Endoperoxides from 1,3-dienes

Possible side-reactions, such as the formation of allyl hydroperoxides from compounds having an allylic hydrogen (a competitive ene reaction Section 6.7.3) or 1,4-endoperoxides from 1,3-dienes, may be suppressed by changing the experimental conditions.1359,1438 For example, the extent of [2 + 2] and [4 + 2] photooxygenation in styrenes, such as the propenylanisole 522 (Scheme 6.253), is controlled by solvent polarity and pH, possibly due to protonation of a perepoxide/zwitterion intermediate.1421,1439 The [4 + 2] product 523 is preferentially produced in non-polar benzene or chloroform, whereas the 1,2-dioxetane 524 is almost exclusively formed in methanol or acidified non-polar solvents. 

Epidioxides (sometimes called endoperoxides) can be prepared from acenes, from suitable steroids, and from dienes of low molecular weight ([Pg.274]

Endoperoxides from acylic 1,3-dienes. 1,4-Endoperoxides have been obtained in the photooxygenation of acyclic 1,3-dienes (5, 488). These are the usual products of oxygenation of cyclic 1,3-dienes. In fact, it was originally suggested that an acyclic 1,3-diene is more reactive to than an isolated double bond. However, more recent results suggest the following order of reactivity of carbon—carbon double bonds toward Oj trisubstituted mono-alkene > 1,3-diene > 1,1-disubstituted monoalkene. ... 

The endoperoxides from 2,5-diphenylfuran and 1,4-diphenylcyclopenta-1,3-diene react with pivaldehyde to give c/s-fused 1,2,4-trioxanes <97H(44)367>. 

In this case, the benzene dioxide 5 is obtained from a [2,2]paracyclophane diene by photooxygenation and rearrangement of the derived endoperoxide. 

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. 

Singlet oxygen reacts through the [4- -2]-cycloaddition mode with the sterically hindered and non-planar 1,3-diene system in l,l -dimenthene (493) to afford the cis-endoperoxide 494, albeit in moderate yield (Scheme 135) ". Cycloaddition of O2 to more planar bis(dialine) (495) proceeds smoothly leading predominantly to the trans-endoperoxide trans-496 (70%), while cis-496, which would be expected to derive from a concerted process, constitutes the minor (15%) product (Scheme 136) . 

In the case of chiral base catalysis of the E2 elimination, enantioenriched 7-hydroxyenones from the corresponding endoperoxides were obtained <2006JA12658> in fact, a one-pot asymmetric 1,4-dioxygenation of 1,3-cyclohepta-diene by sequential reaction with singlet oxygen and 5 mol% chiral catalyst provided the 7-hydroxyenones 80 in 90% yield and 92% ee (Scheme 18). 

Endoperoxides (sometimes called 1,4-epiperoxides or dioxapropellanes) are six-membered ring peroxides, which arise from singlet oxygenations of arene and diene compounds and have been synthesized as pure compounds (e.g., 43-52 and 62-72 Schemes 11.4 to 11.7). Many endoperoxide structures have been reported [38-41] the rosettes of Schemes 11.5 and 11.7 show examples where endoperoxides were formed in good yield. 

Endoperoxides derived from cyclohexa-1,3-dienes are the most abundant in this category due to the greater stability compared to their analogues and to the substrate availability [5c,8d]. This subunit is present in a great number of terpene and steroid products. Most of steroid endoperoxides are 5,8-derivatives in which the oxygen bridge has the a-configuration [l,5c,8d]. The steric demand of the methyl at C-10 appears... 

Photo-oxygenation is a powerful synthetic tool to introduce, simultaneously, oxygenated syn- 1,4-functionalizations in 1,3-diene systems, from alicyclic to cyclic, from carbocyclic to heterocyclic or aromatic. The primary peroxidic products (endoperoxides) can undergo a variety of transformations (Sch. 49), some of them stereospecific, which can either or not involve the cleavage of the 0-0 bond and, often, can be performed in situ and at low temperature overcoming the thermal instability problem of the peroxide intermediate [5b,9b]. 

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