An Efficient Commercial Process for the Preparation of Isotretinoin

Abstract

We describe an efficient process for the preparation of isotretinoin (13-cis isomer of vitamin A acid) in a single step starting from β-ionylidene acetaldehyde ( 5 ). The process conditions are convenient to operate on a commercial scale and afford isotretinoin of excellent quality; levels of related isomeric impurities such as tretinoin (all trans retinoic acid) and 9,13-di- cis -retinoic acid are extremely low. Thus, condensation of dienolate of methyl 3,3-dimethylacrylate with β-ionylidene acetaldehyde ( 5 ) followed by aqueous acidic workup afforded isotretinoin in >95% purity. The condensation reaction proceeds via in situ formation of lactone ( 8 ); furthermore, the reaction conditions have been optimized to exploit in situ generated methoxide anion for lactone ring opening to afford the desired product. Distinct advantages of this process are that it does not require isolation of intermediate lactone and utilizes in situ generated methoxide for lactone ring opening, thus obviating the need for an additional step and base. We also describe an optimized process for the preparation of β-ionylidene acetaldehyde ( 5 ), a key intermediate for isotretinoin.