Acylations regioselective

Akylsilanes undergo highly regioselective acylation to give P,y-unsaturated ketones (177). Acylation of y,y-dialkylallyltrialkylsilane provides a route to the constmction of difficulty accessible quaternary carbon centers. 

Regioselective Acylation of Hydroxy Compounds. AUphatic diols can be selectively acylated at the primary position by a number of Upases in nonaqueous solvents. For example, PPL suspended in solutions of various diols in ethyl carboxylates catalyzes transesterification in a highly regioselective manner, producing primary monoesters in up to 97% yield (93). Similarly, chloramphenicol [56-75-7] (72) (R = NO2) can be acylated by a number of Upases to produce optically pure, water-insoluble 3-0-palmitate in a highly selective manner (94). 

A number of steroids have been regioselectively acylated ia a similar manner (99,104). Chromobactenum viscosum hpase esterifies 5a-androstane-3P,17P-diol [571-20-0] (75) with 2,2,2-triduoroethyl butyrate ia acetone with high selectivity. The hpase acylates exclusively the hydroxy group ia the 3-position giving the 3P-(monobutyryl ester) of (75) ia 83% yield. In contrast, bacillus subtilis protease (subtihsia) displays a marked preference for the C-17 hydroxyl. Candida iylindracea]i 2Lse (CCL) suspended ia anhydrous benzene regioselectively acylates the 3a-hydroxyl group of several bile acid derivatives (104). 

By taking advantage of the remarkable abihty of subtihsia [9014-01-1] to remain catalyticahy active ia anhydrous dimethylformamide, a number of carbohydrates and other sugar-related compounds have been regioselectively acylated with trichloroethyl butyrate (100). In the case of maltose [69-79-4] (76) and sahcia [138-52-3] (77), for example, acylation occurs exclusively at the C-6 positions. 

One particular feature of ionic liquids lies in their solvation properties, not only for hydrophobic compounds but also for hydrophilic compounds such as carbohydrates. Park and Kazlauskas reported the regioselective acylation of glucose in 99 % yield and with 93 % selectivity in [MOEMIM][BF4] (MOE = CH3OCH2CH2), values much higher than those obtained in the organic solvents commonly used for this purpose (Entry 18) [22] (Scheme 8.3-4). 

Some proteases show an esterase activity, especially in their catalytic activity for regioselective acylation of sugars. By utilizing this property, enzymatic synthesis of polyester containing sugar group in the backbone was demonstrated... 

Treatment of the elimination product 107 with triethylamine resulted in smooth isomerization of the olefin, to afford the a,p-unsaturated ketone 108. Ally lie oxidation of 108 then generated the secondary alcohol 109 in 72 % yield. The acetonide and silyl ether functions of 109 were cleaved in one reaction to afford a tetraol intermediate that was regioselectively acylated at the secondary alcohol functions, to provide the triacetate 110 in high yield (89 %). Hydrogenolysis of the benzyl ether... 

Tamarez, M., Morgan, B., Wong, G.S.K., Tong, W., Bennett, E., Lovey, R., McCormick, J.L. and Zaks, A., Pilot-scale lipase-catalyzed regioselective acylation of ribavirin in anhydrous media in the synthesis of a novel prodrug intermediate. Org. Proc. Res. Dev., 2003, 7, 951-953. 

Based on multiple regioselective acylation via the respective starmylene intermediates, followed by the ester-activated inversion, novel and efficient methods to synthesizing p-D-marmopyranoside and P-D-talopyranoside derivatives can be designed. 

Peri, F. Cipolla, L. Nicotra, F., Tin-mediated regioselective acylation of unprotected sugars on solid phase. Tetrahedron Lett 2000,41, 8587-8590. 

This strategy using rapid automated synthesis of libraries of peptides and fluorescent screening of reactivity has allowed Miller to identify specific peptide-catalysts for specific applications such as the KR of an intermediate en route to an aziridomi-tosane [165,169], the KR of certain fert-alcohols [166], the regioselective acylation of carbohydrates [168], and finally the KR of AT-acylated fert-amino alcohols with s values from 19 to >50 (Scheme 21) [166],... 

The 2-benzoxazolinone moiety is also effective for carboxy-activation as a comparable leaving group. In contrast, the saturated 2-oxazolidinone skeleton fails to show such a high leaving ability. Regioselective acylation of the primary... 

The influence of catalysts (AICI3 and SnCU) acid chlorides, and solvents (dichloroethane, nitromethane) in the acylation of methyl 2-methyl-4/f-thieno[3,2- ]pyrrole-5-carboxylate 75 was studied. Conditions for the regioselective acylation processes were found and four types of compounds 76a-f, 77a-e, and 78 were obtained. 

Figure 6.1 Chemo- and/or regioselective acylation of sucrose (1), salicin (2), and adenosine (3) catalyzed by subtilisin.

Figu re 6.3 Complementary biocatalyzed regioselective acylation of the steroid model 5a-androstan-3P,17 3-diol (10). 

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