Deprotection of allyl groups

Scheme 14 Formation of an Allylammonium Species in the Deprotection of Allylic Groups with Secondary...

Scheme 16 Deprotection of Allylic Groups with Dimedone or iV,iV-Dimethylbarbituric Acidl l...

Deprotection of allyl groups from carboxylic allyl esters is also possible using these conditions [31, 33]. In a homogeneous CH3CN/H20 medium, the facility of cleavage of the allyl group follows the order allyl > cinnamyl > dimethylallyl. 

The Baran group has reported an unusual deprotection of allyl esters in micro-wave-superheated water. A diallyl ester structurally related to the sceptrin natural products (see Scheme 6.87) was cleanly deprotected at 200 °C within 5 min (Scheme 6.168) [181]. Other standard deprotection transformations carried out under microwave conditions, specifically N-detosylations [317], trimethylsilyl (TMS) removal [318, 319], and N-tert-butoxycarbonyl (Boc) deprotection [231], are summarized in Scheme 6.169. 

The selectivity and mildness of the Pd(0)-catalyzed deprotection of allyl (All) esters and the allyloxycarbonyl (Aloe) urethanes117 181 reaction also allowed for the successful and efficient application of this blocking group technology in the synthesis of acid- and base-labile lipidated pepti-... 

The reaction mechanism (Scheme 6.25) involves formation of a cationic 7t-allylpalladium complex by the oxidative addition of the substrate onto the catalyst. In case of a dimethylallyloxycarbonyl protecting group this step is disfavoured compared to Alloc and therefore the removal of dimethylallyl groups is slower or requires more catalyst. Accordingly, in homogeneous CH3CN/H2O solutions deprotection of (allyl)phenylacetate proceeded instantaneously with 2 mol % [Pd(OAc)2]/TPPTS while it took 85 min to remove the dimethylallyl group (cinnamyl is an intermediate case with 20 min required for complete deprotection). The reactivity differences are... 

Example 22 removal of allyl group attached to a phosphorus centre with Pd, Pt and Rh complexes is a well established procedure [51] but is inconvenient for synthesis of therapeutic compounds on a large scale. During the deprotection step the palladium catalyst is susceptible to poisoning especially with P-S compounds resulting in loss of catalytic efficiency. Furthermore traces of organometallic compounds remain in the product after deprotection. In the paper of Manoharan et al. other methods of deprotection of allyl... 

Deprotection of allyl or allyloxycarbonyl derivatives of amino acids.1 This combination of reagents effects rapid reductive cleavage of allyl or allyloxycarbonyl derivatives of amino acids in CH2C12 containing a proton donor (p-N02C6H50H, acetic add, H20). The actual catalyst is probably bis(triphenylphosphine)palladium(0). Benzyl, Boc, and Cbo groups are stable to these conditions. This cleavage does not induce racemization. 

Guibe and co-workers took advantage of this high reactivity to use N,N -dimethylbarbituric acid 231 (Scheme 47) as an efficient nucleophilic reagent in the deprotection of functional groups protected with allyl radicals. The recovery of benzylamine from its diallyl derivative 232 is one example (93JOC6109). 

Fleury has described a similar method, in which N-acetyl-N-allyl-glycosylamines protected either with acetyl or with trimethylsilyl groups are grafted by hydrosilylation [23]. Deprotection of trimethylsilyl groups was made by the same method as in Section 5.5. 

A large variety of nucleophiles may be used as allyl group scavengers in the palladium-catalyzed deprotection of allylic derivatives, including oxygen, nitrogen, carbon, or sulfur nucleophiles, and hydride donors. 

Protection and deprotection of functional groups. Deacetylation as well as allylic... 

The palladium-catalyzed deprotection of allyl-based protecting groups has emerged as an important tool in organic synthesis.194 Consequently, special... 

Deprotection of allyl ester in 467 with HC02H/Et3N was highly chemoselec-tive. In the coexistence of a 2,4-enyne system in 467, at first 7r-allylpalladium carboxylate 468, formed from the allyl ester, attacked the triple bond to form the butenolide 469 without reacting with formate. Hydrogenolysis with formate took place only at the final step to afford 470 chemoselectively. Also, the alkenyloxirane group, which is allylic epoxide, stayed intact [175],... 

Deprotection of allyl ethers is usually effected by base-mediated isomerisation of the allyl double-bond to produce the corresponding enol ether, e.g. Figure 2.21 [35] that can then be readily hydrolysed under conditions that leave most acid-sensitive functionality intact. Transition metal catalysts offer an alternative to bases for the initial isomerisation these reagents offer the possibility of isomerisation and hydrolysis in a one-pot process (Method 11). Selectivity between different types of allyl protecting group is possible as the example in Figure 2.22 shows [36]. 

V.2.3.2 Palladium-Catalyzed Deprotection of Allyl-Based Protecting Groups... 

V.2.3.2 Pd-CATALYZED DEPROTECTION OF ALLYL-BASED PROTECTING GROUPS... 

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