Calculated heats of formation of sterol diene isomers compared with synthetic yields of isomerisation reactions of Δ5,7 sterols

Abstract

Heats of formation of five series of diene sterol isomers were calculated and compared with synthetic yields of acid-catalysed isomerisation reactions starting from Δ^5,7 isomers. Calculations were based on molecular mechanics, using the MM3 program package. For each of the five Δ^5,7 starting compounds, three possible reaction paths were considered, in which heats of formation were calculated for theoretically possible intermediate double bond isomers. Similar results are found for all five series. The starting compounds are found to have the unfavourable heats of formation compared to all other isomers considered within one series. In general, isomerisation reactions of diene sterols ultimately yield spiro compounds when allowed to proceed for a sufficient amount of time. These compounds are found to have the lowest heats of formation in each series. However, they were not formed in the reactions considered in this paper, because the reactions were stopped after the desired isomer was formed in excess, before the spiro compounds could occur. Most compounds identified as products in the syntheses have a favourable heat of formation compared to the isomers preceding the (most stable) spiro compounds. However, when the reactions were carried out at low temperature, isomers with less favourable heats of formation could be trapped. We show that calculated heats of formation correspond well with synthetic yields and we suggest they can be a useful tool in planning syntheses.