Preparation of medium-ring cycloalkynes from 1-bromo-trans-cycloalkene derivatives

Abstract

2-Bromo-3-methoxy-trans-cycloalkenes react very rapidly with potassium t-butoxide in anhydrous dimethyl sulphoxide solution at room temperature to give the corresponding 3-methoxycycloalkynes, usually in high yields. The preparation of 3-methoxycyclo-octyne (7a), 3-methoxycyclononyne (12a), 7-methoxycyclonon-1-en-5-yne(18), and 3-methoxycyclodecyne (20) is described. 3-Methoxycyclononyne (12a) readily isomerizes in the reaction medium to give the allene (13).

2-Bromo-3-hydroxy-trans-cycloalkenes may similarly be converted into the corresponding 3-hydroxycycloalkynes, but protection of their hydroxy-groups by tetrahydropyranylation may be necessary before treatment with base. The preparation of cyclo-oct-2-ynol (7b) and cyclonon-2-ynol (12b) is described. The preparation, in modest yields, of the cyclodeca-1,6-diyne derivatives (23a and b) from the corresponding 2,7-dibromo-trans,trans-cyclodeca-1,6-dienes (22a and b) is reported. The preparation of 1-bromo-trans-cyclononene (28) and 1-bromo-trans,cis-cyclonona-1,5-diene (30c) and the conversion of the latter into cyclonon-1-en-5-yne (31) is described. The preparation of cyclononyne (2) from cyclonon-2-ynol (12b) is also described.