Syntheic studies in carolenoids

Abstract

The thesis entitled “Synthetic Studies in Carotenoids” describes a novel route to the synthesis of vitamin A derivatives. It is divided into two chapters. The second chapter is further divided into three sections. Chapter I is a general introduction and gives a brief outline of the history of vitamin A. A fairly comprehensive account of the synthetic methods in the vitamin A field has been given, with emphasis on recent methods and industrial importance. The recent work on the anticarcinogenic effects of vitamin A has been briefly touched upon. Section A of Chapter II starts with a comprehensive survey of the methods available for regiospecific allylic functionalisation of 3,7?dimethyloctadiene derivatives. The selenium dioxide oxidation of geranonitrile, ethyl geranate, and geranyl acetate to yield bifunctional dimethyloctadiene derivatives is described. Other methods of functionalisation tried and the results obtained are also described. Section B of Chapter II starts with a comprehensive survey of the dehydrogenation scheme involving allylic bromination and dehydrobromination. It describes the synthesis of all?trans?dimethyloctatriene derivatives by the technique of brominating with N?bromosuccinimide and dehydrobrominating with 1,5?diazabicycloundecene (DBU). A method of dehydrogenation that was specifically studied was for the preparation of 8?acetoxy?2,4,6?octatrienal, 2,6?dimethyl. This method involved the preparation of an enol acetate which was brominated with bromine and the product dehydrobrominated with DBU. Section C describes a novel synthesis of all?trans?retinonitrile and all?trans?ethyl retinoate. The synthesis is of the (C?? + C??) type, and the novelty arises from the fact that both the C?? units are ultimately derived from citral. It is for the first time in the field of carotenoids that a synthesis of vitamin A derivatives has been described wherein citral alone accounts for all twenty carbons of vitamin A. Since both retinonitrile and all?trans?ethyl retinoate have been converted to vitamin A by previous workers, a formal total synthesis of vitamin A has thus been achieved. Attempts to find alternative schemes of converting retinonitrile to retinal have also been described.