Total synthesis of ancistrotanzanine A.

Abstract

This thesis describes the first total synthesis of ancistrotanzanine A, a member of the naphthylisoquinoline class of natural products. In Chapter 1 the synthetic challenges presented by the naphthylisoquinoline alkaloids are discussed and strategies that have been adopted in previous syntheses of naphthylisoquinoline alkaloids overviewed. Chapter 2 describes the preparation of the key 5,3'-biaryl linkage via the Pinhey-Barton reaction. Studies into forming the linkage atropselectively were investigated using chiral hydrobenzoin acetal auxiliaries. This was found to have limited success with an atropisomeric ratio of 65:35 obtained. Changing the base from the achiral pyridine to the chiral brucine was also investigated and found to give no enhancement in the diastereoselectivity. From the results presented in Chapter 2, it was concluded that hydrobenzoin acetal auxiliaries were not appropriate for the diastereoselective synthesis of the key biaryl linkage of ancistrotanzanine A. As the chiral acetal strategy outlined in Chapter 2 failed to yield an atropselective process, efforts were re-focused on a new approach to the naphthylisoquinolines. In Chapter 3, an overview of all the methods available for the synthesis of chiral 3,4-dihydroisoquinolines is provided. From this, it was decided to apply the alkylation of o-tolylnitriles with chiral sulfinimines, as originally developed by Davis, to the synthesis of naphthylisoquinolines. Synthesis of the o-tolylnitrile lead reagent was readily achieved, but it was found that the amount of lead tetraacetate had to be carefully controlled to avoid side-reactions in the Pinhey-Barton reaction. After careful optimisation, the key 5,3'-biaryl linkage was prepared in high yield. Application of the Davis methodology to the MOM protected biaryl failed, with no reaction resulting. After much experimentation, it was established that the reaction was very sensitive to steric hindrance. A successful reaction was finally achieved by changing the base to lithium diethylamide. However, it was found the diastereoselection of the alkylation was quite low when p-tolyl sulfinimine was used. The use of the t-butane sulfinimine meant that the diastereoselection was significantly improved, with a ratio of 85:15 being obtained. After 3 more steps, the total synthesis was completed and ancistrotanzanine A was obtained, as a 1:1 mixture of atropisomers. Efforts to separate the atropisomers formed failed and even the use of chiral HPLC failed to resolve the material. To complete the Chapter, two analogues of ancistrotanzanine A were prepared – the tetrahydroisoquinoline and the methoxy ether. Chapter 4 summarises the above results and discusses the future potential of this research.