Allyl carbonates hydroxylation

Selenium dioxide is also an oxygen donor to alkenes. In this case, however, the initial reaction of the double bond is with the selenium center followed by two pericyclic steps. After hydrolysis of the organo-selenium intermediate, the result is a hydroxylation at the allylic carbon position65. Thus, limonene (2) yields racemic p-mentha-l,8(9)-dien-4-ol66. The high toxicity of selenium intermediates and prevalence of many rearrangements has limited the widespread use of the reagent in synthesis. 

Allyl carbonate esters are also useful hydroxyl-protecting groups. They are introduced using allyl chloroformate. A number of Pd-based catalysts for allylic deprotection have been developed.54 They are based on a catalytic cycle in which Pd(0) reacts by oxidative addition and activates the allylic bond to nucleophilic substitution. Various nucleophiles are effective, including dimedone,55 pentane-2,4-dione,56 and amines.57... 

The evolution of HBr in the bromination reactions and the uptake of one bromine atom per ring indicate substitution at a secondary allylic carbon atom. The ease of oxidation and cross linking of the polymers and the presence of hydroxyl groups imply the intermediate formation of hydroperoxides on allylic carbon atoms. Treatment of the hydroxyl-containing polymer with benzoyl chloride indicates that the bulk of the hydroxyl groups are on secondary carbon atoms, since tertiary hydroxyl groups would tend to be replaced by chlorine. Although these results do not permit the elimination of structure B, it appears that the bulk of the structural units in the polycyclopentadiene corresponds to 1,2- addition (A). 

Microsomal hydroxylation at allylic carbon atoms is commonly observed in drug metabolism. An illustrative example of allylic oxidation is given by the psychoactive component of marijuana. J -tetrahydnx annabinol J -THC. This molecule contains three allylic carbon centers (C-7. C-6. and C-3). Allylic hydroxylation occurs cxten.sively at C-7 to yield 7-hydroxy- j -THC as the major pla.sma metabolite in humans. " Pharmacological studies show that this 7-hydroxy metabolite is as active as, or even more active than. J -THC per se and may contribute significantly to the overall central nervous system (CNS) p.sychotomimctic effects of the parent compound. Hydroxylation also occurs to a minor extent at the allylic C-6 position to give both the epimeric ba- and 6/3-hydroxy metabolites. " Metaboli.sm does not occur at C-3, presumably becau.se of steric hindrance. 

Intramolecular nucleophilic attack at the allyl carbon terminus via this allyl nitroso cationic species is feasible. Examples are represented by compounds 34 and 36, ° whose side-chain carboxylate or hydroxyl termini undergo cyclization in basic conditions to give the cyclized products 35 and 37 in reasonable yields (> 50%). 

Hydroxylation of allylic carbon Formation of epoxides and oxides Formation of diols Formation of triols... 

A benzyl carbonate was prepared in 83% yield from the sodium alkoxide of glycerol and benzyl chloroformate (20°, 24 h). It was also prepared by a lipase-catalyzed ester exchange with allyl benzyl carbonate. It is cleaved by hydrogenolysis (H2/Pd-C, EtOH, 20°, 2 h, 2 atm, 76% yield) and electrolytic reduction (—2.7 V, R4N X, DMF, 70% yield). A benzyl carbonate was used to protect the hydroxyl group in lactic acid during a peptide synthesis." ... 

Removal of the unsaturated side-chain appendage from C-8 in 22 provides diol lactone 23 and allylic bromide 24 as potential precursors. In the synthetic direction, a diastereoselective alkylation of a hydroxyl-protected lactone enolate derived from 23 with allylic bromide 24 could accomplish the assembly of 22, an intermediate that possesses all of the carbon atoms of PGF2o- It was anticipated that preexisting asymmetry in the lactone enolate would induce the... 

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