Cumulenes epoxidation

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). ... 

Since cumulenes and alkynes are often easily interconvertible, many syntheses discussed above have allenic counterparts, especially base-catalyzed cyclizations of allenic alcohols.77 And, of course, several of the alkyne-based syntheses may well have allenic intermediates. There are, however, a few syntheses based specifically upon allene chemistry. In an important one, due to Stirling and his collaborators,78 an allenic sulfonium salt reacts with an enolate anion. Scheme 12 sketches the main features yields as high as 86% are recorded. Methoxyallene is easily metallated by butyllithium and so converted into an allenic epoxide that can be isomerized by fe/T-butoxide into a furan (Scheme 13) or an exocyclic equivalent similar to 15 clearly this method is particularly suited to the preparation of 3-methoxyfuran... 

The regioselectivity in the epoxidation of ethenylidenecyclopropanes by MCPBA strongly depends on the double-bond substituents. Thus, with alkyl substituents such as methyl 195, the adjacent electron-rich double bond is epoxidized, whereas the diphenyl analogue 196 reacts preferentially on the other double bond both primary products undergo subsequent rearrangements308. The MCPBA oxidation of the hindered cumulene 197 gave the cyclopropanones 198 and 199309. 

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. 

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

The intra- and intermolecular carbon—heteroatom bond forming reactions using alkenyl epoxides and aziridines have been utilized for the synthesis of heterocyclic compounds.261 There are two major processes for the preparation of heterocycles using these substrates, as illustrated in Scheme 192 (a) intramolecular allylation of alkenyl epoxides and aziridines with Y—H and (b) intermolecular cycloaddition of vinyl epoxides and aziridines with the hetero-cumulenes 627, such as isocyanates, carbodiimides, and isothiocyanates.270... 

It is generally accepted that the NCS chromophore intercalates into DNA via its naphthoate side chain, which positions the rest of the molecule within the minor groove [58-60]. Another role for the a-hydroxynaphthoate has been suggested in the activation of NCS chromophore (Scheme 7-9) [61]. Here, the hydroxyester participates in epoxide opening and cumulene formation. Again, the three-dimensional solution structure of intact NCS provides invaluable information, showing that this naphthoate is kept well away from the bicyclic core until the chromophore is released from the apoprotein [42]. 

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