Photooxygenation rose oxide

Scheme 77 Photooxygenation of (-)-p-citronellol (279) conducted in a glass microreactor and the subsequent use of hydroperoxide 278 in the synthesis of (-)-rose oxide (200).

Protopine alkaloids. A notably efficient route to protopanes is the photooxygenation (Rose Bengal or Methylene Blue) of an enamine such as 1 to give an amido ketone (2), which can be converted into 3 by reduction with lithium aluminum hydride in THF followed by oxidation with activated manganese dioxide. The overall yield is about 65%. 

Synthesis of rose oxide (5) by photooxygenation of citronellal as the first step Rose Bengal is used as a photosensitizer Ohioff et al. (1961)... 

The dye-sensitized photooxygenation reaction has to be considered as a biomimetic process 451) involved in the formation of deoxylinalool oxide (132) 459), rose oxide (134) 456) and nerol oxide (135) 459) from linalool (6), citronellol (9) and nerol (8), respectively. 

Oxidation of dihydrocoptisine (124), derived from protopine, with m-chloroperbenzoic acid in dichloromethane afforded coptisinephenolbetaine (125) as the hydrochloride in 78% yield (Scheme 26) (82). This oxidation method was applied to dihydroberberine (90) to produce berberinephenolbetaine (121) (53). Additionally, photooxygenation of 90 in methanol containing Rose Bengal gave 121 in 80% yield, further photooxygenation of which led to the epidioxide 122, a 1,3-dipolar cycloaddition product with oxygen (84,85). 

Kondo et al. (182,183) reported a conversion of a fully aromatized phenolbetaine to a phthalide skeleton through photooxygenation. Reduction of norcoralyne (54) with zinc in acetic acid afforded dihydronorcoralyne (374), which was oxidized with m-chloroperbenzoic acid to the fully aromatized phenolbetaine 375 (Scheme 67). Photooxygenation of 375 in the presence of Rose Bengal, followed by reduction with sodium borohydride, gave rise directly to the phthalideisoquinoline 376 in 70% yield. The same phthalide (376) was also obtained from 2 -acetylpapaveraldine (129) (Section III,B,1). 

Photosensitized oxygenation of a chaicone. Photooxygenation of the chalcone (1) with Methylene Blue as sensitizer gives the flavanol (2). Rose Bengal is a less efficient sensitizer. The reaction may be a model for the biological oxidation of chalcones to flavanoids. 

The photooxygenation of cyclopentadiene with Rose Bengal as photosensitizer was performed using a falling film microreactor in methanol [135]. The endoper-oxide is first generated and then reduced to 2-cyclopenten-l,4-dioI, which is used as an intermediate in pharmaceutical drug synthesis. This route is not easily possible by batch processing since the explosive endoperoxide intermediate is formed in substantial amounts. 

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