Microwave‐Assisted NiCl2 Promoted Acylation of Alcohols

Abstract

A microwave oven acylation of alcohols by carboxylic acid anhydrides has been developed. NiCl₂ has been proven an efficient catalyst for the acylation of primary, secondary, and tertiary alcohols and phenols under microwave conditions.     

Highly Regioselective Synthesis of 3′-O-Acyl-Trifluridines Catalyzed by Pseudomonas cepacia Lipase

3'-O-Acyl-trifluridines were prepared successfully through an enzymatic approach for the first time. Among the ten commercially available lipases tested, Pseudomonas cepacia lipase displayed the highest regioselectivity towards the acylation of 3'-hydroxyl of trifluridine. Furthermore, the effects of some crucial factors on the enzymatic myristoylation of trifluridine were examined. The optimal reaction medium, molar ratio of trifluridine to vinyl myristate and reaction temperature were found to be anhydrous THF, 1:7 and 50 °C, under which the reaction rate, substrate conversion, and 3'-regioselectivity were 63.9 mM/h, >99.0%, and 99%, respectively. Additionally, the enzyme recognition of the chain length of the acyl donors was investigated. The results showed that 3'-regioselectivity of the enzyme maintained 99% with the increment of acyl chain length (C6, C10, and C14). The reason might derive from the strong hydrophobic interaction between 5-CF(3) group of the base moiety and Leu 287 located in the medium-sized pocket of the active site.     

Porcine Pancreas Lipase Catalyzed Ring‐Opening Polymerization of ϵ‐Caprolactone

Abstract

Extensive studies on lipase catalyzed polymerization of ?‐caprolactone showed that porcine pancreas lipase (PPL) gave good conversions and molecular weights of the order of 11,000. Various attempts were also made to prepare higher molecular weight polycaprolactone esters and to increase the molecular weights of polycaprolactone esters by further polymerizing it in the presence of other potent bifunctional monomers. Blends of enzymatically prepared polycaprolactone ester with polystyrene and cellulose acetate yielded very good films, which were characterized in terms of tensile strength, elongation, and optical properties.     

Convenient and Regioselective One‐Pot Solvent‐Free Synthesis of ß‐Hydroxyphosphonates

A simple, efficient, regioselective, and solvent‐free method has been developed for the synthesis of β‐hydroxyphosphonates from epoxides and triethyl phosphite using ZnCl₂ in high yields under mild conditions.     

Practical Synthesis of (S)‐(+)‐3‐Octanol by Lipase‐Catalyzed Enantioselective Acetylation

(S)‐(+)‐3‐Octanol (S)‐1 was prepared in high enantiomeric excess through catalyzed acetylation of racemic alcohol 1 by using lipase from Candida antarctica (Chirazyme L‐2) in the presence of vinyl acetate in toluene at 30°C. The pure (S)‐1 was obtained in 73% isolated yield with 62% conversion. Moreover, acetate (R)‐2 was converted to (S)‐1 via mesylation and followed by hydrolysis using sodium bicarbonate solution in 44% yield.     

Regioselective N‐Arylation of Some Pyrimidine and Purine Nucleobases

N‐Arylation of some pyrimidine and purine nucleobases in the presence of SiO₂ and Cs₂CO₃ under solvent‐free as well as in solution conditions is described. In these conditions, N1‐aryl pyrimidines and N9‐aryl purines have been obtained regioselectively in moderate to high yields with relatively short reaction times.     

Synthesis and Application of Chitosan‐g‐PLLA Copolymers

The purpose of this paper is to study the synthesis and application of a new type of chitosan‐g‐poly(L‐lactide) copolymer with different grafting percentage in the presence of triethylamine. FTIR and ¹H NMR results indicate that grafting percentage of graft copolymers increases with the molar feeding ratio of L‐lactide to chitosan. The measurement of XRD and TG shows that graft copolymer exhibits low crystallinity and thermal degradation temperature. Static water contact angle testing suggests that graft copolymer has superior hydrophilicity compared with PLLA, which can be very useful for biomedical applications. 5‐Fluorouracil loaded copolymer microspheres were prepared by phase separation method. The size and distribution of microspheres were measured by a Laser particle analyzer. The microspheres with LLA:CS feeding molar rotio (15∶1) have a mean diameter of 332 nm with a narrow unimodal distribution. The spherical microspheres were observed by transmission electron microscopy (TEM). The microspheres shows good releasing property from drug release in vitro, and the drug release rate decreases as the increase of microspheres size.     

