Allenoates formation

In 2012, the Sun group developed an efficient NHC-catalyzed internal redox reaction of alkynals bearing a y-leaving group. This process provides a new route to a range of allenoates in good yields. Preliminary results demonstrate that the enantioselective variant can also be achieved (one example with 89% yield and 30% ee). Albeit with low enantioselectivity, this first successful exploitation of an allenolate intermediate for allenoate formation provides a versatile platform for further development of new NHC-catalyzed reactions of alkynals. Without the use of transition metals and the production of phosphine oxide waste, this process serves as a green alternative for the synthesis of allenoates (Scheme 7.92). 

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. 

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]. 

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... 

This section aims to complete the picture on the studies summarized in CHEC-II(1996) on the synthesis of classical cephalosporins, describing the main cephalosporin methodologies developed during the last decade. In CHEC-II(1996), an approach was presented for the formation of the cephalosporin framework bearing various heteroatom and carbon substituents at the C-3 position, which relies on a sequential addition/cyclization of allenoate 70, derived from penicillin G. 

Kang et al. showed that under the catalysis of Pd(PPh3)4 (5 mol%) the reactions of 4,5- or 5,6-allenyl alcohols/toluenesulfonamides and 4,5- or 5,6-allenoic acids with hypervalent iodonium salts led to the formation of five- or six-mem-bered-ring heterocyclic products (Schemes 22 and 23) [15, 16]. With AT-(3,4-pentadienyl)toluenesulfonamide, a mixture of four- and six-membered-ring products was also formed. 

Whereas these transformations require stoichiometric gold compounds, catalytic amounts of both gold and palladium are sufficient for the cycloisomerization of allyl allenoates to allyl-substituted butenolides. Blum and co-workers reported this tandem C-O/C-C bond formation, which is initiated by activation of the distal allenic double bond with PhaPAuOTf (Scheme 4-107). This induces cyclization to an allyl oxonium intermediate, which undergoes deallylation in the presence of Pd2dba3. Nucleophilic attack of the resulting a-vinylgold intermediate at the ti-allylpalladium species and reductive elimination furnish the allylated butenolide and regenerate both catalysts. 

Carbon-carbon bond formation by using metal enolates as synthons in organic chemistry and the protonation, alkylation, arylation, and vinylation of enolates have been reviewed. " The stereoselective carbon-carbon formation of bond through Mannich reaction has been detailed according to the type of Mannich base produced. Phosphine-catalysed asymmetric additions of malonate esters to y-substituted allenoates and aUenamides have been reported. ... 

A TFA-promoted regioselective hydroarylation of 2,3-allenoates R C=C=C (R )C02Et with N-Ts- or A-Ac-indoles has been reported to occur at the electron-rich C=C bond, giving rise to 4-indolyl-4-arylbut-2-enoates. The E Z selectivity was found to be dependent on the reaction temperature and time. A mechanism involving the formation of ( )- and (Z)-allylic carbocations generated in situ on protonation of the allenoate with TEA, followed by Friedel-Crafts attack at the 3-position of the indoles, was proposed. ... 

The nickel-iminophosphine-catalysed 4- -2-cycloaddition of enones with allenes formed highly substituted dihydropyrans. The enantioselective amine-catalysed 4-I-2-cycloaddition of allenoates with oxo-dienes produced polysubstituted dihydropyrans in high yields and with high enantioselectivities. Novel enam-ine/metal Lewis acid bifunctional catalysis has been used in the asymmetric inverse-electron-demand hetero-Diels—Alder reactions of cyclic ketones with Q ,j9-unsaturated a-ketoesters. The 4- -2-cycloaddition of acylketenes (80) with 2-unsubstituted and 2-monosubstituted 3-aryl-2//-azirines (81) produced 1 1 (82) or 2 1 (83) adducts, being derivatives of 5-oxa-l-azabicyclo[4.1.0]hept-3-ene or 5,7-dioxa-l-azabicyclo[4.4.1]undeca-3,8-diene. The formation of the monoadducts proceeds via a stepwise non-pericyclic mechanism (Scheme 25). A-heterocyclic carbene-catalysed 4- -2-cycloaddition of ketenes with 1-azadienes yielded optically active 3,4-dihydropyrimidin-2-ones (93% ee) ... 

