Allenylidene-tetrahydrofurans

Alternatively, novel allenylidene-tetrahydrofurans such as 84 can be prepared by the tri-butyltin hydride-mediated radical cyclization of bromoalkynyl-oxiranes 82, where the epoxide... 

The radical photoisomeiization of iodoacetylenic esters (alkynes) represents a route to iodoalkylidene lactones [68]. Zinc has been added to reduce side reactions and to increase yields of the photolysis reaction. Bromoalkynyloxiranes are photocatalytically (tri-n-butyltin) cyclized to allenylidene tetrahydrofurans [69]. 

In order to overcome all these synthetic drawbacks, an alternative methodology using tetrahydrofuran solvates [M(CO)5(THF)] and deprotonated tris-amino or alkoxo prop-1-ynes was developed by H. Fischer and co-workers (Scheme 3), which allowed the preparation of the bis(amino)- and bis(ethoxy)-allenylidene complexes [M =C=C=C(NMe2)2 (CO)5] (M = Cr, W) [27] and [M(=C=C=C (OEt)2 (CO)5] (M = Cr, W) [28], respectively. Although the latter could not be isolated in pure form, they were conveniently employed as starting materials for further transformations (see below). 

As shown in Scheme 4, alkynyl metallates derived from propynoic acid amides can also be used as source of the allenylidene C3 unit, their reactions with the tetrahydrofuran solvates [M(CO)5(THF)] affording the N/O-substimted allenylidene complexes 6 after selective O-alkylation with [R30][BF4] [29]. 

The selective intramolecular nucleophilic addition of a hydroxy group at Cyof a ruthenium allenylidene generated by activation of propargylic alcohol by RuCl(Cp)(PPh3)2/NH4PF6 provides a ruthenium vinylidene species, which reacts with allylic alcohols as previously described in the section Formation of Unsaturated Ketones (Eq. 11, Scheme 18) [79]. This unprecedented tandem reaction makes possible the construction of tetrahydrofuran derivatives in good yields and has been used as a key step in the synthesis of (-)calyculin A [80]. 

Scheme 23 Ru-allenylidene [3+2]-cycloadditions to tetrahydrofurans by Sakata, Nishibayashi et al. [23]...

---