With Migrating Groups Analogous to Acyl

Hiyama et al.69 showed that monoacetylated 1,4-butynediols nicely afforded acetoxy allenols on silver-catalyzed rearrangement. These compounds were ideal substrates for silver-catalyzed cyclization, so the overall sequence could directly be performed in one pot, leading to substituted 2,5-dihydrofurans in high overall yields (Table 3.5). In this process, 5-10 mol% of silver perchlorate or tetrafluoroborate was used in refluxing benzene. 

TABLE 3.5. Silver-Catalyzed 2,5-Dihydrofurans Enolacetate Preparation 

This rearrangement-cyclization cascade was applied to natural products synthesis. For example, DDQ oxidation of the appropriate dihydro-3 (27T)-furanone enol acetates (Table 3.5, entries 1 and 7) afforded bullatenone and the antitumor agent geiparvarin in a very rapid route (Fig. 3.1).69 70 

TABLE 3.6. Silver-Catalyzed Preparation of Tetrasubstitnted Furans 

The substitution pattern in the product obtained through this sequence was in agreement with the proposed mechanism (Table 3.7). For example, crotyl chloride gave the 2-chloro-3-methyl hex-4-enoates (entries 3 and 4), and propargyl bromides gave the allenic products (e.g., entries 7 and 8). Yields were usually better with the more substituted tert -butyl diazoacetate. 

---