Allene carboxylate

Aside from alkoxycarbonylations, hydroxycarbonylations in the presence of water to yield allenic carboxylic acids [15] (93, Y = OH) and aminocarbonylations in the presence of amines to give the analogous amides [139] (93, Y = NRR ) have also been carried out, respectively (Scheme 7.13). These products of structure 102 can also be obtained if using the propargylamines 101 with R1 = Ph or R3 Z H as starting materials (Scheme 7.15) [140]. Additionally, hydroxycarbonylations, also termed carboxyla-tions, are successful without palladium catalysis by reaction of propargyl halides and carbon monoxide in the presence of nickel(II) cyanide under phase-transfer conditions [141, 142]. 

Several trivial but highly useful reactions are known to convert one acceptor-substituted allene into another. For example, the transformation of allenic carboxylic acids is possible both via the corresponding 2,3-allenoyl chlorides or directly to 2,3-allen-amides [182,185], Allenylimines were prepared by condensation of allenyl aldehydes with primary amines [199]. However, the analogous reaction of allenyl ketones fails because in this case the nucleophilic addition to the central carbon atom of the allenic unit predominates (cf. Section 7.3.1). Allenyl sulfoxides can be oxidized by m-CPBA to give nearly quantitatively the corresponding allenyl sulfones [200]. The reaction of the ketone 144 with bromine yields first a 2 1 mixture of the addition product 145 and the allene 146, respectively (Scheme 7.24). By use of triethylamine, the unitary product 146 is obtained [59]. The allenylphosphane oxides and allene-... 

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. 

Allene carboxylate 144a afforded the product 145 through an intramolecular [4+2]-cycloaddition [123], On the other hand, allene carboxylate 144b gave the [2 + 2]-cycloadduct 146 exclusively. The switching of the reaction pathway is ascribed to the conformational stability of the transition state. 

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],... 

Introduction 877 Alcohols as Nucleophiles 881 Allenyl Ketones 889 Allenic Carboxylic Acids 895... 

Enzymes such as pig liver esterase have been successfully applied in enantioselective hydrolysis of allenyl esters on a scale of 2 mmoles131. This provides the enantiomerically enriched allene-carboxylic acid as well as the ester of opposite configuration, by what is in fact a catalytic kinetic resolution (6-90% oy). Conversely, partial enantioselective esterification of /1-hydroxy-allenes (3-72% oy) employing lipases has been reported132,133. 

Allenic Alder-ene reactions, characteristics, 10, 584 Allenic carbonyl compounds reductive cyclization, 10, 524 silver-catalyzed cycloisomerizations, 9, 559 Allenic carboxylic acids, silver-catalyzed cycloisomerizations, 9, 559... 

A series of allenic carboxylic acids has been converted to funtionalized P-lactones 47 by oxidation-cyclization promoted by pre-prepared solutions of dimethyldioxirane <02T7027>. This transformation is rationalized by the involvement of unisolated spirodioxide intermediates, which give lactones with appropriately situated cx-hydroxyketone moieties. 

The silver-catalyzed cycloisomerization of allenic carboxylic acids to butenolides was also reported by Marshall and co-workers.329 Starting from the enantiomerically enriched propargyl mesylate 390, palladium-catalyzed hydroxycarbonylation led to the chiral allenecarboxylates 391 which afforded the butenolides 392 upon treatment with silver nitrate (Scheme 114). Unfortunately, partial racemization could not be avoided in this two-step sequence. In a related study, Ma and Shi330 have shown that the combination of Pd(PPh3)4 and Ag2C03 promotes the cyclization of allenecarboxylates to the corresponding butenolides, accompanied by the introduction of aryl or alkenyl groups at C3. 

Silver-catalyzed cyclization reactions of allenic carbonyl compounds were applied for the total synthesis of various natural products, including rubifolide,331 kallolide A and B,332,332a and deoxypukalide.333 In their total synthesis of the enantiomer of rubifolide 396, Marshall and Sehon331 took advantage of the silver-catalyzed cycloisomerization of the allenic ketone 393 to the furan 394, as well as of the analogous reaction of the allenic carboxylate 395 to the target molecule 396 (Scheme 115). 

The aqueous ition of conjugated allene carboxylate (252) to typical dienophiles proceeded smoothly between 20 and 50 C to yield predominantly, if not exclusively, C-=0 endo adducts containing a conjugated cw-methylene group, e.g. the methyl vinyl ketone adduct (253) (Scheme 59). Also diene (255), featuring a sodium carboxylate unit attached via a nitrogen atom, reacted efficiently at 25 C with various, even acid-sensitive, dienophiles as exemplified by the [4 + 2] cycloaddition of (255) to meth-acTolein (- 256). ... 

Allene carboxylic acids have been cyclized to butenolides with copper(II) chloride. Allene esters were converted to butenolides by treatment with acetic acid and LiBr. Cyclic carbonates can be prepared from allene alcohols using carbon dioxide and a palladium catalyst, and the reaction was accompanied by ary-lation when iodobenzene was added. Diene carboxylic acids have been cyclized using acetic acid and a palladium catalyst to form lactones that have an allylic acetate elsewhere in the molecule. With ketenes, carboxylic acids give anhydrides and acetic anhydride is prepared industrially in this manner [CH2=C=0 + MeC02H (MeC=0)20]. 

