Evaluation of Chromane-Based Bryostatin Analogues Prepared via Hydrogen-Mediated C-C Bond Formation: Potency Does Not Confer Bryostatin-like Biology

Abstract

The synthesis and biological evaluation of chromane-containing bryostatin analogues WN-2-WN-7 and the previously reported salicylate-based analogue WN-8 are described. Analogues WN-2-WN-7 are prepared through convergent assembly of the chromane-containing fragment B-I with the "binding domain" fragment A-I or its C26-des-methyl congener, fragment A-II. The synthesis of fragment B-I features enantioselective double C-H allylation of 1,3-propanediol to form the C₂-symmetric diol 3 and Heck cyclization of bromo-diene 5 to form the chromane core. The synthesis of salicylate WN-8 is accomplished through the union of fragments A-III and B-II. The highest binding affinities for PKCα are observed for the C26-des-methyl analogues WN-3 (K_i = 63.9 nM) and WN-7 (K_i = 63.1 nM). All analogues, WN-2-WN-8, inhibited growth of Toledo cells, with the most potent analogue being WN-7. This response, however, does not distinguish between phorbol ester-like and bryostatin-like behavior. In contrast, while many of the analogues contain a conserved C-ring in the binding domain and other features common to analogues with bryostatin-like properties, all analogues evaluated in the U937 proliferation and cell attachment assays displayed phorbol ester-like and/or toxic behavior, including WN-8, for which "bryostatin-like PKC modulatory activities" previously was suggested solely on the basis of PKC binding. These results underscore the importance of considering downstream biological effects, as tumor suppression cannot be inferred from potent PKC binding.

Figure 1

PKC binding affinity of bryostatin 1 and 7, selected bryostatin analogues and neristatin 1.^{a a}Binding affinity to PKCα. See reference for PKCα binding affinity of bryostatin 1 and bryostatin 7. ^bCompounds I-V prepared by Wender were reported to function similarly to bryostatin 1 with regard to the pattern of PKCδ-GFP translocation induced in rat basophilic leukemia cells.^9h,i,k Binding affinity refers to a mixture of rat brain PKC isozymes. The initially reported binding affinity of I (0,25 nM) has been revised. ^cFor the indicated Merle bryologs prepared by Keck, PMA-like vs bryostatin-like biology was established via U937 attachment and inhibition of proliferation assays.

Figure 2

Retrosynthetic analysis of WN-7 illustrating C-C bonds formed via hydrogenative coupling.

Figure 3

PKC binding affinity of WN-1 to WN-8.^{a a}Binding affinity to PKCα. See reference for PKCα binding affinity of bryostatin 1 and bryostatin 7. ^bBinding affinity toward PKCδ and PKCβ, respectively.

Figure 4

Evaluation of WN-2, WN-3, WN-6, WN-7, and WN-8 in U937 Human Histiocytic Lymphoma Cells.^{a a}See supporting information for experimental details.

Figure 5

TNFα secretion from U937 cells^{a a}See supporting information for experimental details.

Figure 6

Toledo Cell Growth Assay^{a a}See supporting information for experimental details.

Scheme 1

Fragments A-I to A-III via H₂-mediated reductive coupling of glyoxal 1a and 1b with 1,3-enyne 2a or 2b.^{a a}The indicated conditions apply to Fragment A-I. Similar conditions are used for Fragments A-II and A-III. See reference and supporting information for precise experimental details.

Scheme 2

Synthesis of Fragment B-I via transfer hydrogenative double allylation of 1,3-propane diol.^{a a}See supporting information for experimental details.

Scheme 3

Synthesis of the chromanone-based macrodiolides WN-2 and WN-4.^{a a}See supporting information for experimental details.

Scheme 4

Synthesis of 1,3-enyne 2b via chelation controlled propargylation.^{a a}See supporting information for experimental details.

Scheme 5

Synthesis of the C26 des-methyl chromanone-based macrodiolides WN-3 and WN-5.^{a a}See supporting information for experimental details.

Scheme 6

Synthesis of the C26 des-methyl chromane-based macrodiolide WN-7.^{a a}See supporting information for experimental details.

Scheme 7

Synthesis of Fragment B-II.^{a a}See supporting information for experimental details.

Scheme 8

Synthesis of previously reported salicylate-based macrodiolide WN-8.^{a a}See supporting information for experimental details.