Concise Synthesis and Antimicrobial Evaluation of the Guanidinium Alkaloid Batzelladine D: Development of a Stereodivergent Strategy

You-Chen Lin, Aubert Ribaucourt, Yasamin Moazami, and Joshua G. Pierce*

Article

Concise Synthesis and Antimicrobial Evaluation of the Guanidinium

Alkaloid Batzelladine D: Development of a Stereodivergent Strategy

Cite This: https://dx.doi.org/10.1021/jacs.0c04091

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sı Supporting Information

ABSTRACT: Herein, we describe a stereodivergent route to

(

±)-batzelladine D (2), (+)-batzelladine D (2), (−)-batzelladine D

(2), and a series of stereochemical analogues and explore their

antimicrobial activity for the ﬁrst time. The concise synthetic

approach enables access to the natural products in a sequence of

−12 steps from readily available building blocks. Highlights of the

synthetic strategy include gram-scale preparation of a late stage

intermediate, pinpoint stereocontrol around the tricyclic skeleton, and a modular strategy that enables analogue generation. A key

bicyclic β-lactam intermediate not only serves as the key controlling element for pyrrolidine stereochemistry but also serves as a

preactivated coupling partner to install the ester side chain. The stereocontrolled synthesis allowed for the investigation of the

antimicrobial activity of batzelladine D, demonstrating promising activity that is more potent for non-natural stereoisomers.

9−21

INTRODUCTION

The marine environment is one of the most proliﬁc sources of

chemically complex and biologically active molecular scaﬀolds

strategy,

by Nagasawa and co-workers.

and Evans provide a [4 + 2] and radical cyclization approaches

followed by a 1,3-dipolar cycloaddition approach

■

22,23

More recent eﬀorts by Gin

24,25

to the trans-batzelladine core, respectively.

From a biological point of view, the batzelladines have

received attention due to their reported activity as inhibitors of

such as polycyclic guanidinium alkaloids (PGAs). Since ﬁrst

isolated in 1989, PGAs have been studied extensively, revealing

valuable information regarding their biosynthetic pathways and

,5,7−11,22,26

−6

HIV gp120-human CD4 binding.

These seminal

molecular properties.

Despite these signiﬁcant contribu-

studies revealed the signiﬁcance of the batzelladine side chain

on activity, with batzelladines A (1) and B (3) active in the

protein−protein interaction assay while batzelladine D (2) was

inactive. Subsequent studies by Nagasawa have suggested

batzelladine D (2) does indeed bind to CD4, but a

comprehensive biological examination of this subfamily of

natural products is lacking, particularly for members bearing

tions, surprisingly little information regarding PGAs biological

function and targets is known for most classes of PGAs.

Eﬃcient and modular synthetic approaches to these molecular

scaﬀolds allow not only for comprehensive biological studies

but also for the generation of derivatives with systematic

stereochemical and functional group modiﬁcations.

The batzelladines are a family of PGAs that were isolated in

the mid-1990s from the Caribbean sponge bataella sp. This

simpliﬁed side chains such as 2. Regardless of their potential

as therapeutic agents, these scaﬀolds represent exciting

platforms for the generation of chemical probes to study

protein−protein interactions as well as to explore the broader

biological activities of these PGAs.

family of molecules possess a tricyclic guanidinium core

bearing a guanidine-functionalized side chain of varying

complexity as highlighted by batzelladine A (1), B (3), and

−11

D (2) (Figure 1a).

The guanidinium core of the

batzelladines bears a pyrrolidine motif that is either of the

cis- or trans-stereoconﬁguration anchored to the various side

chains through an ester linkage. The structural complexity of

these scaﬀolds, in addition to antiviral and cytotoxicity activity

reported by the isolation team, has led to signiﬁcant interest

from the synthetic community. Cis-pyrrolidine members of the

batzelladine family, represented by batzelladine B (3), were

RESULTS AND DISCUSSION

■

Retrosynthetic Analysis. As part of a broader program

targeting modular and practical syntheses of PGAs,

became interested in developing a synthesis of batzelladine D

2) that would facilitate access to an array of stereochemical

5,27

(

12−16

ﬁrst to draw the attention of the synthetic community,

with 3 itself succumbing to an elegant approach from Herzon

Received: April 15, 2020

17,18

and co-workers in 2015.

The trans-pyrrolidine bearing

family members, highlighted by batzelladine A (1) and D (2)

represent distinct synthetic challenges. The ﬁrst synthesis of a

member of this subfamily was batzelladine D (2) by Overman

and co-workers through the use of a tethered Biginelli

XXXX American Chemical Society

Journal of the American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Figure 2 ). At this stage, reduction of the methyl ketone was

Article

Development of Stereodivergent Access to β-lactam

Building Blocks. To begin exploring the possibility of

utilizing a series of enantiomeric and diastereomeric β-lactam

building blocks toward the batzelladines, we targeted a racemic

synthesis of a compound such as 7 (Figure 1b). Our initial

eﬀorts toward this goal began with commercially available β-

lactam 8 which was substituted with butenyl Grignard reagent

N-protected with TBS and subsequently acylated to

provide 12 as a single diastereomer in good yield over 3 steps

Figure 1. (a) Representative batzelladine alkaloids; (b) retrosynthetic

strategy presented herein.

and functional derivatives to enable expanded biological

studies. To this end, our synthetic strategy envisioned the

formation of the tricyclic guanidinium core 4 through a double

S_N2 reaction of a suitably functionalized guanylated diol

precursor 5 (Figure 1b). Such a strategy was inspired by

previous eﬀorts but represents a one-pot solution that should

allow for the generation of all possible stereoisomers from a

19,23

common intermediate.

