\documentclass[12pt,border=1pt,crop,multi=false,tikz,class=scrartcl]{standalone}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{chemfig,chemmacros}
\usepackage{lmodern}
\usetikzlibrary{decorations.pathmorphing}

\chemsetup[chemformula]{font-shape=sf,format=\sffamily}
\renewcommand*{\familydefault}{\sfdefault}
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}}
\setatomsep{2.5em}
\setdoublesep{.6ex}
\setarrowdefault{,1,thick}
\setbondstyle{thick}
%\setarrowlabelsep{1em}

\makeatletter
\begin{document}
\definesubmol\nobond{-[,0.25,,,draw=none]}

%\schemedebug{true}
\schemestart[][north]
	\chemfig{RO-[:30](=[2,.75]O)-[:-30]-[:30]Me}
    \arrow{->[\ch{LiNR2}]}
    \subscheme{
    	\chemleft[\chemfig{Li?-[:10]N(-[:-120]R)(-[2]R)-[:-10]H-[:60](-[2]H)(-[:-20]Me)-[:190](-[:60]OR)=[:-190]O?}\chemright]
        \arrow{0}[-90,.1]
        \chemleft[\chemname{\chemfig{Li?-[:10]N(-[:-120]R)(-[2]@{a1}R)-[:-10]H-[:60](-[2]@{a2}Me)(-[:-20]H)-[:190](-[:60]OR)=[:-190]O?}}{\footnotesize unfavourable syn-pentane}\chemright]
        \chemmove{\draw[-, shorten >=2pt, shorten <=2pt] (a1) .. controls +(90:1c	m) and +(130:5mm) .. (a2);}
    }
    \arrow
    \subscheme{
    	\subscheme{\chemname*{\chemfig{RO-[:30](-[2]OLi)=[:-30]-[6]Me}}{favored}\qquad {E--enolate}}
        \arrow(c6--){0}[-90,.5]
        \subscheme{\chemname*{\chemfig{RO-[:30](-[2]OLi)=[:-30]-[:30]}}{unfavored} \,{Z--enolate}}
    }
\schemestop
\end{document}