Microorganisms are able to generate a variety of sugars which, to our current knowledge are absent in vertebrate organisms. Among these, l-glycero-d-manno-heptose (l,d-heptose) has been identified as a major constituent of the lipopolysaccharide (LPS) of Gram-negative bacteria, an important mediator for numerous interactions with the native and adaptive immune system of the host (Holst, 2007; Kosma, 2008). The inner core region is based on l,d-heptoses and 3-deoxy-d-manno-2-octulosonic acid (Kdo) and is exhibited in a highly conserved manner in many enterobacterial strains (exemplified in Figure 1; Holst, 2007). In LPS-substructures O3 and O7 glycosylations are the most important with O4 phosphorylation being a widespread motif. However, O6 and O2 modifications (e.g., deoxy) have also been targeted for specific biological purposes in the past (Kosma, 2008; Tikad and Vincent, 2013).

In Nature, l,d-heptose substructures are produced from the 6-epimeric structure d-glycero-d-manno-heptose. Thus, structures containing both d-manno-heptoses have been targeted and have for example been studied in terms of binding to the cross-reactive antibacterial monoclonal antibody WN1 222-5 (Di Padova et al., 1993; Gomery et al., 2012), for their interactions with C-type lectins (Jaipuri et al., 2008; Wang et al., 2008; Šulák et al., 2011; Marchetti et al., 2012) and in respect to their roles as potential diagnostic tools for bacterial infections (Anish et al., 2013).

Historically, access to the parent l,d- and d,d-heptoses has required 6-8 step synthesis from commercially available shorter chain sugars, thus creating a substantial entry-burden to this increasingly important research area. In the light of increasing antibiotic resistance, the awareness of the importance of heptoside containing fragments has grown rapidly and as such synthetic efforts in the field have evolved from niche-existence into more mainstream focus, reflected by recent reports across a wider application base. However, these new approaches generally stuck with the principle strategy of long synthetic routes, establishing complex protecting group designs alongside the chain elongation steps toward the heptose backbone. These synthetic efforts, although chemically elegant, do not constitute a general solution to the lack of easy access to common heptose motifs being applicable for multiple purposes (Brimacombe and Kabir, 1986; Segerstedt et al., 2004; Ohara et al., 2010; Anish et al., 2013; Inuki et al., 2017).

We strongly believed that what was required was a fundamental alteration in mind-set and revolutionary change of approach to create a simplified entry to this important field. Ideally, the desired heptoses should be available directly off the shelf as are standard hexoses constituting mammalian glycans, for which efficient and partly automated approaches are already state of the art today. This would allow for an entirely different level of sophistication in terms of questions posed and streamlining the methods applied in answering them.

Toward our aspirations in this area we recently reported the first short and fully scalable synthetic approach to l,d-heptose peracetate based on indium mediated acyloxyallylation of l-lyxose and demonstrated it on a >100 mmol (45 g) scale (Stanetty and Baxendale, 2015). We are currently developing methodology for the analogous preparation of d,d-heptose peracetate (Aronow et al., 2019), with the intention of paving the way to make both d-manno-heptoses commercially available as their crystalline, bench-stable pyranose peracetate derivatives. Availability, be it commercial or at least through synthetically straightforward chemistry, is however only the first piece of a much larger puzzle (Scheme 1, bottom-left).

Thus, the focus of this publication is to underline the suitability of the peracetates 1 and 2 as starting points for different types of decorated heptose structures required for molecular probes or as donor/acceptors for oligoheptoside assembly. Despite tremendous advances in glycosylation techniques, it is still common to resort to a systematic trial and error search to come up with optimized conditions to install in high yield a specific glycosidic bond in a stereoselective fashion. Rapid change of decoration pattern can be a decisive element toward ultimate success in high yielding oligosaccharide formation.

We therefore believe that common starting materials for the late-stage decoration of the d,d- as well as the l,d-scaffolds are required and to this need we suggest the tetraisopropyl disiloxyl (TIPDS) protected manno-heptoses (3 and 4) as suitable examples of such compounds. We suggest diversification based on orthoester methodology and a recently introduced regioselective TIPDS-cleavage.

In cis-trans-triols, the cis-diol can be selectively addressed to form orthoesters which upon acidic hydrolysis yield the axial esters in high selectivity (King and Allbutt, 1970). In the presence of the intermediate orthoester several protecting groups can be installed under basic conditions and after orthoester cleavage another group can be installed at the equatorial hydroxyl group. Such an approach was recently demonstrated comprehensively on mannosides (Chang et al., 2013), and has also been used in target oriented syntheses of heptosides (Stanetty et al., 2014; Walter et al., 2017). The regioselective TIPDS cleavage on the other hand was originally introduced by the Ziegler group (Ziegler and Eckhardt, 1992; Ziegler et al., 1994) on standard hexoses and was successfully transferred to the exocyclic diol of the manno-heptose scaffold within a case study with an exceptionally bulky O4 group in place (see Scheme 1, top-right) and demonstrated there at preparative scale (Stanetty et al., 2014).

Within this current work, we set out to demonstrate the general value of unifying these two methods, exemplifying the orthoester methodology by permutating the labile chloroacetate group over the heptose core, and showcasing that the regioselective partial cleavage of the TIPDS at the exocyclic diol is applicable with other smaller 4O-substitutents as well as for both the l,d- and d,d-heptose scaffold (see Scheme 1, bottom-right).

We are convinced that the field of preparative manno-heptose chemistry would benefit greatly from a shift of paradigm toward common parent starting materials (e.g., peracetates) and differentiation thereof. In this light, we present short and scalable protocols to both (l,d, d,d) crystalline STol-heptosides and promote the utilization of the TIPDS-group as a protection for their exocyclic diols. This is due to its attractive partial regioselective cleavage which was proven to be a generally applicable solution for the differentiation between O6/O7. Additionally, we report an efficient 4-steps/1-purification protocol based on orthoester methodology to achieve the prior differentiation of the sugar core triol. The unforeseen stability of the ortho chloroacetates in this context was exploited by showing its applicability as stable protecting group during the TIPDS cleavage. We hope that the presented approach will pave the way towards a more unified starting point into biologically relevant structures based on manno-heptoses.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

CSu and CSt have been preparing and analyzing the materials. MM and IB have contributed with advice. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.