Kondensation der Phosphoniumsalze (II) ‐ zugänglich aus den Aldehyden (I) ‐ mit den Formylestern (IV) bzw. (V) gibt die Retinoesäure‐Analogen (III) bzw. (VI); neben (VIc) entsteht auch das cis‐Isomere (nach 24 h Bestrahlung des Isomerengemisches in CH2Cl2 stellt sich ein 1:1‐trans/ cis‐Gleichgewicht ein).

Retinoids are a class of compounds structurally related to vitamin A. In preclinical studies, all-trans retinoic acid (tretinoin), 13-cis retinoic acid (isotretinoin) and the aromatic retinoids etretinate and acitretin have preventive and therapeutic effects on carcinogen-induced premalignant and malignant lesions. Clinically, chemoprevention with isotretinoin and etretinate has been tested with some degree of success in such indications as basal cell carcinomas, squamous cell carcinomas, superficial bladder tumors and second primary tumors in patients with squamous cell carcinoma of the head and neck. Limited therapeutic success has also been achieved with retinoid treatment of precancerous and cancerous conditions of the skin, oral cavity, larynx, lung, bladder and vulva. Dramatic therapeutic effects have been observed in the treatment of acute promyelocytic leukemia with tretinoin, which leads to very high rate of complete remission. Excellent results were recently reported in the treatment of squamous cell carcinomas of the skin and cervix with a combination of isotretinoin and recombinant interferon alfa-2a (rIFN alfa-2a, Roferon-A). The mechanism of action of retinoids is through modulation of cell proliferation and differentiation. Retinoids vary in their capacity to induce differentiation and to inhibit proliferation in a series of human transformed hematopoietic and epithelial cell lines. Some cytokines potentiate the retinoid-induced cell differentiation and act synergistically with retinoids to inhibit cell proliferation. The pattern of synergism is dependent upon the combination and tumor cell line tested. The discovery of nuclear retinoid receptors has contributed substantially to the understanding of the mechanism of action of retinoids at the molecular level. Further understanding of the molecular biology of retinoids is expected to contribute to a rational design of new retinoids in the future, which in turn may result in improvements in the prevention and therapy of cancer.

Interest has been increasingly focused on all-trans-retinoic acid (tRA) and 13-cis-retinoic acid (13cRA) in cancer chemoprevention and treatment. We have examined the in vitro effects of these 2 retinoic acids (

A review of recent investigations in the retinoid field is presented. Retinoic acid exerts a prophylactic and a therapeutic effect on chemically induced benign and malignant epithelial tumors in mice. In clinical studies positive therapeutic results have been obtained in patients with preneoplastic and neoplastic epithelial lesions. However, treatment with retinoic acid is limited by serious side effects (hypervitaminosis A syndrome). Therefore, the synthesis of analogs of retinoic acid (retinoids) possessing a more favorable therapeutic ratio has been initiated. Among a large series of synthesized compounds, certain aromatic analogs proved to have a particularly favorable therapeutic ratio. The structure-activity relationship of the most active retinoids is discussed including some biological data concerning prophylaxis and therapy of epithelial tumors. The total synthesis of retinoids according to various building schemes is discussed in detail. Methods for the synthesis of the cyclic end group, of the polyene chain component, and of the full retinoid skeleton are described. Metabolic studies of retinoic acid and of the most active retinoid, as well as the synthesis of some isolated metabolites are outlined. Suggestions concerning the mechanism of action of retinoids are made. Some clinical results on the treatment of acne, psoriasis and precancerous conditions are reported.

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