Acidic Activation With Additional Reagents

It has been found that triphenylphosphine oxide reacts with trifluoromethylsul-fonic anhydride to give an ionic substance with the composition of a simple 1 1 adduct. When this substance is added to a solution containing a carboxylic acid, followed by addition of an amine, amides are formed in good yield. Similarly, esters are formed by treating carboxylic acids first with the reagent and then with an alcohol. What is a likely structure for this ionic substance and how can it effect the activation of the carboxylic acids ... 

Acrylamide, C H NO, is an interesting difiinctional monomer containing a reactive electron-deficient double bond and an amide group, and it undergoes reactions typical of those two functionalities. It exhibits both weak acidic and basic properties. The electron withdrawing carboxamide group activates the double bond, which consequendy reacts readily with nucleophilic reagents, eg, by addition. 

Although these allylic stannanes are rather resistant to uncatalysed or Lewis acid-catalysed carbonyl addition , they are valuable, shelf-stable homoenolate reagents (see Section IV.C.5), which are activated by Lewis acids or lithiodestannylation. Titanium tetrachloride converts the allylstannanes stereospecifically with inversion into very reactive intermediates (equation 83) . Both isomers, (R,Z)- and (5, )-315, are transformed... 

There are an extremely large number of reactions of 2-oxetanones with nucleophilic reagents, and space will allow inclusion of only representative examples. /3-Lactones show the interesting Bal.2 mechanism for base-catalyzed hydrolysis and the Aal2 mechanism for acid-catalyzed hydrolysis, according to data on kinetics and optical rotation studies of optically active lactones. The mechanistic interpretations are complicated, however, by the possibilities for subsequent elimination and addition reactions to occur, so that both of the two sites for nucleophilic attack on the 0-lactone skeleton, C-2 and C-4, may become involved. In fact 0-lactones are unusually insensitive to base, as well as acid, catalysis, the slow reaction with neutral water predominating between pH 1 and 9 (74JCS(P2)377). 

Substitution at the terminal position of the allylstannane, as in crotonyltributyl stannane, however, is not tolerated, because hydrogen abstraction from the allylic position is a competing reaction [21], An extension of the method involves the coupling of the anomeric radical precursors 28 with the allyltributyltin reagent 29 [14], In the reagent 29 the double bond is activated toward addition of nucleophilic radicals by the electron-withdrawing t-butoxy carbonyl group. The obtained product 30 has been useful en route to 3-deoxy-D-marmo-2-octulosonic acid (KDO). 

---