Reaction with alkynyl ethers

The addition of (Bu3Sn)(PhS)CuLi to alkynones (acetylenic ketones) proceeds in high yield and with excellent stereoselectivity for the (Z)-isomer of the product (>95%).2" The reaction of alkynyl ethers gives different regioisomers. Both kinetically and thermodynamically controlled products are obtained depending on the reaction conditions (Equation (123)).300... 

The [Z + Z] cycloaddition reactions of various 4-dialkylamino-3-butyn-2-ones 102 with substituted phenyl isothiocyanates 103 provide access to a series of thietimines 104 in moderate yields <2001SL361, CHEC-III(2.07.9.2)415>. Likewise, cycloaddition of A-sulfonylimines 105 with alkynyl ethers affords functionalized 2-azetines 106 in a quantitative yield (Scheme 54) <1991ZOB1389>. 

Oxygenated alkynes also can be ozonated. The reaction of ozone with alkynyl ethers followed by reductive workup provides a convenient method for the production of a-keto esters in moderate yields (eq 19). ... 

Reaction of cyclic acetals with alkynyl ethers promoted by BF3 gives unsaturated lactones in a process proposed to involve initial ketene intermediate 157 which reacts by an intramolecular [2 + 2] cycloaddition giving an in situ generated oxocarbenium ion and then ring expansion (Eqn (4.96)). Both larger and smaller unsaturated lactones can be prepared by the same procedure (Eqn (4.97)). ... 

The reaction uses only a single equivalent of TMS-Br and MeOH, which acts as a trap for the TMS group, to give MeOTMS as a by-product. The reaction is extremely regioselective with E/Z ratios from 11 1 to >33 1.The bromine(orpossiblyiodine)atom installed in the reaction is easily metaUated and reacted with electrophiles (chloroformates, TMS-Cl, etc.) or coupled, under transition metal-catalyzed conditions, with vinyls, CO, or Grignards. Additionally, TMS-Br may also be reacted with alkynyl ethers (eq 31) to produce a-bromo vinyl ethers. 

Alkynyl ethers, RCH2C CCH2OR and CHjGaCCHCRlOR, are converted into 3,1 -en-ynes by treatment with sodamide in liquid ammonia. The reactions may be visualized as 1,4-eliminations of ethanol and subsequent metailanon of the intermediary cumulenes [143] ... 

Reaction of (diisopropylamino)chloroboryl ethers of alkynols (57) with alkynyl-stannanes (58) in the presence of nickel catalysts has been reported to afford formal f/ms-alkynylboration products (60), which could undergo the Suzuki-Miyaura coupling with organic halides. The 2-borylalkenylnickel(II) complex (59) was isolated as an intermediate in a reaction of the chloroboryl ether (57) with 1 equiv. of a nickel(0)-phosphine complex and characterized by X-ray crystallography.61... 

TABLE 10.3. Asymmetric Mannich Reactions of Enol Ethers with Alkynyl Imines... 

The opposite sequence is described with alkynyl silyloxy-tethered enynes. A cascade CM-RCM reaction proceeds in the presence of a second generation ruthenium carbene to give cyclic siloxanes. The initial CM is directed to occur on the alkyne by employing sterically hindered substituted alkenes tethered to the alkyne via a silyl ether group [24] (Scheme 11). 

Pauson-Khand Bicyclization. Alkynyl and enol ether derivatives have been studied in the cobalt-mediated intramolecular Pauson-Khand reaction and found to provide high diastere-oselectivity, superior to previous work with the auxiliary 2-phenylcyclohexanol. The 3-substituted auxiliary alcohol (3) provides higher selectivity than the 2-substituted analog. Also, the alkynyl ether derivatives exhibit higher reactivity and selectivity than the corresponding enol ether derivatives (eq 6). 

Simple a,3-unsaturated aldehydes, ketones, and esters (R = C02Me H > alkyl, aryl OR equation l)i, 60 preferentially participate in LUMOdiene-controlled Diels-Alder reactions with electron-rich, strained, and selected simple alkene and alkyne dienophiles, - although the thermal reaction conditions required are relatively harsh (150-250 C) and the reactions are characterized by the competitive dimerization and polymerization of the 1-oxa-1,3-butadiene. Typical dienophiles have included enol ethers, thioenol ethers, alkynyl ethers, ketene acetals, enamines, ynamines, ketene aminals, and selected simple alkenes representative examples are detailed in Table 2. - The most extensively studied reaction in the series is the [4 + 2] cycloaddition reaction of a,3-unsaturated ketones with enol ethers and E)esimoni,... 

Very few pericyclic reactions of carbene complexes have been studied that are not in the cycloaddition class. The two examples that are known involve ene reactions and Claisen rearrangements. Both of these reactions have been recently studied and thus future developments in this area are anticipated. Ene reactions have been observed in the the reactions of alkynyl carbene complexes and enol ethers, where a competition can exist with [2 + 2] cycloadditions. Ene products are the major components firom the reaction of silyl enol ethers and [2 + 2] cycloadducts are normally the exclusive products with alkyl enol ethers (Section 9.2.2.1). As indicated in equation (7), methyl cyclohexenyl ether gives the [2 -t- 2] adduct (84a) as the major product along with a minor amount of the ene product (83a). The t-butyldimethylsilyl enol ether of cyclohexanone gives the ene product 9 1 over the [2 + 2] cycloadduct. The reason for this effect of silicon is not known at this time but if the reaction is stepwise, this result is one that would be expected on the basis of the silicon-stabilizing effect on the P-oxonium ion. 

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