2.4.6- Tris phenyl protection

Recently McDonald et al. reported the synthesis of (1 —> 5)-linked D-mannoseptanosyl di- and trisaccharides.80 Once again they utilized differentially protected phenyl 1-thio-a-D-mannoseptanoside as donor and methyl a-D-mannoseptanoside as acceptor to synthesize the di- and tri-a-septanosides exclusively. The strategy used for their synthesis mirrored that depicted in Scheme 26. The septanoside structures and their properties in a glycosidase assay are presented in Section IV. 

Both the square planar bis(acetylacetonato)manganese(II) and the octahedral tris(acetyl-acetonato)manganese(III) have been shown to possess marked fungicidal activity in the protection of cotton and linen.49 Recently, the complex of manganese(II) with 4-(5 -phenyl-l, 3, 4 -oxadiazol-2 -yl)thiosemicarbazide (14) has also been shown to exhibit fungicidal properties.50 The structure of this bis-complex also appears to be unknown at present. 

This section will be fairly brief because silabenzene and its congeners are unstable under normal conditions, and can be studied only at low temperatures or by trapping in solid matrices. Only when Tokitoh and coworkers introduced bulky and effective steric protecting groups such as tris[bis(trimethylsilyl)methyl]phenyl or Tbt groups was it possible to obtain kinetic stabilization and study silabenzene and germabenzene derivatives as stable crystalline compounds under normal conditions [218-221],... 

Access to 1,4-anhydro-D-alditols by radical reduction has received only limited attention, but the most detailed work concerns the reduction of protected glycofuranosyl isocyanides on heating for 2 h at 80 °C in toluene in the presence of /J-BU3S11H (2 eq) and AIBN.188 In particular, 2,3,5-tri-0-benzoyl-0-D-ribofuranosyl isocyanide (53) was converted in 78% yield into l,4-anhydro-2,3,5-tri-0-benzoyl-D-ribitol (54) also accessible from phenyl 2,3,5-tri-0-benzoyl-1 -thio-D-ribofuranoside (55).189... 

Like the silyl ethers, the stability of the silyl esters parallels the steric bulk of the substituents on the silicon atom. Tris(2,6-diphenylbenzyl)siiy1 esters confer extraordinary steric protection upon the carboxyl group.234 For example, the tris(2,6-diphenylbenzyl)silyl ester of 4-phenylbutanoic acid 104.1 [Scheme 6.104] does not react with butyllithium (2.5 equiv) after 5 h at -78 °C or methylmag-nesium bromide (2,5 equiv) at room temperature. Nor did it react with lithium aluminium hydride after 30 min at 0 °C l M HC1 in THF at 40 °C, or aqueous sodium hydroxide at 50 °C after 5 h. Ester 104.1 was reduced with diisobutyl-aluminium hydride in 99% yield to give the 4-phenyl-l-butanol (99%) and HF pyridine in THF (1 2) at 50 aC cleaved it back to the acid after 5 h. Unfortunately, the penalty for such unusual stability is high the tris(2 6-diphenyl-... 

The best way to prepare peptide aldehydes from the corresponding N -protected amino acids is by using a handle based on the Weinreb amide.f This commercial handle allows classical solid-phase elongation of peptides using protected Boc or Fmoc amino adds and, at the end of the synthesis, the peptide aldehyde is formed by reduction and concomitant cleavage from the resin with lithium aluminum hydride. Although the 4-hydro-xybenzoic acid handle also allows the preparation of peptide aldehydes by reduction of the resin-bound phenyl ester with lithium tri-tert-butoxyaluminum hydride, a noixture of the aldehyde and the alcohol is always formed. 

The 3- and 4-coordinate stannylenes are protected against self reaction, and show the same structures in solution and the solid state. Bis[2,4,6-tris(trifluoromethyl)phenyl] stannylene (21-10) is classied here as 3-coordinate because it and related ortho-trifluoromethylphenylstannylenes, in the solid state, show a weak interaction between tin and a trifluoromethyl group, and in the crystal the Sn..Sn distance is 364 pm,34 much longer than that in authentic stannylene dimers (ca. 277-291 pm). 

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