Alkynyl epoxides

Hie same autliors also studied tlie alkylation of alkynyl epoxides for fornialion of optically active a-aUenic alcohols under kinetic resolution ctmdilions fSdieme 8.29) [54]. 

Cumulenes have also been prepared by treating alkynyl epoxides with boron trifluoride. 1,4 Elimination of BrC C=C—CBr has been used to prepare conjugated dienes (C=C—C=C). ... 

The Pd-catalysed carbonylation of alkynyl epoxides 60 and alkynyldioxolanones 61 leads to the allenes 62 which can then be converted to the same pyranones through a tandem conjugate addition-cyclisation (Scheme 40) <00JCS(P1)3188>. Carbonylation of allenyl alcohols is catalysed by Ru3(CO)i2 and yields 5,6-dihydropyran-2-ones . 

Alkynyl epoxides have also been shown to be competent nucleophiles in these processes. Both Mo372 and Ru373 catalyst systems have been used for the preparation of furans in this manner (Equation (105)), with lower catalyst loadings in the latter case (2-10 vs. 25-50 mol%). 

In 1974, Vermeer et al. described formation of allenic alcohols 61 by the reaction of alkynyl epoxides 60 with Grignard reagents in the presence of 10mol% of Cul (Scheme 3.33) [71]. In the absence of Cul, a complicated mixture of products was obtained. Furthermore, the Cu-catalyzed reactions exhibited higher yields and higher selectivity than analogous reactions of alkynyl epoxides with lithium dialkylcup-rates [72], This method was applied to a reaction of allylmagnesium bromide with an alkynyl epoxide [73]. 

The alkynyl epoxide 73 undergoes a copper-catalyzed reductive metallation by wBuLi [81, 83]. The resultant allenyllithium compound 74 is a versatile intermediate and reacts with various electrophiles (Scheme 3.37). 

A cuprate prepared in situ from tBuPh2SiLi and Cul has been found to react with alkynyl epoxides to afford allenylsilanes (Eq. 9.43) [50]. Enantioenriched alkynyl epoxides, which are readily prepared in high yield through Sharpless asymmetric epoxidation [51], afford chiral allenylsilanes with anti stereoselectivity. 

Scheme 8.29. Reactions of alkynyl epoxides 47 with R2Zn.

Asymmetric epoxidation of conjugated dienes and enynes catalysed by the chiral man-ganese(EI) complex 232 give monoepoxides exclusively reactions of cw-enynes give /raw.v-alkynyl epoxides as the major products with a high level of asymmetric induction362. 

Another approach to tetrasubstituted furans via allenes also appeared <03TL3263>. In this reaction, cumulenes were produced as an intermediate from alkynyl epoxides and Smij and the allyl group was incorporated regioselectively. 

The alkynyl epoxide 52 cycloisomerized to a furan in the presence of a group VI transition metal catalyst <1994JA9363>. The analogous reaction with alkynyl thiirane 53 was also observed although thermal extrusion of sulfur was the major competing side reaction (Scheme 19) <2000S970>. 

Asymmetric allylation of cinnamyl halides, alkylation of alkynyl epoxides, and 1,4-addition of nitro olefins" are also successfully demonstrated by combination of an organozinc reagent and a chiral copper phosphoramidite. 

The same authors also studied the alkylation of alkynyl epoxides for formation of optically active ct-allenic alcohols under kinetic resolution conditions (Scheme 8.29) [S4],... 

Cumulenes have also been prepared by treating alkynyl epoxides with boron tri-... 

Coupling reactions. Alkynyl epoxides react with organoboronic acids by a formal Sn2 process, yielding aUenyl carbinols. ... 

Wittig reaction of propargylphosphonium salts with aldehydes is (Z)-selective using MesGa as base. From the analogous suUbnium salts, alkynyl epoxides are obtained (base f-BusGa). ... 

Alternatively, carbene-complexed copper hydride species is involved in the transformation of alkynyl epoxides to allenyl carbinols using Cul, t-BuONa, PMHS and a imidazo-lium salt. ... 

Homoallylic alcohols. C-C Bond formation occurs when an co-alkynyl epoxide is treated with (Cp2TiCl)2. The P-alkoxy radical generated by epoxide cleavage adds to the triple bond. A synthesis of ceratopicanol featured this process as an annulation step. 

A regioselective transition metal-catalyzed cycloisomerization reaction of boron-containing alkynyl epoxides toward C-2- and C-3-borylated furans was developed. It was found that the copper catalyst as well as the gold catalyst with a relatively more basic triflate counterion favored boryl migration toward C-3-borylated furans, whereas employment of the cationic gold hexafluoroantimonate afforded C-2-borylated furans via a formal 1,2-hydrogen shift (14JA13146). 

A mini-review focused on the recent achievements in gold-catalysed oxygen tfansfer reactions of tethered alkynones, diynes, or alkynyl epoxides to cyclic enones has described the corresponding mechanisms. ... 

A new diastereo- and enantio-selective copper-catalysed conversion of alkynyl epoxides into a-allenic alcohols has been explored. Sn2 pathways are involved. 

Acetal skeletons are also obtained as products through a highly regio- and diastereoselective intermolecular addition of water and alcohols to alkynyl epoxides catalyzed by gold(I) (Scheme 16) [134, 135]. 

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