How many stereoisomers does 1,2,3-trimethylcyclopropane have? Treat the ring as planar.

Answer: A

Each methyl can be up or down, giving 8 labelled combinations. The ring has a

C_{3}

axis and mirror planes, so all-cis (UUU = DDD) is one isomer and the others (UUD, UDU, DUU and their inverses) are all the same isomer. Both isomers have a mirror plane, so both are meso (achiral) and the total is 2.

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