Access to the fully functionalized

trans-pyrrolidine relies on the use of a key bicyclic β-lactam

intermediate (6, Figure 1b) that would serve not only as a

preactivated coupling partner though the lactam itself but also

to control the stereochemistry of the pyrrolidine core. 6 would

be prepared through an aza-Michael addition of monocyclic β-

lactam 7, controlling the pyrrolidine stereochemistry while

allowing independent control both alcohols through diaster-

eoselective ketone reductions. Importantly, the β-lactam

approach allows for the thermodynamic installation and

retention of the labile 1,3-dicarbonyl stereochemistry and can

be accessed through an array of well-established synthetic

Figure 2. Synthesis of β-lactam building blocks 13 and 14.

required, and we sought to develop conditions to access both

diastereomers, ultimately enabling both natural product

epimers via subsequent displacement (Figure 1b). After

screening a variety of reduction conditions (Figure 2), we

found that K-selectride/KBF in ether provided the desired

diastereomer 13 in 10:1 dr and 80% yield (entry 4) with the

stereochemistry conﬁrmed through X-ray analysis while Li(n-

Bu)(i-Bu) Al−H provided the opposite diastereomer 14

8−31

methods.

This approach is distinct from previous work

required for batzelladine D (2) in 10:1 dr and 64% yield

which utilizes β-lactams as starting materials, as the β-lactam

herein provides a late-state intermediate required for stereo-

control and carbonyl activation. Most importantly, the β-

lactam enables the installation of the trans-pyrrolidine core:

inverting the cis-selectivity observed for typical aza-Michael

reactions and that of elegant routes toward the cis-pyrrolidine

(entry 11).

In pursuing these conditions, we were motivated to explore

additives that could enhance selectivity and fortunately found

that a potassium counterion and KBF additive were critical

to form the desired reduction and promote reactivity, while

that selectivity could be inverted though the use of bulky

aluminum hydride reagents. Further studies are required to

12,13

containing batzelladine families.

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

uncover the mechanistic underpinnings of this selectivity

enhancement, but this reagent combination may prove useful

in other directed diastereoselective reductions.

Table 1. Aza-Michael Optimization Model Study

Cyclization Approaches to Form Bicyclic β-Lactams.

Bicyclic β-lactams are common scaﬀolds in antimicrobial

natural products but have seen limited utility as strategic

6−38

building blocks for synthesis.

As highlighted in our

entry

conditions

results

retrosynthetic approach, we require a [4.5]-fused bicyclic β-

lactam 6 (Figure 1b) bearing a trans-pyrrolidine core.

Straightforward methods to construct such functionalized

scaﬀolds are not currently available,

embarked on studies to append the requisite ring to β-lactams

such as 15. A number of approaches including amino-

ZrCl (20 mol %), CH Cl , 25 °C

AuCl (10 mol %), toluene, 110 °C

PtCl (10 mol %), CH Cl , 25 °C

ReCl (10 mol %), CH Cl , 25 °C

38−44

and we therefore

BF ·Et O (20 mol %), CH Cl , 25 °C

46−48

49−51

HCl (50 mol %), Et

O, 25 °C

complex mixture

trace product

complex mixture

halogenation, aza-Heck,

amination/cross-coupling,

2,53

K-t-butoxide (2 equiv), THF, −78 °C

K-t-butoxide (5 equiv), THF, 45 °C

TBAH (0.2 equiv)/THF, 25 °C

LDA (2 equiv), THF, −78 °C

EtMgBr (2 equiv), THF, 0 °C

NaH (2.2 equiv), THF, 0 °C

LiHMDS (2.2 equiv), 25 °C

LiHMDS (1.2equiv), 25 °C

and carbonylative amidation

resulted in no cyclization but

instead direct intermolecular reactivity (see SI for details).

These results provided an initial glimpse into the strained

nature of these [4.5]-fused bicyclic β-lactams. To highlight

these challenges, we found that Pd-catalyzed aminohalogena-

tion proceeded under standard reaction conditions, but to

our surprise, instead of obtaining 16, the reaction provided

almost exclusively the [4.6]-fused bicycle 17 (Scheme 1). In

trace product

39%

89%

70%

LiHMDS (0.7 equiv), 25 °C

KHMDS (0.7 equiv), 25 °C

Scheme 1. Strategies to Prepare Bicyclic β -Lactams

All reactions were monitored every 0.1, 0.5, 1, 3, 6, 16 h unless

otherwise noted. Reactions were quenched after 10 min. >10:1 trans

to cis diastereoselectivity. NR = no reaction.

exploration of substoichiometric amount of lithium or

potassium bis(trimethylsilyl)amide bases, we were gratiﬁed to

observe high levels of conversion and yield to generate our

desired [4.5]-fused bicyclic β-lactam 20 (entries 13−16, Table

). Further, we were able to conﬁrm the reaction proceeded

to provide the trans-pyrrolidine stereochemistry in >10:1 dr as

conﬁrmed by NOSEY experiments (see SI for details).

The diastereoselectivity of this process is signiﬁcant, as the

β-lactam aza-Michael addition provides the trans-pyrrolidine

core, whereas previous aza-Michael addition strategies provide

12,13

the cis-pyrrolidine.

The profound diﬀerences observed by

altering the stoichiometry and identify of the base employed

points to a reversible reaction due to the added ring strain of

the bicyclic system and the potential instability of the reaction

product upon prolonged exposure to strong base. In attempts

to equilibrate the system to the cis-pyrrolidine, only increasing

decomposition is observed with increases in base, temperature,

or time.

