Diethyl alkyne elimination

Secondary and tertiary dialkylcuprates, lithium dialkenyl-, and even diphenyl-cuprates, add in very good yields to the reactive propionaldehyde diethyl acetal. The syn addition products may be trapped with a variety of electrophiles such as alkyl, alkenyl, alkynyl and aryl halides. The method has been used for the synthesis of several natural products. Substituted alkynic acetals also react with lithium dialkylcuprates in ether to furnish stable dialkenylcuprates of type (128) which do not eliminate to the corresponding alkoxy allenes (129) if the temperature is maintained below -20 C.164-179... 

Starting from (+)-diethyl tartrate (2), bromobutenolide 18 was obtained in nine steps. Three of the four C=C double bonds were built up using a Wittig reaction (11—>12), an Ando- y Q Horner-Wadsworth-Emmons reaction (13— 15) and (3-elimination (16 18). From (-)-actinol (3) stannane 23 and sulfone 24 were synthesized in 9 and 13 steps, respectively. Their common intermediate, alkyne 22, was synthesized using methoxycarbonylation. Sharpless asymmetric epoxidation and Ci-elongation with lithio trimethylsilyldiazomethane. Stannane 23 was obtained upon hydrostannylation and TBS deprotection. Sulfone 24 was obtained after addition to methyl tetrolate, reduction, Mukaiyama redox condensation, acetylation and catalytic oxidation. 

A mild and efficient synthesis of terminal alkynes starts with readily accessible methyl ketones and converts them to the corresponding enolates with LDA. The eno-lates produced are trapped with diethyl chlorophosphate to give enol phosphates, which possess a good leaving group for elimination. Subsequent treatment of the enol phosphates with LDA furnishes the corresponding lithium alkynylides and on protonation of these, the corresponding terminal acetylenes. 

Cycloaddition of thiophene 1,1-dioxides with alkynic dienophiles leads to the formation of benzene derivatives with elimination of sulfur dioxide. Thus, the unstable parent thiophene 1,1-dioxide 1 reacts with diethyl acetylenedicar-boxylate and cyclooctyne to give diethyl phthalate and benzocyclooctene, although in low yields (Scheme 53) [132, 175]. Cycloadditions with alkenic and alkynic dienophiles had been used as evidence for the generation of 1 until spectroscopic evidence became available [46]. Tetrachlorothiophene dioxide 53 reacts with phenylacetylene [35] and a cyclic alkyne 92 [176] to give 1,2,3,4-tetrachloro-5-phenylbenzene and compound 93, respectively (Scheme 54). 

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