It's the addition of Lewis acids that does it! Reaction (a) proceeds with diastereoselectivities ≥ 100:1 if the radical transfer takes place in the presence of Lewis acids such as MgI2, MgBr2, or Sc(OTf)3. The influence of the substituents at position 4 of the oxazolidinone ring on the allylation was also studied.
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Sibi et al. (1996) studied this question.