Alkynyl silyl ethers

Alkenyl or alkynyl silyl ethers. These can be prepared in high yield by reaction of an alkenyl- or alkynyllithium with 1, followed by reaction of the product with an alcohol. 

In the presence of a catalytic amount of Sc(OTf)3, imines and alkynyl sulfides undergo [2 + 2] cycloaddition and successive fragmentation to afford a, -unsaturated thioimidates (eq 21)/ Alkynyl selenides or alkynyl silyl ethers undergo similar reactions in the presence of Sc(OTf)3. 

R = alkyl, alkenyl, alkynyl, aryl, acetate, ester, benzyl or silyl ether groups Scheme 6.15 Cleavage of methoxyphenyl methyl (MPM) ethers using clayan. 

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.

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). 

Alkynyl ketones (88) behave like substituted alkynes (29). Thus, the triple bond reacts first to give vi-nylsilanes (89) and/or (90) (equation 56), which are sometimes reduced further to allyl silyl ethers (91) and/or (92). ° ... 

The reaction of the P-alkynyl silylated phosphoranimine (71) with KP(H)Ph yields a cyclic product which adopts a polymeric structure (72) after careful recrystallization from diethyl ether. The formation of the cyclic structure probably arises from addition of [P(H)Ph] to the phosphoranimine, followed by a 1,3-H shift from phosphorus to carbon. In the structure of (72) each potassium atom is coordinated to one N atom and two P(III) atoms from different ligands with a mean K-P distance equal to 327.3(6) pm. Attack at the carbon atom adjacent to P(V) has been observed for the reaction of (73) with LiP(R)Ph (R = H or SiMe3Bu ), affording the products (74a) and (74b). The tin analogues (75a) and (75b) can be prepared in low yield by the reaction of SnCb with (74a) and (74b), respectively. ... 

In coupling with the silyl-protected 2-pyrimidinone (69) (Scheme 14), hydrolysis of the silyl ether function during the reaction is prevented by hexamethyldisilazane (HMDS), which is added to trap any water in the reaction mixture high yields of the O-protected alkyne (70) result [86ACSA(B)381]. The 5-iodopyrimidin-4-one (71) is alkynylated (72) according to the standard protocol at 50°C. The product reacts further on heating and is cyclized to the fused furo[2,3-d]pyrimidine (73) (82CPB2417). 

Allenamides are functionalized at the a-position by virtue of the regioselective lithiation. Thus, on introduction of an alkynyl chain, valuable precursors of the Pauson-Khand reaction can be prepared. Lithiation of the silyl ethers of propargyl or allenyl alcohols engenders O —>C transsilylation. ... 

Chattopadhyaya and co-workers have also demonstrated that alkynyl groups can be used as radical acceptors when linked through a silyl ether [76c]. Radical cyclization and subsequent oxidative cleavage of the tether provided access to 2 - and 3 -C-branched a-keto-]3-D-ribonucleosides, usually in good to excellent yields. 

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