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Number the OH-bearing carbon C1 and the adjacent brominated carbon C2. In the trans bromohydrin, base generates an alkoxide. A chair conformation can place O at C1 and Br at C2 trans-diaxial, aligning the oxygen nucleophile for backside attack at C2. Intramolecular SN2 closure expels bromide and inverts C2: the retained C1–O bond and the newly formed C2–O bond are both on the original OH face, giving the epoxide shown. ring inversion can provide the reactive chair even if it is not the dominant ground-state conformer.

The cis bromohydrin cannot place OH and Br trans-diaxial: one is axial when the other is equatorial. In the chair with Br axial, the hydrogen on C1 can instead be antiperiplanar to C2–Br. An alkoxide-assisted semipinacol pathway moves the C1–H bonding pair to C2 while the oxygen lone pair forms C1=O and bromide departs. hydride migration gives the ketone without changing the carbon skeleton. The hydrogen is transferred to C2 rather than expelled as a free hydride. The original phenyl-bearing stereocentre is not involved in either bond-reorganization sequence. Configuration controls which properly aligned displacement or migration can occur; a flat ring drawing alone conceals that distinction.

Direct iodide attack at the terminal CH2–OMs would replace the leaving group without disturbing the adjacent stereocentre, so it cannot by itself explain racemization. sulfur participation produces an intermediate with two equivalent methylene paths. Label the original S-adjacent methylene A, the stereogenic carbon C, and its CH2–OMs substituent B. The thioether sulfur attacks B intramolecularly and expels mesylate, forming a four-membered thietanium circuit S–A–C–B–S. The rest of the original six-membered ring remains as the other path between S and C.

In this bridged sulfonium ion, A and B are both CH2 groups joining the same S and C centres. Exchanging them no longer changes the labelled molecular connectivity: the distinction that made C stereogenic has been removed. iodide can open the strained intermediate by backside attack at either A or B. Opening one S–CH2 bond gives one configuration of the six-membered product, while opening its equivalent partner exchanges which methylene is in the ring and which is the iodomethyl substituent, giving the opposite configuration.

The carbon iodinated in each opening is a methylene; racemization appears at the adjacent carbon through exchange of equivalent paths. It is not inversion at a CH2 stereocentre, nor does the mesylate carbon have to form a primary free carbocation. An achiral intermediate in an achiral medium gives equal mirror-related outcomes. nitrogen participation alone in a simple terminal displacement would not provide this specific equivalent-path explanation.

With triethylamine present, proton transfer from thiophenol produces a more nucleophilic thiolate population. sulfur attacks the anomeric epoxide carbon C1 from the face opposite its epoxide C–O bond. Displacement by backside attack opens the epoxide, breaks C1–O, and leaves the former epoxide oxygen attached to C2. protonation of that alkoxide by thiophenol or triethylammonium gives G: SPh at C1 on the front face and OH at C2 on the back face, a trans relationship. This is inversion at C1 with retention of the C2 C–O bond; there is no need to invert every stereocentre. Rapid protonation also suppresses competing silyl transfer by the alkoxide.

Without added base, neutral thiophenol is less nucleophilic. acid-assisted cleavage at the anomeric epoxide bond can instead be accompanied by donation from the ring oxygen, giving an oxocarbenium ion with charge shared with the ring oxygen. thiophenol capture from the face that puts SPh back on the original epoxide face, followed by deprotonation, gives H with SPh and C2–OH both on the back face, the net-retention pathway. An ion-pair or highly asynchronous opening can provide this outcome; the drawing does not establish a long-lived free carbocation.

The distinction is configuration, not a mandatory fixed chair. For the closely matching trisilylated substrate, the primary study obtained both anomers with thiophenol alone, and established an unusual inverted chair for the alpha thioglycoside by NMR and crystallography. Thus H should be explained as an accessible product, not asserted to be the sole product or necessarily to have axial SPh in its preferred conformation.^[ref]

A three-membered ring containing a positively charged sulfur atom and two carbon atoms. Nucleophilic opening at carbon normally involves backside attack and cleavage of one sulfur–carbon bond.