Highly Regioselective One‐Pot Synthesis of 7‐Hydroxy‐6‐methylphthalide from 3‐Hydroxy‐4‐methylbenzylalcohol

7‐Hydroxy‐6‐methylphthalide 2 was synthesized with high regioselectivity and moderate yield using a novel one‐pot synthesis that employed 3‐hydroxy‐4‐methylbenzylalcohol 1 and formaldehyde in the presence of tin(IV) chloride as catalyst and triethylamine as base. The proposed mechanism of the formation of 2 involves ortho‐formylation followed by hemiacetal formation and oxidation.     

Mild and Efficient Solvent‐Free Friedel–Crafts Acylation of Reserpine

The first mild, efficient, solvent‐free Friedel–Crafts acylation of reserpine has been developed, which resulted in the synthesis of 10‐ and 12‐acylated homologues in high overall yields.     

A Versatile and Convenient Regioselective Acylation of 2′-Deoxynucleosides and Ribonucleosides by Supported Lipases of Pseudomonas Cepacia

Pseudomonas Cepacia lipase supported on ceramic particles (lipase PS-C) and on diatomite (lipase PS-D) regioselectively acylated 2′-deoxynucleosides and ribonucleosides to 3′-O-acetyl-2′-deoxynucleosides and 3′-O-acetyl-ribonucleosides with oxime esters in organic solvents at room temperature. This enzymatic reaction was significant because the regioselectivity was total; as any other regioisomer nor the N-acylated product were observed.     

Regioselective Acylation of 2＇- or 3＇-Hydroxyl Group in Salicin： Hemisynthesis of Acylated Salicins

Salicin-based phenolic glycosides（PGs） are important defensive substances against herbivore feeding and have good bioactivities. In this work, a novel approach for the synthesis of salicin-based PGs has been developed, by which PGs of 2＇-O-acetylsaliein（5a）, 3＇-O-acetylsalicin（gb） and 3＇-O-benzoylsalicin（Sd） were hemisynthesized. The effects of acylation reagent, solvent and temperature on the regioselective acylation of 2＇- or 3＇-hydroxyl groups of salicin mediated by dibutyltin oxide were investigated. The optimal conditions under which the best regioselectivity reached for 5a-5d were discovered, respectively. Moreover, a tentative tin-oxygen coordination mechanism was put forward to explain the different regioselectivities shown under different conditions.     

Direct and Convenient Method of Regioselective Benzylation of Methyl α‐D‐Glucopyranoside

A facile and convenient method for the direct preparation of methyl 2,3,6‐tri‐O‐benzyl‐α‐D‐glucopyranoside (2) by the regioselective benzylation of methyl α‐D‐glucopyranoside (1) with benzyl bromide in the presence of mild bases K₂CO₃ and KOH (1∶1) without solvents is reported.     

A Facile,One‐Step Conversion of 6‐O‐Trityl and 6‐O‐TBDMS Monosaccharides into the Corresponding Formate Esters

A convenient method has been developed for a facile and high‐yield conversion of 6‐O‐tert‐butyldimethylsilyl and 6‐O‐trityl protected monosaccharides to their formate esters, which may serve as useful intermediates for the replacement of the primary hydroxyl group of sugars by other functional groups.     

Synthesis of Mesoionic 4‐(para‐substituted Phenyl‐5‐2,4‐dichlorophenyl)‐1,3‐4‐thiadiazolium‐2‐aminides by Direct Cyclization via Acylation of Thiosemicarbazides

The synthesis of ten novel mesoionic 4‐[para‐substituted (H, CH₃, OCH₃, NO₂, Cl, Br, OH, t‐C₄H₉, C₆H₅, C₄H₉) phenyl‐5‐2,4‐dichlorophenyl]‐1,3‐4‐thiadiazolium‐2‐aminides, as hydrochlorides, are described. The synthesis strategy utilized the corresponding para‐substituted isothiocyanates as starting materials to obtain the thiosemicarbazides through reaction with phenylhydrazine (61–98%), which were then submitted to acylation with 2,4‐dichloro benzoyl chloride and direct cyclization to generate the desired substituted mesoionic compounds in good yields (ca. 80%).     

