\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{1.5em}
\setdoublesep{.6ex}
\setarrowdefault{,1.2,thick}
\setbondstyle{thick}
%\setarrowlabelsep{1em}

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

%\schemedebug{true}
\schemestart[][west]
	\chemname{\chemfig{R_{L}-[:-30](<:[6]R_{M})-[:30](-[2]OM)=^[:-30]-[:30]Me}}{\Z-enolate}
    \arrow{->[\ch{RCHO}]}
    \chemfig{R_{L}-[:-30](<:[6]@{a1}R_{M})-[:30](=[2]O)-[:-30](<:[6]@{a2}Me(-[6,,,,,draw=none]{\scriptstyle 1,3-\syn}))-[:30](<:[2]OH)-[:-30]R}
    \arrow(@c1--){0}[-90,.1]
    \chemname{\chemfig{R_{L}-[:-30](<:[6]R_{M})-[:30](-[2]OM)=_[:-30]-[6]Me}}{\E-enolate}
    \arrow{->[\ch{RCHO}]}
    \chemfig{R_{L}-[:-30](<:[6]@{b1}R_{M})-[:30](=[2]O)-[:-30](<:[6]@{b2}Me(-[6,,,,,draw=none]{\scriptstyle 1,3-\syn}))-[:30](<[2]OH)-[:-30]R}
    \arrow(@c3.south west--.north west){0}[-90,.1]{\small\ch{R_{L}}=large group; \ch{R_{M}} = medium sized group}
    \chemmove[-]{
    	\draw (a1) .. controls +(south:8mm) and +(south:8mm) .. (a2);
        \draw (b1) .. controls +(south:8mm) and +(south:8mm) .. (b2);
    }
\schemestop
\end{document}