Furans from allenyl ketones

The analogous transformation of 125, also realized by flash vacuum pyrolysis, gave rise to allenic oximes 126 [165], which are not directly accessible by the classical route starting from allenyl ketones and hydroxylamine (see Section 7.3.2) [122], Because compounds 125 are prepared from allenyl ketones and furan by [4 + 2]-cycloaddition followed by treatment with hydroxylamine, the retro-Diels-Alder reaction 125 —> 126 is in principle the removal of a protecting group (see also Scheme 7.46). 

Tandem coupling and cyclization. Functionalized allenes are converted to a-arylvinylated cyclopropanes and heterocycles including epoxides, tetrahydrofurans, and oxazolidinones. Furan derivatives are formed from allenyl ketones." ... 

Disubstituted furans 62 are obtained from a gold-catalyzed cycloisomerization/dimerization pathway involving terminal allenyl ketones and a,(3-unsaturated ketones <00AG(E)2285>. 

Gevorgyan and co-workers demonstrated that allenyl imines can be formed in situ by treating alkynylimines with a base (see Section 15.8, compound 185) [71, 72]. The same principle also works for the in situ formation of allenyl ketones from alkynyl ketones and their conversion to furans with a copper(I) catalyst [71, 72]. That Cu(I) would catalyze the isomerization of an allenyl ketone was known from work of Hashmi et al. [57, 58],... 

The formation of 2 furane rings was achieved in one transformation by Ma and co-workers. Allenoic acids and allenyl ketons were reacted in the presence of a palladium catalyst to give the unsymmetrical bifuryl product, arising from the cyclization of both allene derivatives mediated by the same palladium centre followed by their coupling (3.73.) 91... 

Isomerization of an a-allenylcyclopentenone, obtained from a propargyl ether and a mopholino a,p-unsaturated amide, in the presence of Hg-catalyst to a furanyl cyclopentenone provided another example of the conversion of allenyl ketones to furans <03OLl 171>. 

Acetylenic ketones are viable substrates as well for gold-catalyzed cyclizations to furans. Whereas alk-3-yn-l-ones readily cyclize to substituted furans in the presence of gold(I) or gold(III) chloride (possibly via isomerization to a-allenyl ketones see Section 5.2), the corresponding transformation of alk-4-yn-l-ones takes place in the presence of the cationic gold catalyst generated in situ from PhsPAuCl and AgOTf in toluene (Scheme 4-83). The cyclization is accelerated by... 

Transition metal-catalyzed cydoisomerization of allenyl ketones into furan products was first introduced by Marshall. In the first report from this group, it was demonstrated that various alkyl-substituted furans 2 could be prepared in high yields via the Ag-[50] or the Rh(I)-catalyzed [85, 86] cydoisomerization of allenyl ketones 1 (Scheme 8.1) [87]. This type of transformation of allenyl ketones [88-90] proved to be highly effident for the synthesis of up-to-trisubstituted furans. Furthermore, Marshall applied this strategy as the key step in the construction of the 2,5-disubstituted furanocycle 2a (Scheme 8.2) [87]. 

The second communication of that year [24] is now cited more than 225 times, which also represents a remarkable number. This communication also comprised new reactivity patterns (Scheme 2). Based on the results from the first communication discussed above, a furan ring is formed from an allenyl ketone. Then the furanyne intermediate underwent a previously unknown transformation to an annelated phenol. Since they are easily accessible, furanynes can also be used directly. 

For example, terminal allenyl ketones undergo cycloisomeriza-tion/dimerization leading to 2,4-disubstituted furans. This process, apart from the C-0 bond formation, also involves C-C bond formation (eq 26). 

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