Deacetylation with piperidine

If (he carboxylic acid concerned is a protected aspartic acid, an N-linked asparagine results, an observation that suggested that the procedure could be used for the preparation of glycopeptides. Accordingly, coupling of NPG 52 and aspartic acid 53 in acetonitrile solvent yields the N,N-diacyl derivative 54, which can be chemoselectively AT-deacetylated with piperidine in DMF to give asparagine 55 [19] (Scheme 13a). 

When a- or 8-D-glucopyranose pentaacetate is treated with piperidine (three molecular proportions are best), deacetylation occurs on Cl and C2, and the product is iV -(3,4,6-tri-0-acetyl-D-glucosyl)piperidine, whose structure was proved by conventional methods, including its conversion into 2-0-methyl-D-glucose. The same triacetate is obtained when piperidine reacts with 3,4,6-tri-O-acetyl-D-glucosyl chloride or with D-glucose 2,3,4,6-tetraacetate. Loss of the acetyl group on C2 must accompany or... 

The synthetic value of the above-mentioned transformation was considerably enhanced when a carboxylic acid, rather than water, was used to trap the pyranosylacetonitrilium ion (O Scheme 12b) [59]. Reaction of 87 with aspartic acid derivative 88, in dry acetonitrile containing NBS, led to a-imide 89 in 61% yield [60,61]. The acetonitrilium ion 90 was trapped by carboxylic acid 88, to give an imidic anhydride 91, which rearranged in situ to give the Af,Al-diacyl derivative 89. The route to 2-acetamido-l-Al-(L-aspart-4-oyl)-2-deoxy-j8-D-glycopyranosyl-amine 92, was completed by selective Ai-deacetylation of 89 with piperidine (O Scheme 12c) [59]. 

Chitosan, derived from crab shell chitin, is —80% deacetylated. It is dissolved in 1 M HO Ac (5 g/L) and freeze dried to yield a white, soft material. The chitosan is washed with 0.9 M A-methylmorpholine (NMM) in DMF followed by DMF. The Rink linker (0.4 mmol) is dissolved in 6 mL of DMF containing N-[(17/-benzotriazol-1 -yl)(dimethylamino)methylene]-A-methyl-methanaminium tetrafluoroborate A-oxide (TBTU) (0.3 M), HOBt (0.3 M), and NMM (0.4 M) and added to 150 mg (dry weight) of chitosan. The mixture is incubated at 45°C for 1 h, washed with DMF, and the chitosan capped with acetic anhydride-dry pyridine (1 1, v/v) for 1 h at 45°C. This procedure yields Fmoc-linker substituted chitin (Fig. 16). After drying in vacuo, the degree of substitution is determined by measuring the Fmoc released after treatment of a sample with piperidine-DMF (3 7) for 30 min at room temperature. Typically, chitosan substitution levels are 0.08-0.35 mmol/g. 

This reaction offers a stereoselective synthesis of a-linked glucosyl asparagine derivatives from both anomers of pent-4-enyl D-glycopyranoside. The pent-4-enyl 2,3,4,6-tetra-0-benyl-a-/p-D-glucopyranoside starting material for this reaction can be prepared easily in two steps from a-/p-glycopyranose [84]. For selective A-deacetylation of the product best results are obtained with piperidine which affords the a-linked asparagine derivate in 89% yield, the yield of the acetamido side product was reduced to 3.5%. The reaction time is also shortened to 5.75 h (when diethylamine was employed, the reaction was complete after 39 h with worse yields). Note that no reaction occurs when diisopropylamine is employed. 

Thus reduction of the 5-thiocyanato group of 151 by zinc (333, 360, 361) or aqueous sodium sulfide (348. 362), hydrolysis of the thiouronium group (7, 363, 364), and deacetylation of the 5-acetylthiothiazole with cold piperidine (365) have been performed to yield the 5-mercapto-thiazole (Scheme 78). It must be pointed out that depending on the experimental conditions, bis(5-thiazolyl(sulfide may be observed as a byproduct (363, 365). Thus 5-amino-4-methylthiazole (152) treated with... 

Synthesis of (+)-desoxoprosopinine [(+)-4] through a key amidoalkylation reaction, which permits stereospecific generation of protected tran5-2,6-disubstituted piperidines, has been achieved (Scheme 6) AUylation of 60, obtained from compound 59, occurred rapidly upon treatment with allyltrimethylsilane in the presence of TiCU to provide 61 in 88% yield, which underwent deacetylation and benzylation to furnish 62. Ozonolysis of the allyl group in 62 followed by olefination of the resnlting aldehyde and subsequent... 

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