A Reagent for Selective Deprotection of Alkyl Acetates

Abstract

The use of magnesium methoxide for selective deprotection of alkyl esters is described. By adjusting the equivalents of the magnesium methoxide reagent, it is possible to selectively cleave primary acetate in the presence of secondary and tertiary acetate and to cleave secondary acetate in the presence of tertiary acetate. A high selectivity can also be obtained for the same primary acetates if the β-positions of the acetates render a different steric bulkiness. This mild reagent has been successfully applied to the selective deprotection of many natural-occurring molecules including hydroxycitronnellol diacetate, trans-sobrerol diacetate, betulin diacetate, and baccatin III.

A New, Non-Iterative Asymmetric Synthesis of Long-Chain 1,3-Polyols

“…The CD spectrum of (À)-24 ( Figure 1 and Scheme 4) showed, as expected for all possible conformations of the cyclohept-5-en-1,3-diyl bis(pScheme 3. methoxybenzoate) system, a double Cotton effect (De 262 À 30, De 243 3) that results from the exiton coupling between the two aromatic chromophores to produce a negative couplet. [24] This interpretation is consistent with the fact that the point of inflection of the CD curve is close to l 253 nm, the wavelength of maximum absorption in the UV spectrum of (À)-24.…”

“…Selective methanolysis of the acetate was possible on treating (À)-10 with Mg(OMe) 2 in methanol. [24] This generated tetrol ()-16 in 70 % yield, the oxidation of which with NaIO 4 in aqueous dioxane afforded a 3:2 mixture of a-and b-pyranose (À)-18 in 98 % yield (Scheme 3). The 2 D NOESY 1 H MNR spectrum of (À)-18 proved that the H4 and H6 protons are axial, thus demonstrating that epimerisation had not occurred at C4 and C6.…”

Chichibabin and IsoChichibabin Pyridinium Syntheses of Isodesmosine, Desmosine, and their Derivatives

“…Condensation using 1‐ethyl‐3‐(3‐dimethylaminopropyl) carbodiimide hydrochloride (EDC · HCl) with 1‐hydroxybenzotriazole hydrate (HOBt · H 2 O) between 11 and alanine derivative 12 gave 13 in 82 % yield. We attempted removal of the acetyl group of 13 using K 2 CO 3 in MeOH, KCN, guanidine, magnesium in MeOH, and PPL (porcine pancreatic lipase) in pH 6.9 buffer; however, none of these conditions afforded the desired 14 and instead either the starting material recovery or the Boc group removal occurred. Use of DIBAL‐H and 1 % I 2 /MeOH provided a trace amount of 14 (data not shown) …”

SAR Studies of the Leupyrrins: Design and Total Synthesis of Highly Potent Simplified Leupylogs

“…[13] Subsequently,r emoval of the tert-butyl ester with TMSOTf/lutidine, [11] conversion of derived crude acids to active esters 28 and 29 andc yclodehydration [3] with amine 12 gave oxazolines 30 and 31 in high yield. In contrast to previously reported route (Scheme 1), applicationo fc onditions for cleavage of tert-butyl ester (TMSOTf)a nd acetate esters (K 2 CO 3 )w ere unreliable or led to decompositiono f30 and 31.A fter considerable attempts, ad eprotection sequence incorporating magnesium methoxide [14] for acetate cleavagea t 0 8Ca nd zinc bromide [15] in dichloromethane at room temperature for tert-butyl removal enabled ar eliable access to secoacids 32 and 33.S ubsequent Shiina macrolactonization gave macrocycles 34 (73 %) and 35 (94 %) in ag ood and excellent yield, respectively.F inally,r emoval of the remaining primary TBS-group proceeded smoothly by utilizing TBAF in THF at 0 8Ct oo btain leupylogs 4 in 96 %a nd 36 in 61 %y ield, which provedtob em ore reliable as compared to TASF.…”