1.2- Diol, cleavage from epoxides

The required chain extension of 12 was accomplished via deprotonation with NaH and condensation with aldehyde 7 to afford the Diels-Alder precursor 13 in 50% yield. Thermolysis of triene 13 and lactam 3 in xylene at 170 C for four days resulted in the desired cycloaddition to 14. Chromatographic purification permitted isolation of pure 14 in addition to a small amount of an exo isomer (>4 1 ratio). Acid treatment induced cleavage of both the silyl ether and acetonide. Reprotection of the diol and selective epoxidation of the A olefin produced 15 in 64% yield from 12. Epoxide 12 was then transformed to the isomeric allylic alcohol 16 by conversion of the alcohol to the bromide followed by reductive elimination. Protecting-group manipulation and subsequent oxidation the gave aldehyde 17, which was homologated and hydrolyzed to give seco acid 18 in 32% overall yield from 16. 

Earlier work (Vol. 21, p. 218-9) related to the mechanism for DNA cleavage by bleomycin has been reinvestigated by other workers, and the earlier conclusions have been reassessed.290 in a study of the interaction of the diastereomeric 7,8-diol 9,10-epoxides derived metabolically from benzo[a]pyrene and DNA constituents, AMP gave a major product from reaction of 0-2 with the epoxide, as well as products from reaction of the amino group.291... 

It was elaborated in connection with an enatioselective synthesis of D-erythro-sphingosine 132, starting from epoxide 126. There was the intention to use an intramolecular ring cleavage with an N-nucleophile to arrive at the amino-diol pattern present in sphingosine [41]. 

High-valent ruthenium oxides (e. g., Ru04) are powerful oxidants and react readily with olefins, mostly resulting in cleavage of the double bond [132]. If reactions are performed with very short reaction times (0.5 min.) at 0 °C it is possible to control the reactivity better and thereby to obtain ds-diols. On the other hand, the use of less reactive, low-valent ruthenium complexes in combination with various terminal oxidants for the preparation of epoxides from simple olefins has been described [133]. In the more successful earlier cases, ruthenium porphyrins were used as catalysts, especially in combination with N-oxides as terminal oxidants [134, 135, 136]. Two examples are shown in Scheme 6.20, terminal olefins being oxidized in the presence of catalytic amounts of Ru-porphyrins 25 and 26 with the sterically hindered 2,6-dichloropyridine N-oxide (2,6-DCPNO) as oxidant. The use... 

Cleavage of Si—C bonds (12,243-245). This oxidation can be used to convert vinylsilanes in three steps to syn- or anti-1,2-diols. Thus Grignard reagents cleave epoxides of vinylsilanes selectively to (3-hydroxy silanes, which can be oxidized with retention of configuration to 1,2-diols. When applied to an (E)-vinylsilane, the sequence results in the syn-l,2-diol the an -l,2-diol is obtained from a (Z)-vinylsilane by the same reactions. 

Carboxylic ookl cleavage of ecmicyelie and alicyclic epoxides is amply exemplified in the literature. Oxidation of methyleneoyolo-hexane with perforraic acid, for example, yields a 1,2-diol mono-formate, from which may be obtained 1-hydroxymethylcyclohexanol... 

In some cases, oxidation of double bonds does not stop at the epoxide, but proceeds further to oxidative cleavage of the double bond. It was reported that the reaction of a-methyl styrene with H2O2 in the presence of TS-1 or TS-2 produces a-methyl styrene epoxide (15%), a-methyl styrene diol (10-40%) and acetophenone (40-60%) (Reddy, J. S. et al., 1992). However, results similar to those obtained with titanium silicates were obtained for many other catalysts, such as HZSM-5, H-mordenite, HY, A1203, HGa-silicalite-2, and fumed Si02. These materials have much different properties and differ significantly from titanium silicates thus, the results cast some doubt on the role of the catalyst in this reaction. Furthermore, the oxidation of styrene is reported to proceed with C=C cleavage and formation of benzaldehyde, in contrast to previous reports of the formation of phenylacetaldehyde with 85% selectivity (Neri et al., 1986). 

Peracids themselves produce epoxides and diols from alkenes but are not powerful enough to oxidize these further by cleaving the carbon-carbon bond. However, in the presence of transition metals they will cleave alkenes and diols to give, usually, carboxylic acids. For example, peracids in combination with ruthenium compounds are well known in this capacity (Figure 3.31).146,147 Warwel and co-workers have reported the cleavage of alkenes using peracetic acid and the ruthenium catalyst, Ru(acac)3.148... 

Chiral all-syn-l,3-polyols. A reiterative route to these polyols from an optically active epoxide (1) involves ring opening with a cuprate derived from vinyllithium and copper(I) cyanide (11, 366-367) to give an optically active homoallylic alcohol (2). This is converted into the fepoxide (4) via a cyclic iodocarbonate (3) by a known procedure (11, 263). Repetition of the cuprate cleavage results in a homoallylic 1,3-diol (5). The ratio of desired syn- (o anfi-diols is 10-15 1. This two-step sequence can be repeated, with each 1,3-diol unit formed being protected as the acetonide. The strategy is outlined in Scheme (I). 

The CUps-O epoxide 33-41 and related substrates are currently the only available selective fluorogenic substrates for EH. On the other hand various indirect assays have been reported to detect either the unreacted epoxide [46] or the carbonyl product resulting from periodate cleavage of the 1,2-diol product [26,47, 48], These assays are suitable for fluorescence or colorimetric assays for the hydrolysis of any epoxide of interest... 

Acid-catalyzed epoxide cleavage takes place by back-aide attack nucleophile on the protonated toxide in a manner analogous to the step of alkene bromtnation, in ndiich a cyclic brentORtunt ion is opened nucleophilic attack (Section 7.2l. When an opoxycycloalkane is opened aqueous acid, a frona-t -diol results just as a traR5 l,2-dibroiiiide from cycloalkene hrominaUun. 

Anhydrous Fe Cl3 catalyzes the stereospecific epoxidation of norbomene, the demethylation of A, A-dimethylaniline, and the oxidative cleavage of PhCMe(OH)CMe(OH)Ph (and other a-diols) by hydrogen peroxide (Table 11 and Scheme 4). For each class of substrate, the products parallel those that result from their enzymatic oxidation by cytochrome P-450. The close congruence of the prodncts indicates that the reactive oxygen in the Fe Cl3/HOOH model system and in the active form of cytochrome P-450 is essentially the same, with strong electrophilic oxene character (stabilized singlet atomic oxygen). 

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