Allene-carboxylic acids

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-... 

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. 

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]. 

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). 

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). 

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). 

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,... 

Some chiral allene carboxylic acids have been cyclized to 4-substituted butenolides with little racemization by heating with acid catalysts. 2-Acyloxy-5-methylfurans (73) rearrange to give the 4-disubstituted butenolides (74), in yields... 

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