A structure-based approach to kinase inhibition based on the natural product (-)-TAN-2483B
The natural product (-)-TAN-2483B (1) is a polyketide derived secondary metabolite isolated from the fermentation broth of a filamentous fungus at Takeda Industries, Japan. This compound possesses a densely functionalised furo[3,4-b]pyran-5-one framework rarely seen in natural products, and has displayed inhibitory activity against the c-SRC kinase and parathyroid-induced bone resorption in osteoclasts. Previous work in the Harvey group led to the synthesis of analogue 2, which significantly inhibits Bruton’s tyrosine kinase (BTK, 83% inhibition at 10 μM), a validated drug target for various diseases. The work presented herein describes the synthesis, molecular modelling, and BTK inhibitory activity of new analogues of 2. Also described is the first total synthesis of 1, and an assessment of its kinase-inhibitory activity.
An efficient and flexible synthetic strategy was proposed (Chapter 2) to facilitate synthesis of the furo[3,4-b]pyran-5-one scaffold, achieve the total synthesis and further explore the structure-activity relationship (SAR) between 2 and BTK. It was envisaged that the furo[3,4-b]pyran-5-one core may be constructed through a [2+2]-cycloaddition of ketene 3 and glycal 4, followed by Baeyer-Villiger (BV) oxidation. However, all attempts to reach the intermediate 5 were unsuccessful.
Attention shifted to the modelling of 2 in the BTK binding site to aid in rationally designing furo[3,4-b]pyran-5-one-based inhibitors. Structure-based methodology, including docking and molecular dynamics, was utilised to develop reasonable hypotheses for analogue binding (Chapter 3). To experimentally test the binding hypotheses and further develop the SAR, a series of new analogues (6-11) was synthesised by modifying and optimising the existing synthetic route, starting from the chiral building block D-mannose (Chapter 4). Generation of the 2,6-trans-disubstituted pyran 12 was enabled by ring-expansion of dibromocyclopropane 13 and subsequent α-selective alkynylation under Lewis acidic conditions. The (Z)-ethyl ester sidechain present in 2 was installed selectively by utilisation of a Still-Gennari reaction. Alkyne derivatisation and palladium-catalysed carbonylation established the lactone with the desired (S)-configuration at the methyl linkage, completing the bicyclic core. Determination of their BTK inhibitory activities revealed the importance of the endocyclic alkene and hydroxyl for retaining activity.
Chapter 5 illustrates the successful total synthesis of 1 which was achieved using methodology related to that described in Chapter 4. The (E)-propenyl sidechain was introduced by a carefully orchestrated Julia-Kocienski olefination. A revision of the protecting group strategy enabled finalisation of the natural product synthesis, where the product spectroscopically conformed to the proposed structure, and the data reported for the natural product. Utilising the putative SAR data and modelling methodology developed herein, future efforts in this project should aim to rationalise the SAR data by further docking and biological studies, as well as targeted analogue synthesis that could aid in developing furo[3,4-b]pyran-5-ones with enhanced binding to BTK.