Lactonization dimerization

DimeriZa.tlon. A special case of the [2 + 2] cyclo additions is the dimerization of ketenes. Of the six possible isomeric stmctures, only the 1,3-cyclobutanediones and the 2-oxetanones (P-lactones) are usually formed. Ketene itself gives predominandy (80—90%) the lactone dimer, 4-methylene-2-oxetanone (3), called diketene [674-82-8], approximately 5% is converted to the symmetrical dimer, 1,3-cyclobutanedione [15506-53-3] (4) which undergoes enol-acetylation to so-called triketene [38425-52-4] (5) (44). 

Aldoketenes also form piedorninantly the lactone dimers, although the ratio of isomers can be influenced by base catalysis. Ketoketenes dimerize symmetrically, and at a slower rate, to 1,3-cyclobutanediones, unless acidic or basic catalysts are present. 

Enamines containing one -hydrogen atom react with the lactone dimer of dimethylketene to form aminocyclohexanediones 116). Polycondensation of acetone diethyl ketal takes place by treating it with morpholine and a catalytic amount of p-toluenesulfonic acid while distilling off the ethanol formed 117-119). The resulting spiran, bicyclo, and cyclooctadienone products differ from the known polycondensation products of acetone, and hence their formation probably involves enamine intermediates 119). 

DIMETHYLKETENE /3-LACTONE DIMER (3-Pentenoic acid, 3-hydroxy-2,2,4-trimethyl- /3-lactone)... 

The /3-lactone dimer of dimethylketene can be prepared by pyrolysis of its polyester, which is formed by the base-catalyzed polymerization of dimethylketene.3"6 In addition to the rearrangement of the normal dimer described above,6 the direct dimerization of dimethylketene in the presence of aluminum chloride3 or trialkyl phosphites7 leads to the /3-lactone dimer. 

The /3-lactone dimer of dimethylketene reacts with alcohols, phenols, mercaptans, and amines to form derivatives of 2,2,4-trimethylvaleric acid.3 In this respect it is a more powerful acylating reagent than the normal dimer, tetramethyl-l,3-cyclo-butanedione. The preparation of 2,2,4-trimethyl-3-oxovaler-anilide, for example, is accomplished easily with the lactone dimer, but is extremely difficult with the normal dimer.8... 

In the presence of catalytic amounts of sodium methoxide, dimethylketene /3-lactone dimer is polymerized at moderate temperature to a polyester.3 At higher temperatures (above 100°), disproportionation to the cyclic trimer, hexamethyl-1.3,5-cyclohexanetrione, takes place.9 Addition of a stoichiometric amount of sodium methoxide to the lactone dimer generates the sodium enolate of methyl 2,2,4-trimethyl-3-oxovalerate. This reaction provides a convenient entry into certain ester anion chemistry that formerly required the use of a strong base like tritylsodium.10... 

Although these reactions can be duplicated in most cases with the normal dimer of dimethylketene,11 the more reactive lactone dimer is the preferred reagent. The liquid form of this dimer is convenient to handle. A distinct difference in behavior of the dimethylketene dimers is noted when they are pyrolyzed. The normal dimer is dissociated at 600° to dimethylketene,12 but the lactone dimer is decarboxylated almost quantitatively at 450° to tetramethylallene.13... 

Hydroxy-2,2,4-trimethyl-3-pentenoic acid 0-lactone, see Dimethylke-tene 8-lactone dimer Hypoiodite reaction, 46, 62... 

Zinc chloride in conversion of dimethyl-ketene /3-lactone dimer to 2,2,4-trimethyl-3-oxovaleryl chloride, 48,126... 

The reaction mixture is then cooled to 35-40° and poured into a stirred solution of 230 g. of sodium chloride and 6.0 g. of sodium acetate in 600 ml. of water at 40°. The mixture is stirred for 15 minutes and then is transferred to a separatory funnel the layers are separated, and the lower, aqueous layer is discarded. The crude product is distilled at reduced pressure through a stainless steel spinning-band column (Note 5). The yield of the /3-lactone dimer of dimethylketene is 122-132 g. (61-67%) b.p. 69-71.5° (14 mm.) (Note 6). The product may be redistilled and the fraction boiling at 119.5-120° (150 mm.), 20d 1.4380, collected. 

Aluminum amalgam in reduction of oximinomalononitrile, 48, 2 Aluminum chloride, 46, 109 in conversion of tetramethy 1-1,3-cyclobutanedione to dimethyl-ketene d-lactone dimer, 48, 72 with lithium aluminum hydride, in reduction of l,4-dioxaspiro[4.5 -decane, 47, 37... 

Caution Dimethylketene -lactone dimer is a mild but deceptively persistent lachrymator. 

The lactone dimer of dimethylketene has been polymerized, a polyeater of the same struotuie... 

Anet oo recently published the nudearmagnetio reeonenoe spectrum of the -lactone dimer of diplienylketene. 

Oxaphospholans.—Full details have appeared of the reactions of the lactone and dione dimers of dimethylketen with a series of tervalent phosphorus esters and amides, and the postulated quinquecovalent intermediate (49) from the lactone dimer has been isolated in one case. Of potential mechanistic significance is the preferred migration of exocyclic substituents in the steps corresponding to (49) (50). 

Reaction of 3,5-di-f-butyl-l,2-benzoquinone with pyridine cupric methoxychloride and anhydrous ammonia in pyridine gives the imide 159 (-50%) (81JA5795). The lactone dimer 160 is formed in 51% )deld from 3,6-di-f-butyl-l,2-benzoquinone and 3,3-dimethylbut-l-ynyl-lithium (99MI350). 

The diazomethane-keten reaction gave no 1,3-dipolar adducts, such as those obtained readily enough from the corresponding thioketen (see Vol. 1, p. 44), even at low temperatures. The 2 1 adduct obtained (165) cannot arise by prior dimerization of the keten, whose jS-lactone dimer (166) reacts to give a y-lactone (167) by methylene insertion at the C—O bond, so that quite different hydrolysis products are obtained (see Scheme 48). ... 

Mascal M, Dutta S, Gandarias I (2014) The angelica lactone dimer as a renewable feedstock for hydrodeoxygenation simple, high-yield synthesis of branched C7-C10 gasoline-like hydrocarbons. Angew Chem Int Ed 53 1854—1857... 

A distinctive feature of ketenes is their formation from ketene dimers and their corresponding tendency for dimerization, and this reaction is often reversible, serving as a source of ketenes and as a characteristic ketene reaction. The parent ketene 3-lactone dimer 35 is commercially available, and upon thermolysis provides convenient access to ketene (Eqn (4.22)). For the parent the assignment of the stmcture of the dimer as either the p-lactone (35) or the 1,3-cyclobutanedione (36) was controversial for a long period. A pioneering depiction of the alternatives for ketene dimerization of perpendicular attack on the C=C bond (a) or in-plane attack on the C=0 bond (b) is shown in Figure 4.11. ... 

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