A mild deprotection procedure for tert-butyl esters and tert-butyl ethers using ZnBr2 in methylene chloride

MgI2‐Mediated Chemoselective Cleavage of Protecting Groups: An Alternative to Conventional Deprotection Methodologies

“…[32,33] Moreover,t he cleavage of alloc under palladium-free conditions is unprecedented (Table 1, entries [8][9][10]24), with all amines being generated with excellent conversion and isolated yield. Since Fmoc (entries 1-17), acetamide (entry 29), or ether PGs (entries 14,16,22,23,27) remained unaffected, we observedt hat N-trityl wasa lso cleaved (entry 25), Boc being certainly the most labile N-PGs (entries [19][20][21][22][23]. These resultso pen Scheme1.Selectiver emoval of ethyl ester from Fmoc-Phe-OEt.…”

“…Then, mild non-transition-metal-assisted deprotections ystems of the most usual PGs of amines (trityl, [11,12] Z, [13][14][15] Boc [14,[16][17][18] )o rcarboxylic acids [19] (tert-butyl, [18,[20][21][22][23] benzyl [24][25][26][27][28] esters) were reported,b esides al ow effectivem ethod for allyl ester, [27] and none for the alloc PGs. [29] Here, we reasoned that Lewis acids in aproticm edium needed to be explored in am ore systematic way for their chemoselective potential, amino-acids being indeeda ni deal model of functional group diversity and for racemizations tudies.D uring our campaign to find selectivec onditions to cleave ethyl esters while preserving the Fmoc group, we discovered the impressivep otentialo f MgI 2 ,e nlarginga fterwards the scopet of acile chemoselective cleavages of various standardsP Gs or PS-resins( Figure 1).…”

A new efficient enantioselective synthesis of (+)‐cis‐2‐methyl‐ 4‐propyl‐1,3‐oxathiane, a valuable ingredient for the aroma of passion fruit

“…A simple method for the cleavage of aromatic and aliphatic t-butyl ethers reported by Bartoli 30 gave unsatisfactory results (entry 6) and so also the treatment with ZnBr 2 (entry 7), a mild procedure for deprotecting t-butyl ethers and esters. 31 So, a great difference of reactivity on passing from ethers to thioethers was observed, at least in this transformation, considering that two very efficient methods for the removal of t-butyl group from aliphatic ethers were completely ineffective in the case of sulfide 4b. Only the treatment with trifluoroacetic acid/H 2 O 95/5 in dichloromethane 25,26 allowed to achieve the removal of t-butyl group, even if in moderate yield and long reaction time.…”

tert‐Butyl Ethers: Renaissance of an Alcohol Protecting Group. Facile Cleavage with Cerium(III) Chloride/Sodium Iodide

“…[5] In an analogous manner, the CF 3 COOH, [5] Me 3 SiI [6] and t-BuMe 2 SiOTf [7] based methods also cleave, under the same reaction conditions, other ether functionalities. The ZnBr 2 -promoted method [8] requires a large amount (5 equivs.) of catalyst to cleave unfunctionalized tert-butyl ethers.…”