Enol ethers cyclopentenyl

Enol ether additives were used to probe the protonation of 3-cyclopen-tenylidene (127). Treatment of A-nitroso-A-(2-vinylcyclopropyl)urea (124) with sodium methoxide generates 2-vinylcyclopropylidene (126) by way of the labile diazo compound 125 (Scheme 25). For simplicity, products derived directly from 126 (allene, ether, cycloadduct) are not shown in Scheme 25. The Skat-tebpl rearrangement of 126 generates 127 whose protonation leads to the 3-cyclopentenyl cation (128). In the presence of methanol, cyclopentadiene (130) and 3-methoxycyclopentene (132) were obtained.53 With an equimolar mixture of methyl vinyl ether and methanol, cycloaddition of 127 (—> 131)... 

Silyl enol ethers can also be used in the cyclopropanation reaction. Reissig showed that the reaction between methyl diazoacetate 53 and various enol ethers 52a-c using bu-box ligand 3 proceeded in moderate yields, as shown in Table 9.5 (Fig. 9.17fl), with trans/cis ratios up to 97 3 and ee between 32 and 49%. Pfaltz showed that cyclic enol ethers can be used as well." Cyclopentenyl enol ether 55 proceeded with methyl diazoacetate 53 and bu-box ligand 3 to afford the cyclopropanation products in 56% yield, a trans/cis ratio of 27 73, trans ee of 87% and cis ee of 92% (Fig. 9.11b, p. 544). 

---