Catalytic Addition of Hetero-Nucleophiles

Schmidt et al. reported similar reactions of 3-butynols with Cr(CO)5(L) and Mo(CO)5(L) [18]. In most cases, a similar tendency to that reported by McDonald was observed, that is, five-membered cyclic carbene complexes were obtained when Cr(CO)5(OEt2) was employed, while dihydrofurans were obtained when Mo(CO)5 (NEt3) was employed, however, in one specific case, a unique difference of the reaction pathway was observed. Thus, when hemiacetal 44 was treated with Cr (CO)5(OEt2), the corresponding carbene complex 45 was obtained, which was further converted to dihydrofuran by treatment with DM AP. On the other hand, when 44 was 

McDonald et al. applied their dihydropyran formation to the oligosaccharide synthesis featuring reiterative alkynol cycloisomerization [19c-f]. Dihydropyran 49 was prepared by the method described above and then NIS-promoted glycosyla-tion with the acyclic alkynol 50 followed by Ph3SnH-promoted dehalogenation and 

Use of the produced carbene complex intermediate for further carbon-carbon bond formation has been achieved by Barluenga and coworkers [22]. Treatment of the alkynol derivatives 69 with 25mol% W(CO)5(thf) in THF at rt gave tricyclic com- 

Computational analyses of these reactions have been carried out by several groups. As the exo versus endo selectivity of the reaction is sometimes dependent on the structure of the substrate, it is not necessarily easy to analyze the reaction in a general manner [22b, 23]. 

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