Process for the ring-opening of a cycloadduct according to formula I, wherein R 1 and/or R 2 are independently selected from the group consisting of H, C 1 -C 20 alkyl, CHO and hydrazones, oximes, hemiacetals and acetals thereof, CH 2 OH and esters and ethers thereof, CO 2 H and esters thereof, and amides and tertiary amines of CH 2 NH 2 and optionally polymer supported; preferably wherein R 1 and R 2 both are H or methyl, or R 1 is hydrogen and R 2 is methyl; R 3 and/or R 4 are independently selected from the group consisting of CH 3, acetals, hemiacetal, hydrazones and oximes of CHO, CH 2 OH and esters and ethers thereof, CO 2 H and esters thereof, amides and tertiary amines of CH 2 NH 2, and an electron withdrawing group that is preferably selected from the group consisting of H, CF 3, CCl 3, CBr 3, CI 3, NO 2, CN, SO 2 Q, SO 3 Q, COQ, COF, COC1, COBr, COI, C Q, CO 2 Q, C NQ, and C(=NT)Q, wherein Q and T are independently H, or linear or branched C 1 -C 8 -alkyl, optionally substituted with halogens and optionally polymer supported, or R 3 and R 4 together represent - X -, wherein X = O, CH 2, NH, NMe, NEt, NPr, NBu, NPh, or S; preferably R 3 and R 4 together represent - O -, - NMe -, - NEt - or - NPr -; or R 2 and R 4 together represent -CH 2 ZC -, wherein Z is selected from the group consisting of O, NH and S; wherein - - - - - - represent a single or double bond, preferably a single bond; wherein said process comprises contacting the cycloadduct with an acidic mixture comprising sulfuric acid and an activating agent to obtain a ring-opened product; wherein said ring-opened product is a ring-opened cyclohexene product according to formula II, an aromatized product according to formula III or a combination thereof wherein OAct represents a remnant of the activating agent; wherein Y and Z are independently H, linear or branched C 1 -C 20 -alkyl, phenyl or benzyl; and R 1 -R 4 as well as the solid-dashed bond - - - - - - - are as defined for formula I and/or hydrolysates thereof. R 3 and/or R 4 are independently selected from the group consisting of CH 3, acetals, hemiacetal, hydrazones and oximes of CHO, CH 2 OH and esters and ethers thereof, CO 2 H and esters thereof, amides and tertiary amines of CH 2 NH 2, and an electron withdrawing group that is preferably selected from the group consisting of H, CF 3, CCl 3, CBr 3, CI 3, NO 2, CN, SO 2 Q, SO 3 Q, COQ, COF, COC1, COBr, COI, C Q, CO 2 Q, C NQ, and C(=NT)Q, wherein Q and T are independently H, or linear or branched C 1 -C 8 -alkyl, optionally substituted with halogens and optionally polymer supported, or R 3 and R 4 together represent - X -, wherein X = O, CH 2, NH, NMe, NEt, NPr, NBu, NPh, or S; preferably R 3 and R 4 together represent - O -, - NMe -, - NEt - or - NPr -; or R 2 and R 4 together represent -CH 2 ZC -, wherein Z is selected from the group consisting of O, NH and S; wherein - - - - - - represent a single or double bond, preferably a single bond; wherein said process comprises contacting the cycloadduct with an acidic mixture comprising sulfuric acid and an activating agent to obtain a ring-opened product; wherein said ring-opened product is a ring-opened cyclohexene product according to formula II, an aromatized product according to formula III or a combination thereof wherein OAct represents a remnant of the activating agent; wherein Y and Z are independently H, linear or branched C 1 -C 20 -alkyl, phenyl or benzyl; and R 1 -R 4 as well as the solid-dashed bond - - - - - - - are as defined for formula I and/or hydrolysates thereof.