Process for the preparation of tetranorlabdane derivatives

A process for the preparation of a furan of formula (I), or (I′)

• in the form of a racemic or optically active diastereoisomer, wherein the substituents in the positions 9a, 9b and 3a are in a relative configuration cis, and the hydrogen atom in position 5a and the oxygen atoms are in configuration trans relative to the methyl in position 9a;
• which comprises cyclizing an alcohol of formula (II), or respectively (II′)
• wherein the dotted lines indicate the presence of one carbon-carbon double bond in one of the indicated positions, said compounds (II) or (II′) being in a racemic or optically active form and, in the case of compound (II), being also in the form of diastereoisomer wherein the methyl in position 9a and the hydrogen atom in position 5a are in the relative trans configuration;
• while promoting the cyclizing with at least one Lewis acid and optionally an additive to assist in increasing selectivity or yield of the cyclizing.