METATHESIS-BASED SYNTHESIS OF COMPLEX BIOACTIVES - Modern Tools for the Synthesis of Complex Bioactive Molecules - page 165

OAc OAc

AcO AcO

1.

O
AcO

O
AcO

AcO AcO

FmocHN CO 2 Bn

OAc OAc

OAc OAc

O

63

O

HG-I (20 mol%)
CH 2 Cl 2 , 38°C, 48 h

O
OAc

O
OAc

Gb 3 -MUC5AC cluster
KLH conjugate

O O O

OAc

O O

AcO O AcO

AcO AcO

OAc

2. Pt/C, H 2 , EtOAc
66% (2 steps)

62 Gb 3 glycosylamino acid ( 64 )

CO 2 H

NHFmoc

SCHEME 5.19 Alkene cross-metathesis in the synthesis of carbohydrate peptide-based
vaccines by Danishefsky and coworkers.

formed double bond, Gb 3 glycosylamino acid in 66% yield over two steps. This

carbohydrate-based antigenwas later incorporatedwithMUC5ACpeptide epitope, and
the Gb 3 -MUC5AC thioester cassette served as a building block in the construction of
KLH conjugate vaccine [40]. Furthermore, this work illustrated the preparation of
“biologics” by chemical means, showcasing the state-of-the-art technology in the total
synthesis of complex molecular systems (Scheme 5.19).
In 2008, Davis and coworkers put this transformation to the ultimate test in
terms of substrate tolerance and effectiveness in aqueous reactionmedia in the context
of bioconjugation. The phosphine-free Hoveyda-Grubbs second generation catalyst
was selected as themetal carbene initiator owing to its compatibility with the disulfide
bonds present in the biological system. Furthermore, S -allylcystein (Sac) was
identified as the optimal substrate in aqueous alkene cross-metathesis, where the
coordination of the sulfur atom to the ruthenium center was believed to facilitate
the reaction by bringing the reacting partners into close proximity. This finding was
surprising in view of the well-documented detrimental effect of thioethers on
ruthenium-catalyzed metathesis reactions. Most important, the use of MgCl 2 was
crucial to the success of the reaction, disrupting the formation of any “nonproductive”
metalloprotein species derived from the chelation of the protein substrate to the
ruthenium catalyst. Under these optimized reaction conditions, bioconjugation of
serine protease SBL-156Sac

64

took place smoothlywith allyl alcohol and a variety of
allyl ether substrates, inclusive of glycosylation and PEGylation. This work opens up
new avenues and represents a significant advance in posttranslational modifications of
proteins through carbon-carbon bond formation (Scheme 5.20) [41].

65

HG-II (2 mM)

30% t -BuOH, pH 8.0
2-5 h, rt to 37°C

S R S

Privileged tag for
cross-metathesis

, MgCl 2 (80-160 mM)

R

50-90% conversion

66

65

SBL-156Sac

HO

(0.01 mM)

HO

OH

O

HO

O

HO

MeO O

=

R

O

2

HO
HO

O

O

HO O

HO O

OH

3

SCHEME 5.20 Alkene cross-metathesis in the posttranslational modification of proteins by
Davis and coworkers.