Stereo‐ and Regioselective Thiolysis of 1,2‐Epoxides in Water

A simple, efficient, stereoselective, and highly regioselective procedure for the synthesis of β‐hydroxy sulfides by thiolysis of various 1,2‐epoxides in water as a solvent, using no catalyst and under very mild conditions, is described.     

Microwave-assisted Regioselective 1-O-Deacylation of Peracylated Glycopyranoses on Alumina

A facile, rapid and regioselective method for the 1-O-deacylation of peracylated glycopyranoses is described which occurs under mild conditions by absorption onto alumina using microwave irradiation.     

Application of Dichlorovinyl Xyloside for the Novel Synthesis of 2,3,4‐Tri‐O‐methyl‐D‐xylono‐1,5‐lactone

A novel synthesis of 2,3,4‐tri‐O‐methyl‐D‐xylopyranose, 4, and its oxidation product 2,3,4‐tri‐O‐methyl‐D‐xylono‐1,5‐lactone, 5, are reported. The new synthesis applies a regioselective Wittig‐like reaction of tetra-O-acetyl-D-xylopyranase, 1, with triphenylphosphine and carbon tetrachloride to yield an O‐dichlorovinyl xyloside protected at C‐1, 2. The protecting group facilitates the permethylation of xylose and is removed under the methylation conditions, to yield tetra-O-acetyl-D-xylopyranase, 3. The anomeric methyl group was removed under mildly acidic conditions to give 2,3,4‐tri‐O‐methyl‐D‐xylopyranose, 4, in good yield. Compound 4 was oxidized using pyridinium chlorochromate to give the title compound, 5, in 95% yield.     

Regioselective Synthesis of Benzofuran‐Annulated Six‐Membered Sulfur Heterocycles by Aryl Radical Cyclization

The tin hydride–mediated cyclization of a number of sulfides and sulfones under mild and neutral conditions has been investigated. The sulfides were in turn derived from 3(2H) benzothiofuranone and 2‐bromobenzyl bromides by phase‐transfer‐catalyzed reaction, and the corresponding sulfones were prepared by treatment of the corresponding sulfides with m‐CPBA at room temperature. The sulfides and sulfones were then reacted with ⁿ Bu₃SnH‐AIBN to afford regioselectively benzofuran‐annulated six‐membered sulfur heterocycles.     

Immobilization of Lipase by Covalent Binding on Crosslinked Allyl Dextran

TowhomcorrespodenceshouldbeaddressedReceivedJanuary15,1998##D1NTRODUCTIONDespitealltheirpotentialadvantages,theuseofenzymesinorganicsynthesisisstillverylimited.Amongthedrawbacksthathaverestrictedtheiruseasorganiccatalystswasthegeneralbeliefthatenzymesonlyworkinaqueoussolutions,whereasorganicsolventsaregenerallyusedasreactionmediainorganicsynthesis.Immobilizationhasbeensuccessfullyusedtostabilizeenzymesworkinginorganic..di.[1--3J,becauseitfixestheconformationofthecatalyticallyactiveenzyme…     

Regioselective Endocyclic Oxidation of Enantiopure 3‐Alkylpiperidines: Synthesis of (3S,5S)‐(‐)‐3‐Ethyl‐5‐Methylpiperidine

An efficient regioselective endocyclic oxidation of enantiopure 3‐alkylpiperidines 1(a–c) with bromine in acetic acid to generate the corresponding 5‐alkylpiperidin‐2‐ones 3(a–c) as main product is described. In addition, starting from 3a or 3b, the synthesis of (3S,5S)‐(‐)‐3‐ethyl‐5‐methylpiperidine 6 · HCl was achieved. Finally, the X‐ray single‐crystal analysis of compound 4 is reported.