Selectivity in formation of protecting

B. Factors Governing Selectivity in Formation of Protecting Groups 1. Protection of carbonyl groups... 

Selectivity in formation of protective groups may also be achieved by a proper choice of reaction conditions and catalyst. Thus formation of the 3-monothioketal from 3,6-diketones is achieved by dilution of the ethane-dithiol-boron trifluoride reaction mixture with acetic acid. 3-Monocyanohydrins are obtained in good yield from 3,20-diketo-(5a)-pregnanes by diluting the exchange reaction with ethanol. Similarly, dilution of the... 

Factors governing choice of protecting groups / 375 Factors governing selectivity in formation of protecting groups / 375... 

Many of the alloys of copper are more resistant to corrosion than is copper itself, owing to the incorporation either of relatively corrosion-resistant metals such as nickel or tin, or of metals such as aluminium or beryllium that would be expected to assist in the formation of protective oxide films. Several of the copper alloys are liable to undergo a selective type of corrosion in certain circumstances, the most notable example being the dezincification of brasses. Some alloys again are liable to suffer stress corrosion by the combined effects of internal or applied stresses and the corrosive effects of certain specific environments. The most widely known example of this is the season cracking of brasses. In general brasses are the least corrosion-resistant of the commonly used copper-base alloys. 

Acetylsalicylic acid and related non-steroidal anti-inflammatory drugs (NSAIDs) selectively inhibit the cyclooxygenase activity of prostaglandin synthase [2] and consequently the synthesis of most eicosanoids. This explains their analgesic, antipyretic, and antirheumatic effects. Frequent side effects of NSAIDs also result from inhibition of eicosanoid synthesis. For example, they impair hemostasis because the synthesis of thromboxanes by thrombocytes is inhibited. In the stomach, NSAIDs increase HCl secretion and at the same time inhibit the formation of protective mucus. Long-term NSAID use can therefore damage the gastric mucosa. 

This revised strategy demonstrates that a simple change in hydroxyl protecting groups can markedly influence reactivity and selectivity. While formation of the Wittig salt 51 could now be achieved at atmospheric pressure, its coupling with aldehyde 45 only proceeded in moderate yield and selectivity. 

Surprisingly, the nonchelating 0-protecting /erf-butyldiphenylsilyl and ferz-butyldimethylsilyl (Table 15, entries 5, 6 and 10) as well as trimethylsilyl (see below) groups only give poor selectivities in favor of the (/n//-diastcreomer4i,S 499. The results show that two effects, 1. the chelation effect, and 2. the preference of ( )-silyl ketenc acetal formation by deprotonation with lithium diisopropylamide in tetrahydrofuran (p 3413) arc contrary to each other. 

Condensation of the benzylsulfonyl-protected donor 27 with the moderately nucleophilic alcohol 28 has led to mannobiose 29 in 49% yield, in a 1 2 a/p ratio (Scheme 10) [100]. In these studies Schuerch has shown that to achieve optimum P-selectivity complete formation of the mannosyl sulfonate is essential before addition of the aglycone component [100]. 

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