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Spiro lactones from LiBH4 and BH3 reductions of cyclobutane half-esters: identical, enantiomers or diastereomers

Two cyclobutane half-esters, each with an ester arm (wedge) and an acid arm (hash) on the same carbon. Press start and the bench does the rest: LiBH4 reduces the ester arm, BH3 the acid arm, and H+ closes each hydroxy compound to a spiro delta-lactone - the ring closes in opposite directions. The two products are then laid on top of each other by a least-squares fit, the mirror image is tried, and each pair is named identical, enantiomers or diastereomers before the answer is revealed. No prediction stage - input, run, watch, result.

Sub-topics covered

• Chemoselective reduction: LiBH4 reduces esters, BH3 reduces carboxylic acids
• Acid-catalysed lactonisation of hydroxy acids and hydroxy esters (delta-lactones)
• Spiro compounds and stereocentres created by ring closure
• Identical, enantiomers and diastereomers: superimposition and mirror tests
• Cis/trans isomerism in 1,3-disubstituted cyclobutanes; pseudo-asymmetric centres
• Chirality and mirror planes

What it teaches

• LiBH4 reduces an ester to a primary alcohol but not a carboxylic acid (given); BH3 does the opposite - it reduces the acid quickly and the ester only slowly - so the two routes reduce different arms
• Each product has CH2CH2OH and CH2C(=O)X on the same ring carbon; O-C-C-C1-C-C(=O) is a six-membered ring, so H+ closes a spiro delta-lactone (losing H2O from the hydroxy acid, EtOH from the hydroxy ester)
• The two reagents close the lactone in opposite directions: with LiBH4 the ring O comes from the wedge arm and the C=O from the hash arm; with BH3 the reverse
• In the lactone the spiro carbon carries two different ring branches (-CH2-C(=O)- and -CH2-CH2-O-), so swapping their faces inverts it
• Top pair: the spiro carbon and the adjacent CH3 carbon are both stereocentres; only the spiro carbon is inverted, so P and Q are diastereomers (both chiral, not mirror images)
• Bottom pair: with the CH3 across the ring a mirror plane runs through C1, C3 and the lactone, so R and S are achiral cis/trans (pseudo-asymmetric r/s) diastereomers
• Two structures are identical only if a rotation superimposes them; enantiomers if the mirror image superimposes; otherwise stereoisomers of the same constitution are diastereomers
• Without the CH3 the spiro carbon is not a stereocentre and both routes give the same lactone

Simulation ID

• ADV-2026-P1-CHE-Q04 · revision 1

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