Diphenylmethyl groups selective

By reversibly blocking position 3 with a diphenylmethyl group (72, 73), Prystas and Sorm also prepared 6-azauridine. The diphenylmethyl group was removed after ribosidization (74) by selective hydrogenolysis. The crystalline 6-azauridine obtained was identical with 6-azauridine prepared enzymatically. ... 

A model consistent with the products obtained is shown in structure 20. The radical attacks the alkene from the face opposite the bulky diphenylmethyl oxazolidinone substituent. The bulk of the diphenylmethyl group is critically important in determining the selectivities obtained. Replacement of this bulky substituent on the auxiliary by a smaller benzyl group leads to a substantial reduction of selectivity observed. 

Cleavage of p-nitrobenzyl group. Protection of carboxylic acids and amines as the esters and carbamates can take advantage of the selective reduction-induced fragmentation by zinc dust, as C=C bonds, S-N bonds, benzyloxycarbonyl, and diphenylmethyl groups are not affected during the operation. 

The reactivity of the isobutyloxymethyl group towards thiocyanogen lies between that of the triphenylmethyl and the diphenylmethyl group. The difference in reactivity is not sufficient to allow the selective oxidative cleavage of the triphenylmethyl group in the presence of the isobutyloxymethyl moiety, or the cleavage of the isobutyloxymethyl group in the presence of the diphenylmethyl moiety . 

Catechols can be protected as diethers or diesters by methods that have been described to protect phenols. However, formation of cyclic acetals and ketals (e.g., methylenedioxy, acetonide, cyclohexylidenedioxy, and diphenylmethyl-enedioxy derivatives) or cyclic esters (e.g., borates or carbonates) selectively protects the two adjacent hydroxyl groups in the presence of isolated phenol groups. 

Table 1.3 provides rate constants for the decay of selected carbocations and oxocar-bocations in H2O, TFE, and HFIP. As a general comment, water, methanol, and ethanol are highly reactive solvents where many carbocations that are written as free cations in standard textbooks have very short lifetimes. The diphenylmethyl cation, with two conjugating phenyl groups, has a lifetime in water of only 1 ns. Cations such as the benzyl cation, simple tertiary alkyl cations such as tert-butyl, and oxocarbocations derived from aldehydes and simple glycosides, if they exist at all, have aqueous lifetimes in the picosecond range, and do not form and react in water as free ions. This topic is discussed in more detail in Chapter 2 in this volume. 

Selectivity data may be also employed to confirm or rule out the existence of a single intermediate in a number of closely related reactions. Thus the ability of the leaving group to influence the selectivity of the competitive attack of water and borohydride in the reaction of a number of diphenylmethyl derivatives testifies to the fact that attack occurs at the ion pair stage and not on the free carbocation (Bell and Brown, 1966). The dependence of the ratio kti/k-w on the leaving group in the solvolysis of a number of triphenylmethyl derivatives leads to the same conclusion regarding the triphenylmethyl system (Hill, 1965). 

The a-naphthyldiphenylmethyl ether was prepared to protect, selectively, the 5-OH group in nucleosides. It is prepared from a-naphthyldiphenylmethyl chloride in pyridine (65% yield) and cleaved selectively in the presence of a p-methoxyphenyl-diphenylmethyl ether with sodium anthracenide, a (THF, 97%, yield). The p-me-thoxyphenyldiphenylmethyl ether can he cleaved with acid in the presence of this group. ... 

Azabuta-l,3-dienes have been prepared in which the imino group is conjugated with an enol ester acylation of carbanions derived from N-(diphenylmethyl)arylmethanimines affords a wide range of the 2-azadienes (Scheme 197). The site selectivity for the attack of the electrophile on the aza-allyl anion depends upon the substituents on the carbanion, and on the hardness of the... 

Trifluoroacetic acid slowly cleaves Cbz groups allowing the selective removal of Boc groups. However, Kiso and co-workers noted a marked acceleration in the presence of thioanisole, and the method is illustrated in Scheme 8.53 by the simultaneous cleavage of a diphenylmethyl ester and a Cbz group in the last step of a synthesis of Acivicin. ... 

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