Synthesis of the C26−C32 Oxazole Fragment of Calyculin C: A Test Case for Oxazole Syntheses
摘要:
The synthesis of the C-26-C-32 oxazole fragment 4 and its C-32 epimer 20 of serine/threonine protein phosphatase PP1 and PP2A inhibitor calyculin C is presented. The syn methyl arrangement in 4 was established through cyclic stereocontrol. Several methods for oxidizing the intermediate oxazolines 18 and 19 to the finished oxazole fragments were explored. The best results were obtained with oxidations proceeding through the corresponding ester enolate when the carbamate NH side chain was temporarily protected with a TMS group, or with CuBr2/DBU/HMTA-based oxidations. The finished oxazole fragment 4 was obtained in 21% overall yield, starting from Boc-D-alaninal.
Stereoselective alkylation of N-Boc-2-pyrrolidinones and N-Boc-2-piperidinones. Synthesis and characterization of disubstituted lactams
作者:Adriano O. Maldaner、Ronaldo A. Pilli
DOI:10.1016/s0040-4020(99)00835-2
日期:1999.11
Alkylation of enolates of monosubstituted N-Boc lactams 4–6 afforded trans-disubstituted lactams as the major isomer. In the pyrrolidinone series, 1,3-induction seems to be ruled by steric interactions and the diastereoselection is low for the alkylation of enolates with small substituents at C-5 (e.g., Me) and methyl iodide. The trans selectivity improves with bulkier substituents at C-2 and/or bulkier