The Diels‐Alder cycloaddition is the most popular pericyclic reaction with numerous applications in synthesis and catalysis. We now demonstrate that we can perform this reaction under mild and metal‐free conditions relying on molecular iodine as the catalyst. Cycloadditions with cyclohexadiene, cyclopentadiene, or isoprene with various dienophiles can be performed typically within minutes in moderate to good yields and high endo selectivity. The mechanistic studies including kinetic and DFT investigations clearly indicate a halogen‐bond activation and rule out other modes of activation. Furthermore, iodine performs equally well as typical metallic Lewis acids like AlCl3, SnCl4, or TiCl4.

Vanadate‐dependent chloroperoxidase from Curvularia inaequalis catalyzes 5‐ endo ‐ trig bromocyclizations of α‐allenols to produce valuable halofunctionalized furans as versatile synthetic building blocks. In contrast to other haloperoxidases, also the more challenging 5‐ exo‐trig halocyclizations of γ‐allenols succeed with this system even though the scope still remains more narrow. Benefitting from the vanadate chloroperoxidase‘s high resiliency towards oxidative conditions, cyclization‐inducing reactive hypohalite species are generated in situ from bromide salts and hydrogen peroxide. Crucial requirements for high conversions are aqueous biphasic emulsions as reaction media, stabilized by either cationic or non‐ionic surfactants.

The oxidative ring expansion of bio-derived furfuryl alcohols to densely functionalized six-membered O-heterocycles represents an attractive strategy in the growing network of valorization routes to synthetic building blocks out of the lignocellulosic biorefinery feed. In this study, two scenarios for the biocatalytic Achmatowicz-type rearrangement using methanol as terminal sacrificial reagent have been evaluated, comparing multienzymatic cascade designs with a photo-bio-coupled activation pathway.