Ultra-High-Resolution NMR and Total Synthesis

: Uncovering the Stereochemistry of the Caylobolides

Abstract

Caylobolide A is a 36 membered polyhydroxylated macrolide originally isolated from Bahamian
cyanobacterium Lyngyba Majuscula from Cay Lobos, Bahamas by Molinski et al., and caylobolide B is
a structural analogue isolated from a Phormidium spp. collection from Key West, Florida by Luesch et
al.
1-3 The key challenge with Caylobolide - and other structurally related macrolides - are their
structural and stereochemical assignments, arising from their distal 1,5-polyol moieties possessing
near-degenerate frequencies in both 1H and 13C NMR.4
1,5-polyhydroxylated macrolides are of particular interest to due to their broad and potent biological
activities, as well as the longstanding difficulties in their stereochemical assignment. Previous methods
of stereochemical analysis of distal hydroxylated stereocentres includes esterification with chiral acids
(Mosher ester analysis)5
, NMR analysis in chiral solvents6 and liposomal circular dichroism.7, 8 Problems
with the latter two methods arose where more than two hydroxylated stereocentres were present, as
well as particular difficulties in NMR assignment arising from degeneracy in their signals. Access to
high-field cryogenic NMR spectrometers, in conjunction with modern pulse sequences, enables the
acquisition of ultra-high-resolution NMR spectra from sub milligram quantities of natural product
samples thus providing new insights into the structures of polyhydroxylated macrolides - previously
overlooked due to their stereochemical complexity.
The synergistic approach of NMR and synthetic methods for the assignment of stereochemistry was
previously applied to natural product Baulamycin.9 This approach involved the synthesis of a known
diastereomeric mixture of natural product fragments for stereochemical analysis by NMR. To
determine the configurations at C35 and C36 of caylobolide A, an analogous strategy was proposed:
the mixture-based total synthesis of the four remaining potential isomers.
Once the stereochemistry of caylobolide A was established, the absolute configuration of caylobolide
B was inferred by structural analogy. Because the two compounds share an identical C9-C40 skeletal
framework, their absolute configurations in this region were predicted to be the same.
To achieve the stereocontrol required, an iterative sequence to install the stereogenic centres of
fragments 1-4 of caylobolides A and B was employed, utilising α-sulfinyl and α-stannyl benzoates and
as carbenoid precursors for sequential homologation reactions.10-12 The 1,5-polyol units were
constructed by repeated homologation of a primary boronic ester, followed by hydroboration, with
subsequent stereospecific oxidation to the corresponding polyols, as was demonstrated in the
syntheses of bastimolide B and bahamaolide.13, 14 1,3-related stereocentres were constructed through
enantioselective Morken diboration, and hydroboration.15, 16 This work discusses the complete skeletal
3
and stereochemical elucidation of caylobolide A and caylobolide B, and their total syntheses,
performed in 17 steps (LLS) and 13 steps (LLS) respectively.

Date of Award	30 Sept 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Varinder K Aggarwal (Supervisor) & Craig P Butts (Supervisor)