Deprotections tetrakis palladium

Palladium-catalyzed, Allylic Amination. Allylic substitution of mono-saccharidic hex-2-enopyranoside 4-acetates with secondary amines in the presence of tetrakis (triphenylphosphine)palladium(O) liad led to a large variety of 4-aminated 2-enosides, with retention of configuration (56-58). The method was applied to the disaccaridic enoside 1 to give, with benzylmethylamine or dibenzylamine, the 4-amino sugar derivatives g in yields of 92 and 67% (46). Studies concerning hydrox-ylation of t)ie double bond and subsequent deprotection are incomplete. 

Practical advantage is gained by generating the Pd(0) catalyst in situ from a stable Pd(ll) spedes because tetraki (triphenyIphosphine)palladium(0) is unstable and is best prepared fresh. Gen6t and co-workers developed a water soluble catalyst from palladium(II) acetate and trisodium 33 3"-phosphinetriyl-tribenzenesulfonate (TPPTS) which enables efficient deprotections of Aloe groups under homogeneous conditions as illustrated in Scheme 8.78. Note the preservation of the dimethylallyl ester under these conditions. 

Chiral allylic cyclic carbonates such as 551 or 552 undergo excellent regioselective alkylation reactions with soft nucleophiles in the presence of palladium(O) in refluxing THF to provide ( )-allylic alcohols. The reaction of 4-0-benzyl-2,3-isopropylidene-L-threose (167) with the appropriate ylid, followed by deprotection of the isopropylidene ring with acidic resin and cyclic carbonate formation, provides a good overall yield of either 551 or 552. Ring opening with diethyl malonate in the presence of tetrakis(triphenylphosphine)palladium(0) provides in excellent yield the allylic alcohols 553 or 554, where the diastereoselectivity exceeds 99%. This reaction represents an efficient method of 1,3-chirality transfer [180] (Scheme 122). 

Alternatively, the C5 hydroxy group can be protected as 5-0-allyloxycarbonyl derivative instead (not shown in Scheme 29.6.1). This is done by reaction of abamectin with allylchloroformate and tetraethylendiamine in t-butyl methyl ether. In this case, the C5 hydroxy group can be deprotected in the last step by treatment with sodium borohydride in ethanol in the presence of catalytic amounts of tetrakis(triphenylphosphine)palladium. 

Polycondensation and deprotection. The polycondensations of dibromides 3b and 4b with diboronic acid 5 were done according to standard procedures using 1.0 mol % of palladium tetrakis(triphenylphosphin) as catalyst precursor (10) (Figure 2). 

For the deprotection of the allyl group (Glu(OAll)), prepare a solution containing phenylsilane (PhSiHs) (14 equiv. with respect to the loaded amino acid), Tetrakis(triphenylphosphine) palladium(O) (Pd(PPh3)4) (0.05 equiv. with respect to the loaded amino acid—see Note 16) in THF (30 mL for 1 mmol of loaded amino acid). 

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