Patai S., Rappoport Z. 1996 The chemistry of functional groups. The chemistry of amino, nitroso, nitro and related groups. Part 2 / 0683 745

Selective generation of 2-aza-1,5-diradicals 146 through nucleophilic addition to the less substituted positions of the cation radicals 145 arising by intramolecular electron transfer was attributed to the cyclization regiochemistry (equation 66).

The efficient photoaddition reactions between pyrrolinium perchlorate (133) and benzyltrimethylsilane (equation 67) or the allylsilane (equation 68) are examples of the initial electron transfer induced desilylation processes.

(133)

(133)

The allylsilane-iminium salt photochemistry has been found useful for the development of novel spirocyclization methodologies (equation 69)124,125.

Me3 Si

Total synthesis of the representative protoberberines, (C)-xylopinine 151 (equation 71) and (C)-stylopine 152 (equation 72), have been achieved using silylarene-iminium salt photochemistry128. The photochemical routes appear to be superior to alternative groundstate methods involving dipolar cyclizations.

(71)

B. The Aza-di-p-methane Rearrangement

The extension of the di- -methane rearrangement from 1,4-dienes and ˇ, -unsaturated ketones to the use of 1-aza-1,4-dienes has been achieved129 (equation 73).

Acyclic ˇ, -unsaturated oxime ethers are found to undergo both cis trans isomerization and N O bond fission130,132 (equations 74 and 75).

Cyclic ˇ, -unsaturated oximes 153 did undergo aza-di- -methane rearrangement131 (equation 76).

(153)

Various substituted ˇ, -unsaturated imines (equation 77) did undergo aza-di- -methane rearrangements via triplet states132,133.

The introduction of an electron-withdrawing group such as acetyl at the oxime oxygen will decrease the intramolecular electron transfer reaction from the nitrogen lone pair and will enhance the aza-di- -methane rearrangement134 138 (equation 78).

R2 = Me, Ph

R3 = H, Me

Other oxime ether derivatives, such as semicarbazones or benzoyl compounds, hydrazones undergo efficient aza-di- -methane rearrangement139 (equation 79).

R = OCOPh , NHCOPh , NHCONH2

C. Photochemistry of Azirines

The irradiation of 2H-aryl azirines yields nitrile ylides which can be trapped by various dipolarophiles to form five-membered ring heterocycles116,140 142 (equations 80 82).

D. Stilbene-type Photocyclizations

N-Phenylbenzylimines undergo stilbene-type photocyclizations (equation 83) to yield heterocyclic compounds143. The reactions usually take place via the iminium salt and need oxidants like oxygen or iodine. Six-electron electrocyclic reactions have been observed for 1-aza-1,3-dienes144 (equation 84).

H2 SO4

N

(60%)