= Solution
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 \b[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 (chemistry)>, neutral <thiophenol> is less nucleophilic. <acid (chemistry)>-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 \b[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 (chemistry)>, 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 (chemistry)>, not asserted to be the sole <product (chemistry)> or necessarily to have axial SPh in its preferred <conformation>.<Reaction of thiophenol with glucal epoxides: X-ray structure of 3,4,6-tri-O-tert-butyldimethylsilyl-1-S-phenyl-1-thio-α-D-glucopyranoside by Caroline Walford and others>[1997, Schemes 1–2 and Figure 1; neutral reaction mixture and configuration/conformation assignment]{ref}
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