Ethers alkynyl enol

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,... 

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],... 

Entry Alkynyl alcohol Endocyclic enol ether Isolated yield Reference... 

Halogen shifts have been found for tungsten, with assumed formation of iodovinylidenes in reactions of 1-iodo-l-alkynes with W(CO)5(thf) en route to cyclization of 2-(iodoethynyl)styrenes to naphthalenes and of iodo-alkynyl silyl enol ethers [147], while more substantial confirmation is found in Mn =C=C(I)CH (OR)2 (CO)2Cp [R = Me, Et (OR)2 = 0(CH2)30], of which the XRD structure of Mn =C=C(I)CH(OMe)2 (CO)2Cp was determined [148]. 

This reaction in the presence of base was applied to a tandem cyclization. When bis-alkynyl silyl enol ether 93a was irradiated in toluene in the presence of 10 mol % W(CO)6 and DABCO with 1 equiv of H2O, the expected tricyclic ketone 94a was obtained in 80% yield. The five-membered substrate 93b also gave the corresponding tricyclic ketone 94b having the basic carbon skeleton of the cedranes. Thus we can prepare synthetically useful tricyclic compounds utilizing this W (CO)5(L)-catalyzed tandem cyclization in the presence of DABCO [25c] (Scheme 5.29). 

Scheme 5.29 Tandem cyclization of bis-alkynyl silyl enol ethers.

Bromination of the enol ether product with two equivalents of bromine followed by dehydrobromination afforded the Z-bromoenol ether (Eq. 79) which could be converted to the zinc reagent and cross-coupled with aryl halides [242]. Dehydrobromination in the presence of thiophenol followed by bromination/dehydrobromination affords an enol thioether [243]. Oxidation to the sulfone, followed by exposure to triethylamine in ether, resulted in dehydrobromination to the unstable alkynyl sulfone which could be trapped with dienes in situ. Alternatively, dehydrobromination of the sulfide in the presence of allylic alcohols results in the formation of allyl vinyl ethers which undergo Claisen rearrangements [244]. Further oxidation followed by sulfoxide elimination results in highly unsaturated trifluoromethyl ketonic products (Eq. 80). 

Intramolecular alkylation, although it is enticing, has not been developed as a method for cyclohexanone construction. Joseph P.A. Harrity of the University of Sheffield reports (J. Org. Chem. 68 4392, 2003) that TiCl smoothly transforms the enol ether 10, prepared from the corresponding alkynyl phosphonium salt, into the 2-aryl cyclohexanone 11. Alkynyl ethers such as 10 are readily prepared in enantiomerically-enriched form. Would the enantiomeric excess be maintained on cyclization ... 

Thus, strong shieldings are observed for fi carbons of enol ethers and alkynyl ethers, as shown for 1,1 -dimethoxyethene (54.7 ppm) and ethoxyethyne (23.4 ppm) in Table 4.26. In 1-alkoxy-l,3-butadienes, transmission of the electron releasing effect along the conjugated double bonds affects alternate carbons similarly, shielding the carbons in / and <5 position as illustrated for l-ethoxy-2-methyl-l,3-butadiene. 

Palladium-mediated addition of silyl stannane reagents to alkynyl ethers has been employed for the synthesis of aliphatic acyl silanes in very good yields via the intermediate a-alkoxy-/J-stannyl vinyl silanes (enol ethers of acyl silanes)82. In a second palladium-catalysed step, the vinyl stannane moiety could be coupled to suitable halides before hydrolysis to the acyl silanes with trifluoroacetic acid (Scheme 11). 

Ethyl enol ethers of acyl silanes have been prepared by the palladium-mediated addition of silyl stannanes to alkynyl ethers. Hydrolysis using trifluoroacetic acid gave very high yields of acyl silanes (vide supra, Section m.A.4)82. 

Diastereoselective aldol coupling of alkynyl aldehydes.3 The Co2(CO)6 complexes (2) of alkynyl aldehydes react with silyl enol ethers to form aldols with... 

Belmont s group87 reported a cycloisomerization reaction on quinolines 77 (Scheme 5.34) bearing a silyl enol ether group on position 3 and an alkynyl group on position 2, leading to acridine derivatives 78. 

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

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]. 

Cycloadditions of alkynyl carbenes. Alkynyl carbene complexes of chromium and tungsten undergo [2 + 2]cycloaddition at room temperature with a wide range of enol ethers. 

It has already been noted that the enolates of unactivated monocarbonyl compounds do not undergo alkynylation with alkynyliodonium salts3. It is therefore particularly intriguing that [bis(phenyliodonium)]ethyne ditriflate reacts with the silyl enol ether (SEE) of acetophenone to give an allenic diketone (equation 151)41. Except for the SEE of cyclohexanone, which gives a black tar with the bisiodonium compound41, similar studies of other SEEs have not been reported. 

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