Rearrangements system, photochemical

For analysis of the photochemical reaction, the interaction of the hydrogen Is orbital with -tt3 of the allyl system is used. The interaction is bonding at both the migration origin and terminus, so the [ 1,3] sigmatropic rearrangement is photochemically allowed. 

The intramolecular insertion into the N—H bond of j8-lactams was used successfully in the synthesis of bicyclic ring systems. Photochemical, in contrast to Rh(II)-catalyzed, decomposition of diazo ester 62 was found to occur far less selectively. In the photolytic reaction, the imide 63 is the major product. It presumably arises by a photolytic Wolff rearrangement to a ketene intermediate, which is trapped intramolecularly. With Rh2(AcO)4 catalyst the Wolff rearrangement is suppressed and 62 undergoes ring closure to 64 nearly quantitatively (80TL31). 

Rearrangement. The photochemical rearrangement of a variety of bicyclic aziridine ring systems has been studied by Padwa and co-workers. Photolysis of the diazabicyclo[3,l,0]hexene (586), obtained from photoinduced dimerization of the azirine (585), gave a mixture of cis- (587) and trans- (588)... 

The parent thionine system 1 up to now has not been prepared probably because the C-S bond in valence isomeric forms is too weak giving rise to facile rearrangement or decomposition. The obvious synthetic route, photochemical transformation of cyclooctatetraenccpisulfide 2 (9-thiabicyclo[6.1.0]nona-2,4,6-triene), does not lead to 1, but intriguingly to another valence isomer, the sulfur-bridged homotropylidene system 3.20... 

Lasubines I and II are alkaloids containing a 4-arylquinolizidine substructure that have been isolated from plants of the Lythraceae family and have attracted the attention of synthetic chemists for some time. While numerous racemic syntheses of these and related compounds have been reported, only a few enantioselective syntheses are known. Some examples of these syntheses are given below, and the strategies involved in these examples are summarized in Scheme 92. Three of these syntheses involve the creation of the quinolizidine system by formation of one bond at the a- or 7-positions, while the fourth approach is based on a ring transformation associated with a photochemical Beckmann rearrangement. 

Compared to the parent system 3a, the barrier for formation of 3d is the highest in this series whereas the formation of 3b should be the most facile according to our computations. Although the reactions of carbenes la-c are initiated photochemically, the observed reactivity seems to be in line with the computed ground state properties. Thus, while methyl substitution in 3-and 5-position inhibits the vinylcarbene-cyclopropene rearrangement, methyl substitution in 2- and 6-position has the opposite effect. 

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