Allenic lactone

The cycloaddition of allenic lactone 124 with cyclopentadiene was carried out under thermal conditions and also in the presence of a Lewis add catalyst [102]. In both cases, the exo-adduct was formed predominantly. 

The key reaction in Jung et al. s proposed assembly of Plaunol B (81a) and C (81b) was an intermolecular Diels-Alder reaction between a diene and an allenic lactone that should give the exo-methylene group in the natural product (Scheme 19.16) [20], The phenyl-substituted lactone 83 was prepared as a model for the eventual furan lactone of the plaunols. Cydoaddition of 82 possessing a TBS enol ether and... 

Scheme 1.21). Transformation of ester 103 into acid chloride 104 then set the stage for the novel macrocyclization step. Deprotonation of 104 induced an intramolecular Wittig condensation followed by immediate loss of HCl to produce allenic lactone 105 in 42% yield. Subsequent [4 + 2] cycloaddition of diene 100 with 105 gave cyclohexene 106 in 55% yield. Aromatization of 106 under basic conditions then afforded a 50% yield of lasiodiplodin (90). 

Carbonylation of the complex 548 proceeds in ethanol gives ethyl 3-chloro-3-butenoate (554), The lactone 555 and the two esters 556 and 557 are obtained by carbonylation of the dimeric complex 549. The oxidative carbonylation of allene in ethanol with PdCl2 gives ethyl itacoante (558), although the yield is low[498]. 

The reaction of CO2 with allene using bis(dicyciohexylphosphino)ethane as a ligand affords a mixture of esters of methacrylic acid 17 and crotonic acid 18, and the lactone 19[12],... 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Allenic alcohols couple with allyl indium reagents at 140°C to give allylic alcohol products. Similarly, (o-hydroxy lactones couple with organoindium reagents. 

Likewise, a-methylene-/3-lactones, 280, can serve as allene equivalents since they cycloadd to conjugated dienes to provide the expected [2 + 4] products, e.g. 281 and 282, which on heating fragment with carbon dioxide loss to furnish the allene adduct 279 again [116]. 

In contrast to lithiated allenes, the corresponding titanium species and carbonyl compounds furnished the regioisomeric y-addition products [68,69]. Thus, reaction of a-aminoaldehydes 63 with the titanated intermediate 75 gave methoxyalkynes 76, which smoothly cydized in the presence of acid and provided lactones 77, again with high anti selectivity (Scheme 8.21) [69]. The regioselectivity depends on the aldehyde used. 

Cyclic alkynyl carbonates undergo carbonylation in the presence of a palladium catalyst and carbon monoxide (5 MPa) in MeOH to give allenic carboxylates (Eq. 9.118) [92], Bu3P proved superior to Ph3P as the catalyst ligand. An enynyl cyclic carbonate underwent double vicinal carbonylation at 80 °C to produce a five-membered lactone product in 52% yield (Eq. 9.119). When the reaction was performed at 50 °C, the bicyclic enone lactone was produced in 75% yield along with 10% of the y-lactone. 

A benzene ring can act as the diene in the intramolecular [4 + 2]-cycloaddition with an activated allene. Thus, aryl allene carboxylates 147 gave tricyclic lactones 148 (Table 12.10) [124],... 

The only substrate of that type with a longer bridge between the carboxylic acid and the allene is 122, which was isomerized in an exo-dig manner to the six-mem-bered lactone 123 on the surface of yellow HgO in benzene (Scheme 15.37) [84]. 

About 20 years ago, Bohlmann and co-workers examined South American plants of the widespread Vemonia species and found the first and (until now) only naturally occurring endocydic allenes (Scheme 18.32) [97]. These are the bicyclic sesquiterpene lactones 100-102 bearing a 10-membered ring [98], in which the E-double bond of the larger ring can also be present in its oxidized form (101, 102). As in other cases, these fasdnating natural products have not been synthesized until now. 

SYNTHESIS OF 4-(2-BROMO-2-PROPENYL)-4-METHYL-v-BUTYRO-LACTONE BY THE REACTION OF ETHYL LEVULINATE WITH (2-BROMO-ALLYL)DIISOPROPOXYBORANE PREPARED BY HALOBORATION OF ALLENE... 

It was reported that Pd(0)-catalyzed coupling reactions of allenic alcohols, amines and acids with hypervalent iodonium salts afforded cyclized heterocyclic tetrahydrofurans, tetrahydropyrans, pyrrolidines, piperidines, or lactones under mild conditions <99SL324>. Intramolecular 1,5-hydrogen atom transfer radical cyclization reaction of pyrrolidine derivatives was examined. Reaction of 3,4-dialiyloxy-JV-(0-bromobenzyl)pyrtolidine gave hexahydro-... 

Propargyl alcohols also react with the ether of an enol of carbonyls and lactones to give allenic carbonyl compounds [79a-d] (Eq. 58). 

Irradiation of cyclopent-2-enones in the presence of allenes affords mixtures of 6- and 7-methylenebicyclo[3.2.0]heptan-2-ones, the latter regioisomers being formed preferentially.4,5 From cyclohex-2-enones and allene, 8-methylenebicyclo[4.2.0]octan-2-ones are formed regiose-lectively.6 In contrast, six-membered a,/i-unsaturated lactones and allene afford mixtures of regioisomers.7 Mechanistic arguments for the stereochemical outcome of allene photocycloadditions to steroidal enones have been discussed.8... 

Holz, R. C. Allen Chang, C. Horrocks, W. De W. Jr. Spectroscopic characterization of the europium(III) complexes of a series of N.N -bislcarboxymethyl macrocyclic ether bis(lactones). Inorg. Chem. 1991, 30,3270-3275. 

Hydroacylation of Allenes The use of tert-butyl esters allowed Shin to lactonize allenoates using AuCl3 as a catalyst [48]. This paper also studies the effect of different phosphine ligands used as co-catalysts. 

The thermal decomposition of ketene1>60 93 142 146 apparently does not occur by the simple unimolecular decomposition CH2CO CH2 + CO. Young146 suggests that CH2 actually results from breakdown of allene, which is an intermediate product. Ketene is known to dimerize readily, even in the gas phase on Pyrex surfaces to form the d-methylene cyclic lactone. 

Behaviour similar to that of butadiene was observed in the reaction of allene with C02, both esters and a lactone being formed.593 The catalyst was prepared in situ from Cy2PCH2CH2PCy2 and [Pd(r 3-C3H5)2]. Scheme 57 gives the products formed from C02. Allene oligomers also occur. 

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