Building blocks from monosaccharides for synthesis of scaffolds, including Application of allylic azide rearrangement, azide-alkyne cycloaddition and ring closing metathesis

Synthesis of compounds with characteristics of natural products are required to increase the diversity and biological relevance of compounds for screening. These include new frameworks/scaffolds, with multiple stereogenic centres and various functional groups. Carbohydrates are renewable and are readily available with stereochemical diversity and functionality. Herein, building blocks derived from monosaccharides with alkene, alkyne and organic azide functional groups are used. The build-couple-pair strategy of diversity oriented synthesis was employed taking advantage of RuAAC, CuAAC and thermally promoted azide-alkyne cycloadditions, allylic azide (Winstein) rearrangement and ring closing metathesis, leading to polyhydroxylated small, medium and macrocyclic ring containing scaffolds. There is potential to graft appendages (e.g. pharmacophoric groups) to further increase diversity of compounds available for screening, or to consider the scaffolds in ligand design.


INTRODUCTION
Drug discovery and chemical biology rely on the identification of at least one hit compound for a target at an early stage. 1 Strategies for hit identification include structure based design, ligand design 2 or screening of chemical libraries, 3 with both synthetic and natural product research being sources of compounds, including providing inspiration for scaffold selection. In an analysis of over 24 million known compounds, Lipkus and coworkers showed that, as of 2008, as few as 143 scaffolds or frameworks accounted for ~50% of all known compounds, with frameworks tending to occur more frequently once reliable and relatively inexpensive synthesis had been established. 4 As well, many synthetic compounds reported have different properties to those of drugs or natural products and may not be as biologically relevant. 5 The most recent Newman and Cragg analysis 6 has shown that in the area of anti-cancer drug discovery from 1981-2014 that 49% of the approved drugs are natural products or directly derived from them.
Natural products are believed to be more successful in drug discovery, partly because they have 'privileged scaffolds'. 7 Many natural products are structurally complex (poly)cyclic rigid molecules, including macrocycles, 8 containing a variety of functional groups and multiple stereogenic centres as well as heteroatoms, such as oxygen or nitrogen that have evolved to have the capability to interact with protein targets. 9 As a consequence of such observations, there have been proposals, including from industry, for chemists to produce synthetically tractable 'natural product like compounds' as a basis to increase the diversity and biological relevance of chemical libraries. 10 These observations have inspired areas such as 'diverted total synthesis', 11 'diversity oriented synthesis' and 'biology oriented synthesis', 12,13,14 all with a goal to improve the quality and quantity of hits for chemical biology and drug discovery. 15,16

Synthesis of building blocks (build)
The use of building blocks containing at least two functional groups, drawn from that of alkene, alkyne and organic azide, was envisaged for the generation of frameworks.
Compound 1, derived from D-glucuronolactone, 28 was converted to 2, containing the alkene and azide groups in addition to protected hydroxyl groups. Thus, reaction of 1 with p-methoxybenzyl chloride under basic conditions, then regioselective acetonide hydrolysis followed by oxidative cleavage of the resulting diol to an aldehyde and subsequent Wittig reaction gave 2 (Scheme 1). Intermediate 3 has been used in the synthesis of peptidomimetics based on macrocycles with embedded carbohydrates (MECs). 29 Here, propargylation of the free hydroxyl group of 3 gave building block 4 with alkene and alkyne groups. Also 3 was used to give 5 and 6 (Scheme 1), with these similarly containing alkene and alkyne groups. Aside from the napththylmethyl ether being a protecting group, 30,31 which can be removed in the presence of other benzyl like protecting groups, it is also pharmacophoric. 32

Scheme 1: Synthesis of building blocks 2, 4 and 6
Intermediate 7, derived from methyl -D-mannopyranoside was used previously in the synthesis of iminosugars with quaternary centres. 33 Here, a propargyl group was grafted to the 2-oxygen atom of 7 giving 8, a building block with allylic azide, alkene and alkyne groups. A regioisomer of 8 was also generated from 9, an intermediate used for the synthesis of 8. 33 Thus, the TES and acetonide groups were first removed from 9 using TBAF-THF in aqueous acetic acid to give a triol. Then reaction of this triol with 2,2dimethoxypropane in the presence of p-toluenesulfonic acid followed by propargylation, reduction of the ester and finally azidation gave 10 (Scheme 2).

Use of the saccharide building blocks in scaffold synthesis (couple and pair)
The joining together of two building blocks (couple) with azide and alkyne groups was envisaged by using either the ruthenium catalysed azide-alkyne cycloaddition (RuAAC) 34 or copper catalysed azide-alkyne cycloaddition (CuAAC) 35 to produce triazoles, to be followed by RCM 36 to effect cyclisation (pair). By having allylic azide functionality in both 8 and 10, the possibility for incorporating Winstein rearrangement 37  The thermally promoted reaction of 10 gave a mixture of dienes 28 and 29, which have quaternary centers adjacent to a triazole nitrogen atom. Either intramolecular azidealkene or azide-alkyne cycloaddition were possible from reaction of 10a, formed via allylic azide rearrangement. However, triazole formation via alkyne-azide cycloaddition were clearly preferred. Ring closing metathesis from the major product 29 was investigated, and while occurring in low yield (not optimised), it did give rise to a product

SUMMARY
Expanding the number and diversity of scaffolds or frameworks is desired in order to provide a basis for synthesis of new compounds for chemical biology and drug discovery. 44 We have demonstrated herein, the synthesis of a set of (poly)cyclic functionalised scaffolds via intermediates derived from carbohydrates. 45 The reactions chosen to generate the scaffolds, such as allylic azide rearrangement used in tandem with various types of cycloaddition, or CuAAC/RuAAC followed by metathesis, have led to small to medium to macrocyclic ring containing products. The frameworks generated incorporate features of natural products (rigidity, complexity, chirality, scaffold diversity, increased aliphatic content, stereogenic centres, increased oxygen content).
They are appendable, meaning pharmacophoric groups can be attached via the hydroxyl groups. 46 This is facilitated by regioselectively protected carbohydrate precursors, which will allow further chemical modification of the framework in due course. Although macrocycles, like many natural products, do not often fit within the Lipinski 'rule of 5' for drug-likeness 47

SUPPORTING INFORMATION
NMR Spectra (word document) X-ray crystal structure (ORTEP, CIF) files for compound 26.

AUTHOR CONTRIBUTIONS
a These authors made equal contributions. PMcA solved the X-ray crystal structure. PVM planned the work with the other authors and wrote the paper.