The synthesis of the naturally occurring bithiazole (+)-cystothiazole E (1e) is described starting from oxazolidinone 2. It proceeded in 10 steps and an overall yield of 37%. The key reaction of the sequence was a Suzuki cross-coupling between bromobithiazole 4 and the (E)-alkenylboronic acid derived from alkyne 18 (94% yield). Prior to the synthesis, more general investigations related to the cross-coupling
Pd-catalyzed cross-coupling reactions. It was thus possible to achieve the first synthesis of cystothiazole E (1) with a Suzuki coupling of the building blocks 2 and 3 as the pivotal C-C bond-formation step (94 % yield, TBDMS=tBuMe2 Si). The bithiazole 3 could be prepared very conveniently from 2,4-dibromothiazole by regioselective cross-coupling reactions.
The antibiotic elansolid B1 was prepared by a convergent strategy that relied on a highly diastereoselective, biomimetic intramolecular Diels-Alder cycloaddition (IMDA) that furnished the tetrahydroindane unit. Other key features are a double Sonogashira cross-coupling and a substrate-controlled Yamamoto aldol reaction.