Singlet oxygen ascaridole

Using a single-channel chip micro reactor, singlet oxygen is generated by photochemical means in presence of catalytic amounts of Rose Bengal [21]. By [4 -i- 2] cycloaddition of this oxygen species to a-terpinene, the product ascaridole is obtained. 

Singlet oxygen adds to cyclic 1,3-dienes to afford 1,4-endoperoxides 609). This type of reaction has first been applied successfully to the synthesis of ascaridole from a-terpinene 610) (6.4) and of the vesicatory compound cantharidine 6U) (6.5) some 25 years ago. 

In the literature, there are also several examples where the specific oxidation reaction is dependent on the presence of a diene. Often the substrates are conjugated dienes where there is simultaneous oxidation of both double bonds. An important prototype of this reaction is the well-known singlet oxygen oxidation of dienes to endoperoxides, for example the oxidation of a-terpinene (4) to ascaridole (5) (equation 1). 

A [4 + 2]-cycloaddition of singlet oxygen to a-terpinene in methanol yields ascaridole in the presence of catalytic amounts of Rose Bengal as photosensitizer. The reaction... 

Figure 23.5 Schematic flow setup for the through-wall singlet oxygen oxidation of a-terpinene to ascaridole.

An early example of a multi-phase microreaction was reported by Wootton et al., who demonstrated the continuous photochemical production of singlet oxygen and its use in organic synthesis for the cydooxidation of a-terpinene to ascaridol (Scheme 16.5) [10]. 

Scheme 16.5 Photochemical generation of singlet oxygen and oxygenation of a-terpinene to ascaridol.

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