O-Protective groups, removal

A pentopyranoside-fused butenolide is the key intermediate for the synthesis of the natural micotoxin patulin [226, 227]. Its synthesis involves Wittig olefination of a 3,4-di-O-protected arabinopyran-2-uloside, followed by protecting group removal and dehydration (Scheme 47). In other research, the glucopyranosid-2-uloside 190 was converted into the butenolide derivative 191 by aldol condensation with diethyl malonate and transesterification [228]. The latter was shown to be prone to autoxi-dation, leading to 192. Subsequent Michael addition with hydroxide ion, followed by decarboxylation, furnishes C-branched-chain sugar 193. 

Benzyl -o-xylopyranoside was converted into the alcohol 54 (a somewhat capricious isopropylidenation) [39] and a Mitsunobu inversion with N-hydroxyphthalimide, followed by protecting group removal, gave the hydro-xylamine 55. Transfer-hydrogenation (ammonium formate and palladium-on-charcoal in refluxing methanol) [40] then gave, on a small scale and in almost a quantitative yield, the enantiomer of the desired tetrahydro-l,2-oxazine 52. We have never been able to repeat this result since Figure 4 shows the NMR spectra acquired at the time [41]. 

As has been demonstrated, 2-(trimethylsilyl)ethoxymethyl (SEM) esters are selectively removed from amino acids and peptide derivatives in the presence of the most common N- and O-protecting groups applied in peptide chemistry including the urethane-type Boc, Z, Fmoc, and Troc as well as Bzl, tBu, and TBDMS ethers.The SEM ester is removed by acidolysis or with a fluoride ion source, e.g. TBAF in THF or HMPA or with aqueous HF in MeCN (—10°C).f l Deprotection with magnesium bromide in EtjO represents an even milder alternative that allows increased selectivity toward fluoride-labile silyl ethers or Fmoc groups. The SEM esters are prepared in 60-80% yield by stirring a mixture of 0.25 M N-protected amino acids in DMF with 0.8 equivalents of SEM-Cl and 1.1 equivalents of lithium carbonate at room temperature for 16 hours. 

Condensation of the D-galactopyranosyl-derived phosphonium salt 60 and aldehyde 61 provides the unsaturated dimer 62 after desilylation and oxidation. Iteration of the sequence twice with phosphonium salt 60 leads to tetramer 63 with, however, low coupling efficiencies (36% for the trimer and 11% for the tetramer) mostly due to the base-induced a,/3-unsaturation of the aldehydic substrates. Protecting group removal and hydrogenation of the carbon-carbon double bonds furnish tetramer 64, a C-mimetic of the /3-(1 6) tetragalactoside similar to the one shown in O Scheme 3. 

Nakajima, N., Tanaka, T, Hamada, T, Oikawa, Y, and Yonemitsu, O., Highly stereoselective total synthesis of pikronolide, the aglicon of the first macrolide antibiotic pikromycin. Crucial role of benzyl-type protecting groups removable by 2,4-dichloro-5,6-dicyanobenzoquinone oxidation, Chem. Pharm. Bull., 35, 2228, 1987. 

In basic media, 1,3-dimethyluracil transferred an a, /8-unsaturated keto moiety to the 3-aminopyrazol-4-yl C-nucleoside 376, providing the complementary carbon fragment to annulate the pyrimidine ring of 377. Removal of the O-protective groups of 377 with methanolic hydrochloric acid caused... 

N-Debenzylation of ammonium salts, quaternary 14,404 O-Debenzylation 24, 723 -, selective 24, 8, 17, 43 Deblocking s. Protective groups, removal... 

Wolff-Kishner - HCitO.v.i. removal of O-protective groups by - 26,177... 

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