The investigation of group selective intramolecular Diels-Alder reactions towards hydrindene moieties

Abstract

We report the combination of an intramolecular Diels-Alder (IMDA) reaction and a desymmetrisation processes to build enantioselectively the steroid CD-ring moiety starting from an achiral precursor.  This novel methodology would be applied in vitamin D total synthesis.  (Fig. 6300)

We describe the synthesis of 5-substittued-nona-1,3,6,8-tetraene, a simplified precursor, and its use in group selective IMDA reaction.  We also describe our progress toward the synthesis of 4,6-dimethyl-5-substituted-nona-1,3,6,8-tetraene,the desired IMDA precursor.  We report the attempts to form a skipped bis(diene) from a skipped bis(acetylene) system.  We also described another attempt for the bis(diene) synthesis based on a double Wittig-Horner reaction which appeared to be unsuccessful.  Eventually, the bis(diene) was prepared in 8 steps from ethyl sorbate.  The second part of the project concerns the survey on the IMDA reaction.  The reaction was studied with an achiral auxiliary and Lewis acid (chiral or achiral). The reaction was also studied out with chiral auxiliary and a Lewis acid.  A wide range of Lewis acids were screened.  It was found that the IMDA reaction proceeded in moderate yield with a good diastereoselectivity and excellent enantioselectivity.

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