Stereoselective Ketone Reduction. With our desired

bicycle in hand, we were in a position to explore the last two

hurdles to preparing the targeted linear precursor 5 that would

undergo cyclization to form the tricyclic guanidinium scaﬀold

(Figure 1b). First, conditions needed to be identiﬁed for mild

β-lactam ring opening to install the ester side chain required

for the natural products. Second, conditions needed to be

identiﬁed for selective reduction to both diastereomers of the

remaining ketone, as this will allow for both natural product

epimers to be generated from a common intermediate (as was

possible for the other alcohol; see Figure 2). Our initial eﬀorts

were focused on conducting the β-lactam ring opening under a

variety of acidic and basic conditions; surprisingly, this reaction

proved challenging with most conditions providing low levels

of reactivity (see SI for details). The best conditions identiﬁed

were use of boron Lewis acids in dichloromethane, but even in

these cases, the reaction proved challenging to reproduce.

parallel to these studies, we were also exploring approaches in

which the alkene was prefunctionalized to promote our desired

-membered ring formation and targeted a cross-metathesis/

aza-Michael sequence. The feasibility of this approach was

uncertain as these reactions have proven highly reversible in

the literature and the strained nature of the bicyclic β-lactam

15,55

8 may prove problematic in that regard.

To explore the potential of this approach, we prepared the

functionalized model β-lactam 19 (see SI for details, Table 1).

We initially utilized substrate 19 for screening that bears an

ethyl side chain due to the straightforward nature of its

synthesis via an aldol reaction with propionaldehyde, but

subsequently conﬁrmed our successful reactions on the methyl

derivative required for 2. With 19 in hand, we began evaluating

56−59

conditions to promote the desired aza-Michael reaction

generate 20. Initially, a variety of Lewis acid and Bronsted

acids were explored with no reaction or complex mixtures

observed, respectively (entries 1−6, Table 1). Turning our

attention to basic conditions, a number of bases including

potassium tert-butoxide, LDA and sodium hydride did not

prove successful (entries 7−12, Table 1); however, upon

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

We hypothesized that the challenges could be due to the

presence of the ketone and potential retro-aza-Michael

reactions and/or other reaction pathways, and selected to

conduct ketone reduction. This rationale was conﬁrmed by

Table 2. Selected Reduction Attempts on Ketone 17

reducing the ketone on test scale with NaBH and then

subsequently opening the β-lactam with BF ·OEt , which

proceeded smoothly by TLC and gave us conﬁdence to delay

the lactam opening step to after stereoselective ketone

reduction.

results

24:25

entry

conditions

DIBAL (0.75equiv), THF, −78 °C to rt, 15 h

observed

24% with 1:1

To evaluate the conditions required for stereoselective

ketone reduction, bicyclic β-lactam 23 was prepared smoothly

LiBH (1.1 equiv), THF, −78 °C, 3 h

via a three step TBS deprotection, cross-metathesis, and aza-

Michael cyclization sequence (Scheme 2). It is worth noting

Li(t-BuO) Al−H (1.2 equiv), THF, −78 °C, 0.5 h 50% with 5:4

Zn(BH ) (1 equiv), THF, −3 °C, 12 h

77% with 1:3

30% 1:1 dr

Scheme 2. Preparation of Bicyclic β-Lactam 23

b,c

CBS (100 mol %), BMS (2 equiv) CH Cl , 0 °C,

min

CeCl (20 mol %), NaBH (2 equiv) MeOH, −25 50% 1:4 dr

C, 1 h

CeCl (20 mol %), NaBH (2 equiv) MeOH, −78 97% 1:6 dr

C, 5 min

L-selectride (1.2 equiv), THF, −78 °C, 30 min

K-selectride (1.2 equiv), THF, −78 °C, 30 min

45% 1:5 dr

60% with 1:1

K-selectride (1.2 equiv), Et O, −78 °C, 30 min

40% 4:3 dr

45% 3:2 dr

K-selectride (1.2 equiv), Et O, 25 °C, 3 min

K-selectride (1.03 equiv), KBF (1.2 equiv) Et O, 25 64% 5:4 dr

°C, 3 min

K-selectride (0.97 equiv), KBF (1.2 equiv) 0.01 M 59% 5:4 dr

that the aza-Michael cyclization proceeds smoothly in the

presence of the free hydroxyl group, whereas in our model, we

employed a TBS protecting group. With ample quantities of 23

in hand, an array of conditions was screened (Table 2). Most

reaction conditions proceeded to provide the opposite

diastereomer 25 required for batzelladine, with Luche

conditions providing 97% yield and 6:1 dr (entry 7, Table

Et O, 25 °C, 3 min

K-selectride (1.15 equiv), KBF (1.2 equiv) 0.01 M 68% 5:3 dr

toluene, 25 °C, 3 min

(PPh ) RhCl 10 mol %, H balloon EtOAc, 25 °C, NR

4 h

₆d

₇d

Pt/C cartridge, 120 bar H EtOAc, 55 °C, 1 h

Ru/C cartridge, 120 bar H EtOAc, 55 °C, 1 h

60% 1:1 dr

All reactions were performed at 0.1 M, unless otherwise noted.

Over reduced product, [M + H] = 326 dominated. Enantiopure

starting material was used. Reaction was performed in H-cube at

.05 M, 1 mL/min. Isolated yield.

