Reduction, followed by BBr3-mediated demethylation to offer stypodiol in 58 yield over two measures (Scheme 3B). Overall, this perform demonstrates the efficient merger of enzymatic oxidations and radical-based methodology toward severalAuthor Manuscript Author Manuscript Author Manuscript Author ManuscriptAcc Chem Res. Author manuscript; obtainable in PMC 2021 May well 21.Stout and RenataPagemeroterpenoid all-natural items and offers the foundation for the synthesis of other drimane-containing structures. b. STEVIOSIDE DERIVATIZATION: ENT-KAURANE, ENT-ATISANE, ENT-TRACHYLOBANE DITERPENOIDS Offered the good results of our meroterpenoid campaign, we looked to extend this paradigm of chiral pool synthesis to the ent-kaurane, ent-atisane, and ent-trachylobane diterpenoids.51 Arising from exceptional carbocationic rearrangements of ent-copalyl pyrophosphate,52 these terpenoid households share many structural options but differ in the architecture of their C and D rings. In addition, members of the family show a wide array of biological activities,52 making them attractive targets for medicinal chemistry evaluation and chemical probe development. Preceding semisynthetic studies53 inspired us to examine the ent-kaurane diterpene stevioside (121) as a potential beginning point for our endeavor. At 0.65/g, stevioside is out there in bulk quantities and can be readily converted towards the aglycones steviol (122) and isosteviol (137), though synthetic methodologies to selectively functionalize its ent-kaurane skeleton are exceedingly restricted. Hence, we sought to create a biocatalytic C oxidation plan that would afford speedy access to not just the entkauranes, but additionally the ent-atisanes and ent-trachylobanes via manipulation of the C and D rings. In the outset, it was essential to determine enzymes that could selectively Adenosine A3 receptor (A3R) Antagonist manufacturer oxidize the A, B, and C rings of steviol and connected structures. Earlier collaboration with all the Shen lab in the characterization on the platensimycin biosynthetic pathway (AMPA Receptor Activator site Figure 7), as well as our aforementioned operate with P450BM3 variants, revealed various possible enzymes for this goal.54 Immediately after a complete screening campaign, 3 of these enzymes P450BM3 variant BM3 MERO1 M177A, the Fe/KG PtmO6, along with the chimeric P450 PtmO5-RhFRed emerged as promising biocatalysts to impact selective hydroxylation on the A, B, and C rings, respectively, of steviol and ent-kaurenoic acid (Figure 7).3 Critically, every single enzyme was amenable to preparative scale and accepted a range of substrates en route to ent-kaurane, ent-atisane, and ent-trachylobane all-natural merchandise. With three effective terpene hydroxylases in hand, we very first pursued divergent syntheses of mitrekaurenone (126), fujenoic acid (128), and pharboside aglycone (129), every of which would call for only B ring oxidation (Scheme 4A). Beginning from ent-kaurenoic acid (123), we performed C7 hydroxylation with PtmO6 to get secondary alcohol 124 in higher yield as a single diastereomer. Toward mitrekaurenone, 124 was oxidized to ketone 125 and submitted to -oxidation to impact intramolecular lactonization, providing 126 in five methods and 36 overall yield. Alternatively, it was discovered that ketone 125 might be oxidized by PtmO6 to C6-alcohol 127, which was then treated successively with NaIO4 and DMP to afford fujenoic acid in seven actions and 26 general yield. Ultimately, access to pharboside aglycone came in three actions from secondary alcohol 124, featuring methyl esterification, dehydration, and dual dihydroxylation.
