the leaves of Nicotiana tabacum, was synthesized from (+)-sclareolide. The furan ring was formed from a P,y-unsaturated ketone via an oxidation-cyclization procedure <02EJO4169>. 

The diterpenoid (24) undergoes29 an unusual intramolecular Diels-Alder reaction to afford (25) in which the furan ring acts as a dienophile adding across the l(10),2(3)-double-bond isomer of the parent diterpenoid. The absolute stereochemistry of the trans-clerodane caryoptin (26) has been determined30 by conversion into a C-6 ketone and comparison of the o.r.d. and c.d. curves with those of similar derivatives obtained from clerodin. 

The addition reaction to the furan ring which has been by far the most studied is certainly the catalytic hydrogenation of the ring. It has been shown in the numerous publications on this reaction that various classes of compounds can be obtained—alcohols, ketones, hydrocarbons, ethers, and others. The extent of the literature on this subject severely limits an exhaustive treatment, which is not possible within the framework of this chapter. We are able to mention only a small part of the work. A complete review, with particular reference to recent Russian work, has been given by Bel skii and Shuikin.184... 

At higher pressure the furan ring opens at either of the C—0 bonds, and the carbonyl group formed is simultaneously reduced to the alcohol. If the temperature is raised, dehydrogenation of the alcohols previously formed to ketones occurs and even larger yields of hydrocarbons are formed.209 The products obtained with Pd-Al skeleton catalysts at 250-275°,210 and with Ni-CdO catalysts at 300°,211 are... 

On the other hand, Gavrilova and Gonikberg opened the furan ring with hydrogen, without the use of catalysts.213 The reaction is carried out at high temperature (350-375°) and at a pressure between 240 and 750 atmospheres, in the absence of a solvent. The following products are obtained from 2-methylfuran acetone, methyl ethyl ketone, methyl butyl ketone, methyl amyl ketone, acetophenone, methylpropylcarbinol, w-pentane, and water. The authors suggest that these products are formed by a free radical mechanism. 

Bailey and Colomb278 described an ozonolysis of 2,5-diphenylfuran in methanol-acetone, with two equivalents of ozone, which gave 14% phenylglyoxal and 81% benzoic acid. Abnormal ozonizations such as this can be explained if we consider that the initial step is the normal 2,5-addition of ozone on the furan ring. Then 26 or the primary ozonides 27 or 28 could give the resulting ketonic product ... 

Fuson et al., in the course of their work on Grignard additions to extremely sterioally hindered ketones,284 observed that 2-phenyl-4-benzoylfuran adds a phenyl group at the 5-position of the furan ring as well as to the carbonyl group ... 

A catalytic, enantioselective approach towards the synthesis of polyol chains has recently been reported by Carreira et al. and has been applied in a synthesis of the polyol subunit of amphotericin B (Scheme 5) [13]. Aldol addition of the silyl dienolate 26 to furfural (27) catalyzed by the Tol-BlNAP-CuF -complex (2 mol-%) gives rise to the addition product 28 in 95 % yield and >99 % ee after one recrystallization. Spectroscopic evidence indicates that a copper dienolate is formed in situ from the silyl dienolate 26 and is actually the active nucleophile [14], Standard transformations including a j-yw-selective reduction of the ft-hydroxy ketone by the method of Prasad and the oxidative conversion of the furan ring to the car-... 

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