). Fortunately, moving to bulky hydride reagents began to

invert this selectivity, and we ultimately identiﬁed K-selectride

in toluene as the optimal conditions yielding 24 in 68% yield

and a 3:5 dr favoring the desired diastereomer (entry 14, Table

). While this dr is modest, the reaction is highly scalable and

provides preparatively useful quantities of material that can be

separated and recycled if desired. Initial attempts at employing

chiral reagents to overcome this selectivity were unsuccessful

overall yield. Utilizing this endgame, (±)-13-epi-batzelladine D

(30) and 31 were prepared from 28 (Scheme 2) and (±)-15-

epi-batzelladine D (S1) was also prepared from the

corresponding alcohol epimer 14 (See SI for details). This

ﬁnal reaction cascade is plagued by the formation of the

elimination byproducts, and we have spent considerable eﬀort

attempting to optimize this process (see Table S9−S10 for full

reaction screening). Overall, it does not appear that a simple

E1CB reaction accounts for the outcomes observed, and we

have been unable to signiﬁcantly improve the product ratios to

date. Given the straightforward reaction conditions and our

ability to readily separate the product from the elimination

product, we have elected to utilize this optimized protocol to

access the enantioenriched natural products and their stereo-

chemical analogues.

Asymmetric Synthesis of (+)-Batzelladine D and

(−)-Batzelladine D. Having a concise synthesis of (±)-bat-

zelladine D (2) in hand, along with access to diastereomers of

the natural product, we sought to also explore the generation

of the enantiomeric series of compounds along with the

generation of gram-scale quantities of our key intermediates

(Scheme 4). To access the non-natural enantiomer, we chose

to start from commercially available β-lactam 32, already

bearing the necessary hydroxyethyl side chain and available on

(

entry 5, Table 2), but further studies are underway to explore

additional systems in this regard. Regardless, the optimized

conditions provide selective access to the unnatural diaster-

eomer and practically useful access to the natural stereoisomer.

Endgame and Synthesis of (± )-Batzelladine D. At this

stage, having access to all 4-alcohol diastereomers, we were

positioned to optimize the end game of our synthetic approach

and gain access to (±)-batzelladine D, (±)-13-epi-batzelladine

D, and (±)-15-epi-batzelladine D. As shown in Scheme 3,

opening of β-lactams 24 and 25 with side chain 26 proceed

smoothly upon activation with BF ·OEt to provide the target

dihydroxy pyrrolidines 27 and 28. Installation of the guanidine

by treatment with N,N-di-Boc-S-methylisothiourea and mer-

cury chloride was followed in the same pot by mesylation of

both alcohols, rapid displacement by the guanidine to install

the tricyclic core, and ﬁnal treatment with formic acid to cleave

both the core and side-chain Boc protecting groups. This one-

pot process installed the three critical bonds of the core in a

stereoselective fashion and provided a separable 1.0:1.2

mixture of (±)-batzelladine D (2) and 29, resulting from β-

mesylate elimination under the reaction conditions with 41%

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

Scheme 3. Completion of the Racemic Synthesis of 2 and 30

with sodium benzenesulﬁnate and subsequently displaced by

butenyl Grignard reagent 9 to generate 34 on 6-g scale

(

Scheme 3a). Cross-metathesis, aza-Michael addition, and TBS

deprotection proceed smoothly, as before, to provide

+)-bicyclic β-lactam 23 on gram-scale, set to undergo

(

diastereoselective reduction and conversion to (+)-batzelladine

D (2) and (+)-13-epi-batzelladine D (30) as previously

developed. The natural enantiomer required denovo synthesis,

and we relied on an Ellman auxiliary approach that readily

provided (−)-β-lactam 10 in three steps (Scheme 3b). This

material of the natural enantiomeric series was advanced to

(

−)-batzelladine D (2) and (−)-13-epi-batzelladine D (30)

through a 9-step sequence as previously described for racemic

β-lactam 10.

Initial Antimicrobial Evaluation. In addition to interest

in having access to a stereochemical library of batzelladine

analogues to study their reported activity as inhibitors of HIV

gp120-human CD4 binding (ongoing collaborative eﬀorts), we

sought to explore their antimicrobial activity against a series of

4−66

ESKAPE pathogens

(Table 3). In initial screening,

(

±)-batzelladine D (2) proved moderately active against

both methicillin sensitive S. aureus (MSSA) and methicillin

resistant S. aureus (MRSA), with an MIC of 8 μg/mL. This

represents the ﬁrst evaluation of batzelladine D’s antimicrobial

properties and gave us reason to evaluate our stereoisomer

library, elimination byproducts, and synthetic intermediates for

their antimicrobial activity against an expanded panel of

bacterial pathogens. Upon systematic evaluation of the

stereoisomers against MSSA and MRSA, it was revealed that

non-natural stereoisomers were more active than the racemic

natural product or natural enantiomeric series. This eﬀect can

be highlighted by comparison of non-natural (+)-13-epi-

batzelladine D (30) and natural (−)-13-epi-batzelladine D

(

30), wherein the unnatural enantiomer is somewhat more

large scale due to its use in antibiotic synthesis. 32 could be

readily converted to the requisite sulfone 33 through treatment

active (Table 3). The eﬀect of stereochemistry on antimicro-

bial activity suggests there may be distinct targets or pathways

Scheme 4. (a) Synthesis of (+)-Batzelladine D and Gram-Scale Access to β-Lactam 37 and (b) Synthesis of (−)-Batzelladine D

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

https://pubs.acs.org/10.1021/jacs.0c04091

Article

Table 3. Antimicrobial Evaluation MIC values (ug/mL) of Batzelladine D and Stereochemical Analogues

racemic batzelladine D. enantiopure batzelladine D (non-natural). enantiopure batzelladine D (natural). racemic 13-epi-batzelladine D.

enantiopure 13-epi-batzelladine D (non-natural). enantiopure 13-epi-batzelladine D (natural). racemic 15-epi-batzelladine D. racemic 13-epi-

elim byproduct. enantiopure 13-epi-elim byproduct (non-natural). enantiopure 13-epi-elim byproduct (natural). enantiopure elimination

byproduct (non-natural). enantiopure elimination byproduct (natural). racemic [4.6]-fused bicyclic β-lactam. racemic [4.5]-fused bicyclic β-

lactam. enantiopure [4.5]-fused bicyclic β-lactam. LZD = linezolid (control). CPL = Chloramphenicol (control). Colistin = CST (control).

involved in the observed activity and warrants additional

studies to enable a more in-depth evaluation. Further studies

on expanded pathogens will also allow determination as to the

signiﬁcance of these ﬁndings more broadly. In addition to the

natural products and their stereoisomers, elimination by-

products (±)-31 and (+)-31 also showed activity against

MSSA and MRSA, although these compounds may function by

an alternate mechanism given their reactive Michael acceptor.

