Isoureas mixed

All other carboxylic acid derivatives in Table 6.1, in which the leaving group is bound to the carboxyl carbon through an O atom, are increasingly better acylating agents than carboxylic acid alkyl esters (entry 3) in the order carboxylic acid phenyl ester (entry 4) < acyl isourea (entry 7) < mixed carboxylic acid/carbonic acid anhydride (entry 8) < carboxylic acid anhydride (entry 9) < mixed carboxylic acid anhydride (entry 10). 

B, so-called O-acyl isoureas. To a certain extent, these constitute diaza analogs of the mixed anhydrides B of Figure 6.14. As can therefore be expected, O-acyl isoureas react with good nucleophiles with the same regioselectivity as their oxygen analogs at the carboxyl carbon of the carboxylic acid moiety. 

A study examined the formation of ethers by substitution at secondary alcohols by the isourea method. (25)-Octan-2-ol provides (7R)-l-methylheptyl phenyl ether by this chemistry. The formation of olefinic by-products suggests the involvement of carbocation intermediates and a mixed 1/5 2 mechanism. 

Mixed phosphorous acid esters s. 12, 310 Mixed phosphoric acid esters from isoureas... 

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