Synthesis of a 35-member stereoisomer library of bistramide A: evaluation of effects on actin state, cell cycle and tumor cell growth

Abstract

Synthesis and preliminary biological evaluation of a 35-member library of bistramide A stereoisomers are reported. All eight stereoisomers of the C1-C13 tetrahydropyran fragment of the molecule were prepared utilizing crotylsilane reagents 9 and 10 in our [4+2]-annulation methodology. In addition, the four isomers of the C14-C18 gamma-amino acid unit were accessed via a Lewis acid mediated crotylation reaction with use of both enantiomers of organosilane 11. The spiroketal subunit of bistramide A was modified at the C39-alcohol to give another point of stereochemical diversification. The fragments were coupled by using a standard peptide coupling protocol to provide 35 stereoisomers of the natural product. These stereochemical analogues were screened for their effects on cellular actin and cytotoxicity against cancer cell lines (UO-31 renal and SF-295 CNS). The results of these assays identified one analogue, 1.21, with enhanced potency relative to the natural product, bistramide A.

Figure 1

A schematic drawing of the actin monomer showing the major subdomains (inset). Cross-section representation of 1 and 3 subdomains of G-actin. The red portion of the molecules insert into the “cleft” of 1 and 3 subdomains of actin. The atoms of the natural products involved in polar contacts with the protein are boxed.

Figure 2

Representative Structures of Potent Stereochemical Analogs of Bistramide A.

Figure 3

Cell-Growth Inhibition of Stereochemical Analogs 1.1–1.10 Against Renal Carcinoma Cell Line UO-31.

Figure 4

Cell Cycle Analysis of the UO-31 Cell Line for the Most Potent Cell Growth Inhibiting Isomers 1.1 and 1.33 and Least Active Isomer 1.7.

Figure 5

Cell Cycle Analysis of the SF-295 Cell Line for the Most Potent Cell Growth Inhibiting Analogs 1.1 and 1.33 and Least Active Stereochemical Compound 1.7.

Figure 6

Confocal Microscopy Using Phalloidin Staining Images of UO-31 Cells. Bistramide 1.1 causes disruption of F-actin in dose dependent manner, while least active epimer 1.7 does not.

Figure 7

Confocal Microscopy Using Phalloidin Staining Images of SF-295 Cells. Bistramide 1.1 and stereochemical analog 1.33 causes disruption of F-actin in dose dependent manner, while least active epimer 1.7 does not.

Scheme 1

Structures of bistramides A–D and K.

Scheme 2

Retrosynthetic analysis of bistramide stereoisomers 1.1–1.36.

Scheme 3

Proposed Mechanism for Formation of 2,6-trans-5,6-trans dihydropyran 12.

Scheme 4

Preparation of enantiomers of silyl glycidols.

Scheme 5

Synthesis of (Z)-crotylsilanes 9.

Scheme 6

Possible mechanism for (Z)-crotylsilane ent-9b.

Scheme 7

Synthesis of Eight Stereoisomers of the C1-C13 Tetrahydropyran Fragment 6.

Scheme 8

Initial Synthetic Approach to the C14-C18 Amino-Acid Fragment 7.

Scheme 9

Preparation of C15-Homoallylic Alcohols 33.

Scheme 10

Synthesis of Four Isomers of γ-amino Acid Subunit 7.

Scheme 11

Reaction Sequence for Synthesis of Phosphine Salt 42.^a

Scheme 12

Coupling of Tetrahydropyran 44 with the Phosphine Salt 42

Scheme 13

Synthesis of C19-C40 Spiroketal Fragment 49

Scheme 14

Coupling of acids 36 and spiroketal 49 and silyl deprotection

Scheme 15

Preparation of C39-epimer of the Spiroketal Subunit 52

Scheme 16

Completion of the Synthesis of Stereoisomers 1.1–1.36