Cyclopentadiene endoperoxide

Pentaarylcyclopentadienols (375) are reported to yield endoperoxides (376) which when heated or treated with acids decompose to tetraarylfurans and the corresponding aryl acids.251 Tetraarylsubstituted cyclopentadienones such as tetracyclone (378) lead to m-diaroylstil-benes and carbon monoxide.252-255 Probably, as with other cyclopentadienes, endoperoxides are the primary products. However, no attempts seem to have been made to elucidate the mechanisms of rearrangements and decompositions involved in these reactions. 

Clavulones I and II are members of an unusual family of marine prostanoids from the coral Clavularia viridis which are biosynthesiied by a cationic (i.e. non-radical, non-endoperoxide) pathway. The total synthesis of clavulones I and II was accomplished from cyclopentadiene as SM goal. 

GL 24] [R 1] [P 26] The feasibility of safely carrying out the oxidation of cyclopentadiene by singlet oxygen to 2-cyclopentene-l, 4-diol was demonstrated [40]. The explosive intermediate endoperoxide was generated and without isolation used onsite for a subsequent hydration reaction. By reduction with thiourea the pharmaceutically important product 2-cyclopentene-l,4-diol was so obtained. 

Arylsubstituted cyclopentadienes such as 1,2,3,4-tetraphenylcyclo-pentadiene (369) give rise to isolable endoperoxides249 which can be thermally rearranged to m-diepoxides. From the corresponding fulvene derivative (372) only the diepoxide 374 was isolated which probably originates from a thermally unstable endoperoxide 37i.249,250... 

Conjugated cycloalkadienes4l3 17 are converted to bicyclic endoperoxides that may undergo thermal or photochemical rearrangement to yield diepoxides.374-377 1,3-Cyclopentadiene and 1,3-cyclohexadiene give endoperoxides exclusively. The first synthesis of the naturally occurring ascaridol (52) was also carried out in this way 418,419... 

The photooxygenation of cyclopentadiene with Rose Bengal as photosensitizer was performed using a falling-film microreactor in methanol [317]. The endoperoxide is first generated and then reduced to 2-cyclopenten-l,4-diol, which is used as an intermediate in pharmaceutical drug synthesis. This route is not easily possible by batch processing because the explosive endoperoxide intermediate is formed in substantial amounts. 

Endoperoxidation is the preferred process for cyclic dienes, and simple five, six, seven, and eight membered ring aliphatic dienes lead, by photooxygenation, only to the corresponding endoperoxides which, except for that of cyclopentadiene, are quite stable (Sch. 17) [28]. 

Schenck and Ziegler170 obtained the naturally occurring endoper-oxide ascaridole (166) by irradiation of an alcoholic solution of a-terpinene in the presence of eosin. Endoperoxides similar to 166 are formed in the photosensitized autoxidation of cyclopentadiene,182 cyclohexa-1,3-diene,179,182 cyclohepta-1-3-diene,183 and their phenyl-... 

The endoperoxide of 1,4-diaryl-1,3-cyclopentadiene (293) reacts in precisely the same way with pivalaldehyde and cyclopentanone affording the cis and trans t-butyltrioxanes (294), and the spiro-cyclic derivative (156) (Equation (42)) <95HCA647>. In all cases, x-ray reveals that the newly formed trioxane ring is c -fused to the original ring. 1,3-Cyclohexadiene peroxides also react with aldehydes and ketones to give cw-fused bicyclic trioxanes. 

Polycyclic aromatic hydrocarbons such as anthracenes, tetracenes, and pentacenes, as well as cyclopentadienes, cyclohexa-l,3-dienes, cyclo-hepta-l,3-dienes, and furans, have been found to be suitable diene systems to which the singlet oxygen adds as a dienophile in a 1,4-cycloaddition reaction. Thus, endoperoxides (transannular peroxides) and, in the case of furans, ozonides of the corresponding cyclobutadienes are the primarily produced, more or less stable addition products (2, 21, 22). 

Selective reduction of unsaturated endoperoxides. 2,3-Dioxabicyclo[2.2.1]-heptane (3) serves as a model of the prostaglandin endoperoxide (PGG2), involved in the biosynthesis of prostaglandins and also of thromboxanes and PGl2 (prostacyclin). One successful synthesis of 3 involved sensitized photooxygena-lion of cyclopentadiene to give the unstable heat-sensitive endoperoxide 2, which... 

A similar addition of singlet oxygen was reported for cyclopentadiene at 10-15 °C [26]. The 32-charmel falling film micxoreactor equipped with a quartz window from the Institut fiir Mikrotechnik Mainz (IMM) was used for efficient gas-liquid contacting. The very reactive endoperoxide intermediate is reduced on-line with thiourea to avoid accumulation. A 20% isolated yield (0.95 g) of the 1,4-diol was obtained. Again, the process time is not given and cannot be estimated from the published data. 

One of the most reactive substrates for endoperoxide formation is 1,3-cyclopentadiene. Rate constants of 1 X 10 (in chloroform) and 3.9 x 10 (in toluene) mol seer have been determined. The kinetics and the solvent dependence of the singlet oxygen [4 -i- 2]-cycloaddition with 1,3-cyclohexadiene have been investigated in detail. - The rate constant decreases about tenfold when the ring size is increased by one methylene unit (Table 25.1). 

Except for the endoperoxide from 1,3-cyclopentadiene, the higher homologues are thermally stable compounds, which have been isolated in moderate to good yields under various photooxygenation conditions. " Cyclic 1,3,5-trienes, which can undergo valence tautomerization, give the respective cycloaddition products (Table 25.2). 

In addition to photolytic or thermolytic conditions, the endoperoxide rearrangement to ihs-epoxides can also be conducted in a catalytic version in the presence of metaloporphyrins (e.g., cobalt(ll) tet-raphenylporphyrin). An illustrative difference between base-catalyzed (Kornblum-DeLaMare reaction) and thermal rearrangement of a cyclopentadiene-derived endoperoxide 13 was described recently. When... 

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