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. Author manuscript; available in PMC: 2018 Oct 9.
Published in final edited form as: Angew Chem Int Ed Engl. 2018 Jun 25;57(29):9003–9007. doi: 10.1002/anie.201804373

Scheme 3.

Scheme 3.

Chemical assembly of core structures 1−5. (a) 2,3-butanedione, trimethyl orthoformate, (±)-camphorsulfonic acid, 110 °C, 12 h, 58%; (b) TBSCl, imidazole, DMF, rt, 1 h, 84%; (c) 14, TMSOTf, 4 Å MS, −50 °C to −20 °C, 83% for 9, 86% for 10, 85% for 11; (d) 14 (1.0 equiv), TMSOTf, 4 Å MS, −50 °C, 1 h, then 15 (1.5 equiv), −20 °C, 1 h, 65% for 2 steps; (e) 15 (1.0 equiv), TMSOTf, 4 Å MS, −50 °C, 1 h, then 14 (1.5 equiv), −20 °C, 1 h, 74% for 2 steps; (f) 1) TFA/H2O (9:1, v/v); 2) NaOMe/MeOH, then 1.0 M LiOH; 3) Ac2O/MeOH, 1 (1.8 g) 87% for 3 steps; 2 (1.88 g) 90% for 3 steps; 3 (1.13 g) 84% for 3 steps; 4 (1.58 g) 91% for 3 steps; 5 (1.41 g) 90% for 3 steps.