\documentclass{beamer}
\usetheme{Copenhagen}
\setbeamercolor{lecture in toc}{parent=structure}
\usepackage{pgffor}
\usepackage{totcount}
\regtotcounter{part}
\makeatletter
\AtBeginLecture{
\mode<beamer>{
\part{\beamer@lecturename}
\expandafter\NR@gettitle\expandafter{\beamer@lecturename}
\label{part:\thelecture}
\begin{frame}
\begin{beamercolorbox}[sep=16pt,center]{part title}
{\Large\textsc{Lecture}\par}
{\large\insertpart\par}
\end{beamercolorbox}
\tableofcontents
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}
\beamer@tocsectionnumber=0
}
\makeatother
\title{Some Title}
\begin{document}
\begin{frame}
\tableofcontents
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\vskip-0.4cm
\protect\usebeamercolor[fg]{lecture in toc}
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\lecture{Lecture1}{lec1}
\section{Section 1}
\subsection{Subsection 1a}
\frame{}
\subsection{Subsection 1b}
\frame{}
\subsection{Subsection 1c}
\frame{}
\mode<article>{\part{part for your book with beamerarticle}}
\section{Section 2}
\frame{}
\lecture{Lecture2}{lec2}
\section{Section 1}
\subsection{Subsection 1a}
\frame{}
\subsection{Subsection 1b}
\frame{}
\subsection{Subsection 1c}
\frame{}
\end{document}