Carbamates benzyl, reduction

The CTFB group was developed to be orthogonal to the 2-naphthyl carbamate. Benzyl esters and aromatic benzyl ethers can also be reduced in its presence. It is introduced via the chloroformate. It is cleaved by hydrogenolysis with Pd-C in sluggish cases, Pearlman s catalyst should be used. It can be cleaved without reduction of an aromatic nitrile group. 

Many carbamates have been used as protective groups. They are arranged in this chapter in order of increasing complexity of stmcture. The most useful compounds do not necessarily have the simplest stmctures, but are /-butyl (BOC), readily cleaved by acidic hydrolysis benzyl (Cbz or Z), cleaved by catalytic hy-drogenolysis 2,4-dichlorobenzyl, stable to the acid-catalyzed hydrolysis of benzyl and /-butyl carbamates 2-(biphenylyl)isopropyl, cleaved more easily than /-butyl carbamate by dilute acetic acid 9-fluorenylmethyl, cleaved by /3-elimination with base isonicotinyl, cleaved by reduction with zinc in acetic acid 1-adamantyl, readily cleaved by trifluoroacetic acid and ally], readily cleaved by Pd-catalyzed isomerisation. 

From intermediate 28, the construction of aldehyde 8 only requires a few straightforward steps. Thus, alkylation of the newly introduced C-3 secondary hydroxyl with methyl iodide, followed by hydrogenolysis of the C-5 benzyl ether, furnishes primary alcohol ( )-29. With a free primary hydroxyl group, compound ( )-29 provides a convenient opportunity for optical resolution at this stage. Indeed, separation of the equimolar mixture of diastereo-meric urethanes (carbamates) resulting from the action of (S)-(-)-a-methylbenzylisocyanate on ( )-29, followed by lithium aluminum hydride reduction of the separated urethanes, provides both enantiomers of 29 in optically active form. Oxidation of the levorotatory alcohol (-)-29 with PCC furnishes enantiomerically pure aldehyde 8 (88 % yield). 

Reductive cleavage of the thiazoline C-S bond in heterocycle 193 with -Bu3SnH <1986T3537> followed by in situ hydrolysis of the resulting hemiaminal and protection of nitrogen as its benzyl carbamate gave 194 in 64% overall yield. This was then converted in several steps to potent marine neurotoxin Kainic acid 195 in racemic form (Scheme 27) <1994JOC2773>. 

Optimization of the previously reported Mannich-type reaction of trimethyl (pent-2-en-3-yloxy)silane with the sulfone Is derived from phenyl acetaldehyde (Table 5, entry 11) led to the corresponding (3-amino ketone in a good yield with moderate diastereoselectivity (2 mol% Bi(0Tf)3-4H20, yield = 84%, 24v/24v syn/anti = 72 28) (Scheme 8). Reduction of the major diastereoisomer 24v with lithium tri-ferf-butoxyaluminohydride afforded 25 as the only one diastereoisomer. Further cyclization of the latter with NaH afforded 4-benzyl-6-ethyl-5-methyl-l,3-oxazinan-2-one 26. The relative configuration of the six-membered carbamate was established as cis-cis by NMR analysis. 

Japanese chemists (96) have reported the chemical conversion of kobusine into the chloramine (95). The latter was treated with sodium methoxide in methanol to afford compound 96 in which the bridged C-14-C-20 bond was cleaved. Reduction of kobusine with sodium in n-propanol, followed by acetylation afforded compound 88. Treatment of 88 with excess phenyl chloroformate in refluxing o-dichlorobenzene gave the carbamate 89. The latter was hydrogenated over Pd-C in methanol to obtain compound 90 in 94% yield. Further hydrogenation of 90 in the presence of platinum in acidic solution gave 91. Acidic hydrolysis of 91 afforded compound 92. The carbamate 92 was converted to the benzyl derivative 93 by treating with... 

Acyclic C-N bonds of allyl or benzyl triflamides,51 pivalamides and carbamate derivatives9 can be reductively lithiated, as can N-benzyl benzotriazoles.72... 

Benzyl esters, ethers and carbamates can be deprotected in the presence of other easily reducible groups by treatment of the substrate with paHadium(H) acetate and triethylamine in the presence of trie thy lsilane.,nR 109 Competing reduction of bromoarenes, cyclopropanes or alkenes is not observed [Scheme 6.43].110... 

The C33-C37-unit of (-F)-calyculin A (a marine natural product) is an amide derived from 5-0-methyl-4-deoxy-4-dimethylamino-D-ribonic acid that has been prepared by Evans and co-workers [250]. A-Protection of sarcosine as benzyl carbamate affords acid 118 which is activated and used to iV-acylate the (5)-phenylalanine-derived oxazolidinone. This gives 119 that is methoxymethylated diastereoselectively (98 2) to give 120. Reductive removal of the chiral auxiliary, followed by Swem oxidation forms aldehyde 121 with little racemization if... 

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