And if those are supposed to be separate fields / attributes, unless I'm missing something, you'll want to ensure you're instantiating that String array properly, with quotes around each disparate attribute.
From:
string[] attr = { "Extended_SF_18_UC_ID__c, Aconex_Key__c, Last_Updated__c" };
To:
string[] attr = { "Extended_SF_18_UC_ID__c", "Aconex_Key__c", "Last_Updated__c" };