Tethered Biginelli condensation of enantioenriched hexahydropyrrolopyrimidines 8 with beta-ketoesters provides efficient asymmetric access to tricyclic guanidines 9 having a syn relationship of the angular C2a and C8a hydrogens. This reaction was employed to realize the first practical enantioselective access to this fragment of batzelladine alkaloids B (2) and E (5). The efficiency of this strategy is illustrated in the synthesis of the dextrorotatory enantiomer of batzelladine B methanolysis product 10 in 10 steps and 25% overall yield from 2-nonanone and methyl acetoacetate. The asymmetric synthesis of 10 establishes that the absolute configuration of the tricyclic portion of batzelladine B (2) is 25aR,28S,30R. The 4-methyl-7-alkyl-1,2,2a,3,4,5,6,7,8,8a-decahydro-5,6,8b-triazaacenaphthalene-3-carboxylic acid subunit, e.g., 29, of batzelladine alkaloids A (1), D (4), F (6), and G was also prepared for the first time by catalytic hydrogenation of tricyclic guanidines 26 having the 2a,8a-anti stereochemistry.

514 1. Total Synthesis of Allopumiliotoxins 267A and 339B 514 2. Total Synthesis of Pumiliotoxin 251D 516 B. Synthesis of Allopumiliotoxin 339B Using Pd(0) Catalysis 517 C. Synthesis of Allopumiliotoxins 267A and 339A Using a Nozaki−Kishi Cyclization 518 V. Conclusion 520 505

[reaction--see text] An asymmetric total synthesis of the unusual siphonariid metabolite, (-)-baconipyrone C (3), is described. Key steps included a tin(II)-mediated aldol coupling for the preparation of the carboxylic acid 17 and two different boron-mediated aldol additions leading to alcohol 8. Ester formation using modified Yamaguchi conditions gave 24, leading on PMB deprotection to (-)-baconipyrone C.

The spirocyclic core of the siphonarins was constructed by a directed cyclization of a linear triketone, prepared using a Sn(II)-mediated aldol coupling and Swern oxidation at C9 and C13. To circumvent a facile retro-Claisen pathway generating a baconipyrone-type ester, a Ni(II)/ Cr(II)-mediated coupling reaction with vinyl iodide was used to complete the first synthesis of siphonarin B and dihydrosiphonarin B. A stable isomeric spiroacetal was also prepared which could not be equilibrated to the siphonarin skeleton.

Induced pluripotent stem cells (iPSCs) and their differentiated neurons (iPSC-neurons) are a widely used cellular model in the research of the central nervous system. However, it is unknown how well they capture age-associated processes, particularly given that pluripotent cells are only present during the earliest stages of mammalian development. Epigenetic clocks utilize coordinated age-associated changes in DNA methylation to make predictions that correlate strongly with chronological age. It has been shown that the induction of pluripotency rejuvenates predicted epigenetic age. As existing clocks are not optimized for the study of brain development, we developed the fetal brain clock (FBC), a bespoke epigenetic clock trained in human prenatal brain samples in order to investigate more precisely the epigenetic age of iPSCs and iPSC-neurons. The FBC was tested in two independent validation cohorts across a total of 194 samples, confirming that the FBC outperforms other established epigenetic clocks in fetal brain cohorts. We applied the FBC to DNA methylation data from iPSCs and embryonic stem cells and their derived neuronal precursor cells and neurons, finding that these cell types are epigenetically characterized as having an early fetal age. Furthermore, while differentiation from iPSCs to neurons significantly increases epigenetic age, iPSC-neurons are still predicted as being fetal. Together our findings reiterate the need to better understand the limitations of existing epigenetic clocks for answering biological research questions and highlight a limitation of iPSC-neurons as a cellular model of age-related diseases.