Lactonium salt

Further support for the importance of secondary electronic effects has recently been obtained (2, 18) from a study of the SN2 displacement by iodide on lactonium salts. It was found that the iodide ion reacts instantly with salts such as 13 to give a mixture of iodoester 14, lactone 2 , and alkyl iodide. For example, in the case of 22 (R=CH3), a mixture of iodoester 14 (R=CHj) (70%), and 6-valerolactone (J5) (30%) was obtained. 

Assuming that these lactonium salts exist in the EZ form 21, the two transition states yielding 21 and 21 can be illustrated by 2j> and ]] respectively. In 2Z> the bond to be broken is antiperiplanar to the non polar ( -Cj bond whereas in 21> the bond to be broken is antiperiplanar to the C- -0- polar bond. In other words, in 26 the electron pair orbital of the C5 - O5... 

More precise information concerning the course of events in the acid hydrolysis of orthoesters was obtained from the study of the four bicyclic orthoesters 77-80 which have two different alkoxy groups. Each orthoester yielded exclusively the hydroxy-ester resulting from the ejection of the axial alkoxy group. Thus, 77, T, and 79 afforded the same hydroxy methyl ester 81 whereas orthoester 80 furnished the hydroxyl ethyl ester 82. The reverse process which occurs under basic conditions, i,e. the addition of alkoxide ion to the corresponding bicyclic lactonium salt, has already been described (cf. p. 71) and it was shown to take place with the same specificity. 

The reaction of lactones with trialkyloxonium salts furnishes lactonium salts, e.g. (404 equation 188), which react with alkoxides to give cyclic orthoesters. - - ... 

Childs RF, Kostyk MD, Lock CJ, Mahedran M (1991) Structural studies on 6-ethoxytetrahydropyrylium cations steneoelectronic control in the reactions of lactonium salts. Can J Chem 69 2024—2032... 

Intramolecular substitutions mediated by Ag" " do not seem to require activated halides. For example, w-chloro amides react with AgBF4, giving products from intramolecular attack of the amide oxygen. Depending on the structure of the amide, imino lactones, imino lactonium salts, or lactone hydrazones (eq 4)" are obtained as products. Huorination of a-bromo ketones using AgBp4 has also been reported. ... 

The trans bicyclic orthoester must exist in the conformation 95 and the primary stereoelectronic effects permit the ejection of the methoxy group Ohly, yielding the bicyclic lactonium ion which after hydration (-97) and cleavage will give the hydroxy-lactone % only. The process 97 - 90 cannot occur with stereoelectronic control its energy barrier must therefore be higher than in other cases. Kaloustian and Khouri (61) have shown that the reaction of sodium methoxide with the bicyclic salt % gave the trans orthoester specifically. 

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