Mono Substituted Dipolarophiles

Besides methyl acrylate and styrene, the reactivity of three other mono-substituted dipolarophiles in the [3 + 2] cycloaddition with nitronate (16b) was investigated. 

---

In [3 + 2] cycloadditions of cyclic nitronates with mono-substituted dipolarophiles cjco-selectivity is generally favored, suggesting that steric interactions dominate the transition structure energy (Scheme 16.25 and Table 16.1) [121]. The ratio of the exo- and cn//o-products correlates with the size of the substiments on the dipolarophile bulkier... 

Regio- and stereoselectivity of the process depend on the nature of its participants and are determined by the character of the approach of the dipolarophile to the dipole. (In Scheme 3.127, this is demonstrated for the reaction of mono-substituted nitronates with monosubstituted olefins.)... 

Studies on the stability of nitrile oxides [299] bound to Wang resin revealed that decomposition started to be detectable only after 3 days of storage in a dry box at r. t. The authors also demonstrated that cleaner products were obtained by generating the 1,3-dipole prior to addition of the dipolarophile. Mono-substituted electron-poor alkenes represent better dipolarophiles (in terms of both yield and regiose-lectivity) than 1,2-substituted electron-poor alkenes. Electron-rich alkenes gave good results vstith electron-poor (carboxy-substituted) nitrile oxides. The latter were more reactive than the corresponding alkoxy-substituted nitrile oxides. 

First, the reactivity of nitrostyrenes (15a and 15b) in combination with methyl acrylate and styrene as the dipolarophile was investigated (Scheme 9.6). With methyl acrylate as the dipolarophile, nitroso acetals (17 and 18) were obtained after 18 h at 15 kbar and room temperature (RT) in 95 and 78 % yield, respectively. Both nitroso acetals (17 and 18) were isolated as a mixture of three diastereomers. The products were formed from a completely regioselective 1,3-dipolar cycloaddition, which is in agreement with reported literature data on related cycloadditions of mono-substituted acrylates with nitrones and nitronates [22]... 

---