Patent ID: 8227630

Claim:
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 selected from the group consisting of (a) metal salts of formula AlCl 2 R or MX 3 , wherein R is a C 1 -C 4 alkyl group, M is a trivalent metal cation selected from the group consisting of Al, Y, Sc and Fe, and X represents a Cl or F atom or is a weakly or non-coordinating mono anion selected from the group consisting of ClO 4 − , C 1-8 sulfonates, BF 4 − , PF 6 − , SbCl 6 − , AsCl 6 − , SbF 6 − , AsF 6 − or BR 4 4− , C 6 F 5 SO 3 − , CF 3 SO 3 − , MeSO 3 − , MeC 6 H 4 SO 3 , wherein R 4 is a phenyl group optionally substituted by one to five groups of halide atoms or methyl or CF 3 groups, and (b) BF 3 adducts with an (OR 1 ) 2 or R 2 COOH, wherein R 1 is a C 1 -C 5 alkyl group, and R 2 is a C 1 -C 20 alkyl group, wherein the cyclizing is optionally conducted with an additive to assist in increasing selectivity or yield of the cyclizing.