Novel semicarbazone-based α-amidoalkylating reagents: general synthesis and reactions with &amp;lt;em&amp;gt;H&amp;lt;/em&amp;gt;-, &amp;lt;em&amp;gt;O&amp;lt;/em&amp;gt;-, &amp;lt;em&amp;gt;S&amp;lt;/em&amp;gt;-, &amp;lt;em&amp;gt;N&amp;lt;/em&amp;gt;-, and &amp;lt;em&amp;gt;P&amp;lt;/em&amp;gt;-nucleophiles