Englerins A and B are guaiane sesquiterpenes that were isolated from the bark of Phyllanthus engleri, a plant indigenous to east Africa. The englerins consist of a 5-6-5 fused tricyclic structure with an ether bridge and two ester-bearing stereogenic centers, including a highly unusual glycolate residue. Englerin A is a potent and selective inhibitor of the growth of six human renal cancer cell lines. We report herein an efficient, eight-step synthesis of englerin A that leverages simple carbonyl-enabled carbon-carbon bond formations. Our route is amenable to the production of a diverse series of analogues for structure-function studies and determination of the mode of action of these natural products.

Protocols for the stereodefined formation of α,α-disubstituted enolates of pseudoephedrine amides are presented followed by the implementation of these in diastereoselective alkylation reactions. Direct alkylation of α,α-disubstituted pseudoephedrine amide substrates is demonstrated to be both efficient and diastereoselective across a range of substrates, as exemplified by alkylation of the diastereomeric pseudoephedrine α-methylbutyramides, where both substrates are found to undergo stereospecific replacement of the α-C-H bond with α-C-alkyl, with retention of stereochemistry. This is shown to arise by sequential stereospecific enolization and alkylation reactions, with the alkyl halide attacking a common π-face of the E-and Z-enolates, proposed to be that opposite the pseudoephedrine alkoxide side-chain. Pseudoephedrine α-phenylbutyramides are found to undergo highly stereoselective but not stereospecific α-alkylation reactions, which evidence suggests is due to facile enolate isomerization. Also, we show that α, α-disubstituted pseudoephedrine amide enolates can be generated in a highly stereocontrolled fashion by conjugate addition of an alkyllithium reagent to the s-cis-conformer of an α-alkyl-α,β-unsaturated pseudoephedrine amide, providing α,α-disubstituted enolate substrates that undergo alkylation in the same sense as those formed by direct deprotonation. Methods are presented to transform the α-quaternary pseudoephedrine amide products into optically active carboxylic acids, ketones, primary alcohols, and aldehydes.Here we describe practical methods for the stereocontrolled construction of quaternary carbon centers using pseudoephedrine as a chiral auxiliary. 1-3 Protocols for the stereodefined formation of α,α-disubstituted enolates of pseudoephedrine amides are presented followed by the implementation of these in diastereoselective alkylation reactions.Equations 1 and 2 illustrate the finding that the diastereomeric α-methylbutyramides 1 and 2 undergo stereospecific enolization with lithium diisopropylamide (LDA) in the presence of lithium chloride at 0 °C to form Z-and E-enolates, respectively, as inferred from 1 NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript dichlorodiisopropylsilane in the presence of 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU)]. 4 These observations can be rationalized within the framework of prior analyses of pseudoephedrine amide alkylation reactions, extended here to enolate formation. 1c-f We propose that in the favored pre-transition state assemblies the alkoxide side-chain and base are positioned on opposite faces of the incipient enolate, with the α-C-H bond aligned for deprotonation (see Figure 1, which illustrates the proposal for the specific case of substrate 1).(1)Alkylation of enolates derived from amides 1 and 2 at −40 °C, initially examined using an excess of the electrophile benzyl bromide (2 equiv), was also stereospecific; replacement of the α-C-H bond with α-C-benzyl proceeded with net retention of stereochemist...

As part of our studies of lethal viral mutagens, a series of 5-substituted cytidine analogues were synthesized and evaluated for antiviral activity. Among the compounds examined, 5-nitrocytidine was effective against poliovirus (PV) and coxsackievirus B3 (CVB3) and exhibited greater activity than the clinically employed drug ribavirin. Instead of promoting viral mutagenesis, 5-nitrocytidine triphosphate inhibited PV RNA-dependent RNA polymerase (K d = 1.1 ± 0.1 μM), and this inhibition is sufficient to explain the observed antiviral activity.Ribonucleoside analogues that enhance the basal mutation frequency of RNA viruses constitute a promising new class of antiviral therapeutics. Such compounds, termed lethal mutagens, accelerate viral mutagenesis to intolerable levels, resulting in "error catastrophe" and loss of viral viability. 1-9 The mechanism of antiviral activity for mutagenic ribonucleoside analogues typically involves (i) in vivo conversion to ribonucleotides facilitated by host cell enzymes, (ii) misincorporation into the viral genome by error-prone viral RNA-dependent RNA polymerases (RdRP a ), and (iii) indiscriminate nucleotide templating during genomic replication. Over successive rounds of replication, the accrual of excessive mutations forces the virus into "error catastrophe", and viral viability is lost. Previously, we demonstrated that ribavirin (1), a clinically employed antiviral drug, functions as a lethal mutagen against poliovirus (PV) 8 and hepatitis C virus. 10 Inspired by the known lethal mutagen for HIV, 5-hydroxy-2′-deoxycytidine (2), 6,9 we report here the antiviral evaluation activity of a suite of 5-substituted cytidine analogues (4-7).* To whom correspondence should be addressed. Phone: (814) 865-2969. Fax: (814) Hydroxylated cytosines, such as 5-hydroxycytosine, are hallmarks of oxygen radical induced DNA damage and early causative factors in genomic mutagenesis. 11 The addition of a hydroxyl moiety to the 5-position of cytosine alters the relative distribution of amino to imino nucleobase tautomers, thereby increasing the abundance of imino 5-hydroxycytosine, which can base-pair with adenine. 12-14 Mispairing of this oxidative lesion with A during DNA replication promotes transition mutations and consequential scrambling of the encoded genetic message. Oxidative DNA damage can also occur by the introduction of oxidized deoxycytidine triphosphates (dCTPs) into the genome during replication. The 5′-triphosphate of 2, a product of dCTP oxidation, is incorporated into DNA by Klenow DNA polymerase I. 15,16 Fortunately, the integrity of DNA is maintained by complex networks of repair machinery that target such forms of DNA damage. 17,18On the basis of its mutagenic capacity toward genomic DNA, 5-hydroxy-2′-deoxycytidine (2) has been evaluated as an antiviral lethal mutagen against the HIV retrovirus. 9 Treatment with 2 confers significant reductions in viral titer, including an increase in G to A substitutions in the gene-encoding reverse transcriptase (RT). 9 In addition,...