Allenes enol triflates

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

In the second example, one carbon atom of the future allene group is contributed by the sulfoxide 72, which is first converted to the ketone 74 by LDA treatment followed by quenching of the resulting carbanion with methyl 3-phenylpropanoate (73). To convert the now complete carbon framework into a central allene unit, the enol triflate of 74, intermediate 75, is produced under the conditions given in the scheme. Ultimate butyllithium treatment of 75 then furnishes the alkylated allene 76 [23],... 

Allenes also react with aryl and alkenyl halides, or triflates, and the 7r-allyl-palladium intermediates are trapped with carbon nucleophiles. The formation of 283 with malonate is an example[l 86], The steroid skeleton 287 has been constructed by two-step reactions of allene with the enol triflate 284, followed by trapping with 2-methyl- 1,3-cyclopentanedione (285) to give 286(187]. The inter- and intramolecular reactions of dimethyl 2,3-butenylmalonate (288) with iodobenzene afford the 3-cyclopentenedicarboxyIate 289 as a main pro-duct[ 188]. 

The Pd-catalyzed three-component reaction of the enol triflate 63, allene (64,) and 2-methyl-l,3-cyclopentanedione (65) gives 66 in high yield, which is converted to the steroid skeleton 67 [31]. 

Dicarborative 1,2-additions of aryl halides, vinyl halides or enol triflates and stabilized carbanions to allenes in the presence of similar palladium catalyst systems reportedly occur with high EfZ stereoselectivity70. They have been applied in an intramolecular version36 and in a two-step synthesis of a steroidal skeleton 1. 

Allenes. Substituted allenes are prq phosphates. Interestingly, enol triflates ten protected aminoalkenylphosphates." Sil> ... 

Enol triflates, which can easily be prepared by trapping regiospecilically generated enolates with triflic anhydride or 7V-phenyl triflimide, have also been applied in car-bopalladation reactions of allenes (Scheme... 

Scheme 2.28 Functionalized allenes obtained by 1,6-cuprate addition to acceptor-substituted enynes and regioselective enolate trapping with methyl triflate (77), aldehydes (78, 79), ketones (80) and silyl halides (81).

A route involving trapping the enolate as a silyl enol ether, subsequent transme-tallation to the corresponding lithium enolate and alkylation turned out to be more efficient (Scheme 18.41) [123]. Thus, treatment of 120 with the cuprate 124 and chlorotrimethylsilane furnished the silyl enol ether 125, which was then converted into the desired enprostil derivative 127 with 68% yield over both steps by reaction with methyllithium and the allenic triflate 126. 

C(2)-C(3) fused polycyclic cephalosporins have received considerable attention as new candidates for /3-lactam antibiotics. An access to tricyclic cephalosporins based on metal-promoted alkenylation of 3-trifloxy-A3-cephem and subsequent Diels-Alder reaction has been published <1996TL5967>. Alternatively, the reaction of a cephalosporin triflate with silyl enol ethers and silylketene acetals has been described to afford tri- and tetracyclic cephalosporins <1996TL7549>. A related process is the formation of fused polycyclic cephalosporins 27 and 28 bearing a wide range of functionalities from the reaction of cephalosporin triflates 26 with unsaturated compounds (alkenes and alkynes) and a base (Scheme 5) <1997JOC4998>. These studies have suggested that the reaction proceeds via the intermediacy of a six-membered cyclic allene which undergoes concerted nZs + K2a cycloaddition with alkenes and acetylenes. 

An interesting reaction between the bis phenyl iodonium triflate of acetylene and the silyl enol ether of acetophenone afforded an allene (PhCOCH=C=C=CHCOPh, 84%) [6], Also, alkynyl iodonium tosylates and carbon monoxide in methanol or ethanol, with palladium catalysis, furnished alkyne carboxylates [53]. 

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