Allyl group cleavage

Three selective methods to remove protective groups are receiving much attention assisted, electrolytic, and photolytic removal. Four examples illustrate assisted removal of a protective group. A stable allyl group can be converted to a labile vinyl ether group (eq. 4) a /3-haloethoxy (eq. 5) or a /3-silylethoxy (eq. 6) derivative is cleaved by attack at the /3-substituent and a stable o-nitro-phenyl derivative can be reduced to the o-amino compound, which undergoes cleavage by nucleophilic displacement (eq. 7) ° ... 

The allyl group was used to protect the nitrogen in a /3-lactam synthesis, but was removed in a four-step sequence. Whether a transition-metal-catalyzed isomerization to the enamide followed by hydrolysis is an effective cleavage procedure remains to be tested and warrants further study. ... 

Osmium tetroxide used in combination with sodium periodate can also effect alkene cleavage.191 Successful oxidative cleavage of double bonds using ruthenium tetroxide and sodium periodate has also been reported.192 In these procedures the osmium or ruthenium can be used in substoichiometric amounts because the periodate reoxidizes the metal to the tetroxide state. Entries 1 to 4 in Scheme 12.18 are examples of these procedures. Entries 5 and 6 show reactions carried out in the course of multistep syntheses. The reaction in Entry 5 followed a 5-exo radical cyclization and served to excise an extraneous carbon. The reaction in Entry 6 followed introduction of the allyl group by enolate alkylation. The aldehyde group in the product was used to introduce an amino group by reductive alkylation (see Section 5.3.1.2). 

Products of this type were not isolated, however, when the non-immobilised metathesis substrates were allyl esters or ethers. In these cases, the combined effect of metathesis and TFA cleavage was simply to remove the allyl group. A modified protodesilylation mechanism was proposed to account for these results (Scheme 3). 

The third method was based on the reductive cleavage of allyl groups from TV-allyl-V-alkylanilines, employing lithium aluminum hydride and nickel chloride.269 Treatment of 230 with these reagents gave a mixture of products, identified as 231-234. 

Malonate was used in the synthesis,2 and it makes sense to add the more reactive alkyl group last in this case the allyl group. The oxidative cleavage was done with ozone. 

We have used a simple allyl group so far but, as the other half of the alkene is lost in the cleavage reaction, it doesn t matter much what it is. So, with a,(3-unsaturated carbonyl compounds 31 and 34, which give rise to 1,2-diCO compounds 32, 33 or 35, it is convenient to use aldol reactions to make the alkene, and, say benzaldehyde is easier to use than formaldehyde. 

Side Reactions. A side reaction that often accompanies the rearrangement of substituted allyl ethers is the cleavage of the allyl group from the oxygen with the formation of a phenol and a diene the cleavage reaction is favored by increased substitution in the allyl group.68 6 i 6 i 61 Thus, a, 7-dimethylallyl 4-carbethoxyphenyl ether (LI) gives a 59% yield of... 

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