Allylic protection groups

The protected nucleoside-3-phosphoramidite monomer units such as 671 are used in the solid-phase oligonucleotide synthesis. In the 60mer synthesis, 104 allylic protective groups are removed in almost 100% overall yield by the single Pd-catalyze reaction with formic acid and BuNH2[432], N,(9-protection of uridine derivatives was carried out under pha.se-transfer conditions[433]. 

Protection for the double bond in the allyl protecting group may be achieved by epoxidation. Regeneration of the allyl group occurs upon... 

The use of various allyl protective groups in complex molecule synthesis has been reviewed. ... 

Allylic protection groups and their removal through catalytic palladium rr-allyl methodology in transformations of heterocycles 98T2967. [3- -3]Benzannelation of heteroaromatics consisting of successive nucleophilic... 

Allylic protection groups and their removal through catalytic palladium Tr-allyl methodology in synthesis of heterocycles 98T2967. 

The rhodium-catalyzed isomerization of allylamine can be used for the deprotection of A-allyl protective groups (Scheme 7).19 20... 

The allyl protecting group has also been demonstrated to be useful for blocking of side-chain functionalities (36). Thus, the palladium(0)-catalyzed... 

Example 20 diallyl-iV,N-diisopropylphosphoroamidite has been prepared by Bannwarth and Kiing and employed in the phosphitylation of the peptide hydroxy function [49]. The phosphitylation by this reagent proceeds in the presence of tetrazole (step a) followed by CBPA oxidation (step b) and removal of allyl protecting groups in the presence of Pd(0)P(C6H6)3 (step c). 

In a preparation213 of 2,3,4-tri-O-benzyl-D-galactopyranose, allyl protecting-groups were removed from 0-1 and 0-6 of allyl 6-O-allyl-2,3,4-tri-0-benzyl- -D-galactopyranoside, and the allyl group has been used214 for the temporary protection of 0-2, 0-3, 0-4, or 0-6 in benzyl... 

F.Guibf and Y. Saint M Leux, The allyloxycarbonyl group for alcohol protection Quantitative removal or transformation into allyl protecting group via w-allyl complexes of palladium, Tetrahedron Lett. 22 3391 (1981). 

The construction of the maleic anhydride moiety 44 in only five steps starts with the conversion of the amide into an appropriate thioester. Upon treatment with DBU, an aldol-type cyclization occurs to provide the /1-hydroxy thiolactone as a single diastereomer. After removal of the allylic protecting group, dehydration and decarboxylation are carried out simultaneously by simple heating. The thiobutenolide is oxidized to the corresponding thio-... 

Other ways to obtain 3-OH acceptors are the use of mannose precursors with a temporary 3-0-allyl protecting group in the Grignard reaction [16,18,20], or phase transfer benzylation [42] of a heptopyranosyl 2,3-diol, or benzylation of a tin-activated [43] heptofuranosyl 2,3-diol, both last methods giving preferentially the 2-0-benzyl derivative [27,15]. However, if the tin-assisted alkylation is performed on a heptopyranosyl 2,3-diol, the 3-0-alkyl derivative is the main product [5,28]. 

To test the validity of their assumption, Mootoo and Fraser-Reid prepared NPGs 12-18 and treated them with NBS in 1% aqueous acetonitrile [16]. Their results, summarized in O Table 1, showed that differently substituted NPGs could be chemoselectively liberated at the anomeric center to yield hemiacetals 19-24. Furthermore, benzylidene, silyl, p-methoxybenzyl (PMB), ethoxyethyl, and allyl protecting groups proved to be compatible with the conditions employed in the deprotection of the anomeric pent-4-enyl group. Diol 18, however, furnished a complex reaction mixture probably related to competing glycosylation processes, vide infra. 

Konradsson and Fraser-Reid [44] reported the conversion of NPGs into glycosyl bromides, e. g. 76, by treatment of the corresponding pentenyl glycoside with a dilute dichloromethane solution of bromine, conditions that favor unimolecular reaction. The reaction was shown to be compatible with acetals, benzyl, silyl, and allyl protecting groups in the NPG (O Scheme 11a). 

Schmidt and co-workers have also independently developed the self-cleavable diene linker 14 and solid-phase synthesis of the branched hexasaccharide 16 has been achieved (O Scheme 6) [12], The ring-closing metathesis of 15 provided the allyl glycoside 16 from which the allyl protecting group was readily removed by Ir-mediated isomerization followed by hydrolysis. 

Yet a third road has been explored where the plan was to introduce the vinyl ether functionality prior to the formation of the pentacycle. This is accomplished via the use of the allyl protecting group in the phlorogjucinol nucleus (155), leading eventually to the tricyclic acetate (156). This is easily transformable to the tosylate (157), which suffers elimination (diazabicycloundecene) in rather dissapointing yield, to give the vinyl ether (iJ<5)167. Selective removal of the phenolic allyl ether... 

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