PURPOSE: To simply and efficiently obtain the chelating compounds each of which contains at least one sulfur donor atom and is capable of being bonded to a metal atom to form a chelate and useful for diagnosis and therapy.
CONSTITUTION: The chelating compounds are those each of which contains at least one donor sulfur atom joined to a sulfur-protecting group such as ethoxyethyl group, to form a hemithioacetal group represented by the formula I (wherein: each of R3 and R4 is a lower alkyl group; and R5 is H or a lower alkyl group), and is capable of being bonded to a metal atom to form a chelate. As an example of these compounds each having a hemithioacetal group(s), such a compound having an ethoxyethyl sulfur-protecting group(s), or the like is preferred. By using this method, the need of any separation process for removing the sulfur-protecting group(s) can be eliminated and also the radiolabeling operation can be simplified and is advantageously performed, in particular at the time of radiolabeling the chelating compound immdediately before using it and further, basic pH conditions and severe operating conditions with respect to the radiolabeling operation, etc., can be avoided.
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