\documentclass[12pt,border=0pt,crop,multi=false,tikz,class=scrartcl]{standalone}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{chemfig,chemmacros}
\usepackage{lmodern}
\usepackage{gensymb}
\usepackage{textgreek}
 
%\chemsetup[chemformula]{font-shape=sf,format=\sffamily}
\renewcommand*{\familydefault}{\sfdefault}
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}}
\setatomsep{2em}
\setdoublesep{.6ex}
\setarrowdefault{,1,thick}
\setbondstyle{thick}

\makeatother
 
\begin{document}
\definesubmol\nobond{-[,0.25,,,draw=none]}
\schemedebug{false}
\begin{tikzpicture}
\node {
\schemestart
	\chemnameinit{\chemfig{\lewis{0:4:,O}=[6]-[:-30](-[6]R)(-[:210]H)-[:30](-[:-30]-[6]R)-[2,,,2]H|\lewis{2:0:,O}}}
	\chemname{\chemfig{\lewis{0:4:,O}=[6]-[:-30](-[6]R)(-[:210]H)-[:30](-[:-30]-[6]R)-[2,,,2]H|@{a1}\lewis{2:0:,O}}}{\parbox{7em}{\centering\footnotesize aldol (lost H shown for clarity)}}
    \arrow{<=>[\chemfig{@{a2}H|\pch}][\chemfig{H_2|@{b1}\lewis{0:6:,O}}]}
    \chemfig{\lewis{0:4:,O}=[6]-[:-30](-[6]R)(-[@{b3}:210]@{b2}H)-[@{b4}:30](-[:-30]-[6]R)-[@{b5}2,,,2]H_2|@{b6}\lewis{2:,O}|\pch}
    \arrow{<->>[$-$\chemfig{H_2O}]}
    \chemname{\chemfig{\lewis{0:4:,O}=[6]-[:-30](-[6]R)=_[:30]-[:-30]-[6]R}}{\parbox{7em}{\centering\footnotesize\textalpha,\textbeta$-$unsaturated\\aldehyde}} 
    \chemmove[-stealth,shorten <=3pt, shorten >=2pt, red]{
    	\draw (a1.east) .. controls ++(east:2em) and ++(north:3em) .. (a2.north);
        \draw (b1.south) .. controls ++(south:2em) and ++(south west:2em) .. (b2.south west);
        \draw (b3.north) .. controls ++(north:2em) and ++(north:2em) .. (b4.north);
        \draw (b5.east) .. controls ++(east:2em) and ++(east:2em) .. (b6.east);
    }
    \chemnameinit{}
\schemestop
};
\end{tikzpicture}
\end{document}