Synthesis and structure-activity relationships of skin ceramides

Ceramides are a complex group of lipids that has gained much attention as cell signaling molecules and skin barrier constituents. In the skin, these sphingolipids form a major part of the stratum corneum intercellular lipid matrix, which is the barrier for penetration of most compounds. The development of such a protective layer was a critical step in the evolution of life on a dry land. Moreover, prominent skin diseases such as psoriasis and atopic dermatitis are associated with diminished ceramide levels and may be effectively improved by exogenous ceramides or their analogues. Since ceramides are not obtained from natural sources in pure form, they are of synthetic interest since 1950's. In this review, we describe sphingosine syntheses from 1998 until 2008, and the synthetic approaches to the unique epidermal ceramides, including the 6-hydroxysphingosine-based ones, the alpha- and omega-hydroxy forms and the omega-acyloxy species. Moreover, the structural requirements of ceramides for a competent skin barrier are discussed, including acyl chain length, trans double bond, acyl alpha-hydroxyl, stereochemistry, omega-linoleyloxy species and ceramide conformation.