Expanding the screening to Gram-negative pathogens revealed

promising activity, particularly for (+)-30 and (−)-2, with

MICs of 32−64 μg/mL. While these are not clinically relevant

potencies against these challenging pathogens, they provide a

starting point for further optimization.

Detailed experimental procedures, spectroscopic data,

and H and C NMR spectra (PDF )

Crystallographic data for rds852 (CIF)

Crystallographic data for rds677 (CIF)

AUTHOR INFORMATION

Corresponding Author

■

Joshua G. Pierce − Department of Chemistry, College of Sciences

and Comparative Medicine Institute, NC State University,

Raleigh, North Carolina 27695, United States; orcid.org/

000-0001-9194-3765 ; Email: jgpierce@ncsu.edu

Authors

You-Chen Lin − Department of Chemistry, College of Sciences

CONCLUSIONS

and Comparative Medicine Institute, NC State University,

Raleigh, North Carolina 27695, United States

Aubert Ribaucourt − Department of Chemistry, College of

Sciences, NC State University, Raleigh, North Carolina 27695,

United States

■

In conclusion, we have developed a platform for the synthesis

of a variety of stereochemical isomers of the batzelladine core

and have utilized this approach to prepare (+)- and (−)

batzelladine D and a panel of selectively prepared diastereo-

meric isomers. Our approach utilizes β-lactam building blocks

as key starting materials but more signiﬁcantly, takes advantage

of [4.5]-fused bicyclic β-lactams as key intermediates that

enable stereocontrol of the core and provide preactivation of

side chain coupling. This strategy eﬀectively relays the single

stereoisomer in the readily available β-lactam starting material

to control the four additional stereocenters in the natural

product. Access to batzelladine D (2) and stereoisomers

allowed for the evaluation of these scaﬀolds as antimicrobial

agents, revealing non-natural isomers with promising levels of

activity. Although it is likely such molecules have multiple

targets and mechanisms of action, further understanding of

these molecules in a variety of biological systems may open the

door to the identiﬁcation of new targets and pathways for small

Yasamin Moazami − Department of Chemistry, College of

Sciences, NC State University, Raleigh, North Carolina 27695,

United States

Complete contact information is available at:

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

We are grateful to the NIH (R01GM117570) for generous

support of this work and to NC State University for support of

our program. Mass spectrometry data, NMR data, and X-ray

data were obtained at the NC State Molecular, Education,

Technology and Research Innovation Center (METRIC).

Kaylib Robinson and Christina Martinez-Brokaw are acknowl-

edged for the training to conduct the antimicrobial assays. The

molecule targeting by these and other classes of molecules.

Future eﬀorts are focused on an expanded analogue library

to explore the SAR of this family, the synthesis of batzelladine

A for study and comparison of HIV gp120-human CD4

binding, and the further study of the scope and mechanism of

these molecules as antimicrobial agents. These investigations

will be reported in due course.

S. aureus strains in Table were provided by the Network on

Antimicrobial Resistance in Staphylococcus aureus (NARSA)

for distribution by BEI Resources, NIAID, NIH.

REFERENCES

■

ASSOCIATED CONTENT

(1) Sfecci, E.; Lacour, T.; Amade, P.; Mehiri, M. Polycyclic

Guanidine Alkaloids from Poecilosclerida Marine Sponges. Mar.

Drugs 2016, 14 (4), 77.

■

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/jacs.0c04091.

(2) El-Demerdash, A.; Tammam, M. A.; Atanasov, A. G.; Hooper, J.

N. A.; Al-Mourabit, A.; Kijjoa, A. Chemistry and Biological Activities

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

of the Marine Sponges of the Genera Mycale (Arenochalina), Biemna

Article

and Clathria. Mar. Drugs 2018, 16 (6), 214.

(+)-Batzelladine A and (−)-Batzelladine D, and Identification of

Their Target Protein. Chem. - Eur. J. 2005, 11 (23), 6878−6888.

(23) Ishiwata, T.; Hino, T.; Koshino, H.; Hashimoto, Y.; Nakata, T.;

Nagasawa, K. Total Synthesis of Batzelladine D. Org. Lett. 2002, 4

(17), 2921−2924.

(

3) El-Demerdash, A.; Atanasov, A. G.; Bishayee, A.; Abdel-Mogib,

M.; Hooper, J. N. A.; Al-Mourabit, A. Batzella, Crambe and

Monanchora: Highly Prolific Marine Sponge Genera Yielding

Compounds with Potential Applications for Cancer and Other

Therapeutic Areas. Nutrients 2018, 10 (1), 33.

(24) Arnold, M. A.; Day, K. A.; Duron, S. G.; Gin, D. Y. Total

Synthesis of (+)-Batzelladine A and (−)-Batzelladine D via [4 + 2]-

Annulation of Vinyl Carbodiimides with N -Alkyl Imines. J. Am.

