Cycloadditions, 55. – Substituent Effects in Tandem Intramolecular Silyl Nitronate Olefin Cycloadditions (ISOC) Leading to Functionalized Tetrahydrofurans†

The intramolecular silyl nitronate olefin cycloaddition (ISOC) leading to substituted, fused-ring tetrahydrofurans 6 was examined with regard to optimum conditions and substituent effects. The required unsaturated nitro ethers 3 resulted from low-temperature, base-mediated Michael addition of allyl alcohols 2 to nitroolefins 1, followed by conversion to unsaturated silyl nitronates 4. Cycloaddition of the latter and elimination of silanol provided 6. One-pot tandem reactions, starting with nitroolefins 1 and allyl alcohols 2 and involving four steps, sometimes proceeded in better yields but often gave unexpected side products. Terminal olefinic electron-donating substituents (Me, MeO) increased the rate of cycloaddition, while an internal olefinic methyl substituent slowed down the reaction. In the case of nitronates possessing ester or nitrile moieties as terminal olefin substituents, tandem Michael addition to produce substituted furans 14, 15 occurred faster than trapping of the nitronate anion by TMSCI.