Cross metathesis approach to retinoids and other β-apocarotenoids

Cross metathesis (CM) reactions between polyenes, such as β-carotene, canthaxanthin or retinyl acetate, and various alkenes or dienes in the presence of second generation Hoveyda–Grubbs (H II) or Grubbs (G II) catalysts were investigated. Depending on the cross partner different apocarotenoids were obtained. Cross metathesis reactions of retinyl acetate proved to be fully regioselective. Carotenoid CM reactions afforded mixtures of two products due to competing cleavage of the C11–C12 and C15–C15′ double bonds. However, regioselectivity can be controlled by choice of appropriate reaction conditions. The reactions of polyenes with dienes worked better in respect of yields and diastereoselectivities than those with monounsaturated cross partners.