Syntheses of thunbergols and α- and β-cembra-2,7,11-triene-4,6-diols

Abstract

Alkylation of the racemic sulfone 25, available from the epoxide 8, using the iodide 24 followed by reduction gives the protected hydroxy acetal 27. Selective deprotection gives the alcohol 28. This is converted into the bromide 29 which is used to alkylate the keto phosphonate 33. Hydrolysis of the alkylated keto phosphonate 30 gives the aldehyde 31 which is cyclised under mild conditions (63%) and the product treated with methylmagnesium iodide, to give the racemic thunbergols 3 and 4, in a ratio of 3∶4 = 10∶90. The laevorotatory sulfone 25 has been prepared by regioselective ring-opening of the epoxide 38 followed by hydrogenation, selective protection and functional group modification. After alkylation of this sulfone using the iodide 24 and conversion into the aldehyde 46, an asymmetric aldol condensation gives the hydroxy amide 47 which is converted directly into the hydroxy keto phosphonate 49 by reaction with an excess of lithiated dimethyl methylphosphonate. After protection of the hydroxy group, selective hydrolysis of the acetal gives the aldehyde 51 which is cyclised as before to give the naturally occurring cembratrienediols 1 and 2 after reaction with methylmagnesium iodide and deprotection.