Revisiting synthetic preparation of the quorum sensing substrate S-d-ribosyl-l-homocysteine (SRH)

• We synthesize S-ribosylhomocystine (SRH) at convenient scale.
• Thioether generation by substitution is influenced by both base and electrophile.
• Single-step global deprotection efficiently accesses the final product.
• A straightforward purification process isolates SRH·TFA at high purity.
• The final SRH·TFA product serves as substrate in the LuxS bioassay.

Cleavage of the thioether bond of S-d-ribosyl-l-homocysteine (SRH) by the enzyme S-ribosylhomocysteinase (LuxS) serves as the final biosynthetic step in the generation of the quorum sensing autoinducer AI-2 by bacteria. Herein, a revised chemical synthesis of SRH is presented at convenient scale and purity for in vitro studies of LuxS. Potassium bis(trimethylsilyl)amide (KHMDS) is identified as a judicious base for the formation of the thioether of the target compound from readily-accessible precursors: a thiol nucleophile derived from l-homocystine and a sulfonate-activated d-ribosyl electrophile. The exclusive use of acid-labile protecting groups allows for facile deprotection to the final product, producing the TFA salt of SRH in five synthetic steps and 26% overall yield. The chemically-synthesized material is isolated at high purity and demonstrated to serve as the LuxS substrate by an in vitro assay.

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