Chem. Soc. 2006, 128 (40), 13255−13260.

(25) Evans, P. A.; Qin, J.; Robinson, J. E.; Bazin, B. Enantioselective

Total Synthesis of the Polycyclic Guanidine-Containing Marine

Alkaloid (−)-Batzelladine D. Angew. Chem., Int. Ed. 2007, 46 (39),

7417−7419.

(26) Olszewski, A.; Sato, K.; Aron, Z. D.; Cohen, F.; Harris, A.;

McDougall, B. R.; Robinson, W. E.; Overman, L. E.; Weiss, G. A.

Guanidine Alkaloid Analogs as Inhibitors of HIV-1 Nef Interactions

with P53, Actin, and P56lck. Proc. Natl. Acad. Sci. U. S. A. 2004, 101

(39), 14079−14084.

(27) Shi, Y.; Pierce, J. G. Synthesis of the 5,6-Dihydroxymorpholin-

3-One Fragment of Monanchocidin A. Org. Lett. 2015, 17 (4), 968−

971.

(28) Pitts, C. R.; Lectka, T. Chemical Synthesis of β -Lactams:

Asymmetric Catalysis and Other Recent Advances. Chem. Rev. 2014,

114 (16), 7930−7953.

(29) Brandi, A.; Cicchi, S.; Cordero, F. M. Novel Syntheses of

Azetidines and Azetidinones. Chem. Rev. 2008 , 108 (9), 3988 −4035.

(30) Hosseyni, S.; Jarrahpour, A. Recent Advances in β -Lactam

Synthesis. Org. Biomol. Chem. 2018, 16 (38), 6840−6852.

(31) Deketelaere, S.; Nguyen, T. V.; Stevens, C. V.; D ’hooghe, M.

Synthetic Approaches toward Monocyclic 3-Amino-β -Lactams.

ChemistryOpen 2017, 6 (3), 301−319.

(32) Rao, A. V. R.; Gurjar, M. K.; Vasudevan, J. An Enantiospecific

Synthesis of the Tricyclic Guanidine Segment of the Anti-HIV Marine

Alkaloid Batzelladine A. J. Chem. Soc., Chem. Commun. 1995, 0 (13),

1369.

(33) Galletti, P.; Quintavalla, A.; Ventrici, C.; Giannini, G.; Cabri,

W.; Penco, S.; Gallo, G.; Vincenti, S.; Giacomini, D. Azetidinones as

Zinc-Binding Groups to Design Selective HDAC8 Inhibitors.

ChemMedChem 2009, 4 (12), 1991−2001.

(34) Nonoshita, K.; Maruoka, K.; Yamamoto, H. Conjugate

Reduction of α ,β -Unsaturated Ketones with Amphiphilic Reaction

System. Bull. Chem. Soc. Jpn. 1988, 61 (6), 2241 −2243.

(35) Bouffard, F. A.; Christensen, B. G. Thienamycin Total

Synthesis: Stereocontrolled Introduction of the Hydroxyethyl Side

Chain. J. Org. Chem. 1981, 46 (11), 2208 −2212.

(36) Aoki, H.; Okuhara, M. Natural Beta-Lactam Antibiotics. Annu.

Rev. Microbiol. 1980, 34 (1), 159−181.

(37) Alcaide, B.; Almendros, P.; Aragoncillo, C. β -Lactams: Versatile

Building Blocks for the Stereoselective Synthesis of Non-β -Lactam

Products. Chem. Rev. 2007, 107 (11), 4437−4492.

(38) Toshimitsu, A.; Terao, K.; Uemura, S. Intramolecular

Amidoselenation of N-Alkenyl Amides: Formation of Nitrogen

Heterocycles. J. Org. Chem. 1986, 51 (10), 1724−1729.

(39) Huang, X.; Li, X.; Xie, X.; Harms, K.; Riedel, R.; Meggers, E.

Catalytic Asymmetric Synthesis of a Nitrogen Heterocycle through

Stereocontrolled Direct Photoreaction from Electronically Excited

State. Nat. Commun. 2017, 8 (1), 2245.

4) Liu, J.; Li, X.-W.; Guo, Y.-W. Recent Advances in the Isolation,

Synthesis and Biological Activity of Marine Guanidine Alkaloids. Mar.

Drugs 2017, 15 (10), 324.

5) Shi, Y.; Moazami, Y.; Pierce, J. G. Structure, Synthesis and

Biological Properties of the Pentacyclic Guanidinium Alkaloids.

Bioorg. Med. Chem. 2017, 25 (11), 2817−2824.

(

6) Kashman, Y.; Hirsh, S.; McConnell, O. J.; Ohtani, I.; Kusumi, T.;

Kakisawa, H. Ptilomycalin A: A Novel Polycyclic Guanidine Alkaloid

of Marine Origin. J. Am. Chem. Soc. 1989, 111 (24), 8925−8926.

(

7) Patil, A. D.; Kumar, N. V.; Kokke, W. C.; Bean, M. F.; Freyer, A.

J.; Brosse, C. D.; Mai, S.; Truneh, A.; Carte, B. Novel Alkaloids from

the Sponge Batzella Sp.: Inhibitors of HIV Gp120-Human CD4

Binding. J. Org. Chem. 1995, 60 (5), 1182−1188.

(

8) Laville, R.; Thomas, O. P.; Berrue, F.; Marquez, D.; Vacelet, J.;

Amade, P. Bioactive Guanidine Alkaloids from Two Caribbean

Marine Sponges. J. Nat. Prod. 2009, 72 (9), 1589−1594.

