Allenes from propargyl esters

An alternative, but related, route to allenic titanium reagents from propargylic esters has been reported recently. Reaction of titanocene dichloride with BuMgCl and Mg yields a reactive titanocene intermediate, formulated as Cp2Ti. This reduced Ti species reacts in situ by oxidative addition to propargylic acetates. The allenyltitanium reagents thus produced add to aldehydes and ketones, as expected, to afford homopropargylic alcohols (Table 9.27) [43]. 

Rearrangement of acetylenic sulphenates to the allenic sulphoxides 151 was discovered when the synthesis of propargylic ester of trichloromethanesulphenic acid 152 was attempted (equation 86). This reaction is of general scope and gives very good yields of allenic sulphoxides (Table 14) from structurally diverse cohols and various sulphenyl chlorides Reaction of alkynols 153 with benzenesulphenyl chloride in the presence... 

Mikami and Yoshida extended the scope of this method considerably by using propargyl phosphates and chiral proton sources [94], The propargylic phosphates thereby have been found to be advantageous owing to their high reactivity towards palladium and the extremely low nudeophilicity of the phosphate group [95]. In some cases, it was even possible to obtain allenes from primary substrates, e.g. ester 194 (Scheme 2.60) [96]. A notable application of this transformation is the synthesis of the allenic isocarbacydin derivative 197 from its precursor 196 [97]. 

The enantioselective synthesis of an allenic ester using chiral proton sources was performed by dynamic kinetic protonation of racemic allenylsamarium(III) species 237 and 238, which were derived from propargylic phosphate 236 by the metalation (Scheme 4.61) [97]. Protonation with (R,R)-(+)-hydrobcnzoin and R-(-)-pantolactone provided an allenic ester 239 with high enantiomeric purity. The selective protonation with (R,R)-(+)-hydrobenzoin giving R-(-)-allcnic ester 239 is in agreement with the... 

Rearrangements of propargyl esters with silver salts were first mentioned by Zakharova in the mid-1940s.49 He described the conversion of 3-chloro-3-methyl-but-l-yne into a mixture of acetates in which the allenic acetate, l-acetoxy-3-methylbut-1,2-diene, was the major compound (Scheme 3.30). Although this product could arise from a silver assisted SN2 reaction, it could also be produced from the substitution product through rearrangement, probably catalyzed by silver ions. 

Dibromoselenuranes 196 can be prepared from the corresponding selenides by reaction with elemental bromine. The a,/ -unsaturated ester 196 is converted into the a-bromo-/ ,7-unsaturated ester 197 by elimination of phenyl-selenenyl bromide (Scheme 58).338 Dibromoselenuranes from propargylic selenides undergo similar reactions leading to either allenes or propargylic bromides.339... 

Two different types of tandem reactions for the synthesis of highly functionalized cyclohexenones cyclopropyl-substituted propargyl esters initiated rhodium-catalysed Saucy-Marbet 1,3-acyloxy migration have been reported (Scheme 152). The resulting cyclopropyl-substituted allenes derived from acyloxy migration followed by 5 -f 1-cycloaddition with carbon monoxide. ... 

Hydride-promoted reactions are also well known, such as the acrylic and vinylacrylic syntheses (examples 7-10, Table VII). Some less-known compounds, which form in the presence of halide ions added to tetracar-bonylnickel, have been described by Foa and Cassar (example 11, Table VII). Reaction of allene to form methacrylates, and of propargyl chloride to give itaconic acid (via butadienoic acid), have been reported (examples 13 and 14, Table VII). 1,5-Hexadiene has been shown to be a very good substrate to obtain cyclic ketones in the presence of hydrogen chloride and tetracarbonylnickel (example 15, Table VII). The latter has also been used to form esters from olefins (example 16, Table VII). In the presence of an organic acid branched esters form regioselectivity (193). 

Air-stable palladium(O) catalyst, [(Cy3P)2Pd(H)(H20)]BF4, catalyses carbonylation of propargylic alcohols to generate dienoic acids and esters (equation 167)286. Since propar-gyl alcohols are obtained from carbonyl compounds by acetyhde addition reactions, this sequence constitutes a three-carbon homologation. a-Allenic alcohols are converted to tt-vinylacrylic acids under similar conditions (equation 168)287. 

Not least for the syntheses of natural products, alkoxycarbonylations with formation of allenic esters, often starting from mesylates or carbonates of type 89, are of great importance [35, 137]. In the case of carbonates, the formation of the products 96 occurs by decarboxylation of 94 to give the intermediates 95 (Scheme 7.14). The mesylates 97 are preferred to the analogous carbonates for the alkoxycarbonylation of optically active propargylic compounds in order to decrease the loss of optical purity in the products 98 [15]. In addition to the simple propargylic compounds of type 89, cyclic carbonates or epoxides such as 99 can also be used [138]. The obtained products 100 contain an additional hydroxy function. 

Additives (cosolvents) which serve as ligands have great influence on the reducing power of Smlj. Allylic and propargylic derivatives are reduced via it-allylpalladium species, and the proton source has important effects on the generation of allenes or alkynes. Chiral allenic esters are obtained when pantolactone delivers a proton to racemic organosamarium species derived from 4-phosphato-2-alkynoic esters. ... 

Several new aspects of organoalane chemistry have been described this year. Orthoesters react with alanes, prepared from a variety of allyl and propargyl halides, to give the corresponding 0,y-unsaturated acetals (Scheme 45). Interestingly reactions involving acetals rather than ortho esters lead exclusively to the formation of allenic ethers (160). ... 

Naturally occurring (l S, 25, 3i )-4-hydroxymethylcyclopent-4-ene-l,2,3--triol (950) plays a central role in the ability of a non-aristeromycin producing mutant strain of Streptomyces citricolor to support production of both aristeromycin and neplanocin. Swem oxidation of readily available 13 from L-tartaric acid provides the aldehyde 943 which, when treated with an excess of propargyl zinc bromide, leads to a 2.3 1 diastereomeric mixture of acetylenic alcohols 944. Silylation of the hydroxyl group with TBSOTf and subsequent saponification of the ester group yields the carboxylic acid 945 in 74% overall yield from 13. Interestingly, Dess-Martin oxidation of 943 provides the allenic ketone 946, which is unstable to base and cannot be used in the subsequent radical cyclizations. 

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