An extensive review of the selective formation of oxetanes using photochemical 2 + 2-cycloaddition reaction (Paterno-Biichi reaction) has been published. Methods that describe the control of stereo- and regio-selectivity of synthetically important oxetanes have been discussed. The bicyclic guanidine, l,5,7-triazabicyclo[4.4.0]dec-5-ene, catalyses the 2 -l- 2-cycloaddition of allenoates with trifluoromethylaryl ketones to yield highly substituted 3-alkyl-oxetan-2-ylidenes in high yields. ... 

A phosphane-catalyzed [4 - -1] annulation between nitroalkenes and Morita-Baylis-Hillman carbonates was performed by He and co-workers. " The authors claimed the in situ formation of an allylic phosphorus ylide as an active intermediate. Allenoates and enones were able to form cyclopentenes via two cycloaddition reactions they underwent a [3 -F 2] or a [2 -F 4] process in the presence of catalytic phosphines or amines, respectively (Scheme 15). To explain such a different reactivity, Huang, Lankau and Yu, carried out M06-2X/6-31- -G calculations to study the role of ylide intermediates. ... 

Recently, 13 was reported to catalyze the asymmetric annulation reaction between allenoates and a,P-unsaturated carbonyl compounds to form substituted 2H-pyrans (Scheme 6.15) [37]. This formal [4+2] cycloaddition afforded exclusively the product with the (E) configuration of the exocyclic double bond, in line with calculations that indicated the ( )-isomer to be about 14.2 kj mol lower in energy than the (Z) isomer. The mechanism of the reaction was formulated as an initial attack by the nitrogen atom of the quinuclidine ring at the allenoate with formation of an ionic intermediate that preferentially reacts at the terminal carbon atom with the a,p-unsaturated carbonyl compound prior to elimination of the catalyst with formation of the 2H-pyran derivative (Scheme 6.15). 

In addition, a C6 thiourea derivative of P-isocupreidine 25 has been prepared and shown to catalyze the Rauhut-Currier reaction between allenoates and maleimides with the formation of products in a high yield but only moderate ee (Scheme 6.30) [72]. 

Miller and coworkers have performed kinetic studies on a pyridylalanine-peptide catalyzed enantioselective coupling of allenoates 16 and N-acyl imines 17 to investigate the mechanism of the aza-MBH reaction [20]. In the catalytic cycle of a typical MBH/aza-MBH reaction, the proton transfer step or C-C bond formation is often considered as the rate-determining step. However, through mechanistic... 

Dinitrogen-fused heterocycles have been formed in high yield by thermal 3-1-2-cycloadditions of two types of azomethine imines with allenoates. Rhodium-catalysed formal 3 -l- 2-cycloadditions of racemic butadiene monoxide with imines in the presence of a chiral sulfur-alkene hybrid ligand have furnished spirooxindole oxazolidines and 1,3-oxazolidines stereoselectively. ° Formation of 1,2-disubstimted benzimidazoles on reaction of o-phenylenediamine with aldehydes is promoted by fluorous alcohols that enable initial bisimine formation through electrophilic activation of the aldehyde. 

DFT study of interception of the allenyl enolate intermediate of Meyer-Schuster rearrangement using aldehydes and imines has shown that the active form of the vanadium catalyst bears two triphenyl siloxy ligands and that vanadium enolate is directly involved in the C-C bond formation." DFT calculations have elucidated the mechanisms and diastereoselectivities of phosphine-catalysed [4-1-2] annulations between allenoates and ketones or aldimines (Scheme 44). 

A-Acyl aminophosphines catalysed the 4 -i- 2-cycloaddition of a-substituted allenoates with dual-activated olefins at -18 °C with up to 97% ee. A key step in the reaction is the formation of a zwitterion between the catalyst and the allenoates. ... 

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