(

9) Patil, A. D.; Freyer, A. J.; Taylor, P. B.; Carte, B.; Zuber, G.;

Johnson, R. K.; Faulkner, D. J. Batzelladines F-I, Novel Alkaloids from

the Sponge Batzella Sp.: Inducers of P56 Lck -CD4 Dissociation. J.

Org. Chem. 1997, 62 (6), 1814−1819.

(

10) Hua, H.-M.; Peng, J.; Dunbar, D. C.; Schinazi, R. F.; Andrews,

A. G. de C.; Cuevas, C.; Garcia-Fernandez, L. F.; Kelly, M.; Hamann,

M. T. Batzelladine Alkaloids from the Caribbean Sponge Monanchora

Unguifera and the Significant Activities against HIV-1 and AIDS

Opportunistic Infectious Pathogens. Tetrahedron 2007, 63 (45),

1179−11188.

11) Gallimore, W. A.; Kelly, M.; Scheuer, P. J. Alkaloids from the

Sponge Monanchora u Nguifera. J. Nat. Prod. 2005, 68 (9), 1420−

423.

12) Black, G. P.; Murphy, P. J.; Walshe, N. D. A. A Short Synthetic

Route to the Tricyclic Guanidinium Core of the Batzelladine

Alkaloids. Tetrahedron 1998, 54 (32), 9481−9488.

(

13) Black, G. P.; Murphy, P. J.; Thornhill, A. J.; Walshe, N. D. A.;

Zanetti, C. Synthesis of the Left Hand Unit of Batzelladine F;

Revision of the Reported Relative Stereochemistry. Tetrahedron 1999,

5 (21), 6547−6554.

14) Snider, B. B.; Chen, J. Synthesis of Batzelladine E and Its E

Isomer. Tetrahedron Lett. 1998, 39 (32), 5697−5700.

15) Snider, B. B.; Chen, J.; Patil, A. D.; Freyer, A. J. Synthesis of the

Tricyclic Portions of Batzelladines A, B and D. Revision of the

Stereochemistry of Batzelladines A and D. Tetrahedron Lett. 1996, 37

(

39), 6977−6980.

16) Cohen, F.; Overman, L. E. Enantioselective Total Synthesis of

Batzelladine F and Definition of Its Structure. J. Am. Chem. Soc. 2006,

28 (8), 2604−2608.

17) Parr, B. T.; Economou, C.; Herzon, S. B. A Concise Synthesis

of (+)-Batzelladine B from Simple Pyrrole-Based Starting Materials.

Nature 2015, 525 (7570), 507.

18) Economou, C.; Romaire, J. P.; Scott, T. Z.; Parr, B. T.; Herzon,

S. B. A Convergent Approach to Batzelladine Alkaloids. Total

Syntheses of (+)-Batzelladine E, (−)-Dehydrobatzelladine C, and

(40) Hamed, R. B.; Henry, L.; Gomez-Castellanos, J. R.; Mecinovic,

J.; Ducho, C.; Sorensen, J. L.; Claridge, T. D. W.; Schofield, C. J.

+)-Batzelladine K. Tetrahedron 2018, 74 (26), 3188 −3197.

Crotonase Catalysis Enables Flexible Production of Functionalized

Prolines and Carbapenams. J. Am. Chem. Soc. 2012, 134 (1), 471−

479.

20) Overman, L. E.; Wolfe, J. P. Synthesis of Polycyclic Guanidines

19) Cohen, F.; Overman, L. E.; Sakata, S. K. L. Asymmetric Total

Synthesis of Batzelladine D. Org. Lett. 1999 , 1 (13), 2169 −2172.

K. A.; Lloyd, E. P.; Townsend, C. A. Definition of the Common and

by Cyclocondensation Reactions of N -Amidinyliminium Ions. J. Org.

(41) Bodner, M. J.; Li, R.; Phelan, R. M.; Freeman, M. F.; Moshos,

Divergent Steps in Carbapenem B-Lactam Antibiotic Biosynthesis.

ChemBioChem 2011, 12 (14), 2159−2165.

(42) Asada, S.; Kato, M.; Asai, K.; Ineyama, T.; Nishi, S.; Izawa, K.;

Shono, T. Enantioselective Synthesis of the Carbapenem Ring System

from (S)-Proline. J. Chem. Soc., Chem. Commun. 1989, 0 (8), 486.

Chem. 2001, 66 (9), 3167−3175.

21) Aron, Z. D.; Overman, L. E. The Tethered Biginelli

Condensation in Natural Product Synthesis. Chem. Commun. 2004,

(3), 253−265.

22) Shimokawa, J.; Ishiwata, T.; Shirai, K.; Koshino, H.; Tanatani,

A.; Nakata, T.; Hashimoto, Y.; Nagasawa, K. Total Synthesis of

(

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

44) Nasr El Dine, A.; Gree, D.; Roisnel, T.; Caytan, E.; Hachem, A.;

43) Pieczykolan, M.; Furman, B.; Chmielewski, M. Formal

Synthesis of Thienamycin. J. Antibiot. 2017, 70 (6), 781−787.

(64) Mulani, M. S.; Kamble, E. E.; Kumkar, S. N.; Tawre, M. S.;

Pardesi, K. R. Emerging Strategies to Combat ESKAPE Pathogens in

the Era of Antimicrobial Resistance: A Review. Front. Microbiol. 2019,

10, 539.

(

Gree, R. Synthesis of Fluorine-Containing Exoalkylidene B-Lactams.

Eur. J. Org. Chem. 2016, 2016 (3), 556 −561.

(65) Tommasi, R.; Brown, D. G.; Walkup, G. K.; Manchester, J. I.;

Miller, A. A. ESKAPEing the Labyrinth of Antibacterial Discovery.

