Synthesis and Thermal Rearrangement of 5-lsopropyl-1- methoxybicyclo 3. 2. 01 hepta-3, 6-diene-2-ones

Abstract

A kinetic study on the thermal rearrangement of 1-methoxy-bicyclo[3.2. O]hepta-3, 6-dien-2- ones [1] to give 3-methoxy isomers [2], has been made.5-Isopropyl-l-methoxy [15] and 3, 7-dimethy1-5-isopropyl-1-methoxybicyclo[3, 2, 0]hepta-3, 6-dien-2-ones [18] were synthesized by irradiation of the corresponding 2-methoxytropones [13] and [17]. The kinetic study was carried out in the temperature range from 140°C to 167°C for Cl 5J and from 175°C to 200°C for [18]. The rate of depletion of the starting materials was determined by measuring the proton area of the methoxy signal of [15] and [18] in their NMR spectra (see Table 1). Arrhenius plots for their earrangement are shown in Fig.2 to give the activation parameters in Table 2. There is little difference in the values of activation energy and entropy of activation between [15], [18] and 4-isopropyl isomer [1c]. From our observation, it seems difficult to discriminate between two previously proposed reaction paths, an antara-antara Cope rearrangement and a cancerted process which proceed vii cis, trans, cis-cycloheptatriene inter- mediate [11] In addition, another possible path in which the rearrangement takes place via a more preferred conformer of [11] such as [21] as the transition state is suggested.