Deacylation regioselective

In contrast to castanospermine, 1-deoxynojirimicine 160 contains a primary hydroxy group as well as a much more nucleophilic amino function. If a small excess of trifluoroethyl butanoate is employed, subtilisin converts this alkaloid preferably into the 6-monoester 161 (Fig. 18-22)1266). However, with 6 equiv. of the acylating agent, the 2,6-diester 162 is formed in 77% yield. This diester 162 may be subsequently deacylated regioselectively at the 6-position by means of several different enzymes. 

CHC(=0)Me, BuNH2, MeCN, reflux, 24 h 16% change in regioselectivity and preservation from deacylation may perhaps be explained by the possibility for Schiff base formation of the synthon and product, respectively, in... 

It has been established" that the leaves of Liriodendron tulipifera convert 1-deoxy-D-xylulose (285) into 2-C-methyl-D-erythritol (286) via a skeletal rearrangement (see Scheme 90) reminiscent of the formation of terpene precursors from 1-deoxy-D-xylulose 4-phosphate. An esterase-catalysed regioselective 6-deacylation of... 

Figu re 6.7 Regioselective deacylation of sucrose octaacetate (27) catalyzed by different hydrolases. 

Figure 6.8 Regioselective deacylation of the polyacetilated flavonoid quercetin (28a) by action of different lipases (CalB, lipase from Candida antarctica Mml, lipase from Mucor miehei f-BME, ferf-butyl methyl ether)...

Regioselective deacylation has also been observed189 in the acid-catalyzed transesterification of some 3, 5 -di-0-acetyl-2 -deoxy-2 -halo-uridines in methanol to give the corresponding 3 -0-acetyl derivatives in yields of 36-66%. The presence of the 2-halogen substituent seems essential for the selectivity in the case of 2, 3, 5 -tri-0-acetyl-uridine, no significant differences in the reaction rates of the acetyl groups towards transesterification were apparent. 

Deacylation of ribonucieosides.1 This salt (1) is useful for regioselective 2 -O-deacylation of 2. 3, 5 -triacyl derivatives of ribonucieosides. Hydra/inium acetate is somewhat less effective. 

The H202-HF-SbF5 system has been applied by Jacquesy and co-workers in the hydroxylation of a variety of functionalized arenes.624 Hydroxylation of phenyl esters has been shown to afford themeta and para isomers as main products625 [Eq. (5.217)]. Substantial amounts of the deacylated derivatives were obtained in the reaction of phenyl formate and diphenyl carbonate. In the hydroxylation of 2-chlorophenyl and 4-chlorophenyl acetate, regioselectivity is controlled by the chlorine substituent with the hydroxyl entering into the meta position to the ester group.626 A similar effect was observed in the hydroxylation of anilines and anilides. 

The enol of 35 adds to the diazonium salt 36 and is deacylated in KOH to give the hydrazone 38 that isomerises to the enamine 39 for the Fischer indole to give the indole (/V-formyl 30) that is finally hydrolysed to 30 in aqueous HC1. The remarkably high regioselectivity in the cyclisation may be due to steric hindrance it is difficult to insert a substituent between two others. 

The already mentioned hydrolase-catalyzed regioselective acylations and deacylations of a wide array of polyhydroxylated compounds has been addressed in numerous cases [13]. 

The isoxazolyloxazoline anion 63 and l-methoxycarbonylpyridinium chloride (62) give a mixture of 1,2- and 1,4-addition products. Oxidation of the intermediate dihydropyridines with p-chloranil provides the substituted pyridines 64 and 65 (85JOC5660). IV-Ethoxycarbonylpyridinium chloride (67) reacts with 2-trimethylsilylthiazole (66) regioselectively to give a 1,2-dihydropyridine intermediate, which on oxidative deacylation affords 2-substituted pyridine 68. In a similar manner, 4-methyl-2-trimethylsilyloxazole and 67 provide the corresponding 2-heteroarylpyridine (84TL3637). 

Thermolysis of 24 at 180-190°C for 6 days gave a 35% yield of the desired endo product 35a contaminated with the exo isomer 35b. The ratio of these adducts was quite acceptable (4 1 endo exo. With the basic cytochalasin ring system in hand, what remained was the introduction of the required unsaturation adjacent to the lactone and oxidation of the cyclohexene ring. The former was achieved in 70-76% yield by selenylation followed by oxidative elimination to afford solely the ( )-olefin 36. Hydroxylation was initiated by nitrogen acylation followed by stereo- and regioselective epoxidation of the A olefin. Deacylation and desilylation then afforded cytochalasin F (37) in 51% yield. Cytochalasin B (20) was obtained in a similar way by isomerization of the intermediate epoxide to the allylic alcohol with aluminum isopropoxide, followed by deacylation and desilylation. 

When acylations of nucleosides with acid anhydrides in the presence of lipase from Pseudomonas fluorescence (PFL) in DMF or DMSO as the solvent first proceeded, the regioselectivity was unsatisfactory 1252l However, this lipase together with subtilisin can be utilized to effect highly specific deacylations of various pyrimidine nucleosides 145 (Fig. 18-18) 1253l Thus, lipase from Pseudomonas fluorescence (PFL) preferably attacks the hexanoyl group on the secondary hydroxy function of the N-... 

Thus, the coumarine 180, the chromanone 181, the chalcone 182, the flavanone 183 as well as several flavones, e. g. 183 and 185 were regioselectively deacylated by employing different lipases in organic solvents (Fig. 18-24). Porcine pancreatic lipase (PPL) predominantly attacks one of the phenolic acetates present in 180-183 with... 

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