Nat. Rev. Drug Discovery 2015, 14 (8), 529 −542.

45) Chemler, S. R.; Bovino, M. T. Catalytic Aminohalogenation of

Alkenes and Alkynes. ACS Catal. 2013 , 3 (6), 1076 −1091.

46) Faulkner, A.; Bower, J. F. Highly Efficient Narasaka-Heck

(66) Santajit, S.; Indrawattana, N. Mechanisms of Antimicrobial

Resistance in ESKAPE Pathogens. BioMed Res. Int. 2016, 2016,

2475067.

Cyclizations Mediated by P(3,5-(CF3)2C6H3)3: Facile Access to N-

Heterobicyclic Scaffolds. Angew. Chem., Int. Ed. 2012, 51 (7), 1675−

47) Race, N. J.; Hazelden, I. R.; Faulkner, A.; Bower, J. F. Recent

679.

(67) Bassetti, M.; Merelli, M.; Temperoni, C.; Astilean, A. New

Antibiotics for Bad Bugs: Where Are We? Ann. Clin. Microbiol.

Antimicrob. 2013, 12 (1), 22.

Developments in the Use of Aza-Heck Cyclizations for the Synthesis

of Chiral N-Heterocycles. Chem. Sci. 2017, 8 (8), 5248−5260.

48) Shuler, S. A.; Yin, G.; Krause, S. B.; Vesper, C. M.; Watson, D.

A. Synthesis of Secondary Unsaturated Lactams via an Aza-Heck

Reaction. J. Am. Chem. Soc. 2016, 138 (42), 13830 −13833.

49) Mai, D. N.; Wolfe, J. P. Asymmetric Palladium-Catalyzed

Carboamination Reactions for the Synthesis of Enantiomerically

Enriched 2-(Arylmethyl)- and 2-(Alkenylmethyl)Pyrrolidines. J. Am.

Chem. Soc. 2010, 132 (35), 12157−12159.

50) Babij, N. R.; Wolfe, J. P. Asymmetric Total Synthesis of

51) Bertrand, M. B.; Neukom, J. D.; Wolfe, J. P. Mild Conditions

+)-Merobatzelladine B. Angew. Chem., Int. Ed. 2012, 51 (17), 4128−

130.

for Pd-Catalyzed Carboamination of N -Protected Hex-4-Enylamines

and 1-, 3-, and 4-Substituted Pent-4-Enylamines. Scope, Limitations,

and Mechanism of Pyrrolidine Formation. J. Org. Chem. 2008, 73

(

22), 8851−8860.

52) Malkov, A. V.; Barłog

Farrugia, L. J.; Kocovsky,

(

, M.; Miller-Potucka, L.; Kabeshov, M. A.;

P. Stereoselective Palladium-Catalyzed

Functionalization of Homoallylic Alcohols: A Convenient Synthesis of

Di- and Trisubstituted Isoxazolidines and B-Amino-δ -Hydroxy Esters.

Chem. - Eur. J. 2012, 18 (22), 6873 −6884.

53) Kocovsky, P.; Backvall, J. The Syn/Anti-Dichotomy in the

Palladium-Catalyzed Addition of Nucleophiles to Alkenes. Chem. -

Eur. J. 2015, 21 (1), 36−56.

54) Manzoni, M. R.; Zabawa, T. P.; Kasi, D.; Chemler, S. R.

Palladium(II)-Catalyzed Intramolecular Aminobromination and Ami-

nochlorination of Olefins. Organometallics 2004, 23 (23), 5618 −5621.

55) Louwrier, S.; Tuynman, A.; Hiemstra, H. Synthesis of Bicyclic

Guanidines from Pyrrolidin-2-One. Tetrahedron 1996, 52 (7), 2629−

646.

56) Barrett, A. G. M.; Graboski, G. G.; Sabat, M.; Taylor, S. J. Beta-

Lactam Annulation Using (Phenylthio)Nitromethane. J. Org. Chem.

987, 52 (21), 4693−4702.

57) Fujimoto, K.; Iwano, Y.; Hirai, K. From Penicillin to Penem

and Carbapenem. IX. C 1 -Unit Introduction and the Carbapenam

Synthesis from the Penicillin Molecule. Bull. Chem. Soc. Jpn. 1986, 59,

58) Garud, D. R.; Ando, H.; Kawai, Y.; Ishihara, H.; Koketsu, M.

887−1896.

Synthesis of Novel Selenapenams, Selenacephems, and Selenazepines

Using a 2-(Trimethylsilyl)Ethyl Protection Approach. Org. Lett. 2007,

59) Garud, D. R.; Makimura, M.; Koketsu, M. Synthetic

(22), 4455−4458.

Approaches to Selenacephams and Selenacephems via a Cleavage of

Diselenide and Selenium Anion. New J. Chem. 2011, 35 (3), 581−

60) Chou, S.-S. P.; Huang, J.-L. Tandem Cross Metathesis and

86.

Intramolecular Aza-Michael Reaction to Synthesize Bicyclic Piper-

idines and Indolizidine 167E. Tetrahedron Lett. 2012, 53 (41), 5552−

61) Vedrenne, E.; Dupont, H.; Oualef, S.; Elkaïm, L.; Grimaud, L.

554.

Dramatic Effect of Boron-Based Lewis Acids in Cross-Metathesis

Reactions. Synlett 2005, No. 4, 670 −672.

62) Kim, K. S.; Qian, L. Improved Method for the Preparation of

Guanidines. Tetrahedron Lett. 1993, 34 (48), 7677−7680.

63) Compound was purchased from Ambeed, Inc. 200 g/$221,

CAS: 76855−69−1.

(