Photocycloaddition enamine

With conjugated dienes, photocycloaddition of carbonyl compounds occurs at one of the double bonds to give vinyloxetanes. An interesting example is the reaction of acetone with 2-methyl-l,3-butadiene, which gave the two oxetanes (60) and (61) in a ratio of 3 1 and a total yield of about 20% (72JA8761). Other alkenes which have been used for photosynthesis of oxetanes include enol ethers, ketene acetals, enamines, allenes and diketene, with the reaction of the last compound with benzaldehyde illustrated in equation (105) (75CPB365). 

When enamines are employed as sulfene traps, thiete 1,1-dioxides (e.g. 142) can be prepared by subsequent Hofmann elimination of amine. An alternative route to thiete 1,1-dioxides involves trapping sulfenes with ynamines, as illustrated by preparation of (144) (73S534). Certain thietes such as (145) (80LA873, 78TL3617, 78TL4839) are available directly from thioketones by 2 + 2 photocycloaddition with alkynes. 

Upon irradiation with ultraviolet light Fischer chromium-carbene complexes 16 react as if they were ketenes, although evidence for the generation of free ketenes has not been observed (see Section 1.4.2.). Photocycloadditions of these complexes onto chiral enamines show significant asymmetric induction.22... 

Cycloadditions on 1,2,3,4-tetrahydropyridines are usually limited to those bearing an electron-withdrawing group at the 5-position or with a 4-oxo group to activate the enamine-like alkene. For example, di-l,2,3,4-tetrahydropyridine 220 in which one of the two 1,2,3,4-tetrahydropyridine units has a 4-oxo group undergoes [2+2] photocycloaddition... 

Bach and coworkers investigated the photocycloaddition of 7V-acyl, 7V-alkyl enamines 125 with benzaldehyde [125]. The 3-amido oxetanes 126 were formed with excellent regioselectivity (analogous to reactions with enolethers—vide supra) and good diastereoselectivity (Sch. 41). Enamines, not deactivated by acylation at the nitrogen atom are poor substrates for Paterno-Buchi reactions due to preferred electron transfer reactivity (formation of the corresponding enamine radical cation and subsequent reactions). 

Several efforts were also made by the same group to achieve chiral auxiliary control in 7V-acyl enamines 155, but only moderate diastereoselec-tivities for the 3-amido oxetanes 156 were reported (Sch. 54) [152]. The photocycloaddition of an axial chiral 7V-acyl enamine 157 with benzalde-hyde resulted in oxetane (158) formation with moderate diastereomeric excess [153],... 

Aliphatic enamine derivatives 198 linked to a chiral oxazolidone allow an almost totally diastereoselective [2 + 2] photocycloaddition of 102 (Scheme 53). This efficiency was attributed to a selective shielding of the lower face by the R substituents in the preferred conformation of the chiral oxazilidone enecarbamate. Interestingly, the de of 199 did not depend on the configuration of the stereocenter on the exocyclic appendage [122]. 

The route employed to prepare indanone 51 involved the cycloaddition-hydrolysis-aldol sequence shown in Scheme 3.9. Accordingly, condensation of cyclopentenone 52 with ynamine 53 (84) afforded the bicyclic enamine 54 which was converted to indanones 51 and 55 by hydrolytic cyclobutane ring opening followed by intramolecular aldol condensation. Interestingly, treatment of 54 with aqueous formic acid yielded indanone 51 which has stereochemistry complementary to that at C(15) and C(20) in reserpine. In contrast, hydrolysis of this substance with aqueous hydrochloric acid afforded the trans-fused indanone 55. Subsequent to this work, the Ficini group found that esterification of 51 followed by photochemically induced addition of methanol afforded adduct 56 which has four of the reserpine stereocenters in place (23). While no further work on this problem has been reported, these preliminary investigations demonstrate a novel use of [2 -h 2] photocycloaddition chemistry in potential approaches to yohimbane alkaloid synthesis. 

Bach and Schroder investigated the photocycloaddition of electron-rich N-acyl enamines 35 with benzaldehyde. The 3-amidooxetanes 36 were formed with excellent regioselectivity (analogous to reactions with enol ethers, vide supra) and good diastereoselectivity. 

The photocycloaddition of an axial chiral N-acyl enamine 71 with benzaldehyde resulted in oxetane (72) formation with moderate diastereomeric excess. ... 

Bach, T. and Schroder, J., Photocycloaddition of N-acyl enamines to aldehydes and its apphcation to the synthesis of diastereomericaUy pure 1,2-amino alcohols,/. Org. Chem., 64, 1265, 1999. 

Bach, T, Schroder, J., Brandi, T, Hecht, J., and Harms, K., Facial diastereoselectivity in the photocycloaddition of chiral N-acyl enamines to benzaldehyde. Tetrahedron, 54,4507,1998. 

---