The results of the following experiments give important clues for mechanistic considerations. Isolation of a trienecarboxylate ester as a hypothetical intermediate is not possible in most cases. As an exception, the trienecarboxylate 127 was isolated (64%) as a mixture with the cyclized product 128 (22%) after 4h reaction of 126. Compound 127 was converted to the cyclized product 128 on further exposure to the carbonylation conditions, showing that the allenecarboxylate 127 is a precursor of the cyclized product 128 (Scheme 11-36). Also, the carbonylation of the 1,4-enyne 129 gave only the allene carboxylate 130 (Scheme 11-37). No further carbonylation to give the cyclic ketone 131 or 132 was observed. In addition, the 1,3-diene ester 133 was recovered intact under the same carbonylation conditions without giving the cyclized product 134 (Scheme 11-38). 

Phosphine-catalysed addition of ethyl allene carboxylate to aldehydes also involves the construction of the 5,6-bond. ... 

Production of the ( )- or (Z)-enoate depends on the configuration of the substrate [( )- or (S)-allene carboxylic acid]. Racemic allenic acids lead to ( /Z) mixtures which can be separated and the reactions show a high stereoselectivity (> 96 % ee). 

The experimental conditions for the carboxylation of allenyllithium are governed by the consideration that introduction of the COOLi group renders the allenic protons more acidic (resonance stabilization). If the normal order addition is used (introduction of C02 into the solution of allenyllithium), the allenic carboxylate primarily formed may easily be deprotonated by the strongly basic allenyllithium. The new species (either LiCH=C=CHCOOLi or H2C=C=C(Li)COOLi) may react also with C02. To avoid this situation the solution of allenyllithium is gradually added to a strongly cooled solution of carbon dioxide in THF. As usual, liberation of the acid occurs by adding mineral acid. In view of the possibility of an acid-catalyzed cyclization to a lactone too strongly acidic conditions should be avoided ... 

Asymmetric and enantioselective olefination reactions continue to be of interest. Wadsworth-Emmons reactions of 4-substituted cyclohexanones with the phosphonate (147), which carries a chiral benzopyrano-isoxazolidine substituent, proceed with diastereomeric excesses of 80-90% and hence provide another example of such an approach to enantiomerically pure, axially dissymmetric cyclohexylidene derivatives. A further example of trapping of in situ generated ketenes by Wadsworth-Emmons reactions to give allene carboxylates has been reported and the reaction has been extended to enantioselective synthesis by use of the optically active phosphonates (148) (Scheme 14). Moderate to good chemical yields and e.e. values up to 84% were obtained depending on the nature of (148) and the reactions conditions. 

Among interesting olefin syntheses with ester phosphonates are those with the allenic aldehydes (141) and with the steroidal epoxy-aldehyde (142). The yield of allenic carboxylic esters has been increased to... 

Allene carboxylates have been synthesized from propargylic carbonates using a palladium-catalysed decarboxylation-carbonylation... 

A number of cycloaddition reactions involving allene derivatives as dienophiles have been recorded. Allene itself reacts only with electron-deficient dienes but allene carboxylic acid or esters, in which a double bond is activated by conjugation with the carboxylic group, react readily with cyclopentadiene to give 1 1 adducts in excellent yield. For example, the allene 12 gave, with very high yield and selectivity, the cycloadduct 13, used in a synthesis of (-)-P-santalene (3.19). An allene equivalent is vinyl triphenylphosphonium bromide, which is reported to react with a number of dienes to form cyclic phosphonium salts. These can be converted into methylene compounds by the usual Wittig reaction procedure (3.20). 

An ethereal soln. of ketene added dropwise at -60° to a soln. of N,N-diethyl-l-propynylamine in dry ether, allowed to stand 80 min. at the same temp., then slowly allowed to warm to room temp. -> 1-methyl-l-allene-carboxylic diethylamide. Y 83%. 

Similarly, but using Se instead of Te, R-(+)- and 5 -(-)-jV,jV-dimethyl-l-ferrocenylethylamine provide [R,S-(Fc Se)2l (m 98-100" from hexane) and [S,R-(Fc Se)2] (m 103" from hexane) respectively as red solids, and these catalysts induce highly enantioselective selenoxide elimination to produce axially chiral allene carboxylic esters with high enantiomeric excess [Nishibayashi et al. Tetrahedron lett 35 3115 1994],... 

Instead of allenic carboxylic acids, the corresponding esters can also be used as substrates for gold-catalyzed cyclization reactions. Thus, heating t-butyl allenoates with gold(III) chloride in dichloromethane afforded butenolides in high yield (Scheme 4-104). In a similar way, Backvall et obtained 5-lactones by... 

BFg-etherate added at 0° to a soln. of trimethylsilylketene and tert-butanol in chloroform, and the product isolated after 5 min. product. Y 93%. - Trimethylsilylketene is a powerful acylating agent. F. e., also prepn. of a-allene-carboxylic acid esters (cf. Synth. Meth. 28, 851), s. R. A. Ruden, J. Org. Chem, 39,3607 (1974). 

It has been shown that chirality does not necessarily need to be located on a tetrahedral carbon atom, as in the case of the fran -epoxy dicarboxylates (Scheme 2.33) [238]. For example, the axial chirality of the racemic iron-tricarbonyl complex [239] and of the allenic carboxylic ester shown below [240], was well recognized by PLE. 

As will be discussed later, halogen alkyls can assume the function of the acid, thus, the presence of acids is not necessary. However, if an acid is present, allenic carboxylic acids formed tend to react to lactones... 

Allenic carboxylic acids, 33 - (7) 2-Hethyl-2,3-butadienoic acid, 35 - (5) 6,6-Dimethyl-3,4-heptadien-l-ol, 36 - (9) 4,4-Dimethyl--l-triraethylsilyl-l,2-pentadiene, 36 - (20) 3-Hethoxy-l,2-heptadiene,... 

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