2.3- allenoate

A Baylis-Hillman type product has been obtained through a ring-opening reaction of an epoxide with an allenoate <06OL2771>. The reaction of MgL, with ethyl propiolate provides the iodo allenoate 32. This nucleophile reacts with an aryl epoxide to provide the homoallylic alcohol 33. The Z iodide is the major product formed. 

The product of the previous reaction provides a Baylis-Hillman type product via an intermolecular addition of an allenoate to an epoxide. The first example of a true Morita-Baylis-Hillman reaction of an epoxide has recently been reported <06CC2977>. Treatment of enone 34 with Me3P provides a good yield of the epoxide-opened product 35. The reaction must be carried out at low concentrations in order to avoid the generation of a variety of side products. When the terminal end of the epoxide is substituted (e.g. 34) the exo-mode of cyclization is the only product observed. When the terminal end of the epoxide is unsubstituted (e.g. 36), the endo-mode of cyclization predominates providing 37. 

The relatively electron-rich terminal carbon atom in 2,3-allenoates can be dibro-minated to afford 3,4-dibromo-2-butenoate 465 with the E-isomer being the major product [209-211]. 

The bromolactonization product 470 was obtained by the reaction of y-monoalkyl-substituted allenoates 467 with NBS in H20 or Br2 at room temperature. Here the carbonyl oxygen served as an intramolecular nucleophile. The Z-isomer of the bro-mohydroxylation product was also formed in 4% yield (R Me, R2 = H) [211, 213, 214], The absence of the Z-isomer may be ascribed to the easy conversion of the E-isomer to butenolide 470. 

Hydroxyl-2,3-allenoate 496 can also react with H to reduce the a,(5-C=C bond highly selectively [224],... 

An excess of sodium azide reacts with 2,3-allenoates to give ,/)-unsaturatcd eno-ates 499. No reaction was observed with 4,4-disubstituted 2,3-allenoates [225]. 

Morpholine, pyrrolidine and sodium azide can also be also used as nucleophiles in the synthesis of 3-substitututed A3-cephems 515 via the conjugate addition of the 2,3-allenoate moiety in 513 [233]. 

Based on this, Lu et al. developed the PPh3-catalyzed [3 + 2]-cydoaddition of 2,3-allenoates with electron-deficient alkenes [242, 243] and imines [244, 245], leading to the formation of five-membered compounds 538 and 539. 

This reaction principle has even been applied to the [3 + 2]-cycloaddition of 2,3-allenoates to 60-fullerene [246, 247]. 

Zhang et al. developed the enantioselective chiral phosphine-catalyzed addition of 2,3-allenoates with carbon-centered nucleophiles and electron-deficient olefins leading to efficient enantioselective C-C bond formation [248, 249]. 

Conjugate additions of thiols to a 2,3-allenoate proceed easily [236b, 249, 250],... 

The addition reaction of HBr to 2,3-allenoic acids afforded 2-bromo-2-propenoic acid [251],... 

Substituted 2,3-allenoates react with HX to afford 3-halo-3-enoates [254],... 

The hydrohalogenation reaction can also be performed with MX in HOAc [207a], The reaction of 4-substituted-2,3-allenoates affords a mixture of ( )- and (Z)-/3-halo-/8,y-unsaturated enoates [87]. With 2-substituted-2,3-allenoates, the reaction should be carried out in H0Ac-CF3C02H (1 1) or CF3C02H to form /8-halo-/3,y-unsaturated enoates with high regioselectivity [255],... 

This approach is applicable to cross-coupling reactions of allenoic acids and acrolein, which lead to the synthesis of lactones (Scheme 16.98) [104]. The reaction proceeds via oxypalladation of an allenoic acid to give rise to a vinylpalladium species. 

Scheme 16.98 Cross-coupling reaction of an allenoic acid and acrolein.

In an approach to FR182877, Sorensen and co-workers generated allenoate intermediate 289 from an ce-bromo-ce,/j-unsaturated lactone and effected an intramolecular acylation to afford a-alkylidene-/3-keto-y-lactone 290 as a single stereoisomer (Scheme 19.52) [61]. Evidence for the allenoate intermediate was established by the treatment of a-bromoenoate 291 with tBuLi, which afforded the cydized compound 292 as a single stereoisomer. Treatment of the isomeric bromoenoate of compound 291 also gave 292. Hence this demonstrated a stereoselective but not stereospecific process. 

Enantiopure quinolizidinones and indolizidinones were obtained by Ma and Zhu in an intramolecular conjugate addition of the secondary amine of 293 to the alkynoate ester to provide the intermediate allenoate 294, which was subsequently trapped in a condensation reaction to afford 295 (Scheme 19.53) [62], In some instances, intermediate 296 was isolated, which could also be converted to 295. This quinolizidinone intermediate was then converted in a concise manner to lasubine II (297). 

The formation of 2 furane rings was achieved in one transformation by Ma and co-workers. Allenoic acids and allenyl ketons were reacted in the presence of a palladium catalyst to give the unsymmetrical bifuryl product, arising from the cyclization of both allene derivatives mediated by the same palladium centre followed by their coupling (3.73.) 91... 

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. 

Alleno, E P. Berger, E. Leroy and C. Godart, 2001, in Rare Earth Transition Metal Borocarbides (Nitrides) Superconducting, Magnetic and Normal State Properties, eds K.-H. Muller and V.N. Narozh-nyi (Kluwer Acad. Publ., Dordrecht) p. 265. 

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