Allylic oxygenation

Benzylic or allylic oxygen functions react with Lewis acids such as trifluoroacetic acid to generate allyl or benzylic cations which abstract a hydride from silanes such as triethylsilane 84 b to result in the removal of the oxygen function in a process which has been called ionic hydrogenation and which has been reviewed [34-38]. 

The stereoselectivity is accounted for by a TS in which the allylic oxygen is coordinated to the zinc. 

The Alder-ene cyclization of allylic silyl ethers represents a clever use of cycloisomerization chemistry, as the enol ether products can be easily unmasked to yield aldehydes. Palladium-catalyzed cycloisomerization of 1,6- and 1,7-enynes containing an allylic oxygen most often gives rise to 1,3-dienes (see Section 10.12.4.1). However, enynes of type 63 underwent facile Alder-ene cyclization to the corresponding five- or six-membered rings (Equation (40)) using both [CpRu(MeCN)3]PF6 41 and the Cp analog ([Cp Ru(MeCN)3]PF6, 64).53... 

Iodocyclization to hydroxy tetrahydrofurans. Cyclization of y,8-unsaturated alcohols to tetrahydrofurans (12, 254-256) can be directed by an allylic oxygen substituent, which also can increase the rate of cyclization. Thus derivatives of 4-pentene-l,3-diol undergo iodocyclization mainly to dr-3-hydroxy-2-iodomethylte-trahydrofurans. 

Allylic oxygenations Oxetanes Dioxolanes N-based ligands Dioxygen ... 

The presence of an internal salt, a zwitterion or betaine, in cephalosporins enhances their solubity in water, making such agents particularly suitable for parenteral administration. The preparation of one such dmg first involves the replacement of allyl oxygen in the tert-butylcarbonyloxy protected 7-ACA derivative (23-1) by nitrogen in azaindan (23-2) to afford the betaine (23-3). The protecting group is then removed so as to free the amine on the azetidone (23-4) by treatment with trifluoroacetic acid. Reaction with the thiazole free acid (23-5) in the presence of DCC then affords cefpirone (23-6) [26]. 

The HDO and isomerization reactions were previously described as bimolecular nucleophilic substitutions with allylic migrations-the so-called SN2 mechanism (7). The first common step is the fixation of the hydride on the carbon sp of the substrate. The loss of the hydroxyl group of the alcohols could not be a simple dehydration -a preliminar elimination reaction- as the 3-butene-l-ol leads to neither isomerization nor hydrodehydroxyl at ion (6). The results observed with vinylic ethers confirm that only allylic oxygenated compounds are able to undergo easily isomerization and HDO reactions. Moreover, we can note that furan tetrahydro and furan do not react at all even at high temperature (200 C). 

This olefin oxidative cleavage is in contrast to the allylic oxygenation observed with less oxidizable olefins. Presumably, small amounts of superoxide generated in an inefficient photoreduction initiate a radical chain oxygenation in which allylic activation is ultimately observed, for example, eq. 90 ... 

Table 9 Control of Stereochemistry by Allylic Oxygen Substituents (Equation 38)...

Steric effects similar to those shown in equations (39) and (40) are found when the substitution pattern leads to tetrahydropyran systems through 6-endo cyclization. Cyclizations of systems with an allylic oxygen and a syn alkene substituent give products rationalized by cyclization through H-in-plane conformations as shown earlier in equations (31) and (32).105 128 Examples with allylic methyl substitution have been reported also.1040... 

The bromocyclization of A/,jV-dialkylaminomethyl ethers of allyl and propargyl alcohols to form oxa-zolidinium salts has been reported, but not used in synthesis.255 The heterocyclization of /V-acylamino-methyl ethers with mercury salts has been used for stereoselective synthesis of a variety of 1,2-amino alcohol systems. These cyclizations form rans-4,5-dialkyl oxazolidine products with good to excellent stereoselectivities (equation 120 and Table 33). As shown by entry 5, 6-endo cyclization predominates (6 3) with an internal double bond of ( )-configuration, but this mode of cyclization is reduced with substrates containing a (Z) double bond and/or allylic oxygen substitution (Table 33, entries 6-9). 

Irreversible acetoxyselenenylation of terminal and disubstituted olefins has been achieved on addition of PhSeBr in an acetate-buffered solution. Styrenes afford only Markovnikov adducts, while simple terminal olefins and olefins containing an allylic oxygen substituent (RCO2 or ArO group) furnish 50-80% of the anti-Markovnikov isomer. The product mixture can be isomerized to contain 90-97% of the Markovnikov product by a catalytic amount 6-41%) of BF3.Et20 in CHCI3259. 

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