Meso-endoperoxides

In this chapter, we attempt to review the current state of the art in the applications of cinchona alkaloids and their derivatives as chiral organocatalysts in these research fields. In the first section, the results obtained using the cinchona-catalyzed desymmetrization of different types of weso-compounds, such as weso-cyclic anhydrides, meso-diols, meso-endoperoxides, weso-phospholene derivatives, and prochiral ketones, as depicted in Scheme 11.1, are reviewed. Then, the cinchona-catalyzed (dynamic) kinetic resolution of racemic anhydrides, azlactones and sulfinyl chlorides affording enantioenriched a-hydroxy esters, and N-protected a-amino esters and sulftnates, respectively, is discussed (Schemes 11.2 and 11.3). 

The enantiomerically enriched 4-hydroxyenones 32 are an important class of chiral building blocks. This class of molecules has generally been obtained by the enzymatic or nonenzymatic catalytic desymmetrization of meso-cyclic diols 31 (or their derivatives) that can be prepared from the meso-endoperoxides 30 [32]. It is also well known that achiral bases such as NEt3 can promote the Kornblum-DeLaMare rearrangement of the meso-endoperoxides 30 [33]. This process is believed to proceed via the E2... 

The preparative usefulness of this protocol was highlighted by the one-pot synthesis of the y-hydroxyenone 41 by the 1,4-dioxygenation of 1,3-cycloheptadiene (40) with singlet oxygen, followed by the 35-catalyzed desymmetrization of the resulting meso-endoperoxides. The desired y-hydroxyenone was obtained in 90% yield with 92% ee (Scheme 11.22). 

Endoperoxide 77 was subjected to UV irradiation under various conditions. Under these conditions five main products, 78,306, 307, 308, and 309, which are produced concurrently in variable proportions, were detected. Perhaps 306 is derived from the meso-diepoxide intermediate 310. This diepoxide has been trapped by inclusion of a large excess of Af-methylmaleimide (NMM) during irradiation of 77 in benzene at 20°C, using excitation of wavelengths... 

The influence of meso-phenyl substituents on the thermal behavior of naphthacenic endoperoxides is illustrated by a diversity of isomerization... 

In 1989, the irradiation of (E,E)-2,4-hexadiene S3 sensitized by meso-porphyrin IX dimethyl ester led to the formation of cis-3,6-dimethyl-l,2-dioxene (62), which was the major product detected at — 78 °C in Freon 11 [69]. Endoperoxide 62 was purified under vacuum at 0.75 mmHg, and collected in a trap (98% isolated yield). Dienes that can adopt a cisoid conformation, such as 53 or ( , )-l,4-di phenyl butadiene, were photooxidized by the corresponding endoperoxides in high or quantitative yield in a suprafacial Diels-Alder reaction [60, 70], Dienes that cannot readily adopt cisoid conformations, such as (fc, Z)-2,4-hexadienes and (Z, Z)-2,4-hexadienes, lose their stereochemistry in the singlet oxygen [2 + 4]-cyclo-addition [60], (E,Z)- and (Z,Z)-dienes give a complex mixture of hydroperoxides and aldehydes, which suggests the intervention of intermediate zwitterions or 1,4-diradicals [71]. 

The thermal stability of the endoperoxides of aromatic compounds enhances going from the, only spectroscopically detectable, endoperoxides of benzene derivatives to those from acenes, which are stable enough to be stored for several days. The main rearrangement is reversion to singlet oxygen and the starting materials and depends on the aromatic system and on the type of substituents in the meso positions when they carry the... 

Similar endoperoxides108 of anthracene189 to hexacene170 and of many meso-substituted aromatic hydrocarbons are known.9,10,188 The nature of the meso substituent has a strong effect on peroxide formation. Endoperoxides are not formed by compounds with cyano, carboxy, or carboalkoxy substituents in the meso position.171 Endoperoxides, e.g., 165, have also been obtained from acenes containing a condensed heterocycle.172-174... 

Rearrangement of 1,4-endoperoxides.1 CoTPP catalyzes the rearrangement of 1,4-endoperoxides to syn-1,2 3,4-diepoxides at -78°. Neither meso-tetraphenylporphine nor zinc meso-tetraphenylporphine catalyzes this rearrangement. N,N,N, N -Tetramethyl-ethylenediamine catalyzes the reaction, but only very slowly. The yields from the catalyzed reaction are much higher than those obtained by thermolysis. 

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