documentclass{report}
usepackage{chemnum}
usepackage{titlesec}

titleformat{section}{slshape}{thesubsection}{1em}{}{}

begin{document}

    section{Preparation of cmpd+{compoundA}}

    We made cmpd{compoundA}.

    section{Preparation of cmpd+[format=slshape]{compoundA}}

We made cmpd{compoundA}.

section{Preparation of cmpd[format=slshape]{compoundA}}

We made cmpd{compoundA}.

end{document}