Strain Release Chemistry of Photogenerated Small‐Ring Intermediates

Abstract

Photochemical processes, such as isomerizations and cycloadditions, have proven to be very useful in the construction of highly strained molecular frameworks. Photoinduced ring strain enables subsequent exergonic reactions which do not require the input of additional chemical energy and provides a variety of attractive synthetic options leading to complex structures. This review covers the progress achieved in the application of sequences combining excitation by ultraviolet light to form strained intermediates, which are further transformed to lower energy products in strain‐release reactions. As ring strain is considerable in small ring systems, photogenerated three‐ and four‐membered rings will be covered, mainly focusing on examples from 2000 to May 2020.

Reversible and Irreversible [2+2] Cycloaddition Reactions of Heteroallenes to a Gallaphosphene

“…Strained ring systems are valuable synthons in organic synthesis [26a] . Introducing heteroatoms in such rings led to charge‐induced asymmetry, hence providing reactive reaction sites [26b] .…”

Concise Total Synthesis of Agarozizanol B via a Strained Photocascade Intermediate

“…Starting from commodity chemicals (phenol, γ‐butyrolactone, prenol), agarozizanol B has been prepared in eleven steps in racemic (4 %) and enantiopure form (2 %). The comparably low overall yields are mainly due to the fact that the initial formation of compound rac ‐ 5 [16] proceeded in our hands in only 27 % yield. The pivotal photochemical reaction cascade holds promise for further applications in the synthesis of natural products.…”

“…We envisaged a facile and concise access to compounds with a prezizane skeleton I (Scheme 1) from a strained tetracyclic precursor which we hoped to access by a recently discovered photochemical cascade reaction [14, 15] . Cleavage of the indicated bond [16] would allow for immediate formation of the skeleton with all 15 carbon atoms already present in the key intermediate. We now report the successful implementation of this strategy which allowed us to access agarozizanol B ( 4 ) [6] both in racemic and in enantiopure form.…”

Reversible und irreversible [2+2]‐Cycloadditionen von Heteroallenen an ein Gallaphosphen

“…Gespannte Ringsysteme sind wertvolle Synthone in der organischen Synthese [26a] . Die Einführung von Heteroatomen in derartige Ringe verursacht eine ladungsinduzierte Asymmetrie und die Ausbildung reaktiver Zentren, [26b] was die gespannten viergliedrigen Heterocyclen in 2 – 6 zu vielversprechenden Kandidaten für die Aktivierung kleiner Moleküle macht.…”