Lactonization/benzylic solvents

Reaction of 5-0-Benzyl-l,2-0-isopropylidene-a-D-glucofuranurono-6,3-lactone (45) with Sodium Borohydride in Various Aprotic, Dipolar Solvents to Yield 2-0-Benzyl-3-deoxy-L-[Pg.220]

Oxone has been successfully used in aprotic solvents for oxidation reactions by dispersing it on an alumina surface. Thus, the oxidation of secondary aliphatic, alicyclic and benzylic alcohols using Oxone/wet alumina oxide in CH2CI2 or CH3CN afforded ketones in good to excellent yields (70-96%). Similarly, the conversion of cycloalkanones to lactones is also reported. 

Reactions of the alkaloids lythrine (15 R = H) and lythridine (17) have been studied (Scheme l).17 Mercuric-acetate-induced stereoselective addition to the unsaturated lactone group of lythrine (15 R = H) results in introduction of a methoxy-group at the benzylic / -position to give a product (16) that is epimeric at C-13 with the hydroxylated alkaloids, cf. (17). Benzylic substituents undergo ionization and trapping with solvent to give more stable epf-products cf (17) - (16). 

Dihydropyrans need only a moderate excess of PCC to be converted into unsaturated lactones (Table 8, entry 6)." However, the oxidation of normal allylic and benzylic C— H groups requires a large excess of PCC (Table 8, entries 7 to 9). " The amount of PCC needed is lower if benzene" " or DMSO are used as solvents for oxidation, but the reactions still need to be heated to obtain reasonable conversion. 

Benzylic oxidation. Dihydroisobenzofuran and isochroman are readily converted to the lactones. Solvent effects are manifested in the cleavage of styrenes, since EtOAc favors the formation of aldehydes, whereas in THF, the major products are the benzoic acids. 

Che and co-workers expanded the scope of the Ir-porphyrin complex catalyzed asymmetric C—H bond insertion reaction to intramolecular variants. A catalytic amount of (+)-C5 (L = H2O or solvent) converted an array of benzyl a-aryl-a-diazo esters 91 into ds-p-lactones via intramolecular asymmetric C—H bond insertion reaction in good yields (up to 87%) and enantioselectivity (up to 78% ee) (Scheme 1.30). It is noteworthy that common dirhodium car-boxylate catalysts such as Rh2(S -PTAD)4 or Rh2(S -MEPY)4 could not catalyze this reaction effectively. 

Cation and solvent hydrogen bonding to the hydrophilic phosphate group of the anion BNPPA (l,l-binaphthyl-2-diyl phosphate) (71) leads to bilayers with an interior hydrophilic region and with hydrophobic binaphthyl groups on both exterior sides.A simple thiophosphate-based method for 2-alkylidenation of lactones has been described. Phosphorus-derived chiral auxiliaries for a-alkylation of secondary amines by anodic oxidation has been elaborated. lodotrimethyl-sUane, which is routinely used for the dealkylation of ethers and esters, may be used to iodinate allyl or benzyl diethyl phosphates. ... 

In 1998, Peterson and Larock showed that Pd(OAc)2 in combination with NaHCOs as a base in DMSO as solvent catalyzed the aerobic oxidation of primary and secondary allylic and benzylic alcohols to the corresponding aldehydes and ketones, respectively, in fairly good yields [70]. In both cases, ethylene carbonate and DMSO acted both as the solvent and as the ligand necessary for a smooth reoxidation [71]. Similarly, PdCl2, in combination with sodium carbonate and a tetraalkylammonium salt, Adogen 464, as a phase transfer catalyst, catalyzed the aerobic oxidation of alcohols for example, 1,4- and 1,5-diols afforded the corresponding lactones (Eq. (5.11)) [72, 73]. 

The yield of 2-0-benzyl-3-deoxy-L-/Areo-hex-2-enono-1,4-lactone obtained when 5-0-benzyl-l, 2-0-isopropylidene-a-D-glucofuranurono-6,3-lactone reacted with sodium borohydride (Scheme 105) has been improved by variation of the dipolar aprotic solvent and temperature. Five-membered 2-acetamido-2,3-dideoxy-D-hex-2-enonolactones have been synthesized from 2-acetamido-2-deoxy-D-aldono-1,4-lactones by treatment with dicyclohexylamine in ethanolic... 

Practically the only known solvents at room temperature are fluorinated solvents, such as hexafluoroacetone sesquihydrate and hexafluoroisopropanol. Several strongly polar substances, such as benzyl alcohol, y-butyro lactone, n-methyl-pyrro-lidone, as well as dimethyl formamide dissolve POM at temperatures near its melting range [656]. 

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