Reactions of terpenoids in strong acids. Part 2. Novel cyclisations of citronellol and the isomers of citral

Abstract

Treatment of citronellol (3,7-dimethyloct-6-en-1-ol) with fluorosulphonic acid at –78 °C led to recovery of 2,2,5-trimethyl-and 2-ethyl-2,5-dimethyl-oxacycloheptane (40%; 1 : 1 w/w). The former resulted from an unexpected fragmentation, and ¹⁴C tracer studies revealed that a methyl group was lost from C-7 of the substrate. Similar treatment of citral (3,7-dimethylocta-trans-2,6-dienal and its cis-isomer; 7 : 3 w/w) or α-cyclocitral (2,6,6-trimethylcyclohex-2-enecarbaldehyde) gave low (ca. 17%) yields of the novel products 1,5-dimethyl-2-oxabicyclo-[3.3.1]non-3-ene and 7-methoxy-2,2,8-trimethyl-6-oxabicyclo[3.2.1]octane. β-Cyclocitral (2,6,6-trimethyl-cyclohex-1-enecarbaldehyde) was largely unchanged. ¹H N.m.r. spectra of all substrates in fluorosulphonic acid or fluorosulphonic acid–antimony pentafluoride at –78 to +27 °C revealed carbocations or derived species that corresponded to some of the isolated products and were essentially uncontaminated by the tars that in most cases predominated after quenching.