Allyl carbonates oxidative rearrangement

The Cope rearrangement of 24 gives 2,6,10-undecatrienyldimethylamine[28], Sativene (25j[29] and diquinane (26) have been synthesized by applying three different palladium-catalyzed reactions [oxidative cyclization of the 1,5-diene with Pd(OAc)2, intramolecular allylation of a /i-keto ester with allylic carbonate, and oxidation of terminal alkene to methyl ketone] using allyloctadienyl-dimethylamine (24) as a building block[30]. 

Oxidation of the allylic carbon of alkenes may lead to allylic alcohols and derivatives or a, 3-unsaturated carbonyl compounds. Selenium dioxide is the reagent of choice to carry out the former transformation. In the latter process, which is more difficult to accomplish, Cr(VI) compounds are usually applied. In certain cases, mixture of products of both types of oxidation, as well as isomeric compounds resulting from allylic rearrangement, may be formed. Oxidation of 2-alkenes to the corresponding cc,p-unsaturated carboxylic acids, particularly the oxidation of propylene to acrolein and acrylic acid, as well as ammoxidation to acrylonitrile, has commercial importance (see Sections 9.5.2 and 9.5.3). 

The process was later improved by the use of a p-toluenesulfonyl substituent at the allylic carbon atom (equation 42). The authors claim that this modification has a powerful influence on both the selectivity and mechanism of the oxidation, exclusive oxidative rearrangement then being observed. Several other methods of achieving allylic oxidation using palladium catalysts have also been reported,although these are generally of less importance. 

However, the conversion to the transposed a,p-unsaturated carbtxiyl compound is by far the more useful reaction. The fiill sequence serves both to form carbon-carbon bonds as well as to adjust the functional group array in tlw synthetic intermediate. Thus, starting with the enone (15), organometallic addition generates a tertiary allylic alcohol (16) and oxidative rearrangement yields a P-a]kyl-a,p-enone... 

This weakening of the bond between the allylic carbon and the neighboring atom can lead to bond rupture as illustrated in the photoisomerization of 2H,6//-thiin-3-one-l-oxides to 3//,7H-l,2-Oxathiepin-4-ones [14], These seven-membered ring systems undergo further thermal ring contraction to thietanones via a 2,3-sigmatropic rearrangement followed by a Pummerer reaction (Scheme 10). 

Carbon-Oxygen Bond Formation. CAN is an efficient reagent for the conversion of epoxides into /3-nitrato alcohols. 1,2-cA-Diols can be prepared from alkenes by reaction with CAN/I2 followed by hydrolysis with KOH. Of particular interest is the high-yield synthesis of various a-hydroxy ketones and a-amino ketones from oxiranes and aziridines, respectively. The reactions are operated under mild conditions with the use of NBS and a catalytic amount of CAN as the reagents (eq 25). In another case, N-(silylmethyl)amides can be converted to A-(methoxymethyl)amides by CAN in methanol (eq 26). This chemistry has found application in the removal of electroauxiliaries from peptide substrates. Other CAN-mediated C-0 bondforming reactions include the oxidative rearrangement of aryl cyclobutanes and oxetanes, the conversion of allylic and tertiary benzylic alcohols into their corresponding ethers, and the alkoxylation of cephem sulfoxides at the position a to the ester moiety. 

A related oxidative rearrangement of cephem dioxides has been reported in which an alkene is oxidized steteospecificaUy with rearrangement to the allylic alcohol in good yield by simple exposure to a palladiutn/carbon catalyst, as depicted in equation (12). Adventitious oxygen preadsorbed on the catdyst seems the likely oxidant The reaction fails on the patent cephem or its monoxide, or on the firee acid of the dioxide. This reaction would seem to hold some promise for further utility in the cephem field and otiwr related systems. 

If the insertion step following oxidative addition occurs on one of the two fragments resulting from oxidative addition, an intramolecular catalytic reaction (C—O — C—C rearrangement) takes place (example 40, Table III). It is interesting to note that two different products—2,6- and 3,6-heptadienoic acids—can be obtained from allyl 3-butenoate. Their ratio can be controlled by adding 1 mole of the appropriate phosphine or phosphite to bis(cyclooctadiene)nickel or similar complex. Bulky ligands favor the 2,6 isomer. It is thus possible to drive the reaction toward two different types of H elimination, namely, from the a or y carbon atoms. 

Products from the electrochemical oxidation of cyclohexene (Scheme 2.1) illustrate the general course of reaction [28, 29]. The radical-cation either undergoes loss of an allylic proton or reacts, at the centre of liighest positive charge density, with a nucleophile. Either reaction leads to a carbon radical, which is oxidised to the carbonium ion. A Wagncr-Meerwein rearrangement then gives the most stable carbonium ion, which subsequently reacts with a nucleophile. 

In the course of investigations on allylic and acetylene-allene rearrangements of 3-substituted quinuclidines, it was found that by oxidation and ozonolysis of compounds with functional groups at positions allylic to the double bond, not only the double bonds but also the adjacent carbon-carbon bonds are broken. For example, in the oxidation of 3-hydroxy-3-vinylquinuclidine (119), with potassium permanganate under mild conditions, and in its ozonolysis, qui-nuclidin-3-one (2) is formed along with 3-hydroxy quinuclidine-3-carboxylic acid.161 The positions of double bonds in such systems can be firmly established by NMR spectroscopy, but not by oxidative methods.101... 

---